Polymeric Solar-Thermal Flat-Plate Collectors - Dora.dmu.ac.uk

Polymeric Solar-Thermal Flat-Plate Collectors - Dora.dmu.ac.uk
Polymeric Solar-Thermal
Flat-Plate Collectors
Christoph Nikolaus Reiter
A thesis submitted in partial fulfilment
of the requirements of De Montfort University
for the degree of Doctor of Philosophy (PhD)
November 2014
Institute of Energy and Sustainable Development
De Montfort University Leicester
Institute of new Energy Systems
Technische Hochschule Ingolstadt
Declaration
Declaration
I declare that the content of this submission is my own work. The contents of the
work have not been submitted for any other academic or professional award. I
acknowledge that this thesis is submitted according to the conditions laid down
in the regulations. Furthermore, I declare that the work was carried out as part of
the course for which I was registered at De Montfort University, United Kingdom
from January 2010 until June 2014. I draw attention to any relevant considerations of rights of third parties.
III
Abstract
Abstract
State-of-the-art solar-thermal flat-plate collectors suffer from a limited potential to
decrease production costs for the necessary higher economic benefit of solarthermal systems. Costly metallic materials and corresponding manufacturing processes prevent further cost reductions. For that issues, plastic materials can offer
a promising approach.
The main hurdle for the use of cost-effective plastics lies in the high thermal loads
on the collector components — absorber and insulation — which were identified
in a field-testing. The necessary overheating protection approaches to lower
these thermal loads were investigated in a literature review. A large number of
relevant concepts was evaluated related to achievable temperature reduction,
influence on solar yield, additional costs and intrinsic safety. Therefore, a mathematical model was developed to determine the solar-thermal collector´s behaviour in a solar-thermal system for hot water and space heating. This way, the
most promising overheating concepts were simulated and analysed with regard
to component temperatures and system performance. Omitting the selective absorber coating and reducing the backside insulation was found to be the most
suitable solution for component materials with limited temperature resistance like
polypropylene.
In the second part of the research, collector design concepts were developed on
the basis of the characteristics of plastic material processing. The identified unit
costs showed savings of more than 50 % in comparison to state-of-the-art collectors. The analysis regarding temperature loads and annual solar yield by simulation proved the performance of the concepts. The collector costs and the simulation results were used to define the total costs of the solar-thermal systems and
to evaluate the economic benefits by means of the collector concepts.
The benefits were similar to state-of-the-art set-ups. Thus, further adjustments at
system level are necessary to lower the total costs. Therefore, the system set-up
has to be harmonised with the collector requirements and investigated in detail.
V
Acknowledgements
Acknowledgements
This research was carried out at the Institute of Energy and Sustainable Development at De Montfort University Leicester in cooperation with the Institute of
new Energy Systems at Technische Hochschule Ingolstadt.
I want to express my gratitude to Prof Vic Hanby, Dr Simon Rees and Prof Wilfried
Zörner for having initiated this research work, for their excellent support and guidance as well as for the revision of this thesis.
I also wish to thank Carina Dittenhauser for the linguistic revision of this thesis.
Moreover, I am very grateful to the colleagues who accompanied me during the
years at the Institute of new Energy Systems, especially to Sebastian Brandmayr,
Holger Müller, Tobias Bader, Hermann Riess and Dr Christoph Trinkl.
Finally, I would like to thank my mother and my grandmother for their unequivocal
support and their encouragement throughout the work. I would also like to thank
my deceased father for his support and guidance throughout my childhood, which
was the essential basis for my future work and career.
VII
Table of Contents
Table of Contents
Declaration ......................................................................................................... III Abstract .............................................................................................................. V Acknowledgements .......................................................................................... VII Table of Contents.............................................................................................. IX List of Figures ................................................................................................. XIII List of Tables................................................................................................... XXI Abbreviations ................................................................................................. XXV Symbols ....................................................................................................... XXVII Subscripts ..................................................................................................... XXXI 1 Introduction ..................................................................................................... 1 1.1 Materials and Production Costs of Flat-Plate Solar-Thermal Collectors ... 1 1.2 Overview of Polymeric Materials in Solar-Thermal Collectors .................. 6 1.2.1 Absorber ........................................................................................... 6 1.2.2 Housing ............................................................................................ 8 1.2.3 Glazing ........................................................................................... 10 1.2.4 Fully Polymeric Collector ................................................................ 11 1.3 Overview of Research Activities of Polymeric Materials in Solar-Thermal
Collectors ...................................................................................................... 12 1.3.1 Collectors and Parts ....................................................................... 13 1.3.2 Materials ......................................................................................... 14 1.3.3 Overheating Protection ................................................................... 15 1.4 Formulation of the Research Topics ....................................................... 16 2 Collector Loads ............................................................................................. 17 2.1 Field-Testing System and Measurement Instrumentation ....................... 17 2.2 Thermal Collector Loads ......................................................................... 24 2.2.1 Temperature Profiles of Collector Parts ......................................... 25 2.2.2 Annual Temperature Load Profiles ................................................. 30 2.3 Pressure Loads in the System ................................................................ 37 3 Overheating Protection ................................................................................. 41 3.1 Functional Analysis of Collector and System .......................................... 41 IX
Table of Contents
3.2 Overview on Approaches for Overheating Protection ............................. 46 3.3 Changing the Transmission Properties ................................................... 48 3.3.1 Thermally Switchable Layers ......................................................... 48 3.3.2 Electrically Switchable Layers ........................................................ 53 3.3.3 Chemically Switchable Layers ........................................................ 55 3.3.4 Translucent Appliances .................................................................. 56 3.3.5 Mechanical Appliances................................................................... 61 3.3.6 Hydraulic Appliances ...................................................................... 64 3.4 Changing the Absorption Properties ....................................................... 67 3.4.1 Changing the Absorption Coefficient .............................................. 67 3.4.2 Changing the Absorbing Area ........................................................ 70 3.5 Changing the Convective Losses ........................................................... 71 3.5.1 Natural Convection in the Air Gap .................................................. 71 3.5.2 Forced Convection in the Air Gap .................................................. 77 3.5.3 Convection Combined with Evaporative Cooling ............................ 77 3.5.4 Varying the Insulation Properties ................................................... 78 3.6 Changing the Radiation Losses .............................................................. 80 3.6.1 Thermochromic Layers with Switchable Emission Coefficient ........ 80 3.6.2 Contact between Glazing and Absorber ......................................... 81 3.7 Active Cooling ......................................................................................... 83 3.7.1 Cooling Measures in the Collector ................................................. 83 3.7.2 Cooling Measures in the Solar Circuit ............................................ 85 3.7.3 Cooling Measures in the Heating System ...................................... 92 3.8 Adjustment of the Collector Efficiency .................................................... 93 3.8.1 Increase of the Emission Coefficient of the Absorber ..................... 95 3.8.2 Decrease of the Insulation Properties of the Collector Backside .... 96 3.9 Buffering Critical Collector States ........................................................... 97 3.9.1 Integrated Collector Storage .......................................................... 98 3.9.2 Slurry Phase Change Materials as Heat Carrier ............................ 99 3.10 Preselection of Overheating Protection Approaches .................... 100 4 Collector and System Modelling ................................................................. 107 4.1 State-of-the-Art Flat-Plate Collector Modelling ..................................... 108 X
Table of Contents
4.1.1 Description of the Collector .......................................................... 110 4.1.2 Simplifying Assumptions .............................................................. 114 4.1.3 Model Build-up and Energy Balance ............................................ 118 4.1.4 Optical Properties ......................................................................... 120 4.1.5 Convection ................................................................................... 126 4.1.6 Radiation ...................................................................................... 134 4.1.7 Heat Conduction........................................................................... 137 4.1.8 Edge Losses................................................................................. 138 4.1.9 Overall Heat Transfer Coefficients Between the Nodes ............... 138 4.1.10 Implementation in the Simulation Tool ......................................... 141 4.1.11 Validation...................................................................................... 143 4.2 System and Test Stand Modelling ........................................................ 147 4.2.1 System ......................................................................................... 148 4.2.2 Test Stand .................................................................................... 149 5 Thermal Collector Analysis by Simulation ................................................... 151 5.1 Evaluation Basis ................................................................................... 151 5.1.1 Temperatures ............................................................................... 151 5.1.2 Solar Yield .................................................................................... 153 5.2 Reference Results ................................................................................ 154 5.3 Thermotropic Layers ............................................................................. 156 5.3.1 Application at the Absorber Surface ............................................. 157 5.3.2 Application at the Glazing ............................................................. 159 5.3.3 Summary ...................................................................................... 160 5.4 Micro-structured Layer .......................................................................... 161 5.4.1 Standard Configuration ................................................................ 161 5.4.2 Combination with Service Water Cooling ..................................... 162 5.4.3 Variation of the Collector Position ................................................ 163 5.4.4 Summary ...................................................................................... 164 5.5 Thermochromic Layer with Switchable Emission Coefficient ................ 164 5.6 Adjustment of the Collector Efficiency .................................................. 165 5.6.1 Reduction of the Absorber Emissivity ........................................... 165 5.6.2 Reduction of the Backside Insulation ........................................... 167 XI
Table of Contents
5.7 Evaluation of the Approaches ............................................................... 169 6 Development and Analysis of Polymeric Collector Designs........................ 173 6.1 Overview of Polymeric Production Techniques .................................... 173 6.1.1 Injection Moulding ........................................................................ 174 6.1.2 Extrusion ...................................................................................... 176 6.1.3 Blow Moulding .............................................................................. 178 6.1.4 Thermoforming ............................................................................. 180 6.2 Polymeric Collector Designs ................................................................. 182 6.2.1 Non-transparent Materials ............................................................ 182 6.2.2 Glazing ......................................................................................... 184 6.2.3 Cost Basis for Evaluation ............................................................. 186 6.2.4 Absorber Concepts....................................................................... 187 6.2.5 Casing Concept ............................................................................ 191 6.2.6 Double-Walled Casing Concept ................................................... 193 6.2.7 Evaluation of the Unit Costs for the Collector ............................... 196 6.3 Thermal Analysis .................................................................................. 199 6.3.1 Adjustment of the Reference Results ........................................... 200 6.3.2 Adjustment of the Collector Model................................................ 200 6.3.3 Casing Concept ............................................................................ 202 6.3.4 Double-Walled Casing Concept ................................................... 205 6.3.5 Optional Efficiency Increase ......................................................... 206 6.4 Economic Evaluation of the Systems with Polymeric Collector Approaches
208 7 Conclusions and Outlook ............................................................................ 213 7.1 Thermal Collector Loads ....................................................................... 213 7.2 Polymeric Collectors ............................................................................. 215 7.3 Recommendations for Further Investigations ....................................... 216 References ..................................................................................................... 219 XII
List of Figures
List of Figures
Figure 1.1: Typical frame (left) and deep-drawn casing (right) designs of a
flat-plate collector............................................................................ 2 Figure 1.2: Materials mass percentage of a state-of-the-art flat-plate
collector (according to Hochreiter and Trinkl 2008) ....................... 3 Figure 1.3: Price development of copper (red; LME copper 3-month) and
copper stock (orange; LME copper stock) between 2002 and
2014 (Westmetall n.d.).................................................................... 3 Figure 1.4: Price development of aluminium (red; LME aluminium 3-month)
and aluminium stock (orange; LME aluminium stock) between
2002 and 2014 (Westmetall n.d.) .................................................... 4 Figure 1.5: Typical breakdown of the production costs of solar-thermal flatplate collectors (according to Hochreiter and Trinkl 2008;
Treikauskas and Zörner 2005b) ...................................................... 4 Figure 1.6: Increase of the produced collector area in Europe until 2020
(Müller and Zörner 2008) ................................................................ 5 Figure 1.7: Unglazed and non-insulated absorber for pool heating (OKU
Obermaier GmbH n.d.) ................................................................... 7 Figure 1.8: Flexible pool heater from MAZDA-SOLAR (MAZDA-SOLAR
n.d.) ................................................................................................ 8 Figure 1.9: Pool absorber from Heliocol (Heliocol n.d.)....................................... 8 Figure 1.10: Collector frame constructions from Fasalex and Bosch
Thermotechnik GmbH (Meir 2008) ................................................ 9 Figure 1.11: Thermoformed PC casing with honeycomb structure from Roth
Werke GmbH (Roth Werke GmbH n.d.) ......................................... 9 Figure 1.12: Flat-plate collector from Stibetherm S.A. (Stibetherm S.A. n.d.)
and ICS collector from Solarpower GmbH (Meir 2008) with
dome-structured glazing ............................................................... 10 Figure 1.13: Solar collector with a polycarbonate twin-wall sheet (Solar
Twin Ltd. n.d.) ............................................................................... 11 Figure 1.14: Sectional view of the polymeric collector from Aventa AS
(Aventasolar n.d.) ......................................................................... 12 Figure 1.15: Absorber with a clamping profile (Bartelsen et al. 1999) ............... 13 Figure 2.1: Temperature sensors at the back side (left) and at the frame of
the housing (right; Reiter et al. 2012)............................................ 20 Figure 2.2: Temperature sensors at the absorber sheet ................................... 21 XIII
List of Figures
Figure 2.3: Part of the collector array with the system integrated test
collector (top, left) and the test collector which is permanently
stagnating (bottom, left; Reiter et al. 2012) ................................... 22 Figure 2.4: Chosen metering points at the collector in the system.................... 23 Figure 2.5: Chosen metering points at the dry collector .................................... 24 Figure 2.6: Temperature profile of the collector in the system during a
summer day (16th August 2009) .................................................. 25 Figure 2.7: Temperature profile of the dry collector during a summer day
(16th August 2009) ........................................................................ 26 Figure 2.8: Temperature profile of the collector in the system (9th January
2009) ............................................................................................ 27 Figure 2.9: Temperature profile of the dry collector (9th January 2009) ............ 28 Figure 2.10: Temperature increase of the dry collector (14th December
2008; Reiter et al. 2012) ............................................................... 29 Figure 2.11: Temperature increase and decrease of the dry collector (26th
September 2008; Reiter et al. 2012) ............................................. 30 Figure 2.12: Histogram of the temperatures of the absorber sheet of the
collector in the system (according to Reiter et al. 2012) ............... 31 Figure 2.13: Histogram of the temperatures of the absorber sheet of the dry
collector (according to Reiter et al. 2012) .................................... 32 Figure 2.14: Histogram of the temperatures of the glazing of the collector in
the system (according to Reiter et al. 2012) ................................ 33 Figure 2.15: Histogram of the temperatures of the glazing of the dry
collector (according to Reiter et al. 2012) .................................... 33 Figure 2.16: Histogram of the temperatures of the frame of the collector in
the system (according to Reiter et al. 2012) ................................ 34 Figure 2.17: Histogram of the temperatures of the frame of the dry collector
(according to Reiter et al. 2012) ................................................... 35 Figure 2.18: Histogram of the temperatures of the backside sheet of the
collector in the system .................................................................. 36 Figure 2.19: Histogram of the temperatures of the backside sheet of the dry
collector ........................................................................................ 36 Figure 2.20: Operating conditions of the membrane expansion vessel
(according to Hadamovski and Jonas 2000)................................. 38 Figure 2.21: Increase and decrease of the pressure in the solar circuit
during operation and stagnation (1st August 2009) ....................... 38 Figure 2.22: Histogram of the fluid pressure in the solar circuit ........................ 39 XIV
List of Figures
Figure 2.23: Histogram of the fluid pressure in the solar circuit during
stagnation (detailed view) ............................................................. 40 Figure 3.1: Function structure of the solar-thermal collector with overall
function and flow of material, energy and signals ......................... 42 Figure 3.2: Function structure of the solar collector with main functions of
transparent cover, absorber and back side insulation................... 43 Figure 3.3: Sub-functions of the transparent cover ........................................... 43 Figure 3.4: Sub-functions of the absorber ......................................................... 44 Figure 3.5: Sub-functions of the back side insulation ........................................ 44 Figure 3.6: Function structure of the solar-thermal system (without collector)
with overall function and flow of material, energy and signals ...... 45 Figure 3.7: Function structure of the solar-thermal system with main
functions of the assembly groups fluid circuit and thermal
storage.......................................................................................... 45 Figure 3.8: Sub-functions of the fluid circuit ...................................................... 46 Figure 3.9: Sub-functions of the thermal storage .............................................. 46 Figure 3.10: Systematisation of overheating protection measures
(according to Reiter et al. 2012) ................................................... 47 Figure 3.11: Overview on mechanisms reducing the optical efficiency
(according to Reiter et al. 2012) ................................................... 47 Figure 3.12: Overview on mechanisms and solutions for the removal of
excess thermal energy (according to Reiter et al. 2012)............... 48 Figure 3.13: Groups of technical solutions for the reduction of the
transmission properties (according to Reiter et al. 2012)............. 48 Figure 3.14: Transmission of insolation through a plane, transparent glazing .. 57 Figure 3.15: Description of the beam penetration at different incidence
angles (blue region: full transmission; red region: reduced
transmission) ................................................................................ 58 Figure 3.16: Applications of micro-structured films in solar control glazing
for buildings (Nitz et al. 2008) ...................................................... 59 Figure 3.17: Beam penetration through micro-structured layers in summer
(yellow) and winter (cyan) for vertical solar control glazing for
buildings (Nitz et al. 2008) ............................................................ 60 Figure 3.18: Comparison of the solar transmittances of two glazings with
micro-structured layers against the incidence angle (according
to Nitz et al. 2007)......................................................................... 60 Figure 3.19: Compound parabolic concentrators with metallised ridges for
the reflexion of vertical direct radiation (Nitz 2004) ....................... 61 XV
List of Figures
Figure 3.20: Design of a flat-plate collector for high flow temperatures with
film insulation and shading element (according to Beikircher
und Schmidt 2009) ....................................................................... 62 Figure 3.21: Uncovered and covered evacuated tube collectors (Radiant
Floor Company n.d.) ..................................................................... 63 Figure 3.22: Relation of radiation density at vertical (green) and inclined
(red) incidence .............................................................................. 64 Figure 3.23: Schematic of a glazing in scattering (left) and in lighttransmissive (right) state .............................................................. 66 Figure 3.24: Groups of technical solutions for the reduction of the
transmission properties (according to Reiter et al. 2012) ............ 67 Figure 3.25: Thermochromic colours at 25°C, 40°C and 90°C (Huot et al.
2008) ............................................................................................ 68 Figure 3.26: Schematic design of a polymeric collector with ventilation slots
(Meir et al. 2008) .......................................................................... 72 Figure 3.27: Schematic design of a polymeric collector with additional
convective heat losses caused by a chimney behind the
absorber (Harrison et al. 2004) ..................................................... 73 Figure 3.28: Trends of the absorber temperatures with and without
ventilation by natural convection (Harrison et al. 2004) ................ 73 Figure 3.29: ICS system with closed cooling loop during operation (left) and
during activated overheating protection (right; according to
Roberts et al. 2000) ...................................................................... 76 Figure 3.30: Cross section of collector with variable air gap and inflatable
backside insulation during collector operation (top) and
stagnation (bottom; Sharpe 1984) ................................................ 79 Figure 3.31: Collector with lifting elements for the glazing during stagnation
(top) and during collector operation (bottom; Moore 1983) .......... 82 Figure 3.32: Collector with additional cooling circuit at the absorber driven
by the thermosyphon effect (Cummings 1978) ............................. 84 Figure 3.33: Solar-thermal system with overheating protection from service
water (according to Buckley and Guldman 1983) ........................ 88 Figure 3.34: Solar-thermal system with direct flow using overheating
protection with service water (according to Laing 1985) .............. 89 Figure 3.35: Drain-back system with service water overheating protection
(according to Harrison and Harrison 1979) ................................... 90 Figure 3.36: Efficiency curve and collector losses of a conventional flatplate collector ............................................................................... 94 XVI
List of Figures
Figure 3.37: Efficiency curve and collector losses of a flat-plate collector
without selective coating ............................................................... 95 Figure 3.38: Efficiency curve and collector losses of a flat-plate collector
with reduced insulation thickness ................................................. 97 Figure 3.39: Integrated collector storage solar water heaters (Souliotis et al.
2013) ............................................................................................ 98 Figure 3.40: Gradient of the temperature in dependency of the absorbed
thermal energy of a phase change material .................................. 99 Figure 4.1: Cross section of a state-of-the-art collector with aluminium
frame .......................................................................................... 110 Figure 4.2: Design types of harp absorbers (left: harp design; right: double
harp design) ................................................................................ 111 Figure 4.3: Schematic build-up of an absorber stripe...................................... 115 Figure 4.4: Schematic temperature distribution at the cross section of an
absorber in a one-node (left) and a seven-node (right)
absorber model in comparison to the analytical temperature
gradient ....................................................................................... 115 Figure 4.5: Schematic temperature distribution at the cross section of an
absorber along the pipe in a one-node (left) and a five-node
(right) absorber model in comparison to the analytical
temperature gradient .................................................................. 116 Figure 4.6: Schematic temperature distribution at the cross section of the
casing or insulation during unsteady heat conduction in an
analytical model (left) and a one-node model with additional
steady-state calculation of the temperature gradient .................. 117 Figure 4.7: Cross section of the state-of-the-art collector model with nodes
and energy fluxes ....................................................................... 119 Figure 4.8: Basic optical effects at glazing (left) and absorber (right) ............. 120 Figure 4.9: Polarisation directions parallel and vertical to the plane of
incidence (according to Eicker 2003) ......................................... 121 Figure 4.10: Relations between beam angle and refractive indices
(according to Eicker 2003; Duffie and Beckman 2006) ............... 122 Figure 4.11: Transmission, reflection and absorption of a single glazing
(according to Duffie and Beckman 2006).................................... 123 Figure 4.12: Effective transmission absorption product (τα)eff of a single
glazed collector (according to Duffie and Beckman 2006) .......... 125 Figure 4.13: Wind at the inclined collector glazing .......................................... 129 Figure 4.14: Free convection at the glazing with heat transfer upwards (left)
and downwards (right) ................................................................ 130 XVII
List of Figures
Figure 4.15: Upper limit of the critical Rayleigh number depending on the
collector slope............................................................................. 131 Figure 4.16: Free convection at the casing with heat transfer downwards
(left) and upwards (right) ........................................................... 132 Figure 4.17: Free convection in the enclosed gas layer between absorber
and glazing with heat transfer upwards (left) and downwards
(right) .......................................................................................... 133 Figure 4.18: Heat fluxes via radiation between two elements (according to
Eicker 2003) ............................................................................... 135 Figure 4.19: Heat conduction between the absorber nodes............................ 137 Figure 4.20: Schemes of serial (left) and parallel (right) heat transfer ............ 139 Figure 4.21: Thermal circuit of a serial connected system .............................. 140 Figure 4.22: Thermal circuit of a parallel connected system ........................... 140 Figure 4.23: Build-up of the collector block with embedded S-function (grey)
in Simulink .................................................................................. 142 Figure 4.24: Mask of the collector block with embedded S-function of the
collector in Simulink .................................................................... 143 Figure 4.25: Comparison of the simulated efficiency curves of the
parameter model and the developed dynamic collector model .. 144 Figure 4.26: Comparison of the measured and simulated efficiency of the
collector ...................................................................................... 146 Figure 4.27: Build-up of the test stand in Matlab/Simulink (left) and
parameter mask of the fluid block (right) .................................... 149 Figure 5.1: Basic energy flow of a conventional heating system (left) and a
solar-thermal heating system (right) ........................................... 153 Figure 5.2: Histograms of the absorber temperatures with (black) and
without (grey) selective absorber coating over one year ............ 155 Figure 5.3: Temperature-dependent gradient of the transmittance of two
thermotropic layers for simulation (according to Resch et al.
2009) .......................................................................................... 156 Figure 5.4: Histograms of the absorber temperatures without selective
coating (grey) and with thermotropic absorber layer C-4-5
(green) over one year ................................................................. 158 Figure 5.5: Histograms of the absorber temperatures without selective
coating (grey) and with thermotropic absorber layer C-6-7
(green) over one year ................................................................. 158 Figure 5.6: Moulding of a negative asymmetric CPC structure (left) and
simulation of the transmission of metalized asymmetric CPC
XVIII
List of Figures
structures depending on the incidence angle (right; according
to Walze 2005) ........................................................................... 161 Figure 5.7: Histograms of the absorber temperatures with selective coating
(black) and with micro-structured layer (green) over one year ... 162 Figure 5.8: Histograms of the absorber temperatures with selective coating
(black) and with thermochromic absorber layer (green) over
one year ...................................................................................... 165 Figure 5.9: Histograms of the absorber temperatures with selective coating
(black) and with TISS paint coating (purple) over one year ....... 166 Figure 5.10: Histograms of the absorber temperatures with selective
coating (black) and with 30 mm backside insulation (purple)
over one year .............................................................................. 168 Figure 5.11: Histograms of the absorber temperatures with selective
coating (black) and with 15 mm backside insulation (purple)
over one year .............................................................................. 168 Figure 5.12: Histograms of the absorber temperatures with selective
coating (black) and without backside insulation (purple) over
one year ...................................................................................... 169 Figure 5.13: Rating diagram of the overheating protection measures............. 170 Figure 6.1: Overview on the main groups of relevant manufacturing
processes for collector components ........................................... 174 Figure 6.2: Injection moulding cycle (according to Michaeli 2006) .................. 175 Figure 6.3: Capabilities of components manufactured by injection moulding .. 176 Figure 6.4: Schematic build-up of a plastic extruder (according to
Kaiser 2007) ............................................................................... 177 Figure 6.5: Capabilities of components manufactured by extrusion ................ 178 Figure 6.6: Blow moulding cycle (CustomPartNet 2008) ................................. 179 Figure 6.7: Capabilities of components manufactured by blow moulding ....... 179 Figure 6.8: Vacuum forming cycle (DUROtherm Kunststoffverarbeitung
GmbH (n.d.)) ............................................................................... 180 Figure 6.9: Capabilities of components manufactured by thermoforming ....... 181 Figure 6.10: Twin sheet thermoforming cycle (DUROtherm
Kunststoffverarbeitung GmbH n.d.) ............................................ 181 Figure 6.11: Capabilities of components manufactured by twin sheet
thermoforming ............................................................................ 181 Figure 6.12: Heat conduction in a sheet-pipe (above) and a volumetric
absorber (below) ......................................................................... 188 Figure 6.13: Sectional view of a twin sheet thermoformed absorber............... 189 XIX
List of Figures
Figure 6.14: Sectional view of an extruded absorber with serpentine
structure ..................................................................................... 191 Figure 6.15: Sectional view of a polymeric collector casing with exemplary
clipped double glazing ................................................................ 191 Figure 6.16: Cross section of the collector with casing and insulation ............ 193 Figure 6.17: Sectional view of a double-walled casing with exemplary fixed
double glazing ............................................................................ 194 Figure 6.18: Cross section of the collector with double-walled casing ............ 194 Figure 6.19: Comparison of the unit costs of the polymeric collector
concepts ..................................................................................... 197 Figure 6.20: Build-up of the subsystem of the weather block for the
stagnation test in Simulink .......................................................... 199 Figure 6.21: Cross section of the polymeric collector model with nodes and
energy fluxes .............................................................................. 201 Figure 6.22: Occurrences of thermal loads depending on duration and
temperature level on a polymeric absorber with 10 mm mineral
wool insulation and air enclosure during stagnation over one
year ............................................................................................ 203 Figure 6.23: Occurrences of thermal loads depending on duration and
temperature level on a polymeric absorber with 30 mm mineral
wool insulation during stagnation over one year ......................... 204 Figure 6.24: Occurrences of thermal loads depending on duration and
temperature level on a polymeric absorber with double-walled
housing during stagnation over one year .................................... 206 Figure 6.25: Typical breakdown of the total costs for end-users of the
reference system ........................................................................ 208 Figure 6.26: Development of the fuel oil price................................................. 210 XX
List of Tables
List of Tables
Table 2.1: Description of the investigated heating system in the field-testing ... 18 Table 2.2: Description of the sensors used in the field-testing for analysing
the weather data ........................................................................... 19 Table 2.3: Description of the sensors used in the field-testing for analysing
the solar circuit ............................................................................. 20 Table 2.4: Description of the sensors used in the field-testing for analysing
the collector temperatures ............................................................ 21 Table 2.5: Description of the sensors used in the field-testing for analysing
the heating system........................................................................ 22 Table 3.1: Implementations of overheating protection by natural convection .... 75 Table 3.2: Classes of the overheating concepts regarding intrinsic safety ...... 100 Table 3.3: Assessment criteria for overheating protection concepts ............... 101 Table 3.4: Evaluation and feasibility of the overheating approaches
changing the optical efficiency — part 1: changing the
transmission properties............................................................... 102 Table 3.5: Evaluation and feasibility of the overheating approaches
changing the optical efficiency — part 2: changing the
absorption properties .................................................................. 103 Table 3.6: Evaluation and feasibility of the overheating measures removing
the thermal energy ...................................................................... 104 Table 3.7: Evaluation and feasibility of the overheating approaches by
adjusting the collector efficiency ................................................. 105 Table 3.8: Evaluation and feasibility of the overheating approaches by
buffering critical collector states .................................................. 105 Table 3.9: Chosen overheating protection concepts for simulation ................. 106 Table 4.1: Basic design data of the state-of-the-art collector .......................... 111 Table 4.2: Design data of the absorber ........................................................... 112 Table 4.3: Design data of the insulation and casing........................................ 113 Table 4.4: Optical properties of the collector ................................................... 113 Table 4.5: Test conditions and efficiency curve of the collector derived from
testing ......................................................................................... 114 Table 4.6: Collector component temperatures during stagnation of a
conventional flat-plate collector in testing and simulation .......... 147 Table 4.7: Description of the simulated solar-thermal heating system ............ 148 XXI
List of Tables
Table 5.1: Temperature limits of opaque materials for absorber, insulation
and casing regarding defined tests (Domininghaus et al. 2012) 153 Table 5.2: Description of the simulated reference heating system.................. 154 Table 5.3: Fractional energy savings and relative decrease of the reference
systems in annual simulations .................................................... 155 Table 5.4: Fractional energy savings and relative decrease of the reference
systems as well as of the systems with thermotropic absorber
layers in annual simulations........................................................ 157 Table 5.5: Fractional energy savings and relative decrease of the reference
systems as well as of the systems with non-selective absorber
and thermotropic layers on the glazing in annual simulations.... 159 Table 5.6: Fractional energy savings and relative decrease of the reference
systems as well as of the systems with selective absorber and
thermotropic layers on the glazing in annual simulations .......... 160 Table 5.7: Fractional energy savings and relative decrease of the reference
systems as well as of the system with micro-structured layer
on the glazing in annual simulations ........................................... 162 Table 5.8: Fractional energy savings and maximum absorber temperature
of the system with micro-structured layer on the glazing in
annual simulations depending on the collector position .............. 163 Table 5.9: Fractional energy savings and relative decrease of the reference
systems as well as of the system with thermochromic
absorber layer in annual simulations .......................................... 164 Table 5.10: Fractional energy savings and relative decrease of the
reference systems as well as of the system with reduced
emission coefficients in annual simulations ................................ 166 Table 5.11: Fractional energy savings and relative decrease of the
reference systems as well as of the system with reduced
backside insulation thicknesses in annual simulations ............... 167 Table 5.12: Sorted list of maximum absorber temperatures occurring in the
annual simulations ...................................................................... 171 Table 6.1: Properties of PE (Domininghaus et al. 2012;
Kunststoffinformation n.d.) .......................................................... 183 Table 6.2: Properties of PP (Domininghaus et al. 2012;
Kunststoffinformation n.d.) .......................................................... 183 Table 6.3: Properties of PP GF30 (Domininghaus et al. 2012;
Kunststoffinformation n.d.) .......................................................... 184 Table 6.4: Properties of PMMA (Domininghaus et al. 2012;
Kunststoffinformation n.d.) .......................................................... 184 XXII
List of Tables
Table 6.5: Properties of PC (Domininghaus et al. 2012;
Kunststoffinformation n.d.) .......................................................... 185 Table 6.6: Costs and weight of various PMMA glazing types
(EVONIK 2011) ........................................................................... 186 Table 6.7: Costs and weight of a single glazing made of solar glass .............. 186 Table 6.8: Boundary conditions for the calculation of the manufacturing
costs ........................................................................................... 187 Table 6.9: Comparison of the heat conduction in a metallic sheet-pipe and
a polymeric volumetric absorber ................................................. 188 Table 6.10: Unit costs of a twin sheet thermoformed absorber ....................... 190 Table 6.11: Unit costs of a thermoformed casing ............................................ 192 Table 6.12: Unit costs of an injection moulded casing .................................... 192 Table 6.13: Unit costs of a thermoformed double-walled casing ..................... 195 Table 6.14: Unit costs of a blow moulded double-walled casing ..................... 195 Table 6.15: Typical breakdown of the unit costs of the reference collector ..... 196 Table 6.16: Composition of collector concepts for the evaluation ................... 197 Table 6.17: Composition and decrease of the unit costs of the collector
concepts ..................................................................................... 198 Table 6.18: Test conditions for stagnation according to DIN EN 129752 (2006) ...................................................................................... 199 Table 6.19: Reference results from simulation of two different state-of-theart flat-plate collectors................................................................. 200 Table 6.20: Fractional energy savings and efficiency decrease of the
reference system as well as of the systems with polymeric
collectors in annual simulations .................................................. 202 Table 6.21: Stagnation temperatures according to DIN EN 12975-2 (2006)
of the polymeric collector concepts with mineral wool
insulation .................................................................................... 204 Table 6.22: Fractional energy savings and efficiency decrease of the
reference system as well as of the system with polymeric
collectors with double-walled casing in annual simulations ....... 205 Table 6.23: Stagnation temperatures according to DIN EN 12975-2 (2006)
of the polymeric collector concept with double-walled casing ..... 206 Table 6.24: Influence of the emission coefficients on the heat transfer
coefficient for radiation................................................................ 207 Table 6.25: Influence of the aluminium film on the fractional energy savings
and the stagnation temperatures of the absorber according to
DIN EN 12975-2 (2006) .............................................................. 207 XXIII
List of Tables
Table 6.26: Influences of the polymeric collector concepts on the system
costs for customers .................................................................... 209 Table 6.27: Energy costs of the reference system over 20 years ................... 210 Table 6.28: Energy costs of the systems with polymeric collector concepts
over 20 years .............................................................................. 211 Table 6.29: Influences of the polymeric collector concepts on the total costs
for customers with linear fuel price development over 20 years . 211 Table 6.30: Influences of the polymeric collector concepts on the total costs
for customers with quadratic fuel price development over 20
years ........................................................................................... 212 XXIV
Abbreviations
Abbreviations
CPC
compound parabolic concentrator
EPDM
ethylene propylene diene monomer
GBP
Great Britain Pound
GdMg
gadolinium magnesium
HDT
heat deflection temperature
ICS
integrated collector storage
IEA
International Energy Agency
IL
interference lithography
LC
liquid crystals
LME
London Metal Exchange
MP
mechanical processing
PA
polyamide
PC
polycarbonate
PE
polyethylene
PMMA
polymethylmethacrylate
PP
polypropylene
PPE
polyphenylene ether
PPE+PS
polyphenylene ether/polystyrene (blend)
PPS
polyphenylene sulphide
PU
polyurethane
PV
photovoltaics
PVC
polyvinyl chloride
PVD
physical vapour deposition
XXV
Abbreviations
r.h.
relative humidity
RTD
resistance temperature detector
SHC
Solar Heating and Cooling Programme
TISS
thickness-insensitive spectrally selective
VAT
value added tax
WO3
tungsten oxide
XXVI
Symbols
Symbols
′
area
[m2]
collector efficiency factor
[-]
heat removal factor
[-]
Nusselt number
[-]
solar irradiance
[Wm-2]
electrical current
[A]
extinction coefficient
[m-1]
characteristic length
[m]
Prandtl number
[-]
energy consumption of an auxiliary heater
[kWh]
energy consumption of heater of a conventional heating system
[kWh]
total system load of the house (for domestic
hot water and space heating)
[kWh]
heat flow rate
[W]
Rayleigh number
[-]
angle adjusted Rayleigh number
[-]
electrical resistance
[Ω]
thermal resistance
[KW-1]
critical Rayleigh number
[-]
Reynolds number
[-]
absorbed solar radiation
[W]
temperature change
[K]
temperature
[K]
overall heat loss coefficient of the collector
[Wm-2K-1]
XXVII
Symbols
Φ
XXVIII
electrical voltage
[V]
linear heat loss coefficient
[Wm-2K-1]
quadratic heat loss coefficient
[Wm-2K-2]
specific heat capacity
[kJkg-1K-1]
diameter
[m]
per time unit
[s]
friction factor
[-]
Prandtl number correction factor
[-]
fractional energy savings
[%]
collector slope correction factor
[-]
acceleration due to gravity
[ms-2]
heat transfer coefficient
[Wm-2K-1]
heat conductivity
[Wm-1]
pipe length
[m]
mass
[kg]
mass flow rate
[kgs-1]
refraction index
[-]
reflection factor
[-]
distance
[m]
time
[s]
velocity
[ms-1]
form factor
[-]
absorption coefficient
[-]
collector slope
[°]
Symbols
intermittency factor
[-]
emission coefficient
[-]
zero loss efficiency
[-]
incidence angle at collector surface
(in relation to surface normal)
[°]
kinematic viscosity
[m2s-1]
reflection coefficient
[-]
Stefan-Boltzmann constant
[-]
transmission coefficient
[-]
transmittance without reflection
[-]
XXIX
Subscripts
Subscripts
absorber
air
ambient
casing
collector
heat conduction
convection
direct
diffuse
collector edge
fluid
forced (convection)
free (convection)
glazing
inlet / previous node
laminar flow
maximum value
mineral wool
average fluid temperature of the collector
item
inner surface of the riser pipe
reflected
radiation
sky / atmosphere
transmitted
XXXI
Subscripts
total
turbulent flow
cross-section area of the pipe wall
wind
vertical
∥
XXXII
parallel
1 Introduction
1 Introduction
Considering the rapidly increasing costs of aluminium and copper the solar-thermal flat-plate collector industry is showing increasing interest in polymeric materials, which have a remarkable potential for cost reduction. Apart from moderate
material costs, especially for commodity plastics and an outstanding flexibility in
the scope of the design regarding product development, highly efficient production processes provide an interesting application for the use of polymeric materials (Zörner 2004). Treikauskas and Zörner (2005a) have already shown this potential for collector casings made of alternative materials.
However, several technological challenges have to be met prior to the use of
polymeric materials in a wide range of applications for collectors, ranging from
limited mechanical and thermal properties to low UV-durability and low heat conductivity of various polymers in comparison to copper or aluminium. Therefore,
both the component and the system designs need to be adapted to polymer characteristics.
1.1 Materials and Production Costs of Flat-Plate Solar-Thermal
Collectors
Solar-thermal flat-plate collectors are typically used for solar domestic hot water
preparation and space heating in Central European climates. Currently, the stateof-the-art flat-plate collector design contains the following major parts: absorber,
glazing, thermal insulation and casing (Duffie and Beckman 2006).
The absorber is the main element of the collector and is in most cases fabricated
from copper. It transmits the incoming solar radiation, consisting of a direct and
a diffuse part, and converts the radiant energy into thermal energy. Most absorber
types are of the fin-and-tube design. The plate with a black or a selective surface
receives radiation. On the back of the plate fluid pipes dissipate the thermal energy.
1
1 Introduction
The glazing above the absorber reduces the heat loss of the absorber plate by
convection and radiation. The glazing is typically made of special break-proof
glass with a very good light-transmitting characteristic.
An insulation typically consisting of rock wool reduces the heat losses on the
backside and the edges. The insulation is between 40 and 60 mm thick on the
backside and about 15 mm on the edges.
The absorber, the glazing and the insulation of the collector are embedded in a
casing to protect them against environmental influences and to fix the collector to
an underground. Common casings are fabricated from aluminium in riveted frame
or deep-drawn casing designs (Figure 1.1).
Figure 1.1: Typical frame (left) and deep-drawn casing (right) designs
of a flat-plate collector
In recent years, numerous industrial development projects in the field of flat-plate
collectors have focused on the enhancement of collector efficiency, for example
by non-reflective glass, double-glazing or highly selective absorber coating. This
led to an increase in production costs and eventually to an increase of collector
prices.
Furthermore, soaring costs for the commonly used raw materials, mainly copper
and aluminium, intensified this trend. Solar-thermal flat-plate collector design has
a very high mass percentage of the materials copper, aluminium and glass. Figure 1.2 illustrates the materials mass percentage of a state-of-the-art collector of
a German manufacturer.
2
1 Introduction
Figure 1.2: Materials mass percentage of a state-of-the-art flat-plate collector
(according to Hochreiter and Trinkl 2008)
Figure 1.3 demonstrates the price development of copper between January 2002
and December 2013 with the price soaring from 1,530 US dollars to 7,060 US dollars per ton (Westmetall n.d.). The price hit a local all-time high in February 2011,
surging to 10,120 US dollars per ton.
Figure 1.3: Price development of copper (red; LME copper 3-month) and copper stock
(orange; LME copper stock) between 2002 and 2014 (Westmetall n.d.)
Figure 1.4 demonstrates the price development of aluminium between January 2002 and December 2013 with the price skyrocketing from 1,380 US dollars
to 1,770 US dollars per ton and reaching a record high of 3,340 US dollars (Westmetall n.d.).
3
1 Introduction
Figure 1.4: Price development of aluminium (red; LME aluminium 3-month) and aluminium stock
(orange; LME aluminium stock) between 2002 and 2014 (Westmetall n.d.)
Against this background, the markets as well as the national and European policymakers are calling for a decrease of consumer prices for solar-thermal systems
in order to reduce the subsidies for these systems in the future. As a consequence, the solar collector industry for example increasingly replaces copper by
less cost-intensive aluminium for the absorber (Zörner 2004). As shown in Figure 1.5, the absorber, the glazing and the casing represent a considerable share
of collector production cost.
Figure 1.5: Typical breakdown of the production costs of solar-thermal flat-plate collectors
(according to Hochreiter and Trinkl 2008; Treikauskas and Zörner 2005b)
4
1 Introduction
The efforts of manufacturers to reduce production costs are undermined by the
development of the price curves. Another reason for the high production costs of
metal parts is the limited suitability for mass production of the processes involved.
These processes require a large degree of assembly work as well as manual
work such as mounting the aluminium frame. This is a particularly negative drawback when bearing in mind that the market for solar-thermal collectors is subject
to major changes to the advantage of high volume production. The production
costs of solar-thermal flat-plate collectors are much higher when producing copper and aluminium manufactured parts, i.e. with the absorber causing half of the
costs and the housing a fifth respectively (Figure 1.5).
A market volume of about 8.5 million m² per year for solar-thermal collectors is
expected in Europe by 2020 (Müller and Zörner 2008). The annually produced
collector area in Europe was 1.7 million m² per year in 2009 and is predicted to
increase within the course of the next 10 years by a factor of about 5 (Figure 1.6).
This development will be a great challenge for the manufacturers to supply the
market.
25%
10,0
Annual produced collector area
20%
6,0
15%
4,0
10%
2,0
5%
0,0
0%
Annual market growth [%]
Collector area [million m²]
Annual market growth
8,0
Figure 1.6: Increase of the produced collector area in Europe until 2020
(Müller and Zörner 2008)
5
1 Introduction
The highly improved production processes in conventional assembly lines have
no further potential of being optimised. Hence, production processes of state-ofthe-art collectors lead to limited cost advantages by economies of scale. On the
contrary, production techniques for polymeric parts have a great potential for
mass production. These machines — or to be more precise production
lines — are linked to high investment costs. Additionally, a large number of units
is required for an adequate depreciation. However, if large production quantities
can be reached, low production costs will be achievable.
In this regard, polymeric materials show a remarkable potential for cost reduction.
However, the replacement of copper and aluminium by polymers for absorber or
casing is not possible within the current collector design. Polymers can generally
only be a cost-effective alternative if the product design is adapted to both the
characteristics of the polymeric material and the polymer production processes.
The sole replacement of materials in a metallic material optimised design will not
show satisfactory solutions, and certainly not when targeting at mass production.
1.2 Overview of Polymeric Materials in Solar-Thermal Collectors
Polymeric materials are already used in several solar-thermal applications. These
are usually not fully polymeric collectors, but rather conventional materials of single parts replaced by polymers. Polymers are mainly used in unglazed absorbers
such as for pool heating. There were also polymeric materials used in simple
applications like thermosyphon or integrated collector storage (ICS) systems. A
general overview on polymeric materials in solar-thermal applications is given by
Meir (2008). More details as to when polymeric collector parts are already available on the market will be described below.
1.2.1 Absorber
Polymeric absorbers are currently used for pool heating applications by various
manufacturers, since pressure and temperature are low within those systems.
6
1 Introduction
These unglazed solar absorbers can supply the system with heat on a very low
temperature level (30–40°C). Hence, high thermal and mechanical stability are
not required for the absorbers. Additionally, pool water is pumped directly through
the part and therefore a resistance against glycol is not needed. The required
properties for absorber materials are ideal for the use of low-cost polymers.
Mainly, low-cost polymers like polyethylene (PE), polypropylene (PP) and EPDM
are used. These rather inefficient unglazed and non-insulated pool heating absorbers cannot be used for hot water preparation and space heating under Central European climates.
Two types of absorbers are available on the market, volumetric absorber structures on the one hand and absorbers with parallel channels on the other hand.
Volumetric absorbers with holohedral flow are rigid and frost-resistant. They are
produced from black polyethylene (PE) by means of extrusion blow moulding (cf.
e.g. Meir 2008; OKU Obermaier GmbH n.d.; Roth Werke Gmbh n.d.). Figure 1.7
shows in an exemplary way the product of OKU Obermaier GmbH. Both the British company Solco Ltd. as well as the German company Roth Werke GmbH offer
a similar range of absorbers.
Figure 1.7: Unglazed and non-insulated absorber for pool heating (OKU Obermaier GmbH n.d.)
Another kind of the above-mentioned absorber type is made from flexible material. Texsun AB manufactures the unglazed absorber El Nino from black elastomeric flexible polyvinyl chloride (PVC) by means of welding two sheets together
(TEXSUN AB n.d.), characterised to be frost-resistant.
7
1 Introduction
The second type of absorbers, which are extruded as sheets with intrinsic channels or as single pipes, has straight channels. Sheets with intrinsic channels
made of polypropylene (PP) are produced by the US company FAFCO, Incorporated. The German company MAZDA-SOLAR pursues a similar design and uses
ethylene propylene diene monomer (EPDM), an extremely flexible material. Figure 1.8 shows the elastic, coextruded absorber with a red surface matching the
colour of the roof (MAZDA-SOLAR n.d.; Meir 2008).
Figure 1.8: Flexible pool heater from MAZDA-SOLAR (MAZDA-SOLAR n.d.)
The pool heater from Heliocol is a bundle of single PP tubes added together.
Figure 1.9 shows the pool absorber during the production process in the assembly line on the left hand and on the right it demonstrates the absorber when already mounted on a roof and connected to the system.
Figure 1.9: Pool absorber from Heliocol (Heliocol n.d.)
1.2.2 Housing
When considering glazed and insulated solar collectors for hot water preparation
and space heating, polymers are still rarely used in collector parts. Nevertheless,
polymeric housing provides a wide scope of flexibility and lightweight design of
8
1 Introduction
polymeric housings due to polymer production processes, which can be attributed
to the major advantages in comparison with housings made of metals.
The Austrian company Fasalex produces parts from composites of wood and
plastic material. A frame of this composite is used for a prototype similar to the
common aluminium frame construction for collector housings. The collector made
by Bosch Thermotechnik GmbH, a German company, has a frame of glass-fibre
reinforced plastics. Figure 1.10 illustrates the described collector frames.
Figure 1.10: Collector frame constructions from Fasalex and Bosch Thermotechnik GmbH
(Meir 2008)
The company Roth Werke GmbH produces thermoformed collector casings
made of black polycarbonate (PC) by means of a thermoforming process. The
honeycomb structure on the bottom of the housing increases mechanical stability
(Meir 2008; Roth Werke GmbH n.d.). Figure 1.11 illustrates the PC part and its
honeycomb structure.
Figure 1.11: Thermoformed PC casing with honeycomb structure from Roth Werke GmbH
(Roth Werke GmbH n.d.)
9
1 Introduction
1.2.3 Glazing
Partly, transparent polymeric covers are used in collectors. Three types of polymeric glazing are available. The basic version consists of a plane sheet analogue
to standard shatterproof glass. Exactly this sheet which is made from PMMA is
used for the flat-plate collectors from Stiebel Eltron GmbH & Co. KG. When it
comes to achieving higher mechanical stability, the bended plate represents another alternative to a single-sheet glazing. In particular, collectors of larger sizes
benefit from the curvature. The British company Imagination Solar Ltd. uses exactly this type of curved PC plates for collectors. The thermosyphon collector
manufactured by Stibetherm S.A. as well as the ICS collector produced by Solarpower GmbH are comprised of bended acrylic glazing. Figure 1.12 shows the
glazing with dome structure.
Figure 1.12: Flat-plate collector from Stibetherm S.A. (Stibetherm S.A. n.d.)
and ICS collector from Solarpower GmbH (Meir 2008) with dome-structured glazing
The twin-wall sheets, a tremendous choice of which is available on the market,
are also used as polymeric collector glazing. Most of all, these plates can be
found in carports or terrace roofs. Transparent plates are extruded from PC or
PMMA combining both low weight and high stability and show, therefore, great
potential for light weight construction. The correspondent product of Solar Twin
Ltd., a collector manufacturer, is equipped with PC twin-wall sheet amounting to
10 mm thickness (Figure 1.13).
10
1 Introduction
Figure 1.13: Solar collector with a polycarbonate twin-wall sheet (Solar Twin Ltd. n.d.)
1.2.4 Fully Polymeric Collector
Most previous work has been limited to single parts of the collector. A fundamental investigation of how to apply polymeric materials not only for single parts but
in integrated polymeric collectors is currently not available.
The Norwegian manufacturer Aventasolar AS presented a collector almost fully
manufactured from polymers (Figure 1.14). However, the collector was designed
for a cost-effective drain-back system for domestic hot water and space heating
being integrated into low-temperature heating systems. The collector’s efficiency
is rather low in comparison to state-of-the-art collectors in order to limit the maximum temperature of the absorber. Basically, the industrial developments remain
on a low technical level and focus on the replacement of materials for single parts.
The cover sheet of the collector is an extruded twin-wall sheet made of PC with
an additional UV protective layer. The absorber consists of an extruded twin-wall
sheet with fixed end caps made of the high-quality material polyphenylene sulphide (PPS). The extruded aluminium frame establishes stiffness and a modular
application of the collectors. The collector has the following size: a width of
600 mm, a variable length of up to 6 m and a specific weight of 5 kg/m² (Aventasolar n.d.). Figure 1.14 shows a sectional view of one collector module.
11
1 Introduction
Figure 1.14: Sectional view of the polymeric collector from Aventa AS (Aventasolar n.d.)
The polymeric collector from Solar Twin Ltd. was developed by the Edinburgh
Napier University and can be integrated additionally into an existing heating system. The collector module with an aperture area of 2.8 m² is designed for small
system applications (Figure 1.13). Within the heat carrier the light design of the
collector has a weight of 33 kg. Solar Twin Ltd. offers this solar-thermal system
as an installation kit containing all necessary parts for the integration to in the
heating system. The solar circuit pump is driven by a small photovoltaic cell for
an off-grid power supply. The cover is an extruded PC twin-wall sheet and the
flexible absorber pipes are made of silicone rubber. This absorber is resistant
when ranging between -60°C and 200°C. The volume expansion of the freezing
fluid inside the absorber cannot break the elastic pipes. Hence, this flexible absorber system is also frost-resistant. The required stiffness of the collector is
achieved by an aluminium frame (Solar Twin Ltd. n.d.).
1.3 Overview of Research Activities of Polymeric Materials in
Solar-Thermal Collectors
Additionally, in the field of solar-thermal flat-plate collectors, research has been
conducted. Research work about polymeric collector parts, material science and
overheating protection is presented in the following paragraphs.
12
1 Introduction
1.3.1 Collectors and Parts
Bartelsen et al. (1999) investigated elastomer-metal-absorbers for the use in roof
and facade elements. The metal plates of these absorbers have integrated clip
profiles for the application of an elastomeric tube (Figure 1.15). The main advantage of this absorber is the inherent freeze resistance making an operation
possible without any antifreeze additives. Moreover, the resistance against corrosion enables the option to operate directly as a solar desalination system for
sea water.
Figure 1.15: Absorber with a clamping profile (Bartelsen et al. 1999)
Concerning research work in the field of polymeric solar-thermal collectors, an
investigation of selectively coated polymer absorbers was carried out by Rommel et al. (1997). They proved the capability of alternative absorber materials in
combination with adequate geometries. The efficiency of the investigated absorbers reached is comparable to state-of-the-art metal fin-and-tube absorbers
proved in calculations and in prototype tests.
Brunold (2010) also described an approach for a cost-effective polymer collector.
This concept was adjusted to polymeric production processes and a first analysis
of the manufacturing costs was conducted. The measured efficiency curve of a
prototype with non-selective absorber and glass cover showed satisfactory results in comparison to a state-of-the-art collector thanks to the optical properties
and the collector efficiency factor in particular. However, the estimated maximum
temperatures were too high for the proposed polymeric materials. Also the tested
13
1 Introduction
thermotropic absorber layer for overheating protection was not sufficient to solve
the issue completely.
Simulations in order to evaluate the thermal performance of polymeric collectors
and future prospects of these materials in solar collectors were also carried out
by Cristofari et al. (2004) and O’Brien-Bernini and McGowan (1984). The simulation results showed that polymeric flat-plate collectors have been able to achieve
comparable efficiencies like state-of-the-art metal collectors.
Kaiser et al. (2012) investigated solar-thermal systems by means of simulation in
order to derive requirements for polymeric materials in collectors. In his analysis
a basic polymeric collector design with twin-wall sheet glazing was implemented
in the simulation tool. A thermotropic layer and a surface cooler for overheating
protection were also analysed in this simulation. The results of the system simulation were compared to a conventional system to estimate the influence on the
system efficiency and the part temperatures.
1.3.2 Materials
Ruesch and Brunold (2008) investigated the ageing behaviour of glazing materials, particularly transparent plastics, for solar-thermal collectors. 52 polymeric
glazing types have been exposed in Switzerland over 20 years. Therefore, the
conditions of solar-thermal collectors have been imitated by building small collectors with a selective steel plate inside. The PMMA sheets showed good transmission results even after 20 years of exposure. The brittleness of these PMMA types
proved to be a problem, but the current materials are expected to be more resistant. The PC types of 1984 can resist high thermal loads and show favourable
transmission properties. However, the low UV-resistance caused a relatively high
degradation. Therefore, a UV-protection layer is required for solar-thermal collector glazing made of PC. For that kind of application the other transparent polymers tested are not suitable for application.
Kahlen and Wallner (2008) tested the ageing behaviour of different polymeric
materials for the use in absorbers of solar heating systems. The material samples
14
1 Introduction
of PC, polyphenylene ether/polystyrene (PPE+PS) blend, PP and polyamide 12
(PA12) were exposed to hot air (140°C) for 500 h as well as to hot water (80°C)
for 16,000 h and were then analysed in regular time intervals.
Kurzböck et al. (2012) investigated black pigmented PP materials with high solar
absorbance for the use in absorbers, focusing mainly on the test of the developed
compounds with regard to their ageing behaviour. The requirements of the materials were improved mechanical properties at operating temperatures of up to
90°C for more than 10 years and a solar absorbance of at least 90 %.
Orel et al. (2010) investigated thickness-insensitive spectrally selective (TISS)
paints for the use on metal as well as plastic absorbers. Moreover, the optical
properties of the paint were analysed. The degradation of the paint under common absorber conditions was also tested. Hence, the paint is a profitable option
to conventional selective absorber coatings. In particular the spraying process for
the application on the surface is interesting for polymeric substrates.
An analysis of polymeric insulation materials for solar-thermal applications was
carried out in Zauner et al. (2011). Rigid foam as well as flexible foam of polyurethane (PU) were tested amongst others for the use in the backside insulation
of collectors. The analysis embraced the investigation of thermal properties, degradation and outgassing of the materials.
1.3.3 Overheating Protection
Some research work was done on thermotropic layers on the glazing to reduce
high temperatures in flat-plate collectors for example by Resch et al. (2007). Furthermore, thermochromic films on the absorber that switch their optical characteristics depending on the temperature were investigated by Huot et al. (2008)
and Marty et al. (2008). Triggered ventilation is another way of controlling critical
temperatures. Most investigations concentrated on arrays with venting dampers
at the ends of the collector, which allow an air exchange above or below the absorber to carry the heat away from the radiation exchanger (Meir et al. 2008;
15
1 Introduction
Harrison et al. 2004; Kearney et al. 2005). Further overheating protection concepts are shown in detail in the following chapters.
1.4 Formulation of the Research Topics
Several products proved the feasibility of polymer use in collectors and showed
that there is a huge technical as well as economic potential. Most concepts, however, only substituted the material of single collector parts without changing the
design. Thus, the full potential of polymeric production processes could not be
tapped and the unknown component temperatures were only solved by choosing
more temperature resistant, hence more expensive materials. The projects in the
field of polymeric materials for solar-thermal collectors focused on single concepts or topics without considering all details and options for a polymeric collector
approach with regard to manufacturing cost in comparison to the system yield.
Hence, the application of polymers is still not possible on an integrated level as
major gaps in the knowledge exist. This thesis addresses these gaps by using
experimental measurement, computer simulation and design analysis. It covers
the four major topics relevant to the conceptual design of a polymeric flat-plate
collector:

Identification of the thermal loads in solar-thermal flat-plate collectors

Analysis of concepts for overheating protection in solar-thermal flat-plate
collectors

Development of polymeric collector design concepts with regard to manufacturing costs

Analysis of polymeric collector design concepts relating thermal loads to
system performance and economic potential
16
2 Collector Loads
2 Collector Loads
Solving the hurdles of the use of low-cost polymers with their limited thermal and
mechanical stability, overheating protection measures for the solar-thermal collector are necessary. Analysing the loads occurring in a state-of-the-art solarthermal system and adjusting this system will be the basis for the use of such
polymers.
In an initial step, a modern system for domestic hot water and space heating with
state-of-the-art solar-thermal flat-plate collectors was analysed in a field-testing
building (Reiter et al. 2012). The measurement data were assessed including the
following aspects: maximum temperatures at the collector components, dynamic
behaviour of thermal loads and accumulated exposition times, i.e. thermal load
profiles. By measuring the fluid pressure of the solar-thermal system the mechanical loads were also investigated. The collected data provide the fundamental
basis for polymeric material selection or development, for an evaluation of material property changes over the life time as well as for an adoption of solar-thermal
systems and collector designs to polymer requirements.
2.1 Field-Testing System and Measurement Instrumentation
For the field-testing building a single-family house inhabited by four persons was
chosen. The house is equipped with a modern solar hot water and a space heating system with standard flat-plate collectors reaching approximately 19 m² of
collector surface. Table 2.1 shows the boundary conditions of the field-testing.
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2 Collector Loads
Table 2.1: Description of the investigated heating system in the field-testing
Design data
System:
Building type
Single family house
Location
Near Munich (Germany)
Total system load
approx. 23,500
[kWh]
Space heating load
approx. 22,000
[kWh]
Domestic hot water load
approx. 1,500
[kWh]
Heat distribution system
Floor heating + radiators
Auxiliary heater
Condensing oil boiler 18 kW
Buffer storage volume (nominal)
1,000
[l]
Aperture area
19.03
[m²]
Collector azimuth
-39 (South=0)
[°]
Collector slope
48
[°]
Optical efficiency
0.798
[-]
Linear heat loss coefficient
3.34
[Wm-2K-1]
Quadratic heat loss coefficient
0.0075
[Wm-2K-2]
Heat capacity
9.5
[kJK-1]
Solar collectors:
The living space is heated by floor heating and radiators. The section for space
heating in the buffer storage has the set-point temperature of the radiators. A
three-way valve mixes flow and return in the heating circuit to supply the floor
heating with the required set-point temperature. Domestic hot water is provided
by an external heat exchanger connected to the storage. The condensing oil
boiler directly supplies the hot water section, the section for space heating and
the heat distribution system. The collector array consists of 10 collectors connected in line and operates with a water-glycol mixture. The volume flow rate
through the collector array amounts to 16.5 lh-1m-2. During the summer period
18
2 Collector Loads
some radiators in the basement operate to cool down the buffer storage and to
avoid stagnation in the collector array.
The installed measurement instrumentation consisted of a unit for recording
weather data as well as sensors for measuring the thermal loads on the collector
parts. Additionally, sensors to measure flow and return temperatures as well as
volume flow rates were installed in all heating circuits of the system.
The measurement devices for recording the weather data to evaluate the ambient
conditions consisted of a silicone pyranometer in the collectors’ plane as well as
a combined temperature and humidity sensor in a ventilated housing. Moreover,
a cup anemometer was installed in the collectors’ plane to measure the wind
speed along the glazing. Table 2.2 lists the sensors for measuring weather data
of the field-testing.
Table 2.2: Description of the sensors used in the field-testing for analysing the weather data
Measurand
Sensor
Accuracy
Solar irradiance
Silicone pyranometer
0–2.000 Wm-2
±(1.0 + 0.15T) %
Relative humidity
Capacitive humidity sensor
0–100 % r.h.
±(2.0 + 0.1T) % r.h.
and temperature
and resistance temperature
-30 – +70°C
±(0.3 + 0.0073T) K
0–50 ms-1
±0.5 ms-1
detector (RTD) – Pt-100
Wind velocity
Cup anemometer
For the evaluation of the mechanical loads on the solar circuit and therewith on
the absorber, a sensor for measuring the absolute fluid pressure was integrated
in the basement. In order to be able to analyse the operation of the collector array
and the volume flow rate, an electromagnetic flow meter was installed in the solar
circuit. In combination with two temperature sensors (calibrated by pair) in the
flow and the return of the solar circuit near the external heat exchanger at the
buffer storage, the heat flow rate of the solar array was determined. Table 2.3
demonstrates the sensors and their properties.
19
2 Collector Loads
Table 2.3: Description of the sensors used in the field-testing for analysing the solar circuit
Measurand
Sensor
Fluid pressure
Piezo-resistive pressure
(absolute)
transducer
Volume flow rate
Temperature
Accuracy
0–106 Pa
+0.2 %
Electromagnetic flow meter
0.8–3.39 m³h-1
[-0.17; +0.11]
RTD – Pt-100
-30 – +250°C
±(0.15+0.002T) K
The measured temperatures from the sensors at the parts of the collectors were
the most important results from the field-testing. For the latter, a new collector
equipped with measurement instrumentation was embedded in the existing solar
system. Furthermore, a dry collector without any connection to the solar-thermal
system and fluid was installed to refer to the maximum loads during continuous
stagnation year.
The last collector in the line of the array represents at the same time the hottest
collector during the system operation. Consequently, this collector was equipped
with temperature sensors at each part. Figure 2.1 shows the temperature sensors
at the back side and at the frame of the collector housing fixed with rivets using
heat-conductive paste.
Figure 2.1: Temperature sensors at the back side (left)
and at the frame of the housing (right; Reiter et al. 2012)
The three temperature sensors at the backside were distributed over the full
length of the absorber. Even two temperature sensors were fixed at the top edge
20
2 Collector Loads
of the frame near the inlet and outlet pipe. Moreover, two sensors were installed
at each side of the frame and at the bottom edge.
In total, the absorber sheet consists of nine riveted sensors. The sensors were
fixed in the middle between two riser pipes. Hence, the sensors were measuring
the highest temperatures at the end of the fin. Figure 2.2 shows the fixed temperature sensors at the absorber sheet in detail and in general view.
Figure 2.2: Temperature sensors at the absorber sheet
The temperatures at the glazing of the collector (top, middle and bottom) were
measured inside the collector by bonded surface sensors. Aluminium foils were
fixed around the sensors to shield them from incoming solar radiation. Table 2.4
shows the sensors used in the test collectors.
Table 2.4: Description of the sensors used in the field-testing for analysing
the collector temperatures
Measurand
Sensor
Temperature
RTD – Pt-100; riveted type
Temperature
RTD – Pt-100;
bonded foil type
Accuracy
-30 – +250°C
±(0.15+0.002T) K
-30 – +130°C
±(0.15+0.002T) K
In addition to the test collector in the system, an identical collector, which was,
however, not embedded in the system had been equipped with sensors and then
mounted on the roof. This collector which was permanently stagnating was used
as a reference for maximal temperature loads. The collector parts were provided
21
2 Collector Loads
with analogical temperature measurement instrumentation. Figure 2.3 shows the
installed test collectors on the roof.
Figure 2.3: Part of the collector array with the system integrated test collector (top, left)
and the test collector which is permanently stagnating (bottom, left; Reiter et al. 2012)
In addition to the solar circuit, the other heating circuits connected to the buffer
storage had embedded temperature sensors in flow and return (calibrated by
pair).In the return, a sensor for measuring the volume flow rate was installed to
analyse the transferred heat flow rates in the system. Additionally, a temperature
sensor was integrated in the buffer store at 80 % in relative height from the bottom. Table 2.5 shows the sensors used in the heating circuits.
Table 2.5: Description of the sensors used in the field-testing for analysing the heating system
Measurand
Sensor
Accuracy
Volume flow rate
Ultrasonic flow meter
0–10 m³h-1
±1.0 %
Temperature
RTD – Pt-100
0 – +100°C
±(0.15+0.002T) K
During the two and a half years testing period, the signals from the sensors were
logged and saved directly in a data base at the testing object. In a further step
22
2 Collector Loads
the results were analysed and visualised on a semi-automated way by using a
programmed script. As a consequence of that, the different stages of operation
or to be more precise the collector loads could be analysed. The evaluation of
the thermal loads was carried out by analysing the temperature gradient at certain
stages of the system. A detailed analysis on a day-to-day basis as well as annual
histograms for load profiles of the collector parts provided further information.
For analysing the results the sensors at absorber, glazing, frame and backside
with the most meaningful temperature data have been chosen. Figure 2.4 and
Figure 2.5 display the metering points of the collector in the system and the dry
collector out of operation used for the evaluation of the temperature loads on the
collector parts.
Figure 2.4: Chosen metering points at the collector in the system
23
2 Collector Loads
Figure 2.5: Chosen metering points at the dry collector
2.2 Thermal Collector Loads
The temperature loads of the collector parts were investigated regarding their
time dependent behaviour on the one hand and their annual temperature loads
on the other hand.
The time dependent temperature profiles show the maximum temperatures of the
parts at different weather conditions and operating points of the system to figure
out the potential points to act for overheating protection and an adoption of the
solar-thermal system to polymer needs. Furthermore, the dynamic of the temperature change and temperature differences between the parts provide an overview
for advices in design because of the thermal expansion for example.
The annual load profiles provide information about the duration at the various
temperature levels as well as about the maximum loads. This will be the basis for
the selection of materials and design details beside the development of overheating protection measures.
24
2 Collector Loads
2.2.1 Temperature Profiles of Collector Parts
The following evaluation of the collector part temperatures in detail is visualised
as a time dependent gradient.
The 16th August 2009 was chosen as an exemplary day for high temperature
loads on the parts during the testing period. On that clear, hot day the global
irradiance at the collector plane reached 1.000 W/m² and the ambient temperature rose to 34°C. Figure 2.6 shows the temperature profiles of the parts of the
1000
Insolation
220
800
Absorber
200
600
Temperature [°C]
180
400
160
200
Backside
140
0
2
240
Insolation [W/m ]
collector in the system.
Glazing
120
100
Frame, near outlet
80
60
Ambient
40
20
0
Volume flow rate
8:00
10:00
12:00
14:00
16:00
Figure 2.6: Temperature profile of the collector in the system during a summer day
(16th August 2009)
As early as in the operating phase of the solar-thermal system the absorber
reached final temperatures of up to 130°C. The sensors at the glazing and at the
frame near the outlet measured temperatures of about 60°C. Meanwhile temperature rose to approximately 50°C at the backside. After 11 o´clock a.m. the circulation in the solar circuit stopped and the stagnating state of the system set in.
25
2 Collector Loads
Therefore, the final temperature of the absorber soared to 190°C and even surpassed that level. During this period the temperature of the glazing rose to up to
80°C and the frame showed almost the same results. Furthermore, at the backside a peak value of 68°C was recorded. The graphic also demonstrates the dynamic movements of the temperature level of the absorber when the insolation
on the collector shortly decreased.
In Figure 2.7 the temperature profiles of the dry collectors’ parts on the abovementioned day are shown. With rising intensity of the sun the temperatures of the
absorber increased and reached a maximum of 195°C. The respective figure for
the glazing peak on this day amounted to 85°C. While the sensor at the frame
near the outlet was reaching temperature as high as approximately 70°C, the
1000
Insolation
220
800
Absorber
200
600
Temperature [°C]
180
400
160
200
Glazing
140
120
0
2
240
Insolation [W/m ]
backside was heating up to 65°C.
Frame, top
100
80
60
40
20
0
Ambient
Backside
8:00
10:00
12:00
14:00
16:00
Figure 2.7: Temperature profile of the dry collector during a summer day (16th August 2009)
Figure 2.8 visualises the temperature profile of the collector in the system on 9th
January 2009 while at the same time insolation was varying and low ambient
temperature prevailed. This diagram is a perfect example to demonstrate the dependency on the insolation. The figures for the absorber fluctuated heavily as the
26
2 Collector Loads
absorber was subject to solar irradiance, whereas the parts of the housing varied
only marginally. The temperature of the glazing was also affected slightly by these
changes. At about 1 o´clock p.m. the absorber temperature was hot enough to
supply the storage and the circulation set in. The fluctuations in fluid and absorber
temperature were obviously caused by a chopping of the pump in the solar system while the collector flow temperature was ranging around the set-point temperature of the controller. The absorber temperature was increasing until the collector operation started off. Afterwards, the temperature level was almost constant. The temperatures of the casing and the glazing had risen to 16°C and 19°C,
before the temperature dropped due to decreasing insolation (Reiter et al. 2009a,
2009b).
Absorber
80
500
Frame, near outlet
2
Temperature [°C]
600
Insolation
400
60
Backside
40
Insolation [W/m ]
100
300
Glazing
Ambient
20
200
0
100
Volume flow rate
-20
0
8:00
9:00
10:00
11:00
12:00
13:00
14:00
0
15:00
Figure 2.8: Temperature profile of the collector in the system (9th January 2009)
Figure 2.9 displays the temperature profiles of the dry collector for the same period. When having an insolation of only 600 W/m² and an ambient temperature of
-5°C, frame temperatures as high as 15°C and a backside temperature of 10°C
27
2 Collector Loads
were reached. During the peak sun-hours starting at about 12.30 o’clock the absorber temperature increased well above the level of 100°C whereas the absorber temperature of the collector in the system reached only 67°C. Under these
ambient conditions the temperature differences at the housing between the dry
collector and the collector in the system were only marginal. Moreover, the influence of the ambient air at these conditions was crucial for the housing parts.
Insolation
Absorber
80
500
Frame, top
2
Temperature [°C]
600
Insolation [W/m ]
100
60
400
Glazing
40
300
Backside
20
200
0
100
-20
8:00
Ambient
9:00
10:00
11:00
12:00
13:00
14:00
0
15:00
th
Figure 2.9: Temperature profile of the dry collector (9 January 2009)
Beside the maximum values of the temperatures at the collector, the dynamic
behaviour of collector parts is of significant importance. On the one hand the
quick temperature changes have to be considered in the construction of the collector because the parts have different thermal expansion properties. On the
other hand time intervals for the reaction of overheating protection measures can
be estimated. Figure 2.10 exemplary shows a rapid increase of the absorber temperature in the dry collector on 14th January 2008. Back then the absorber temperature rose to 140 K within 40 minutes although the ambient temperature was
slightly above 0°C. In this case, the housing parts saw only a slight increase in
28
2 Collector Loads
temperature to about 25 K. As a reason for this, housing parts can be characterised as relatively inert.
900
140
800
120
700
Absorber
100
Frame, top
Glazing
80
Backside
60
600
500
400
Frame, bottom
300
2
145°C
Insolation
Insolation [W/m ]
Temperature [°C]
160
40
Ambient
20
5°C
100
0
10:50
200
11:00
11:10
11:20
11:30
0
Figure 2.10: Temperature increase of the dry collector (14th December 2008; Reiter et al. 2012)
Figure 2.11 clearly shows the heavy fluctuations in the temperature of the absorber during stagnation. The chart demonstrates that the absorber reacted
heavily within the given period to boundary conditions whereas the frame and
backside sheet fluctuated only marginally. Within 2.5 h the temperature has risen
to 125 K and again decreased to 121 K. The sequence of the cooling curve is
larger than the expected cooling curve with constant cloudiness caused by short
periods of high insolation. The longer-than-expected cooling curve has been
caused by short periods of high insolation while the sky was largely cloudy. During this period the temperature of the housing rose from 12°C to 35°C. The temperature of the bottom edge reached its maximum level of almost 26°C. The ambient temperature varied between 9°C and 14°C (Reiter et al. 2009a, 2009b).
29
2 Collector Loads
180
1000
173°C
Insolation
160
800
120
2
Temperature [°C]
140
600
Glazing
100
Backside
Frame, top
80
60
Insolation [W/m ]
Absorber
400
Frame, bottom
48°C
53°C
40
200
20
Ambient
0
9:30
10:00
10:30
11:00
11:30
0
12:00
Figure 2.11: Temperature increase and decrease of the dry collector
(26th September 2008; Reiter et al. 2012)
2.2.2 Annual Temperature Load Profiles
The annual occurring thermal loads were evaluated using histograms with a bin
width of 10 K by applying the temperatures of the collector parts from 1st January
2009 to 31st December 2009 (Reiter et al. 2010a and 2010c). Due to problems
with the data logger in March and June 30 days of the measurement period are
disregarded. The coordinated systems in the diagrams are scaled to display the
high temperatures. Hence, the data at low temperatures near the ambient temperature level are outside of the array and cannot be considered in detail.
The absorber in the collector in the system reached temperatures of up to 140°C
during the operation of the solar circuit. On the contrary, the absorber sheet in
the stage of stagnation temperatures rose to 192°C. The high temperatures were
measured near the flow of the absorber in the upper part of the collector. Figure 2.12 shows the annual evaluation of this metering point. The solar-thermal
30
2 Collector Loads
system hardly came to a halt although the collector array has large dimensions.
This result was caused by the operation of the radiators in the basement. These
were used as an overheating protection measure to avoid the stagnation of the
system because the maximum storage temperature will not be exceeded. The
general scaling of the y-axis was chosen from 0 to 500 hours to highlight the
relevant temperature levels of the collector parts in the analysis. The cut-off durations at moderate temperature levels near the ambient temperature have a minor importance for the investigation of critical thermal loads for polymeric materials.
500
1,850
1,940
988
400
300
200
100
1.5
0
-20
0
20
40
60 80 100 120 140 160 180 200
Temperature [°C]
Figure 2.12: Histogram of the temperatures of the absorber sheet
of the collector in the system (according to Reiter et al. 2012)
The hottest measurement point in the dry collector was situated in the middle of
the sheet. The logger recorded a maximum temperature of 208°C. Figure 2.13
demonstrates that the absorber has reached high temperatures several times.
For more than 1,100 h in the year the absorber was at temperature levels above
95°C.
31
2 Collector Loads
500
1,925
1,978
897
Duration [h]
400
300
200
0.07
100
0
-20
0
20
40
60 80 100 120 140 160 180 200
Temperature [°C]
Figure 2.13: Histogram of the temperatures of the absorber sheet of the dry collector
(according to Reiter et al. 2012)
The temperature of the transparent cover was measured with a bonded sensor
at the inner surface installed at the centre of the sheet. Thermal load above 55°C
mainly occurred during stagnation periods of the system (Figure 2.14). The highest measured temperature has been 82°C. Almost always the cover temperature
for the use of transparent polymeric materials has been ranging within harmless
temperature ranges.
The temperature sensor of the cover in the dry collector had the same position.
The metered values were significantly higher than the values of the collector in
the system (Figure 2.15). Within the stated period the cover temperature was
under consideration 585 h over 55°C with the maximum temperature of 86°C.
32
2 Collector Loads
500
2,347
2,357
1,169
730
559
Duration [h]
400
300
200
100
1.5
0
-20
0
20
40
60 80 100 120 140 160 180 200
Temperature [°C]
Figure 2.14: Histogram of the temperatures of the glazing of the collector in the system
(according to Reiter et al. 2012)
500
2,337
2,379
1,055
514
Duration [h]
400
300
200
100
0.08
0
-20
0
20
40
60 80 100 120 140 160 180 200
Temperature [°C]
Figure 2.15: Histogram of the temperatures of the glazing of the dry collector
(according to Reiter et al. 2012)
33
2 Collector Loads
In order to identify the temperatures of the frame sensors have been riveted on
the inner surface of the aluminium frame. On each edge two sensors have been
fixed between the side insulation and the frame. The investigated and displayed
sensor has been positioned at the top edge near the flow. Consequently, with this
position it was the hottest measurement point of the frame. Figure 2.2 shows the
sensor in the right picture. At this position the temperature reached a value of
79°C. The major part of the measured period, however, the frame temperature
remained below 65°C due to the influence of the ambient temperature. Figure 2.16 displays the common low temperature level near the flow.
500
2,251
2,387
1,332
691
400
300
200
100
1.2
0
-20
0
20
40
60 80 100 120 140 160 180 200
Temperature [°C]
Figure 2.16: Histogram of the temperatures of the frame of the collector in the system
(according to Reiter et al. 2012)
The metering point at the dry collector was positioned at the centre of the upper
frame part. The temperature reached slightly lower values as the hot header pipe
did not have any impact on the absorber (Figure 2.17).
34
2 Collector Loads
500
2,337
2,467
1,257
579
Duration [h]
400
300
200
100
0
-20
0
20
40
60 80 100 120 140 160 180 200
Temperature [°C]
Figure 2.17: Histogram of the temperatures of the frame of the dry collector
(according to Reiter et al. 2012)
The temperature of the backside plate of the collector in the system was measured in central position. Figure 2.18 shows that relatively low temperatures of this
part were recorded. The temperature level of 68°C was not exceeded. Generally,
this level is very low and is not considered to have negative impacts on the process of the material selection for this array.
In comparison to another metering point in the upper part of the collector in the
system below the header, the dwell at temperatures with around 50°C and 60°C
was significantly higher. The maximum temperature of 72°C differed only marginally. The metering point of the dry collector was also in central position of the
backside sheet. The maximum temperature of the sensor in the collector which
was not installed in the system was 66°C (Figure 2.19). Finally, during the whole
year no harmful temperatures for low-cost polymers were recorded.
35
2 Collector Loads
500
2,287
2,449
1,427
752
Duration [h]
400
300
200
100
0.7
0
-20
0
20
40
60 80 100 120 140 160 180 200
Temperature [°C]
Figure 2.18: Histogram of the temperatures of the backside sheet
of the collector in the system
500
2,297
2,449
1,518
667
Duration [h]
400
300
200
100
0.4
0
-20
0
20
40
60 80 100 120 140 160 180 200
Temperature [°C]
Figure 2.19: Histogram of the temperatures of the backside sheet of the dry collector
36
2 Collector Loads
In summary, the field-testing measurement shows that the material selection depends on the specification of each part of the collector regarding to the temperature gradient. Hence, a common maximum temperature of the collector as it is
currently determined in standardised measurement procedures e.g. according to
DIN EN 12975-2 (2006) is not an adequate approach. As a result from field-testing the measured data clearly show that temperatures at the housing do not represent major obstacles for low-cost polymers, even in state-of-the-art collectors.
The absorber as well as the arrays with contact to the absorber such as the insulation reach high temperatures and are at the same time subject to considerable
temperature fluctuations.
2.3 Pressure Loads in the System
The pressure loads were measured in the solar circuit near the store in the basement. The solar circuit is equipped with a membrane expansion vessel (nominal
volume 60 l) and a safety valve, which allows a maximum pressure of 600,000 Pa.
The measured absolute pressure in the circuit combines the static pressure and
the dynamic pressure in the system. The value of the static pressure consists of
the sum of the pressure of the fluid from filling the system as well as of the weight
of the hydrostatic head in the pipe up to the collector (Hadamovski and Jonas 2000).
There are three elements contributing to an increase in the system pressure,
namely the dynamic pressure caused by the pump as well as by the pressure
drop through friction at the walls of the pipes and the third component is the viscosity of the fluid.
The pressure mainly depends on the fluid temperature. The hot fluid has an expanded volume and fills the membrane expansion vessel while the gas buffer is
compressing and the pressure is rising steadily. Under extreme weather conditions the hot fluid even evaporates and its volume rises enormously. This state
37
2 Collector Loads
occurs during stagnation of the system and as a consequence of which the pressure reaches the peak value. Figure 2.20 shows the three described system
states in the membrane expansion vessel.
Figure 2.20: Operating conditions of the membrane expansion vessel
(according to Hadamovski and Jonas 2000)
Figure 2.21 shows the gradient of the system pressure during the night, system
Pressure [Pa]
Temperature [°C]
operation and stagnation (red array).
Figure 2.21: Increase and decrease of the pressure in the solar circuit
during operation and stagnation (1st August 2009)
38
2 Collector Loads
In the case of the above-mentioned field-testing the system pressure during the
night was lower than the primary pressure in the membrane expansion vessel.
Hence, the vessel was empty and the pressure of the fluid in the system rose
along with the temperature. The system pressure was rising between 6.00 and
8.00 o’clock a.m. until the system pressure is equal to the primary pressure in the
expansion vessel. Afterwards, the fluid filled the vessel and compressed the gas
pocket. During this process the system pressure was relatively constant because
the fluid volume changed only slightly in comparison to the gas volume of the
vessel. In the interval between about 8.00 and 12.00 o’clock a.m. the solar-thermal system was in operation. After operation the fluid temperature rose abruptly
and the fluid evaporated. During stagnation the volume change of the evaporated
fluid was large and the gas pocket was compressed strongly. Hence, the pressure
was subject to rapid changes and reached its maximum.
Figure 2.22 demonstrates that under low temperatures the static pressure was
between 100,000 Pa and 150,000 Pa during the night. The pressure under the
conditions during the operation of the solar-thermal system was around
290,000 Pa.
2000
System in operation
and
at low temperatures
Duration [h]
1600
System in stagnation
(durations <10 h)
2,210
2,980
1200
800
400
0
0
1x10 5
2x10 5
3x10 5
Pressure [Pa]
4x10 5
5x10 5
Figure 2.22: Histogram of the fluid pressure in the solar circuit
39
2 Collector Loads
Stagnation in the system only occurred when the fluid or absorber temperatures
rose above 140°C. Figure 2.23 shows the duration of the system pressure during
stagnation in a detailed view. The pressure rose in a range between about
Duration [h]
300,000 Pa and a peak value of 462,000 Pa.
Figure 2.23: Histogram of the fluid pressure in the solar circuit during stagnation (detailed view)
As expected the system pressure rises to high values and is a main obstacle for
plastics with limited mechanical strength. In particular at temperatures near the
maximum temperature of the material the pressure load is at its maximum value.
The mechanical strength, however, decreases with the material temperature.
Moreover, most volumetric absorbers designs used for polymeric absorber constructions are even more deformable under these conditions because of their disadvantageous large channel width. Hence, a non- or extremely low-pressurised
system is necessary for the use of plastics in absorber.
40
3 Overheating Protection
3 Overheating Protection
The identified critical high temperature loads — especially at the absorber — have to be avoided to realise polymeric collector designs. Therefore,
potential concepts for the necessary overheating protection have to be developed
and investigated according to the conceptual design process described in
Pahl et al. (2007).
In a functional analysis of the collector and the solar-thermal system, the main
functions and sub-functions of the components were analysed in order to identify
and systematise basic operating principles which avoid critical component temperatures. Accompanied by a worldwide literature and patent research in various
fields of technology, this was followed by the development and the investigation
of applicable overheating protection approaches. Finally, a preselection of the
most promising approaches according to their obvious properties was conducted.
This selection is the basis for an extensive simulation analysis regarding thermal
collector loads and solar yield.
3.1 Functional Analysis of Collector and System
For the development of overheating protection concepts, the complete system
was analysed with regard to its sub-functions and was mapped onto a function
structure based on thermal processes. The function structure consists of several
levels, where the top level shows the overall function considering the flow of material, energy and signals. This can then be divided into main functions and subfunctions of the different parts of the collector. The advantage of this methodology
is the illustration of the entire thermal processes regarding components and system in the detailed function structure, where potential possibilities for overheating
protection and their operating principles can be identified systematically. The system was therefore divided into two parts, where on the one hand the collector and
on the other hand the solar system without the collector were analysed.
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3 Overheating Protection
On the top level of the function structure of the collector only the flows of material,
energy and signals crossing the system boundary of the collector were considered (Figure 3.1). The overall function of the collector was defined as "conversion
of radiation into heat’’. The heat is removed from the collector by a cold medium
flow through the collector. A part of the collector’s energy is released due to
losses from the collector to the ambient. A control signal is led from the collector
to the solar system controller. Figure 3.1 illustrates that the collector is supplied
with energy by solar radiation and dissipates the energy via losses and fluid circulation. Thus, there are two obvious principles to prevent overheating: Either the
incidence respectively the conversion of solar radiation in the collector is reduced
or the thermal energy in the collector is removed.
Control signal
to system
Medium - Cold
Radiation - Short wave
Medium - Hot
Conversion of
radiation into heat
Heat to system
Energy losses
to ambient
Figure 3.1: Function structure of the solar-thermal collector with overall function
and flow of material, energy and signals
The main functions of the collector refer to the parts of the collector (Figure 3.2).
The transparent cover has the functions to let pass the incoming short-wave radiation and to reduce the energy losses of the absorber at the front side. The
functions of the cover sheet are obviously connected to energy losses. The absorber converts the solar radiation into heat, which is removed from the collector
by the fluid, and as well causes energy losses. The control signal contains the
temperature of the absorber. The back side insulation reduces the heat losses of
the absorber.
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3 Overheating Protection
Control signal
to system
Medium - Cold
Medium - Hot
Heat to system
Transmission of radiation
+
Reduction of heat losses
at the front side
Conversion of
radiation into heat
Reduction of the
heat losses
at the back side
Radiation
Short wave
Energy losses
to ambient
Heat losses
to ambient
Figure 3.2: Function structure of the solar collector with main functions
of transparent cover, absorber and back side insulation
Figure 3.3 shows the next deeper level of the function structure for the cover with
its sub-functions.
Transmission
of radiation
Radiation
Short wave
Removal
of the heat
Heat losses
to ambient
via radiation
Heat
conduction
Heat losses
to ambient
via convection
Energy losses
to ambient
via reflection
Absorption
of the heat
Reduction
of heat losses
via convection
Energy losses
from absorber
via reflection
Heat losses from absorber via convection
Heat losses from absorber via radiation
Figure 3.3: Sub-functions of the transparent cover
Possibilities for overheating protection at the transparent cover can for example
be found in the functions “transmission of radiation” and “reduction of convection”.
Hence, the collector could be protected by reducing the amount of radiation transmission or by increasing the convective losses at the front side.
Figure 3.4 shows the function structure of the absorber with its sub-functions. In
the operating state of the solar-thermal system a cold medium flows through the
absorber while the absorber converts the incident solar radiation into heat and
transfers the heat to the medium. Potential overheating protection at the absorber
could be the variation of the absorption characteristics for solar radiation or the
variation of the emission characteristics regarding infrared radiation. Apart from
that, unused heat could be transported out of the collector by the fluid loop.
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3 Overheating Protection
Control signal
to system
Medium - Cold
Medium - Hot
Removal
of heat
Absorption
of heat
Conveyance
of the medium
Heat to system
Radiation
Short wave
Absorption
of radiation
Conversion
into heat
Heat
conduction
Temperature
Measurement
Heat losses to back side insulation
Energy losses to glazing
via reflection
Heat losses to glazing via radiation
Heat losses to glazing via convection
Figure 3.4: Sub-functions of the absorber
Figure 3.5 illustrates the function structure of the back side insulation with its subfunctions. At the back side insulation for example the function “reduction of heat
transfer” could actively be influenced depending on the operating state.
Absorption
of heat
Heat
conduction
Removal
of heat
Reduction
of heat losses
Heat losses from absorber
Heat losses to ambient
(mainly via convection)
Figure 3.5: Sub-functions of the back side insulation
Apart from the collector, the solar-thermal system was analysed where the overall
function is to supply the consumer with heat (Figure 3.6). A typical system has a
thermal storage that absorbs and emits thermal energy through several fluid circuits to execute this overall function. The system controls the fluid circuit of the
collector and extracts thermal energy from the medium. Additionally, heat is supplied by a back-up system. The control signal of the collector is transferred to the
controller, which activates or deactivates the fluid circulation. Electricity is required by the system to operate the circulation pumps as well as the measurement and control equipment. Thermal energy is removed from the system by heat
44
3 Overheating Protection
supply to the consumer and by losses. Thus, the system provides two basic possibilities for the removal of thermal energy from the collector: The energy can be
removed by thermal losses to the ambient or by transferring the heat to a sink.
Control signal
from collector
Medium - Cold
Medium - Hot
Heat from collector
Heat to consumer
Supply with heat
Heat from auxiliary heater
Electricity
Energy losses Control signal to
to building
back-up heating
Figure 3.6: Function structure of the solar-thermal system (without collector) with overall function and flow of material, energy and signals
Figure 3.7 shows the function structure of the solar-thermal system with its main
functions related to assembly groups. The first group consists of the fluid circuit
connected to the collector and the controller, which activates the fluid circuit and
transfers heat from the collector to the thermal storage. For managing the operation mode of the pump, the controller uses the temperature signals from the
collector and the thermal storage. The second assembly group contains the thermal storage unit transferring heat to the consumer and absorbing heat from the
back-up heating if required.
Control signal
from collector
Medium - Cold
Heat to consumer
Medium - Hot
Heat from collector
Electricty
Transport of medium and heat
+ Transfer of heat
+ Control of the solar circiut
Energy losses
to building
Storage of heat
Heat losses
to building
Heat from auxiliary heater
Control signal to
back-up heating
Figure 3.7: Function structure of the solar-thermal system with main functions of the assembly
groups fluid circuit and thermal storage
Figure 3.8 shows the function structure of the fluid circuit with its sub-functions.
The pipes of the fluid circuit convey the medium from the solar collector to the
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3 Overheating Protection
heat exchanger in order to transfer the heat of the medium to the thermal storage.
There are thermal losses in the piping through heat conduction to walls and convection at the insulation of the pipes.
Energy losses
to building
Control signal
from collector
Medium - Hot
Medium - Cold
Heat from collector
Conveyance
of the medium
Circulation
of the medium
Heat
conduction
Pump
control
Heat
transfer
Heat to storage
Electricity
Removal
of the heat
Control signal
from storage
Heat losses to building
(mainly via convection)
Figure 3.8: Sub-functions of the fluid circuit
The thermal storage shown in Figure 3.9 absorbs the heat from the collector and
the back-up heating and emits the heat to the consumer, when it is requested.
When heat is stored, there is a loss of thermal energy due to heat conduction
through the storage wall and convection at the surface of the insulation. The integrated temperature measurement transmits information for the controller of the
fluid circuit and the back-up heating regarding charging of the thermal storage.
Heat from
solar circuit
Control signal
to solar circuit
Heat to consumer
Storage
of heat
Temperature
measurement
Control signal to
back-up heating
Heat
conduction
Heat from auxiliary heater
Removal
of heat
Heat losses
(mainly via convection)
Figure 3.9: Sub-functions of the thermal storage
3.2 Overview on Approaches for Overheating Protection
Based on the derived function structure of component and system, where fundamental mechanisms regarding overheating could be identified systematically, applicable operating principles and solution approaches for overheating protection
of collectors were developed (Reiter et al. 2010b; Reiter et al. 2012). Additionally,
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3 Overheating Protection
a worldwide literature and patent research in various areas of technology was
carried out. Finally, approaches were summarised and classified aiming at the
creation of a systematised overview on known, studied or potential overheating
protection measures for solar collectors.
For an overall systematisation of overheating protection measures, two basic
principles have to be differentiated: firstly, the optical efficiency can be reduced
or, secondly, the removal of excess thermal energy can be increased (Figure 3.10).
Overheating protection
Reduction of
optical efficiency
Reduction of
transmission
properties
Removal of
thermal energy
Reduction of
absorption
properties
Increase of
thermal losses
Active cooling
of collector parts
Figure 3.10: Systematisation of overheating protection measures
(according to Reiter et al. 2012)
A reduction of the optical efficiency means that either the collector front side
transmits less solar radiation or alternatively the absorber surface converts less
solar radiation into heat. Hence, these measures change the effective transmission-absorption product. Figure 3.11 shows the potential mechanisms that can
reduce the optical efficiency of a collector.
Reduction of optical efficiency
Reduction of transmission properties
Absorbing effects
Scattering effects
Reflection effects
Reduction of absorption
properties
Reduction of absorption
coefficient α
Reduction of absorbing
area
Figure 3.11: Overview on mechanisms reducing the optical efficiency
(according to Reiter et al. 2012)
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3 Overheating Protection
In addition to reducing the optical efficiency, the removal of excess thermal energy is an option (Figure 3.10) realised either by thermal losses of the solar collector or by cooling with fluid circuits. Figure 3.12 shows the categories for the
removal of thermal energy, which can be applied in the collector or in the solarthermal system.
Removal of thermal energy
Increase of thermal losses
Increase of convective
losses
Active cooling of collector parts
Increase of emission
coefficient of absorber
Heat sink at the collector
Heat sink within the solar
hydraulic loop
Heat sink within the
heating system
Figure 3.12: Overview on mechanisms and solutions for the removal of excess thermal energy
(according to Reiter et al. 2012)
For any mechanism shown, several technical solutions are in principle available
and described below.
3.3 Changing the Transmission Properties
The transmittance of a collector can be reduced by measures having absorbing,
scattering or reflecting effects. These measures can be characterised into their
technical solutions presented in Figure 3.13.
Thermally
switchalbe
layers
Electrically
switchable
layers
Chemically
switchable
layers
Translucent
appliances
Mechanical
appliances
Hydraulic
appliances
Figure 3.13: Groups of technical solutions for the reduction of the transmission properties
(according to Reiter et al. 2012)
3.3.1 Thermally Switchable Layers
Thermal switchable layers for changing the transmission properties of the front
side of the collector are clear for solar irradiation in the normal state and reduce
their transmittance above a certain temperature range. Therefore, the material
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3 Overheating Protection
properties become partially absorbing or scattering for sun light. Due to these
effects it must be differentiated between thermochromic and thermotropic layers.
3.3.1.1 Thermochromic Layers
After having reached a certain temperature, thermochromic layers change their
colour and/or its absorption or reflection behaviour. As an application to the
glazing, the above-mentioned layers are able to absorb the incoming solar
radiation which normally increases during periods of high temperature.
Afterwards, the converted heat is transferred to the ambient air as well as to the
cold sky.
Babulanam et al. (1987) investigated a layer for “smart” windows eliminating the
incidence of sun light at high ambient temperatures while solar gains at low ambient temperatures are accepted. The layer mainly consists of VO2 and rapidly
changes its semiconductor state into a metallic state at 68°C. For solar irradiance,
the layer is very transparent in its semiconductor state. Should the layer convert
into a metallic state, the main infrared part of the insulation is reduced by reflection and absorption. For the production of the layer, physical vapour deposition
(PVD) is used. In this process an evaporated coating material condenses on the
substrate in an evacuated space. In this case, the coating material is evaporated
by sputtering. Therefore, the target or rather the coating material is bombarded
with ions to extract vaporised material. The sputter deposition is carried out at
temperatures between 300°C and 500°C. Hence, this layer is not suitable for an
application on polymeric parts.
3.3.1.2 Thermotropic Layers
A thermotropic layer is a thin film suitable for deposition on a glazing or an absorber being transparent up to a certain trigger temperature. Above the trigger
temperature this layer changes its state into a milky white state and is scattering
or rather reflecting for incoming irradiance due to the particles which have evolved
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3 Overheating Protection
and/or released during the process, crystals or droplets having a different refraction index when compared to the remaining material of the matrix. On bounding
surfaces with different refraction indexes, the incoming beam changes its direction. Hence, the layer reduces its transmission coefficient since the light is deflected in all directions at a multitude of bounding surfaces. Several approaches
for layer materials are possible to get this effect. These approaches can be divided into two main groups with phase separating systems on the one hand and
systems with fixed domains on the other hand (Nitz and Hartwig 2005; Nitz and
Wilson 2008).
The advantage of this overheating protection measure lies in the autonomous
activation caused by the actual temperature. Thus, the protection is directly connected to the temperature of the component. This factor, however, needs to be
controlled. The layer works automatically and does not have to be started by any
mechanism and is intrinsically safe since it also functions during blackouts of the
solar-thermal system or of the grid.
The phase separating systems consist of different components decomposing
their phases above a trigger, hence a demixing temperature. Therefore, the material changes into a light scattering state. The principle occurs in various systems:

Hydrogels have been developed for the use in smart windows. The layers
consist of an aqueous polymer gel containing another long-chained polymer. Such polymer chains fade to a light scattering state above a certain
trigger temperature. The gels are produced in a sandwich construction between two transparent plates to protect them against dehydration. Therefore, high standards have to be applied for the sealing. Also, the long-term
stability level is very low and the layer has to be protected against UV
radiation by a protective film (Nitz and Hartwig 2005; Resch and Wallner
2009). Hydrogel layers can be adjusted for trigger temperatures between
5°C and 60°C. A maximum transmission coefficient of 82 % is possible.
After the opaque state has been reached, the transmission coefficient can
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3 Overheating Protection
be reduced by up to 77 % (Resch and Wallner 2009). The costs vary significantly depending on the use of natural or synthetic materials. For windows in the building sector, a Japanese product with such a layer available
has already been launched. The high requirements for sealing and UVprotection as well as the heavy weight and the material costs for the double
glazing sandwich make the application in polymeric low-cost collectors almost impossible although the switching properties (change of transmission
and trigger temperature) are promising (Resch and Wallner 2009).

Liquid Crystals are used for layers to reach a light scattering level above
a certain trigger temperature, achieved by means of a phase change or
phase separation. Until now, no appropriate product has been launched
onto the market (Nitz and Hartwig 2005).

Polymer Blends are layers made of two liquid components turning into a
homogenous mixture below a certain temperature. In the mixture, there
are no local differences of the refraction indexes, whereas the layer appears clear. Above the set temperature, the components begin to separate
and local regions with different refractive indexes can occur. As a result,
the incoming radiation disperses, lacking clear orientation. Only partially
suitable switching properties are provided for protecting the layer against
overheating. The trigger temperature varies between a level of 30°C and
130°C, whereas the transmission coefficient for solar irradiation shows a
level of 85 %. Transmission coefficients in the opaque state below 38 %
are possible. However, switching from the clear state to the opaque state
only occurs when wide temperature ranges are involved. Furthermore, the
change from an opaque to a clear state has a large hysteresis of 15 hours.
The layer is also damageable by humidity, causing an opaque state. Since
there is still no suitable product available on the market and based on the
above-mentioned problems, the use of such a layer type for overheating
protection in collector is not possible (Resch and Wallner 2009; Nitz and
Hartwig 2005).
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3 Overheating Protection
Systems with fixed domains consist of two components with one component being the polymer matrix that contains the second component. The embedded component changes its refractive index when changing from a solid to a liquid state.
Both materials show the same refractive index in the solid condition. If the component in the matrix melts, the refraction index changes and the material mixture
scatters light (Resch and Wallner 2009; Nitz and Hartwig 2005). Layers with a
thermotropic resin system significantly change their transmission within a transition region of 15 K. The maximum transmittance of these systems lies at 84 %
with a reachable change of 25 %. The trigger temperatures can be varied by additives in an array between 25°C and 100°C (Resch and Wallner 2009).
Wallner et al. (2008) investigated possible applications of thermotropic glazings
in solar-thermal collectors by means of simulation. The efficiency curves of various design approaches at 0°C and 30°C ambient temperature and 1,200 Wm-2
were computed. The approaches of the investigation were carried out by flat-plate
collectors with single glazing and black absorber, single glazing with selective
absorber, double glazing with black absorber and double glazing with selective
absorber. The results indicated a large influence of the ambient temperature on
the performance of the collector approaches with selective absorber. The lower
heat radiation from the absorber to the glazing reduced the overall heat flow rate
through the collector front side. Hence, the influence of the ambient conditions on
the glazing as well as the thermotropic layer temperature rose enormously. In
that case, it is not possible to define a suitable trigger temperature of the layer to
protect the collector parts and to reach an efficient collector operation in all situations over the year. There is also an influence of the ambient temperature on
the layer performance at the set-up with black absorber and single glazing. Therefore, a collector with black absorber and a layer on the inner surface of the double
glazing is regarded as most suited because of the thermal insulation by the embedded air volume.
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3 Overheating Protection
3.3.2 Electrically Switchable Layers
Electrically switchable layers change their optical properties by means of an electric current or an electric field. These technologies have not found their way onto
the market yet as they are still in the beginning stage of being established and
are thus also very expensive. For blocking the transmission of solar irradiation
through transparent material the following technologies are feasible.
3.3.2.1 Electrochromic Layers
Electrochromic layers are used as sun protection glazings in facades/windows in
the building sector as well as in car windows (Flachglas Markenkreis; Saint-Gobain Sekurit; GESIMAT). An intended blue colouring of the layer absorbs a part
of the incoming insolation which helps to decrease the transmittance. The effect
can be compounded by building up a multi-layer. Two transparent conductive
layers at outer surfaces include a film made of tungsten oxide (WO3), a film containing charge carriers (mostly lithium ions) and a separating, conductive polymer
film in the middle. In the transparent state tungsten oxide and lithium ions only
occur in their own separated film. After having closed the electric circuit of the two
outer conductive layers the lithium ions penetrate into the interlayer, enter the film
with tungsten oxide and build molecules out of the tungsten oxide. These molecules cause the blue, absorbing colour in the multi-layer build-up. The intensity
of the colouration can be varied by control from outside. When being in a steady
state, an electrical field and/or electric supply become completely irrelevant. Electric energy is only expanded to colour and discolour (BINE Informationsdienst
2002). Switching between clear and coloured state can take up to 15 minutes.
The electric current flow cannot be increased unlimitedly since high current flows
cause irreversible reactions in the tungsten oxide film (Dittrich 2003).
Granquist (1992) tested a WO3 layer with a transmittance level between 15 % and
80 %. The layer was damageable by dehydration of the liquid electrolytes and
showed a strong dependency to the temperature. Thereby, the excellent switching properties could only be reached at room temperature. Moreover, it is only
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3 Overheating Protection
possible to deposit the layer on small areas like shading rear mirrors in cars or
displays. Also Mathew et al. (1997) investigated an electrochromic layer on the
basis of tungsten oxide with a transmission factor of 70 % to 80 % in clear state
and 5 % to 10 % in modified state. The layer was tested at temperatures of up to
85°C but showed an insufficient long-term stability in this temperature range. The
shifting time was less than 5 minutes and in an endurance test 20,000 switches
without any defects were conducted.
Due to the limited thermal resilience and the high costs of these layers, this approach is not capable of being integrated as overheating protection in polymeric
solar-thermal collectors.
3.3.2.2 Electrotropic Layers
These electro-optical layers have been developed for screen glazing (Interpane;
Saint-Gobain Glass). The layers have optical characteristics similar to thermotropic layers. The layer can switch from a transparent to a milky white state and
appears light scattering. This technology is used for screens in outdoor and indoor applications of buildings and means of transport as well as for projection
surfaces. Such so-called LC (liquid crystals) layers consist of a laminate of two
plastic films with indium tin oxide layer and an interlayer of liquid crystals. These
crystals are normally non-directional oriented and scatter the incoming light. At
each film there is a pole of the power supply connected. In case of a voltage
application, an electric field is generated between the layers, which aligns liquid
crystals and change the layer into a transparent state. The layer is only transparent during an operation voltage supply. Hence, the system is intrinsically safe for
overheating protection in case of power blackouts. However, the layers only
achieve a low reduction of the solar transmission and have a low maximum operation temperature. According to these disadvantages, the application in solarthermal collectors is not feasible.
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3 Overheating Protection
3.3.2.3 Switchable Mirrors
Huiberts et al. (1996) report investigations with regard to hydrides of yttrium, lanthanum and trivalent rare earths changing from a clear state to a reflecting state
due to the admittance of hydrogen and the triggered reaction. A thin palladium
coating protects the layer from oxidation with ambient air and catalyses the hydrogen molecule adsorption and hydrogen atom absorption. A layer consisting of
gadolinium magnesium alloy (GdMg) with a reflection of 70 % is considered to be
an advancement. The layer is black in the transition region and absorbs light similarly to electrochromic layers. The clear state has a transmittance of more than
70 %. The hydrogen for the switching operation is extracted from a reservoir between the GdMg layer and a transparent counter electrode. The voltage from the
power supply between GdMg layer and transparent counter electrode drifts the
hydrogen in the GdMg layer and converts the layer into a clear state. If hydrogen
is to be removed in order to get a reflective state, the voltage has to be switched.
Thus, the hydrogen is pushed back to the reservoir (van der Sluis et al. 1997).
Another improvement is a so-called all-solid-state layer consisting of a WO3 layer
as hydrogen reservoir, which is by far easier to produce (Griessen und van der
Sluis 2001). This kind of layer is developed for architecture, displays and optical
devices. However, a low-cost, marketable solution for the requirements as overheating protection in solar-thermal collectors is still not available.
3.3.3 Chemically Switchable Layers
A chemically switchable or rather gasochromic glazing consists of two glass
sheets with a gap. On the inner surface, a WO3 coating and a catalyst layer are
placed. By means of a gas supply, hydrogen flows into the gap. The catalyst layer
converts the hydrogen molecules into hydrogen atoms reacting with the oxide of
the WO3 layer to water. The water vapour passes from the surface and leaves
oxide defects in the WO3 coating changing the layer´s colour (Georg et al. 1998).
This leads to a transmission decrease from 50 % to 15 % (BINE Informationsdienst 2002). This technology is used for controlling solar transmission of
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3 Overheating Protection
windows in buildings and is still in an early stage of development. For that reason,
the devices are very expensive and not suitable for overheating protection in collectors.
3.3.4 Translucent Appliances
Translucent appliances are devices with a partial light transmission, which cannot
be directly controlled. In case of an overheating protection for collectors, translucent appliances partially block the incoming sun light. Increasing the efficiency of
a collector by using insulations, covers and selective layers while permanently
decreasing the efficiency by reducing the transmittance of the front side seems
to be a conflict at first glance. Therefore, the translucent element is adjusted to
reduce the insolation on the absorber only when heat is not required or rather
when an excessive supply of solar energy is present. Most of the time such scenarios occur during summer which is characterised to have a small zenith angle
of the sun. It is recommended to have translucent elements to control the solar
transmission depending on adjusted angles. Hence, translucent elements are a
seasonal overheating protection approach. The irradiation of the sun in autumn,
winter and spring shall smoothly penetrate through the window. In summer time,
however, incoming irradiance with small zenith angles is reflected, scattered or
refracted to reduce the amount of solar energy on the absorber. Depending on
the configuration of the solar-thermal system it may be a disadvantage to reduce
the optical efficiency permanently during summer. Critical thermal operational
states for polymeric parts can easily also occur in spring and autumn when this
measure is not applicable. Hence, for translucent elements there is no intrinsic
safety. Furthermore, the unknown influence of different collector slopes and different zenith angles of various sites on temperature reduction and system efficiency have to be considered. The fact that the operational state of the solarthermal system does not have any influence on this kind of overheating protection
turns out to be major advantage. This measure can also protect the collector during blackouts and hydraulic system failures. It is also possible to combine the
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3 Overheating Protection
above-mentioned measure with another measure in order to reach a more effective protection. Translucent elements can be realised by two different approaches. On the one hand, the transparent cover can be structured directly by
shaping. On the other hand, the surface of the transparent cover can be adjusted
to translucent behaviour.
3.3.4.1 Geometrical Structuring of the Glazing
Nitz (2006) described in a patent application how a geometrical structured glazing
worked for seasonal reduction of thermal loads on solar-thermal collectors.
Thereby, the transmission of insolation is decreased at certain incidence angles.
If a light beam through a boundary surface of transparent glazing penetrates another refractive index compared to the ambient air, the incidence angle of the
beam changes. With a second boundary surface of the glazing the opposite is
the case. Despite the refraction, this leads to the same beam angles before and
after the glazing (Figure 3.14).
Air
Θ1
Glazing
Boundary surface 1
Θ2
Θ2
Boundary surface 2
Air
Θ1
Figure 3.14: Transmission of insolation through a plane, transparent glazing
If this glazing is structured geometrically meaning that the boundary layers are
no plane parallel surfaces, the incidence angle to the absorber can be changed
in certain cases. As an example, the left side of Figure 3.15 shows the direction
of the sunbeam under vertical or almost vertical incidence where the largest part
of the light leaves the glazing under flat angles. That means that most of the
sunbeams are screened off by total reflection on the second boundary surface of
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3 Overheating Protection
the glazing. In temperate zones like in Europe, such situations will mainly occur
at noon and during midsummer. This leads to a reduction of solar energy on the
absorber surface and prevents the collector from overheating under these conditions. The remaining times of the days the direct radiation under flat incidence
angles at the structured glazing occur where the major part of the light transmits
without any losses. Figure 3.15 shows an exemplary beam transmission under
the above-mentioned conditions on the right side.
Air
(refraction
index n=1)
Air
(refraction
index n=1)
Glazing
(refraction

index n 1.5)
Air
(refraction
index n=1)
Glazing
(refraction

index n 1.5)
Air
(refraction
index n=1)
Figure 3.15: Description of the beam penetration at different incidence angles
(blue region: full transmission; red region: reduced transmission)
A structured glazing provides several advantages for the use in solar-thermal collectors. The glazing lowers the reduction by reflexion under flat incidence angles
during the heating period compared to planar cover sheets. The glazing reduces
the irradiance on the absorber in midsummer when heat supply for domestic hot
water preparation is less needed. Hence, the glazing avoids situations of possible
overheating. The glazing is also cheap in manufacturing by extrusion or thermoforming. The correct operation of the measure is independent from the hydraulic circuit of the solar-thermal system and the power grid. The fact that the
reaction cannot be controlled, however, constitutes the main disadvantage. Thus,
critical temperatures occurring in spring and autumn cannot be prevented by a
structured glazing adjusted for protection in midsummer. Another problem is the
unknown effectiveness of a standard adjustment of the structure for different angles of the collectors’ azimuth and slope. In principle, the approach is of interest
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3 Overheating Protection
for the use in flat-plate collectors but there is no product available on the market.
Furthermore, it has neither been investigated nor documented so far of how effective the procedure would be for overheating protection in solar-thermal collectors.
3.3.4.2 Microstructures on the Surface of the Glazing
Nitz et al. (2007) investigated micro-structured layers for controlling the solar irradiance in transparent elements for buildings. These layers are supposed to increase the available natural light in autumn, spring and winter. On the other hand,
the layers will reduce overheating and glare in summer. This technology is implemented on polymer films which are spanned or fixed within insulation glass (Figure 3.16).
Figure 3.16: Applications of micro-structured films in solar control glazing for buildings
(Nitz et al. 2008)
Figure 3.17 describes the beam penetration through vertical micro-structured layers in summer (yellow) and winter (cyan). The light beams from the sun in summer with flat incidence angles on the surface are directed by the layer to the second surface which reflects the major part backwards. The light beams from the
sun in winter with steep incidence angles on the surface pass the application.
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Figure 3.17: Beam penetration through micro-structured layers in summer (yellow)
and winter (cyan) for vertical solar control glazing for buildings (Nitz et al. 2008)
Figure 3.18 presents the curve progressions depending on the incidence angle
on the microstructures produced by interference lithography (IL) and mechanical
processing (MP).
Figure 3.18: Comparison of the solar transmittances of two glazings with micro-structured layers
against the incidence angle (according to Nitz et al. 2007)
Mechanical processed films are relatively cost-efficient to manufacture by roll embossing. The rolls are produced by ultra-precision machining like diamond milling
or turning. However, these layers developed for vertically mounted building windows have contrariwise performance characteristics.
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3 Overheating Protection
Nitz (2004) investigated a micro-structured layer for inclined surfaces to reflect
vertical sunbeams made from PMMA. Figure 3.19 shows the structure of the socalled compound parabolic concentrators (CPC) in detail. The reflecting metallisation is deposited by a PVD process. The layer is designed for roof-lights to
reflect insolation with incidence angles in the array of the perpendicular and to
enable high transmission under flat incidence angles. Thus, this layer has suitable performance characteristics for overheating protection at the inner surface of
the collector glazing.
Figure 3.19: Compound parabolic concentrators with metallised ridges
for the reflexion of vertical direct radiation (Nitz 2004)
3.3.5 Mechanical Appliances
The field of mechanical protection devices comprises all measures reducing or
preventing transmission through the collector front by a mechanically controlled
process.
3.3.5.1 Shading Elements
Shading elements are all devices reducing or disabling the transmission at the
collector front by impenetrable materials. These devices are either mounted fixed
or flexibly on the collector front. Such elements are already used at inclined and
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3 Overheating Protection
vertical glazings in buildings to control light intensity and indoor climate. Rotating
and contracting lamellas are an example of flexible elements. Due to rotating lamellas, there are permanently shaded areas on the absorber whose position has
to be found in order to prevent a further reduction of the optical efficiency of the
collector. Moreover, all kind of curtains or roller shutters being moved by a mechanical drive may be applied. The reflecting outer surface further improves the
overheating protection. It can avoid absorption and residual heat inside the collector. In the majority of cases, flexible shading elements have no significant influence when it comes to the efficiency of the collector. They ensure a completely
controllable thermal protection. However, the complex set-up (shading element,
motor, controlling unit) is accompanied by various problems such as a reduced
lifespan, moveable parts, a high collector weight and high initial costs. There is
also a dependency on electrical power supply which could only be solved by additional costs for PV elements or storage batteries.
Beikircher und Schmidt (2009) developed the prototype of a high-efficiency collector by using this technology with a shading film (Figure 3.20). The collector has
a transparent film for the reduction of the convective losses or a reflecting film for
the prevention of solar absorption stretched in front of the absorber depending on
the operation mode. The film unit is implemented into the collector by the film reel
principle. Therefore, a roll with a PV-driven motor moves the film. A barrel arbor
to stretch and to roll up the film complements the unit.
Figure 3.20: Design of a flat-plate collector for high flow temperatures with film insulation and
shading element (according to Beikircher und Schmidt 2009)
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3 Overheating Protection
Apart from automatically controlled shading elements, Radiant Floor Company
describes a simple system regarding the overheating protection of evacuated
tube collectors (Radiant Floor Company n.d.). Therefore, a knitted polymer
screen, for example, can be used to cover the collectors when the weather conditions cause high absorber temperatures (Figure 3.21). The basic intention of
this approach is the reduction of the solar yield during periods of high solar irradiance and low demand of heat in order to prevent stagnation of the system. The
approach ensures a sufficient temperature reduction and causes low additional
costs. However, a missing intrinsic safety due to the manual triggering of the
measure and a difficult accessibility of the collectors on a roof are major drawbacks.
Figure 3.21: Uncovered and covered evacuated tube collectors (Radiant Floor Company n.d.)
3.3.5.2 Infilling of Shading Material (Bulk Goods)
Harrison (1975) patented a device for the transmission reduction of windows and
flat-plate collectors. In this case, granulate is pushed, by means of an air blower,
into the gap of a double glazing or a collector to reduce or prevent the light transmission. The main advantages are the controllability and the high temperature
reduction. However, the large and complex design of the measure leads to high
initial and maintenance costs. The appliance also has no intrinsic safety to protect
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the collector during a power blackout. Due to the above-mentioned disadvantages, the measure is not suited as overheating protection in cost-efficient
flat-plate collectors.
3.3.5.3 Rotation of the collector
The change of the incidence angle on the collector surface changes the optical
efficiency. This is how the collector can be repositioned to follow the sun and
absorb solar radiation reaching the vertical incidence area. This leads to low
losses by reflection and the highest radiation density (Figure 3.22). In cases of
potential stagnation the incidence angle can be changed until the reflexion wanes
and the radiation density reaches temperatures regarded as not harmful to the
collector parts.
G
β Figure 3.22: Relation of radiation density at vertical (green) and inclined (red) incidence
Such tracking devices are already used in solar-thermal power plants and PV
systems. The complex construction and high initial costs, however, outweigh the
advantages of making use of this method for overheating protection in flat-plate
collectors.
3.3.6 Hydraulic Appliances
Hydraulic protection devices consist of a double glazing in which a fluid is pumped
in order to alter the optics of this specific part. The device uses scattering and
absorbing effects to control the transmittance.
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3 Overheating Protection
3.3.6.1 Shading Fluid between a Double Glazing
In this device, a dark fluid is pumped into the gap of a double glazing to absorb
the incoming solar irradiation and to convert it into heat. The thermal energy is
quickly transferred to the ambient air by convection. Dockery (1972) patented a
system to defrost and to vary the transmittance of a windshield for cars. In that
system, a heated, coloured fluid is pumped from a reservoir into the gap of a
double glazing. Taking the reservoir, the pumping, the double glazing and the
control unit into consideration, the whole procedure turns out to be complex and
expensive. Moreover, it is not intrinsically safe during power blackouts. In addition, the shading fluid has strong requirements to UV-resistance as well as to
temperature stability. This limits the variety of suitable fluids. If the fluid is being
left in an empty mode, the optical efficiency of the collector decreases. The advantages of a controllable measure with high possible temperature reduction in
the switched mode cannot compensate these disadvantages.
3.3.6.2 Light Scattering Double Glazing
The patented appliance with double glazing by Stephens (1981) uses a light scattering surface inside the gap. The scattering effect is caused by unevenness of
the surface and the different refractive indexes of the air and the interlayer material. As a result, the refraction of the light is non-directional regarding the whole
surface. This leads to a decreased transmission. To accomplish a clear state, a
transparent fluid with a refractive index comparable to the interlayer is filled inside
the gap of the double glazing. Hence, there is no change of the refractive index
at the rough surface and the scattering boundary layer vanishes for the light
beam. Figure 3.23 shows the schematic device in scattering and transmissive
state.
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Air (n=1)
Boundary surface 1
Air (n=1)
Glazing (n=1.5)
Boundary surface 1
Glazing (n=1.5)
Boundary surface 2
Air (n=1)
Fluid (n=1.3)
Boundary surface 3
Interlayer (n=1.3)
Interlayer (n=1.3)
Air (n=1)
Boundary surface 4
Air (n=1)
Boundary surface 4
Figure 3.23: Schematic of a glazing in scattering (left) and in light-transmissive (right) state
Stephens (1981) also described a test of a conventional flat-plate collector with
selective absorber coating made of black chrome. This collector was heated in a
solar simulator up to a stagnation temperature of the absorber of 100°C. Afterwards, a scattering layer was implemented between absorber and glazing. This
led to a temperature of 79°C under the same test conditions. In a third test phase,
a fluid with a similar refractive index was filled into the gap between glazing and
scattering layer to negate the scattering mode of the device. The final absorber
temperature was 97°C. The test proved that the temperature can reasonably be
reduced while the efficiency of the collector is only slightly influenced. The transparent fluid in this collector is adjusted to vaporise at a certain temperature, to
change the refractive index and to leave the gap. As a result the transmittance of
the collector front is reduced. This device is independent from the operation of
the hydraulic circuit and the power grid. The measure also shows a minimal collector loss in the clear state of system operation. An elaborate installation with a
scattering interlayer, a suitable fluid and a reservoir for the vapour reduces the
applicability of the measure. Additionally, a temperature controlled valve between
the return of the reservoir and the collector is necessary to avoid a permanent
reflow and renew evaporation. Also the resistance of the fluid against UV radiation and further climatic influences like frost has to be looked at critically. Against
this background an implementation in collectors seems to be unfeasible.
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3.4 Changing the Absorption Properties
The absorbance of a collector can be reduced by measures having absorbing,
scattering or reflecting effects. The measures are separated by their technical
solutions presented in Figure 3.13. These functions control the two absorption
properties with the absorption coefficient α and the absorbing area.
Thermally
switchalbe
layers
Electrically
switchable
layers
Chemically
switchable
layers
Translucent
appliances
Mechanical
appliances
Hydraulic
appliances
Figure 3.24: Groups of technical solutions for the reduction of the transmission properties
(according to Reiter et al. 2012)
3.4.1 Changing the Absorption Coefficient
The absorption coefficient α describes the capability of a body to receive radiation
and to convert it into thermal energy. For an optimal conversion of the irradiance
into heat, the absorption coefficient α has to be close to 1 for the solar spectrum.
Typical absorbers with selective coating reach an absorbance value of 0.95. This
means that 5 % of the solar irradiance will be reflected and not be converted into
heat. For overheating protection measures it is necessary to have an absorption
coefficient α nearly 1 during system operation and a low absorption coefficient
during stagnation. This prevents the radiation being converted into heat during
system states where heat cannot be transferred by the heat transfer medium.
3.4.1.1 Thermochromic Layers with Switchable Absorption
Thermochromic colours are normally used as indicators for hot surfaces and for
packaging materials. These colours fade above a certain temperature due to a
change of their absorption coefficient for visible light. Thus, the capability to convert irradiance into heat decreases; this effect is caused by an organic dissolver.
The dissolver is in the solid state below the trigger temperature. The pigments as
well as an activator are embedded into the latter. In this state the layer is visible.
When the organic dissolver melts, the layer becomes transparent and the surface
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behind the colour coat visible. The primer is white for the use on an absorber to
get the lowest absorbance. The transition range of such a colour is smaller than
6 K and the colour changes within seconds (Printcolor AG 2009; Gem’innov
2008). There are inks on the market with trigger temperatures of up to 80°C where
a disappearance of 90 % of the colour is realised (Gem’innov 2008). Huot et al.
(2008) investigated the colour gradient of two thermochromic paints (Figure
3.25). The paints showed a suboptimal colour but can be improved by another
dissolver matrix.
Figure 3.25: Thermochromic colours at 25°C, 40°C and 90°C (Huot et al. 2008)
The low UV-resistance turns out to be a major disadvantage of the pigments. The
admixing of metal flakes or UV-stabilisers can lower this effect. The application
of a UV-protection layer is also possible (Huot et al. 2008). The potential of the
layers is to be investigated further since low costs, the intrinsic safety and the
switching characteristics are advantages for these layers.
3.4.1.2 Thermotropic Layers
The thermotropic layers described in Chapter 3.3.1.2 can also be applied on the
absorber. The function and the effect is the same as with the glazing. The advantage of the application of such a layer on the absorber is the direct switching
behaviour, since there is no influence of the ambient temperature and a short
length of the control path. The application of the layer on a selective absorber is
not possible because of the layers’ non-selective characteristics (Wallner et al.
2008). Resch et al. (2009) described promising layers for the use of overheating
protection measures on the absorber. More detailed investigations have to be
carried out as to how local temperatures and the system efficiency are influenced.
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3.4.1.3 Electrically Switchable Layers
In principle, the use of electrically switchable layers to control the surface properties is applicable. Therefore, an electrochromic layer on a bright or reflecting
base layer is black and absorbent to change solar irradiance into heat during
system operation. In case of an impending overheating, the layer rises its transmission coefficient to let the irradiance pass the reflecting base layer and to reduce the absorbance of the part. Electrotropic layers can be deposited onto the
black absorber surface. They change their transparent state into a milky white
state to reflect the insolation and to prevent further heat conversion before overheating of the parts. However, the previously described disadvantages of these
layers described in Chapter 3.3.2 are also valid for an application on the absorber.
3.4.1.4 Chemically Switchable Layers
Chemically switchable layers, as described in Chapter 3.3.3, can also be used on
an absorber layer. Since the above-mentioned disadvantages also apply for this
kind of application, further investigations are not required for overheating protection in solar-thermal collectors.
3.4.1.5 Microstructures on the Surface of the Absorber
The principle of the described micro-structured layers in Chapter 3.3.4.2 can also
be fixed directly at the absorber to reduce the transmission of solar radiation depending on the incidence angle. Thus, the conversion of radiation into heat by
the subjacent absorber coating can be reduced or avoided. For this layer application, the temperature resistance of the layer needs to be investigated in more
depth. Furthermore, a selective absorber behaviour is not possible due to the
high emission coefficient of the micro-structured plastic film. Hence, the application of such a layer on the absorber has no advantages compared to the application on the glazing.
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3.4.2 Changing the Absorbing Area
The reduction of the light absorbing surface of a collector decreases the transfer
of irradiance into heat and lowers the maximum component temperatures. The
implementation of this principle can be designed mechanically as well as hydraulically.
3.4.2.1 Mechanical Appliances
The reduction of the light absorbing surface can be realised by rotatable risers of
the absorber. Also, sliding of the risers about each other can reduce the absorbing area. Another option is a grid of absorber pipes above CPC mirrors being
movable to each other. During operation pipes and mirrors are positioned to concentrate the focal lines on the pipes. Thus, the whole irradiance from the mirrors
is focused on the absorber pipes for a maximum energy gain. In case of overheating, the pipes or mirrors are moved one-dimensionally to differentiate the
positions of pipes and focal lines. Hence, the absorbing surface of the collector
is reduced. However, using mechanical appliances in collectors is not reasonable
due to the high technical complexity of mechanisms for turning or moving as well
as the susceptance to failure of movable parts.
3.4.2.2 Hydraulic Appliances – Black Fluid
In the 1970s and 1980s the principle of the so-called “black fluid” was investigated. In particular, in the United States approaches of solar-thermal systems
with black fluid were the subject of patents. Polymeric collectors with black fluid
dispose of a transparent absorber and drain-back system as solar circuit. A drainback system has an open hydraulic circuit. Hence, during operation the fluid is
pumped from the storage to the collector and backwards. If the pump stops, the
fluid flows out of the collectors and is collected in the storage or an external tank.
In such a system the black fluid coloured by carbon serves as the heat transfer
medium as well as the absorber which converts irradiance into heat. The empty
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absorber is fully transparent or has a reflecting backside plate, hence cannot convert the insolation into heat. Because of the missing absorber area of the black
fluid in the empty collectors, the system has an intrinsic safety. Heat inside the
collector is only generated during a fully operating solar-thermal system. The difficulty of launching coloured fluids onto the market, their absorption properties as
well as their long-term stability constitute are the main drawback of this approach.
3.5 Changing the Convective Losses
Flat-plate collectors dispose of an insulation at the backside and a housing with
a transparent cover to minimise the heat losses from the absorber to the ambient
air. The backside insulation reduces the heat transmission with low heat conduction to the outer surface of the casing or backside sheet where the thermal energy
is emitted mainly by convection. The convective heat loss on the front side is
reduced by an embedded air layer between glazing and absorber. Thus, the heat
flow has to be transferred by natural convection from the absorber to the glazing
and from the glazing to the ambient atmosphere. If convective losses of the absorber are to be extended in order to reduce the collector temperatures, thermal
resistances of the front side or the backside have to be decreased.
3.5.1 Natural Convection in the Air Gap
The fluid flow of a natural convection to transport thermal energy is driven by the
density differences caused by a temperature gradient within the fluid.
An air stream from the outside of the collector through the air gap between glazing
and absorber causes the rise in the loss coefficient of the front side by transporting additional thermal energy to the ambient air. Meir et al. (2008) experimentally
investigated a polymeric collector with non-selective absorber and overheating
protection by means of a lockable chimney in the front side of the collector. Therefore, a permanently opened ventilation slot on the bottom edge and ventilation
controlled by the thermal expansion of the absorber on the top edge were installed in the collector (Figure 3.26).
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Figure 3.26: Schematic design of a polymeric collector with ventilation slots (Meir et al. 2008)
Closing the upper ventilation slot is sufficient for suitable collector efficiencies
because hot air in the collector moves upwards. Thus, the natural convection of
the stack-effect is blocked and the air layer shows a similar behaviour such as
the air layer enclosed. The reference collector without overheating protection in
the outdoor testing reached a maximum absorber temperature of 145°C in the
upper part of the collector. The collector with opened ventilation dampers showed
a maximum absorber temperature of about 120°C. The measurements were carried out at 30°C ambient temperature.
Harrison et al. (2004) tested a conventional flat-plate collector with selective absorber coating and ventilation dampers on bottom and top edge of the backside
sheet (Figure 3.27). The air stream was controlled by the upper damper through
a spring made of a shape memory alloy.
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Figure 3.27: Schematic design of a polymeric collector with additional convective heat losses
caused by a chimney behind the absorber (Harrison et al. 2004)
The test results of a conventional and an adjusted collector were monitored at a
collector slope of 18° during dry stagnation. First tests at 1,150 Wm-2 irradiance
and 25°C ambient temperature showed absorber temperatures of 160°C at the
reference collector and 170°C at the adjusted collector with closed slots. The difference can be attributed to the fact that backside losses reduced thanks to the
additional air layer of the closed chimney. Figure 3.28 shows the results of a testing day at 1,100 Wm-2 and 25°C ambient temperature.
Figure 3.28: Trends of the absorber temperatures with and without ventilation
by natural convection (Harrison et al. 2004)
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The absorber temperatures are similar up to a value of approximately 100°C.
Above this level, the dampers of the adjusted collector opened and the stagnation
temperature reached only 122°C while the reference collector had a temperature
of 155°C.
Kearney et al. (2005) published a survey in which he calculated the temperature
reduction during stagnation for polymeric collectors with ventilation by natural
convection in front as well as behind the absorber and both together. The model
was simulated with an air gap and a slot of 25 mm and a collector slope of 45°
under extreme weather conditions with an insolation of 1,100 Wm-2 and an ambient temperature of 43°C. As reference he took a conventional flat-plate collector
with selective layer und single glazing. Under these conditions an absorber temperature of 205°C was reached. A second reference was a collector without selective coating representing a black polymeric absorber. In this case the maximum temperature was 149°C. The collector disposing of ventilation at the front
side showed a reduced temperature of 123°C. For the ventilation at the backside
part of the insulation was removed. The removed insulation also reduced the collector efficiency in the case without ventilation. The air gap behind the absorber
was painted black to reach the highest distribution of thermal energy at the complete chimney surface for an effective convective heat transfer. By means of ventilation at the backside, the collector reached a temperature reduction from 144°C
to 113°C under the above-mentioned conditions. Ventilation in both air gaps
causes a reduction from 144°C to 102°C.
The mentioned examples show that ventilation by natural convection is only useful for collectors without selective absorber coating when commodity plastics are
required.
Table 3.1 presents an overview on documented implementations of overheating
protection by natural convection in solar-thermal collectors.
Venting slots can be implemented at the backside, at the frame or at the border
of the glazing. The convective cooling can be switched by various elements at
the slots at the top edge and possibly at the bottom edge of the collector. The
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elements can be controlled by bi-metal springs, shape memory alloys, pistons,
inflatable elements or linear expansion of the absorber. Except for the inflatable
elements all systems are self-regulating and intrinsically safe against system failures and blackouts.
Table 3.1: Implementations of overheating protection by natural convection
Vents
Closing Elements
Actuator
Reference
End faces housing
Not chosen
Not chosen
Kearney et al. 2005
End faces housing
Damper, top
Linear expansion of
Meir et al. 2008
the absorber
End faces housing
Damper or slide gate, Bi-metal spring
Haticon GmbH 2006
bottom
End faces housing
End faces housing
--
Dampers
Temperature-depend-
UFE SOLAR GmbH
ent air stream
1999
Piston with phase
Palmer 1980
change material (wax)
End face housing,
Damper, top
Bellows with phase
bottom
change material (alco-
+ Glazing, top
hol etc.)
End face housing,
Buckley 1979
Inflatable tubes
Pump, blower
Koenig 1983
Damper, top
Shape memory alloy
Harrison et al. 2004
bottom
+ Glazing, top
Backside,
bottom and top
spring
Glazing,
Gaps between
Bending of the glazing Rich 1995
bottom and top
glazing and housing
opens slots caused by
linear expansion
Glazing,
bottom and top
Glazing, liftable
Linear expansion of
Scott 1977
the absorber
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Ventilation on the front side of the absorber can reduce the optical efficiency of
the absorber by dust and insects. Therefore, a protection element like a grid has
to be mounted.
Roberts et al. (2000), for example, investigated a venting measure for an ICS
system with a double-walled housing being separated to the air gap of absorber
and glazing by dampers. At high absorber temperatures the dampers opened and
the hot air from the absorber circulated through the housing (Figure 3.29). This
measure increased the surface for convective heat transfer to the outer shell and
reduced the absorber temperature from 115°C to 106°C. The system was protected against pollution from outside by the closed cooling loop.
Housing
Dampers
closed
Housing
Dampers
open
Figure 3.29: ICS system with closed cooling loop during operation (left) and during activated
overheating protection (right; according to Roberts et al. 2000)
On the basis of the different approaches, two common conclusions can be made.
For front ventilation, the collector design and height must not be changed because the air gap between glazing and absorber already exists. Ventilation behind the absorber requires a suitable air gap between absorber and insulation.
This causes changes in collector design, height and efficiency.
The measures described for ventilation triggered by natural convection have a
high intrinsic safety as well as a low efficiency decrease compared to collectors
without overheating protection. The movable parts pose problems due to their
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limited endurance and high production costs. Due to the fact that temperatures
can only be slightly reduced, it is recommended to combine this with another
method.
3.5.2 Forced Convection in the Air Gap
The use of heat transfer by forced convection means transporting of thermal energy carried by a fluid stream driven by an external force. The use of this function
as overheating protection measure for collectors requires the same design as for
venting by natural convection plus an additional ventilator blowing air through the
collector.
Scharfman (1977), for example, patented a system with a central ventilator and
collectors connected to each other by ventilation slots at the top and bottom of
the backside of the casing. Air is blown through a closable damper to the front
side of the absorber to cool down the surface.
Schmidt (1992) patented a collector with transparent insulation and overheating
protection. Because of the high efficiency and the polymeric material of the transparent insulation, an overheating protection measure became necessary. Therefore, a temperature controlled axial fan at the end face of the casing was installed.
During fan operation an air stream flowed through an air gap of 30 mm distance
between absorber and transparent insulation and left the collector through
opened dampers.
Intrinsic safety of such systems is only possible by means of an additional electric
power supply in the form of an accumulator or a PV module. The complex design
for forced ventilation with a ventilation unit, an electric energy supply and further
parts in the collector housing (slots, dampers etc.) are adverse conditions for a
failsafe and economic overheating protection measure.
3.5.3 Convection Combined with Evaporative Cooling
Besides cooling by natural convection, Kearney et al. (2005) also analysed the
influence of natural convection in combination with evaporative cooling by means
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of mathematical modelling. A standard flat-plate collector with selective coating,
a standard flat-plate collector without selective coating and a non-selective flatplate collector with controlled venting by natural convection at the backside of the
absorber plus a wetted pad were simulated and compared to each other. The
underlying conditions included a solar irradiance of 1,100 Wm² and an ambient
temperature of 43°C. Under these conditions, the stagnation temperatures of the
collectors were 205°C, 149°C and 65°C respectively.
Evaporative cooling, hence, proves to be considerably more effective than mere
natural convention. Nevertheless, the complex design of the venting mechanism
is exacerbated by the water pad and the temperature controlled water tap. Moreover, it is highly probable that the measure can fail due to calcification and frost
damage.
3.5.4 Varying the Insulation Properties
Changing the insulation properties of the collector can also vary the heat transfer
from the absorber to the ambient air. These heat losses can be controlled by
modifying the geometry of the air gap in front of the absorber or the backside
insulation.
Changing the insulation property of the air gap between glazing and absorber
depends on the distance between the parts. Reducing the space between glazing
and absorber decreases the heat transfer via convection but significantly increases the heat conduction leading to an increased heat transfer. Sharpe (1984)
patented a collector concept comprising an air-tight housing blown up by an airpump. The increased pressure inside curves the glazing whereupon the distance
between glazing and absorber rises. The measure is intrinsically safe since the
application of this measure only becomes necessary during the operation of the
solar-thermal system. A thermally controlled valve deflates the collector at a certain trigger temperature. In consideration of coming up with cost-effective collectors, the design of an air-proof and pressure resistant housing as well as the
valve, air pump and controller appear to be too sophisticated. Furthermore, the
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failure probability of the parts over the life time of the collector has to be considered. Figure 3.30 shows the cross section of the collector during regular collector
operation and active overheating protection.
Figure 3.30: Cross section of collector with variable air gap and inflatable backside insulation
during collector operation (top) and stagnation (bottom; Sharpe 1984)
The change of the heat transfer properties of the backside can also control the
local temperatures of the collector. Therefore, the heat transfer coefficient of the
backside insulation can be varied by using a switchable insulation. ZAE Bayern (2001) tested a prototype of a switchable insulation for building facades. The
prototype consisted of an evacuated insulation panel with a glass fibre filling. A
capsule with metal hydride reactor released hydrogen by means of heat which
reduced the heat transfer of the panel. When the getter cooled down, the hydrogen was reversibly embedded in the getter. The heat transfer rose by a factor of
40 in the heated getter state but the panel required 5 Wm-2 of electricity. Hence,
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the prototype panel requires an electric power supply. Due to the complex design,
this measure for collectors can also be discounted for further use.
Besides the variable air gap, Sharpe (1984) also patented an inflatable backside
insulation to control the absorber temperature. The collector shown in Figure 3.30
has an inflatable matting to reduce the thermal losses. An air pump and a thermally controlled valve regulate the air content of matting. The number of parts
and the use of flexible materials under high temperatures and ambient conditions
are two of the main drawbacks to be named for the application in collectors.
3.6 Changing the Radiation Losses
In addition to the heat losses caused by conduction and convection, also thermal
radiation between absorber, glazing and the cold sky reduces the part temperatures of the collector. The change of the emission properties of the absorber can
be arranged by the surface properties of the absorber and by the collector design.
3.6.1 Thermochromic Layers with Switchable Emission Coefficient
An absorber consisting of a thermochromic layer with switchable emission coefficient has selective absorption properties in the temperature array of ordinary
collector operation. The surface absorbs a large part of the incoming short-wave
radiation from the sun and emits only a small fraction of the converted heat to the
glazing or rather to the cold sky. Above a certain absorber temperature the layer
changes its emission coefficient in the long-wave region and the heat losses by
means of radiation through the front side increase.
Huot et al. (2008) investigated an inorganic, thermochromic coating to protect the
water-glycol mixture of a state-of-the-art flat-plate collector against overheating
and resulting degradation. The ternary compound conduct changes a phase from
a semiconductor state to a metallic state when a certain temperature is exceeded.
In simulations a layer was identified with an emission coefficient of 5 % during
operation and an emission coefficient of 40 % to keep the maximum absorber
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temperature below 160°C. A similar coating with emission coefficients between
5 % and 35 % was developed. The layer had a switching temperature of about
65°C and had an absorption coefficient for solar insolation of 97.3 %. First samples of the layer were produced on laboratory scale with a PVD-sputtering process.
The application of such a coating on polymeric substrates has to be further investigated. On the one hand, the process for application has to be approved for
the chosen substrate regarding its material properties such as electrical conductivity and the temperature resistance. On the other hand, the polymeric materials
need a diffusion barrier protecting the layer from corrosion and flaking. The main
advantage of the layer is the complete and independent intrinsic safety. For the
implementation of the layer in a polymeric collector, however, the temperature
reduction is too low. A further development of the switching properties or the combination with another overheating protection measure is considered to be necessary.
3.6.2 Contact between Glazing and Absorber
Moore (1983) patented a collector with either single or multiple glazing being able
to be elevated by lifting elements of the absorber. In the collapsed condition when
the system is not collecting solar energy, the glazing directly rests on the selective
absorber. In this case, the heat from the absorber is transferred with low resistance to the glazing. The glazing emits the heat as a black body. Hence, the
selective behaviour of the absorber is bypassed and the heat loss surges. Figure
3.31 shows a collector concept with an inflatable absorber as lifting element for
the cover plates. During system operation the pump pressurizes the solar circuit
and blows up the flexible absorber regions elevating the cover plates and creating
an air gap between glazing and absorber.
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3 Overheating Protection
Figure 3.31: Collector with lifting elements for the glazing during stagnation (top)
and during collector operation (bottom; Moore 1983)
In addition, the increased losses by radiation the thermal resistance in the air gap
between glazing and absorber are also erased. Thus, the collector shows unglazed pool absorber characteristics at the front side during active overheating
protection.
The intrinsic safety of the solution is provided because high temperatures occur
only during measures intended to actively reduce heat losses. Apart from the fact
that the measure represents a cost-effective, efficient approach to protect the
collector parts from high temperatures and ensure suitable collector efficiency, all
movable parts within the collector have adverse effects with regard to their purpose and their life span.
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3.7 Active Cooling
For active cooling of the collector a heat carrier is used to carry away the heat
from the absorber and other collector parts. Various measures with heat sinks at
the collector, at the solar circuit and at the heating system are conceivable. Different kinds of measures for active cooling depending on their position are described in detail as follows.
3.7.1 Cooling Measures in the Collector
Cooling measures in the collector are measures for active temperature control
directly placed in the housing of the collector. Two different systems are possible.
One system has an additional emergency cooler integrated in the collector; the
other system has an external open cooling circuit.
3.7.1.1 Collector Integrated Emergency Cooler
The collector integrated emergency cooler system consists of an external
mounted surface radiator connected to the piping system at the absorber plate.
During stagnation the heat carrier circulates between the radiator and the absorber driven by the thermosyphon effect and transports the heat from the absorber to the ambient air. Cummings (1978) patented a collector design with two
heat carrier circulation systems at the absorber (Figure 3.32). One piping system
is connected to the solar circuit and transports the converted heat to the buffer
storage. The second circuit is connected to a fin-tube heat exchanger at the top
edge outside the collector. The heat and fluid transport in the system is driven by
the thermosyphon effect. A thermally switchable valve controls the heat transport
and avoids the heat exchange by the overheating protection measure during temperature levels in the operational range of the solar-thermal system.
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Figure 3.32: Collector with additional cooling circuit at the absorber
driven by the thermosyphon effect (Cummings 1978)
The design of absorber and piping is complex and an external heat exchanger
requires additional space. Furthermore, the thermosyphon circuit requires a unit
to buffer the thermal expansion of the fluid to prevent damage due to extreme
pressures. The measure is intrinsically safe to blackouts, but the costs and complexity for the surface radiator and the control valve remain a serious problem for
the realisation.
Another design of the measure is a direct connection between the absorber and
the external heat exchanger. Hence, only one piping system is implemented on
the absorber and a second circuit with expansion unit is not required. Hence, the
design and the production of collector are simpler, but a leakage in the solar circuit causes a breakdown of the measure. Also the behaviour of the solution during filling, draining or venting can be difficult.
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3.7.1.2 Open Cooling Circuit
An open cooling circuit for overheating protection has to be placed above the
absorber. This measure overflows the part with a cold fluid to transport the thermal energy away from the collector. The use of service water for being independent from blackouts and failures of the solar circuit constitutes a simple approach.
A mechanical valve with a bi-metal actuator is used as a controlling unit. The hot
medium draining out of the collector is collected by the roof gutter and conducted
to a drain. The temperature reduction of the measure is quite large. However, the
high water consumption lowers the ecological as well as the economic advantages of the solar-thermal system. The freeze resistance of the measure has
to be taken into consideration as well. The calcification of the absorber surface
decreases the optical efficiency of the collector. Thus, this measure cannot be
implemented in modern solar-thermal collectors.
3.7.2 Cooling Measures in the Solar Circuit
Cooling measures in the solar circuit use the pipes and the collector fluid to
transport the thermal energy of the collector array to its heat sink. The heat sink
can be implemented as active apparatus or passive elements controlled by the
pump of the solar circuit.
3.7.2.1 Additional Heat Sinks
The temperature reducing heat sinks in the solar circuit can be integrated as dissipaters or heat exchangers supplying external customers.
A dissipater or an emergency cooler in the solar circuit is an additional element
in the solar-thermal system to transfer the thermal energy from the collector fluid
to the ambient air. The radiator can be implemented in various designs.
A cooler driven without auxiliary energy and without pump operation in the solar
circuit works in the same way as an emergency cooler being integrated in the
collector. With this principle a radiator is connected to the whole collector array.
The design is simpler because the collectors do not require any additional parts.
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3 Overheating Protection
The piping and the valves are additionally clamped to the collector array. Problems arising during filling, venting and in case of leakages have to be viewed
critically.
A cooler driven without auxiliary energy and with pump operation in the solar circuit is integrated parallel as surface cooler in the flow pipe of the collector field.
The surface cooler can consist of a fin-tube heat exchanger, a radiator or an uninsulated pipe. A three-way valve controls the bypass of the overheating protection measure. The measure is only useful in an operational system. Hence, there
is no intrinsic safety for the overload protection in the system. This measure for
reducing the temperature is very efficient. However, a permanent power supply
for the operation of the pump is necessary. The additional costs for the hydraulic
integration and the surface cooler as well as a suitable place for the heat sink
have to be taken into account.
A further enhanced version is a cooler driven without the need of any auxiliary
energy and with pump operation in the solar circuit. The surface cooler with air
blower has an increased cooling capacity. Hence, the size of the element can be
reduced. Becker et al. (2006) analysed a solar-thermal system with 300 m² of
conventional flat-plate-collectors and 27 m³ of thermal storage for Freiburg (GER)
by simulation. The objective of the analysis was the prevention of high collector
temperatures to protect the water-glycol mixture from degradation. In an annual
system simulation the cooling unit limited the maximum collector temperature to
90°C with an operation time of about 100 hours. The required electric power supply was 1.7 kW for the air blower plus the power for pump operation during the
prevented stagnation times. The cooling unit was also implemented in a bypass
of the collector flow controlled by a three-way-valve. The temperature reduction
reached was very high and the system´s efficiency was not impaired. However,
the measure has no intrinsic safety against blackouts and failures in the hydraulic
circuit. Furthermore, the investment and operational costs for a system with no
intrinsic safety have to be considered.
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Another measure is a heat exchanger at the flow of the collector array connected
to external heat sinks. This can be the regeneration of the ground of a heat pump
system, the heating of a green house or a pool as well as the exchange to the
groundwater for example. However, since these measures do not provide any
intrinsic safety, they do not represent a standard for implementation.
3.7.2.2 Open Cooling Circuit
Direct cooling in the solar circuit with water requires an open system. In case of
overheating, an external water supply decreases the temperature in the collectors
and solar circuit. The cold water can be provided in several designs and from
different sources.
One possibility to cool down the collector array is the use of service water. Therefore, the system requires a discharge to drain-off the excess heat carrier. Buckley and Guldman (1983) patented solar-thermal systems with overheating protection by service water to prevent vapour formation and to use non-metallic materials in collectors. One option is a non-frost-resisting system with a closed solar
circuit during operation as shown in Figure 3.33. The thermal storage is connected to the water conduit by a corrugated pipe as heat exchanger. A pressurecontrolled valve fills the storage, the solar circuit and the collectors until a pre-set
pressure. A temperature-controlled valve connected to the absorber regulates a
discharge near the flow pipe of the collector array. At high absorber temperatures
the valve opens and the water flows out of the solar-thermal system. The decreasing system pressure opens the valve at the water conduit. The cold water
flows from the storage through the collectors to the discharge and reduces the
collector temperature. A check valve in the collector outlet pipe ensures the correct flow direction in the system. When the absorber underruns the pre-set temperature, the valve at the discharge closes and the system pressure rises again.
Afterwards, the pressure-controlled valve of the water conduit closes also and the
normal operational state in the system is restored.
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Flow
Check valve
Temperature‐controlled valve
Absorber
Discharge
Heat exchanger
Return
Water tap
Pressure‐controlled valve
Heat storage
Pump
Figure 3.33: Solar-thermal system with overheating protection from service water
(according to Buckley and Guldman 1983)
Laing (1985) patented an open, frost-resistant system concept with overheating
protection for polymeric collector parts (Figure 3.34). The service water flows directly through the collectors and is stored in a tank for the hot water supply. The
water tap is connected at the inlet of the collector array at the bottom edge and
is controlled by a temperature-sensitive valve. The sensor of the valve has its
position at the outlet near the absorber at the top edge gaining almost the highest
collector temperatures possible. Above a pre-set temperature the valve opens
and the cold service water flows through the collector array and transfers the heat
to the thermal storage. Hence, the collectors cannot reach critical temperatures.
The storage also has an overflow to drain-off excessive water produced during
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the overheating protection. A frost sensor controls a discharge valve to drain-off
the collector array for the protection against frost damage.
Temperature sensor Thermal storage Overflow
Collectors
Inlet valve
Absorber pipe
Frost sensor
Water conduit
Discharge valve
Figure 3.34: Solar-thermal system with direct flow using overheating protection with
service water (according to Laing 1985)
Harrison and Harrison (1979) patented a drain-back system with service water
overheating protection for polymeric collectors. During normal system operation
the water is pumped from the storage to the header of the absorber at the top.
The water flows from the header through a glass fibre mat behind the absorber
plate. The header pipe at the bottom edge of the collector conduits the heated
water back to the thermal storage. At high absorber temperatures a fluid at the
absorber sensor evaporates and the actuator opens the three-way valve at the
water tap. The cold service water flows also from the top edge of the collector
through glass fibre mat to the thermal storage. An overflow conveys the excess
water to the sewerage system or other tanks. If the absorber temperature decreases, the actuator closes the water tap. The three-way valve enables draining
of the service water pipe to the collector. Thus, the measure is freeze-resisting.
An additional service water pipe to the collector provides an emergency measure.
The pipe with an enclosed air volume for freeze resistance is closed by a meltable
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safety device. In case of complete system failures during critical absorber temperatures, the pipe opens irreversibly and supplies the collector array with cold
service water. A vortex chamber acoustically signalizes the operation of the overheating protection measures and the service water consumption. The design of
the system is shown in Figure 3.35. This system provides a quite high temperature reduction and is protected against blackouts and system failures. Even the
system efficiency is not impaired by this measure.
Figure 3.35: Drain-back system with service water overheating protection
(according to Harrison and Harrison 1979)
However, the ecological and economic influence of the service water consumption has to be further considered. A combination with another measure to reduce
the water consumption seems to be promising.
The use of groundwater is a cost-effective alternative to service water. The setup is similar and only requires a pump for the groundwater supply. Nevertheless,
the missing intrinsic safety against blackouts reduces the practicability of the approach.
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3.7.2.3 Intelligent System Control
The dissipation of the thermal energy of the storage during night can prevent
stagnation caused by excessive supply of solar energy. The operation of the solar
circuit during night transfers the heat from the thermal storage to the collector
array. The collector loses heat via convection and radiation to the ambient and
cold air. Hence, the storage is prepared for a permanent operation without being
overloaded the next day. This measure is quite useful for solar-thermal systems
and for space heating during summer. In particular, since the solar energy available and thermal consumption will largely not be required. The measure requires
a control unit to identify a potential storage overload and to start an operation of
the solar circuit in order to reduce the storage temperature.
Becker et al. (2006) investigated the stagnation behaviour of a large-scale solarthermal system (300 m² collector area, 27 m³ buffer storage volume, Freiburg GER) with an implemented system control for collector operation during night by
simulation. This measure completely avoids the storage overloading over the
year and enabled a maximum absorber temperature of about 90°C. For the operation of pumps in the solar station an additional power consumption of 21 %
was found to be necessary to transfer the heat from the storage to the collector
array during night. The simulation showed that there was a strong dependency
between the annual solar gain and the additional power consumption from the
pre-set controller parameters. This included the cut-off temperature of the storage
in cooling operation, the target collector temperature and the flow rate during
night. Furthermore, the reverse flow direction of the solar circuit is necessary to
reach a high temperature difference between heat carrier and ambient air.
This measure is independent from the set-up and the size of the solar-thermal
system. Only the parameters have to be adjusted. On the basis of the documented findings, a relatively large temperature reduction of the collector parts
may be possible. The additional parts and the changes in the set-up of the solarthermal system are also relatively low. However, there is no intrinsic safety provided by this measure neither against blackouts nor system failures. The missing
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overheating protection in case of an incorrect insolation prediction provides another disadvantage. Furthermore, the dissipation of thermal energy in the storage
can reduce the system efficiency due to a potential mismatch of overheating protection and heat consumption. However, active overheating protection measure
is only needed when an excess supply of solar-thermal energy and low heat consumption exist. Due to the increased operating costs and the absence of the intrinsic safety this measure is not deemed to be adequate as single protection
element. A combination of additional measures, however, provides potential.
3.7.3 Cooling Measures in the Heating System
The cooling measures in the heating system include options to dissipate the thermal energy from the storage. Therefore, the measures or rather heat sinks are
integrated in the storage or connected to the storage periphery. Such measures
can only be used as additional, supporting steps because they provide no intrinsic
safety for the collector array. The two possible versions of cooling measures are
additional heat sinks at the thermal storage or the use of present heat sinks in
the heating system by an intelligent system control.
3.7.3.1 Additional Heat Sinks
The additional heat sinks extract thermal energy from the storage to prevent thermal overloading. This measure can operate simultaneously with the solar circuit
for a direct or time-delayed heat removal for a storage preparation. Therefore,
heat exchangers are implemented at the thermal storage or in the heating system
to transfer the thermal energy to additional consumers or to apparatuses for heat
dissipation. Ground regeneration for heat pump systems, the green house, the
pool, the washing machine or the dishwasher are only a few examples for additional heat sinks. The lack of standardisation of such measures is one of the main
drawbacks since the implementation depends on the installation environment to
a great extent. Apparatuses for heat dissipation are all kind of surface heat exchangers transferring the heat to the ambient air inside and outside the building.
The additional power consumption for the operation of the solar-thermal system
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and the circulation pumps as well as investment are among the major problems
besides the difficulty of providing intrinsic safety.
3.7.3.2 Intelligent System Control
The discharge of the thermal storage with an intelligent system control uses the
existing heat sinks in the heating system. In contrary to additional heat sinks, no
retrofit or mounting of extra parts is necessary. Only the controller strategy is to
be adjusted to the operation of the overheating protection measure.
Streicher (2008) suggests a flow through the inactive boiler of the burner to reduce the storage temperature. The boiler transfers the heat to the occurring air
flow in the chimney. The system set-up requires a back-up heating from a burner.
Hence, this measure is not independent from the installation environment. Furthermore, the performance of the overheating protection measure is too low in
relation to the energy output of a collector array during stagnation.
The operation of certain radiators or heating areas in the building reduces the
storage temperature and avoids stagnation in the collector array. The controller
activates the pump operation of the heater circuit in the basement for example to
discharge the storage parallel to the collector operation or during night for storage
preparation for the next day. Such strategies are already implemented in stateof-the-art solar-thermal systems.
3.8 Adjustment of the Collector Efficiency
Beside active or passive measures controlling the performance of the collector
and the part temperatures, the adjustment of the collector efficiency as well provides overheating protection. The efficiency reduction by increased losses can
be divided into two effects. On the one hand, there are constant losses of the
optical performance caused by the transmission properties of the glazing and the
absorption properties of the absorber. On the other hand, there are temperature
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dependent thermal losses caused by convection, conduction and radiation. Figure 3.36 shows the collector losses of a conventional flat-plate collector being
Collector efficiency [-]
referenced to the collector efficiency curve.
Figure 3.36: Efficiency curve and collector losses of a conventional flat-plate collector
In order to adjust the efficiency, the collector performance is to be reduced with
regard to reasonable system efficiencies. During system operation, the collector
efficiency of an adjusted collector must therefore be close to the efficiency of a
conventional collector and should decrease at higher collector temperatures.
Thus, thermal losses only should be used to reduce the collector efficiency and
the temperatures since the effects increase hand in hand with the component
temperatures.
As a second aspect for the efficiency adjustment, the costs for having the collectors manufactured should also be taken into consideration. Due to the lowered
solar yield of the system, the collector costs are even more important to compete
with state-of-the-art collectors. Thus, the approaches for cost reduction have to
compensate the efficiency reduction.
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By means of those requirements, efficiency reduction has to be realised by omitting manufacturing steps or material savings. There are two relevant approaches
for such a performance adjustment especially with regard to polymeric collectors.
3.8.1 Increase of the Emission Coefficient of the Absorber
Performance can be adjusted by substituting or omitting the selective absorber
coating. A surface with a higher emission coefficient has an increased heat transfer via radiation to the glazing and causes higher collector losses. For example,
Figure 3.37 shows the collector efficiency curve and the collector losses of a collector without any selective absorber surface in comparison to the collector effi-
Collector efficiency [-]
ciency curve with selective coating.
Figure 3.37: Efficiency curve and collector losses of a flat-plate collector
without selective coating
The radiation losses are significantly increased and show a non-linear behaviour.
This steep gradient in dependence of the reduced temperature can be used to
gain a relatively high efficiency in the typical operational range of collectors for
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hot water preparation and space heating and to lower the part temperatures during stagnation.
Most state-of-the-art absorber plates have coatings produced in PVD processes.
The substrates for those coating processes are coils made from copper and aluminium. Polymeric absorbers produced as single pieces cannot be coated in such
production lines. The polymeric material, too, is not as suitable as coating substrate. Thus, a cost-effective TISS colour (thickness-insensitive spectrally selective) with an emission coefficient of 0.34 (Orel et al. 2009) or only a black surface
instead of state-of-the-art coatings with emission coefficients of about 0.05 are
possible. In case of a black surface, a black-pigmented polymer can be used to
make further treatment of the surface dispensable.
However, Figure 3.37 also shows a considerable efficiency decrease in the operational range of the collector or rather of the system. Therefore, beside the decrease in temperature, the influence on the system behaviour has to be further
investigated.
3.8.2 Decrease of the Insulation Properties of the Collector
Backside
Higher backside losses in combination with an advanced casing design are also
a possible option to balance collector efficiency and manufacturing costs. Less
insulation material or substituting the insulation by air in new casing designs are
also thinkable. Figure 3.38 shows the collector efficiency curve and the collector
losses of a collector with a reduced backside insulation thickness by one third in
comparison to the reference collector efficiency curve. The increase of backside
losses is not as drastic as the increase of the radiation losses due to the missing
selective coating. In this case, the limited influence on the collector efficiency also
has little influence on the maximum part temperatures. The interaction between
collector efficiency, maximum temperatures and insulation properties has to be
investigated by parametric analysis to identify the potential of the concept.
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Collector efficiency [-]
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Figure 3.38: Efficiency curve and collector losses of a flat-plate collector
with reduced insulation thickness
The low cost savings of mineral wool for insulation decreases the potential of the
approach. However, the production of a casing with a moderate insulation in a
single step represents an interesting alternative. There are two options for improved concepts ranging from casings made from rigid foam combining housing
and insulation to designs with enclosed air chambers to inhibit convection losses
at the backside. A further advantage of this approach is the avoidance of fogging
in the collector and moisture in the mineral wool causing increased heat conductivity.
3.9 Buffering Critical Collector States
Active or passive approaches focus on keeping the part temperatures of the collector permanently on a low level. However, critical weather conditions causing
high collector temperatures only occur for a certain time interval (cf. Chapter 2.2.1). Thus, the time dependent temperature rise can be delayed by increased or additional heat capacities.
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3.9.1 Integrated Collector Storage
Additional heat capacities within collectors are realised in integrated collector
storage solar water heaters. The storage volume is integrated in the absorber
and is heated automatically by the sun. A heat exchanger removes the absorbed
heat from the collector. Such systems are normally used for hot water preparation. ICS solar water heaters can be built as CPC collectors (Souliotis et al. 2013)
as well as flat-plate collectors (Gertzos 2007). Figure 3.39 shows examples of
CPC collectors.
Figure 3.39: Integrated collector storage solar water heaters (Souliotis et al. 2013)
The heat capacity of such collectors can be increased by using phase change
materials instead of water (Chaabane et al. 2014).
Phase-change materials have their melting point at a certain temperature within
the utilisation temperature range of heat storages. The absorbed thermal energy
within a phase is temperature dependant and defined as sensible heat. The absorbed thermal energy during the phase change occurs at a constant temperature and is defined as latent heat (Figure 3.40). Due to the phase transition, the
material is able to absorb larger amounts of heat within a limited temperature
range compared to storage materials that are using sensible heat only. Commonly used materials in heat storages for solar-thermal systems have a solidliquid phase change. Materials that show this reversible effect in heat storages
are waxes or hydrated salts (Farid et al. 2004).
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Temperature [°C]
Sensible heat (liquid)
Latent heat (phase change)
Sensible heat (solid)
Internal energy [J]
Figure 3.40: Gradient of the temperature in dependency of the
absorbed thermal energy of a phase change material
The approach of ICS collectors delays the moment of reaching critical temperatures in collectors and also works during blackouts. However, there is no overheating protection in case of a full thermal storage. Therefore, the system has to
be equipped with an intelligent system control, as described in Chapter 3.7.3.2,
in order to keep the storage on a suitable temperature level and to ensure the
highest potential of the concept. Despite the combination with a control approach,
it is not possible to realise an intrinsically safe collector because of the limited
heat capacity of the storage or rather the dependency on the operational state of
the system. Apart from that, the cost benefit has to be regarded as critical when
using expensive phase change materials.
3.9.2 Slurry Phase Change Materials as Heat Carrier
Serale et al. (2013) analysed the use of a slurry phase change material as heat
carrier in flat-plat collectors. The latent heat of the phase change keeps the mean
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fluid temperature on a lower level and thus causes lower heat losses of the collector. Depending on the climate efficiency increases between 5–10 % were identified.
This effect can also be used in absorbers to delay the temperature increase during stagnation. Especially, volumetric absorbers with larger fluid content have an
increased effect for this reason. However, the limited heat capacity in comparison
to the incoming solar irradiance during stagnation, the missing intrinsic safety
and, especially, the high costs for the heat carrier are drawbacks preventing the
realisation as overheating protection approach in cost-efficient collectors.
3.10 Preselection of Overheating Protection Approaches
The described overheating protection approaches were evaluated and preselected as basis for the extensive simulation analysis with respect to the reachable
temperature reduction and the influence on the collector or system performance.
Therefore, the measures and approaches have to be classified regarding their
intrinsic safety. Table 3.2 lists the different classes for the system.
Table 3.2: Classes of the overheating concepts regarding intrinsic safety
Class
Description
A
Measure protects the system permanently
B1
Measure protects the system during power blackouts
B2
Measure protects the system during system failures
C
Measure protects the system only in the operational state
In a second step, the concepts were evaluated regarding their pertinent properties being identified in the description in order to specify and to remove concepts
in the evaluation process. Thus, the feasibility provides a first overview on approaches being suitable for polymeric collectors as well as for a further analysis.
Afterwards, the evaluation of the single approaches and the combinations for an
effective protection was conducted by assessment criteria from the technical and
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the economic point of view listed in Table 3.3. The evaluation by these criteria is
based on the literature review and additional estimations to identify the most
promising approaches for the detailed investigation.
Table 3.3: Assessment criteria for overheating protection concepts
Assessment criteria
Description
Technical criteria
Temperature reduction
Reachable temperature reduction at the parts
Intrinsic safety
Efficiency of the measure under certain conditions
Universal applicability
Dependence of the measure from installation and system set-up
Economic criteria
Operational costs
Additional costs for the operation of the measure
Efficiency reduction
Influence on the system efficiency
Manufacturing costs
Additional costs for the implementation of the measure
Table 3.4 and Table 3.5 show the evaluation process of the measures which
change their optical efficiency. Therefore, a scale from 1 to 5 were chosen, where
1 the worst and 5 the best rating of the criteria represents. The approaches shown
in red are removed because of the missing intrinsic safety or other pertinent disadvantages like e.g. ageing behaviour or reliability. Most of the approaches provide an intrinsic safety, but only several measures are suitable for a reasonable
implementation in collectors.
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Temperature
reduction
Universal
applicability
Operational
costs
Efficiency
reduction
Manufacturing
costs
Overheating approach
Intrinsic safety
Table 3.4: Evaluation and feasibility of the overheating approaches
changing the optical efficiency — part 1: changing the transmission properties
3.3.1.1 Thermochromic Layers
A
4
5
5
4
3
3.3.1.2 Thermotropic Layers
A
4
5
5
4
3
3.3.2.1 Electrochromic Layers
A
4
5
4
4
0
3.3.2.2 Electrotropic Layers
A
4
5
4
4
0
3.3.2.3 Switchable Mirrors
A
5
5
4
4
0
3.3.3 Chemically Switchable
Layers
A
4
5
3
4
0
3.3.4.1 Geometrical Structuring of the Glazing
A
2
3
5
3
4
3.3.4.2 Microstructures on the
Surface of the Glazing
A
4
3
5
3
4
3.3.5.1 Shading Elements
B2
5
5
4
5
1
3.3.5.2 Infilling of Shading Material (Bulk Goods)
B2
5
3
3
4
1
3.3.5.3 Rotation of the collector
B2
4
5
4
5
1
3.3.6.1 Shading Fluid between
a Double Glazing
B2
4
4
3
3
1
3.3.6.2 Light Scattering Double Glazing
B2
3
4
3
2
1
(sorted by chapters)
3.3 Changing the Transmission
Properties
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Temperature
reduction
Universal
applicability
Operational
costs
Efficiency
reduction
Manufacturing
costs
Overheating approach
Intrinsic safety
Table 3.5: Evaluation and feasibility of the overheating approaches
changing the optical efficiency — part 2: changing the absorption properties
3.4.1.1 Thermochromic Layers with Switchable Absorption
A
4
5
5
2
3
3.4.1.2 Thermotropic Layers
A
4
5
5
2
3
3.4.1.3 Electrically Switchable
Layers
A
4
5
4
3
0
3.4.1.4 Chemically Switchable
Layers
A
5
5
3
3
0
3.4.1.5 Microstructures on the
Surface of the Absorber
A
4
3
5
3
4
3.4.2.1 Mechanical Appliances
B2
3
5
4
5
1
3.4.2.2 Hydraulic Appliances –
Black Fluid
A
5
4
4
4
4
(sorted by chapters)
3.4 Changing the Absorption
Properties
Table 3.6 lists all measures which remove the thermal energy from the collector.
The measures being implemented in the collector show a great potential. However, the most measures being integrated in the system are not intrinsically safe.
Hence, they can only be used as additional, supporting approach.
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Temperature
reduction
Universal
applicability
Operational
costs
Efficiency
reduction
Manufacturing
costs
Overheating approach
Intrinsic safety
Table 3.6: Evaluation and feasibility of the overheating measures
removing the thermal energy
3.5.1 Natural Convection in
the Air Gap
A
3
3
5
4
3
3.5.2 Forced Convection in
the Air Gap
B2
5
5
3
5
2
3.5.3 Convection Combined
with Evaporative Cooling
A
4
3
4
3
1
3.5.4 Varying the Insulation
Properties
B2
3
4
4
4
3
3.6.1 Thermochromic Layers
with Switchable Emission Coefficient
A
3
5
5
4
4
3.6.2 Contact between Glazing and Absorber
A
4
5
4
5
3
3.7.1.1 Collector Integrated
Emergency Cooler
C
5
5
5
4
2
3.7.1.2 Open Cooling Circuit
A
5
4
1
2
3
3.7.2.1 Additional Heat Sinks
C
5
4
3
5
2
3.7.2.2 Open Cooling Circuit
B1
5
5
1
5
3
3.7.2.3 Intelligent System
Control
C
3
4
2
4
5
3.7.3.1 Additional Heat Sinks
C
5
3
3
5
4
3.7.3.2 Intelligent System
Control
C
3
4
2
4
5
(sorted by chapters)
3.5 Changing the Convective
Losses
3.6 Changing the Radiation
Losses
3.7 Active Cooling
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3 Overheating Protection
Table 3.7 shows the evaluation of the approaches applicable for the adjustment
of the collector efficiency.
Temperature
reduction
Universal
applicability
Operational
costs
Efficiency
reduction
Manufacturing
costs
Overheating approach
Intrinsic safety
Table 3.7: Evaluation and feasibility of the overheating approaches
by adjusting the collector efficiency
3.8.1 Increase of the Emission
Coefficient of the Absorber
A
4
5
5
3
5
3.8.2 Decrease of the Insulation Properties of the Collector
Backside
A
3
5
5
4
5
(sorted by chapters)
Table 3.8 shows the evaluation of the approaches for buffering critical collector
states.
Temperature
reduction
Universal
applicability
Operational
costs
Efficiency
reduction
Manufacturing
costs
Overheating approach
Intrinsic safety
Table 3.8: Evaluation and feasibility of the overheating approaches
by buffering critical collector states
3.9.1 Integrated Collector
Storage
B1
3
5
5
5
1
3.9.2 Slurry Phase Change
Materials as Heat Carrier
B1
2
5
5
5
1
(sorted by chapters)
Depending on the class of intrinsic safety, different concepts can be combined to
fulfil the overheating protection in operational state as well as during failures. On
the basis of this evaluation, the concepts of Table 3.9 were chosen for the more
detailed investigation. This preselection will be used for the final qualitative evaluation based on the results from the simulation analysis.
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3 Overheating Protection
Table 3.9: Chosen overheating protection concepts for simulation
Concept
Thermotropic Layers
Thermochromic Layers with Switchable Emission Coefficient
Thermochromic Layers with Switchable Emission Coefficient + Service Water Cooling
Microstructures on the Surface of the Glazing
Microstructures on the Surface of the Glazing + Service Water Cooling
Increase of the Emission Coefficient of the Absorber
Decrease of the Insulation Properties of the Collector Backside
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4 Collector and System Modelling
4 Collector and System Modelling
The detailed analysis and evaluation of the approaches for overheating protection
regarding their temperature reduction as well as their influence on the system
efficiency require an extensive simulation model. On the one hand, a mathematical model of the collector being able to calculate the temperatures of the components is needed. The model has to be able to implement the properties of the
components as well as the overheating protection measures. On the other hand,
a solar-thermal system for space heating and domestic hot water preparation
similar to the field-testing system has to be implemented into a simulation.
The simulation tool Matlab/Simulink (MathWorks n.d.) was used for the implementation of the collector and system models. Simulink is a simulation tool on the
basis of Matlab being able to build mathematical systems from single blocks by
connecting each other and using “drag and drop”. The block libraries embrace
blocks of basic maths and common functions, interfaces for input and output as
well as a multitude of build-ups of various systems. Furthermore, Simulink is able
to embed programmes written in a lower-level programming language like C, for
example. Thus, complex systems can be programmed and integrated easily into
the system environment of the blocks.
Hafner et al. (1999) developed the CARNOT toolbox containing blocks for the
build-up of heating systems. This library provides models for the simulation of
various heat sources, heat consumers, storages, controllers, pipes and pumps.
Furthermore, models for providing weather data, fluid properties and user functions are available. Also user-defined interfaces for the output of the relevant simulation data are programmed.
The detailed build-up of the collector models and the predefined heating systems
as well as the test stand are described as follows.
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4 Collector and System Modelling
4.1 State-of-the-Art Flat-Plate Collector Modelling
Commonly used parametric models in simulation tools are based on collector test
results from the solar simulator or outdoor testing. The function of the collector
efficiency curve from the test results is derived by quadratic regression
(ASHRAE 2003, Cooper and Dunkle 1981). Equation (4.1) describes the simple
parameter model.
′
The linear heat loss coefficient
(4.1)
and the quadratic heat loss coefficient
char-
acterise the thermal collector´s behaviour depending on the temperature difference between fluid and ambient air. The fraction of the converted solar energy
by the absorber is limited by the so-called optical efficiency of the collector. When
the mean fluid temperature is equal to the ambient temperature, almost no heat
losses occur at the collector. Thus, the thermal output of the collector can only be
reduced by the optical characteristics of the glazing and the absorber
as well
as by the collector efficiency factor ′ caused by a temperature gradient in the
fins, which results in heat losses.
This one-dimensional approach can be used as a multimode model to reproduce
the temperature gradient of the fluid along the absorber pipe between inlet and
outlet. Hafner et al. (1999) enhanced the basic equation of the parameter model
by considering the heat capacity of the collector, heat losses dependent on the
wind speed as well as additional radiative heat losses. However, these models
are not suitable for the investigation of new collector designs. On the one hand,
the parameters describe a certain measured collector but are not able to represent adjustments to the collector build or the implementation of overheating protection measures. On the other hand, the model only provides a fluid temperature.
However, the thermal investigation of collectors requires a broad output of the
component temperatures with respect to polymeric materials.
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4 Collector and System Modelling
The steady-state model by Hottel and Whillier (1958) only provides information
on the collector performance by means of the fluid inlet temperature
,
. Equa-
tion (4.2) describes the collector model.
(4.2)
,
The optical characteristics
loss coefficient of the collector
, the heat removal factor
and the overall heat
describe the performance of the collector.
These characteristics are based on the physics of the collector considering dimensions, shapes and materials. The temperature dependent heat transfer coefficients of the overall heat loss coefficient
vary subject to the conditions.
Therefore, the component temperatures and the heat transfer coefficients have
to be calculated by iteration. The reproduced physics of the model enable a
change into various designs or build-ups. Also, all the described overheating protection measures can be implemented into this model. Klein et al. (1974) investigated the transient behaviour of collectors or rather systems and identified significant differences to steady-state model approaches under certain conditions. During daily morning heating and rapidly changing ambient conditions like shading
by clouds, neglecting the thermal capacity of the components led to different results. Therefore, a model on the basis of equation (4.2) was extended with two
heat capacities. Fluid, absorber and half of the insulation show almost the same
temperature and represented the first heat capacity. The second heat capacity
was the glazing. Klein et al. (1974) and also Wijeysundera (1978) found that the
difference between one-node models, two-node models and experimental data
for normal collector build-ups and systems according to the yield became less.
However, taking into consideration that overheating protection measures can
most probably be influenced by quick reaction times, there are significant differences between steady-state and transient models, in particular with regard to
component temperatures. The main disadvantage of the model is the consideration of average component temperatures for the entire collector. Hence, the increase in temperature along the risers during operation causing maximum and
minimum temperatures is not taken into account. The collector properties, too,
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4 Collector and System Modelling
are considered as homogenous for the entire model. This makes the approach
incapable of considering fractional changes (e.g. at the upper half) of the properties or of the design in the collector model.
Thus, a dynamic, numerical model of a state-of-the-art flat-plate collector was
developed under consideration of the relevant capacities deriving information on
individual component temperatures, heat fluxes and collector performance. Moreover, the model can be adjusted to new collector parameters or designs and enables an implementation of overheating protection measures. By implementing
the multi-node collector model, the capacity of absorber, fluid, glazing and casing
was taken into account. The network connecting the nodes as well as the ambient
are physical functions depending on geometry, size, materials and operating conditions.
4.1.1 Description of the Collector
The field-test collector was chosen for the performance simulation of state-of-theart types. This type represents typical flat-plate collectors for solar-thermal systems for space heating and domestic hot water preparation in Central Europe.
The cross section of the collector in frame design is shown in Figure 4.1.
Figure 4.1: Cross section of a state-of-the-art collector with aluminium frame
The basic design data of the collector is listed in Table 4.1. The dimensions are
comparable to most collectors for heat supply in the sector for residual buildings.
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4 Collector and System Modelling
Table 4.1: Basic design data of the state-of-the-art collector
Gross length
1.987
[m]
Gross width
1.046
[m]
Gross area
2.078
[m²]
Aperture area
1.903
[m²]
Absorber area
1.903
[m²]
Weight (empty)
39
[kg]
Liquid content
1.12
[l]
The sheet-pipe absorber has a double harp design as shown in Figure 4.2. This
design provides an improved flow distribution in the riser pipes in contrast to the
conventional harp design. The following factors such as the reduced ratio between the total flow area of the riser pipes and the flow area of the header as well
as the increased pressure drop in the riser pipes which is caused by the doubling
of the length and the velocity of the stream, have a positive effect on the flow
through the absorber (Bassiouny und Martin 1984a and 1984b).
Figure 4.2: Design types of harp absorbers
(left: harp design; right: double harp design)
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4 Collector and System Modelling
The ultrasonic welded absorber is made from copper. The plate is only welded to
the riser pipes. The design data of the absorber is listed in Table 4.2.
Table 4.2: Design data of the absorber
Plate:
Thickness
0.0002
[m]
Length (pipe)
0.926
[m]
Length (plate)
0.985
[m]
Width (plate)
0.034
[m]
Inner pipe diameter
0.016
[m]
Wall thickness
0.001
[m]
Number
10
[-]
Length
1.857
[m]
Width (plate)
0.0985
[m]
Inner pipe diameter
0.007
[m]
Wall thickness
0.0005
[m]
Headers:
Risers:
The casing consists of an aluminium frame and mineral wool insulation (Figure
4.1). The extruded frame profile is plugged together with an aluminium backside
sheet. The mineral wool insulation is placed at the backside as well as at the
frame. The design data is shown in Table 4.3.
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4 Collector and System Modelling
Table 4.3: Design data of the insulation and casing
Casing:
Frame height
0.093
[m]
Backside sheet thickness
0.001
[m]
0.045
[m]
0.013
[m]
Backside insulation:
Thickness
Side insulation:
Thickness
The collector is also equipped with a structured low-iron solar glazing and a selective absorber coating. The optical properties are shown in Table 4.4.
Table 4.4: Optical properties of the collector
Glazing:
Thickness
0.0032
[m]
Transmission coefficient τ
0.918
[-]
Absorption coefficient
0.95
[-]
Emission coefficient
0.04
[-]
Absorber:
The collector was tested by the Institut für Solartechnik SPF (2005), a Swiss research institute of solar energy technology. The conditions and the results of the
outdoor collector performance test are listed in Table 4.5.
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4 Collector and System Modelling
Table 4.5: Test conditions and efficiency curve of the collector derived from testing
Testing conditions:
Water-glycol mixture
33.3
[%]
Volume flow rate
280
[l/h-1]
Irradiance
800
[Wm-2]
Efficiency curve (referenced to absorber area):
η0
0.798
[-]
a
3.34
[Wm-2K-1]
b
0.0075
[Wm-2K-2]
C (heat capacity)
9.5
[kJK-1]
4.1.2 Simplifying Assumptions
Modelling technical components or products requires a number of simplifying assumptions to limit the complexity of the model. Various small effects in components or products are very difficult to implement into the model and can be neglected due to their limited influence on the entire system. Thus, the following
assumptions described for collector modelling were made. These assumptions,
however, do not alter the basic physical situation of the collector.
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4 Collector and System Modelling
1. Heat transfer below and above the absorber is one-dimensional in zdirection (Figure 4.3).
z
y
x
Fluid
Figure 4.3: Schematic build-up of an absorber stripe
2. Headers and their plates make up only a small area of the collector and
can be neglected.
3. There is no difference in the fluid flow rates in the riser pipes.
4. The temperature gradient of the absorber plate (x-direction) caused by
the fin efficiency of the plate-pipe design cannot be described by any
transient model with a limited number of nodes. Moreover, a one-node
model with an average temperature is not sufficient since the effects of
the heat losses via convection and radiation are non-linear. This is why
the absorber plate has to be reproduced by a multi-mode model enabling
a non-linear temperature progression. The corresponding schematic
temperature gradients are shown in Figure 4.4.
T(x)
T(x)
x
x
Figure 4.4: Schematic temperature distribution at the cross section of an absorber in a
one-node (left) and a seven-node (right) absorber model in comparison to the analytical
temperature gradient
5. Around the riser pipes no temperature gradients occur.
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4 Collector and System Modelling
6. The temperature gradient of the absorber along the riser pipes (y-direction) is non-linear and cannot be described by any transient model with
a limited number of nodes. A one-node model with an average temperature is not sufficient since the maximum temperatures are lowered and
the effects of the heat losses via convection and radiation are non-linear.
This has an impact on the other temperature nodes of the absorber as
well. Therefore, a multi-node model along the fluid flow is necessary.
The corresponding schematic temperature gradients are shown in Figure 4.5.
T(y)
T(y)
Fluid
y
Fluid
y
Figure 4.5: Schematic temperature distribution at the cross section of an absorber along
the pipe in a one-node (left) and a five-node (right) absorber model in comparison to the
analytical temperature gradient
7. The riser pipe is only connected to the fluid and the absorber nodes. The
heat transfer to the air gap can be neglected since the pipe surface is
relatively low. Furthermore, the temperature gradient between pipe and
air is relatively low.
8. The two harps of the absorber do not influence each other thermally.
Hence, there is no heat transfer between neighbouring risers of the first
and second harp, neither is there any convection between the two arrays
with different temperature.
9. Unsteady heat conduction in the insulation can be neglected. The analytical temperature distribution in a solid material during time-dependent
temperature changes is influenced by the heat capacity of the system.
Thus, a multi-node model can represent this effect. However, the required number of nodes of the model would be significantly increased.
Therefore, the heat capacity of the insulation is represented by a single
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4 Collector and System Modelling
node. In a second step, the linear temperature gradient through the insulation is calculated under steady-state conditions. Figure 4.6 shows
the described approaches.
T(z,t)
T(z,0)=T0
T(0,t)=T1
T1
T1
T(z,t)
t
t
T0
z
T0
z
Figure 4.6: Schematic temperature distribution at the cross section of the casing or
insulation during unsteady heat conduction in an analytical model (left) and a onenode model with additional steady-state calculation of the temperature gradient
10. The heat capacity of the backside sheet of the casing can be neglected
due to the low mass of the component as well as the low temperature
changes near the ambient air.
11. The frame of the collector is neglected. Due to the one-dimensional heat
transfer through the collector, the effects in the frame cannot be taken
into consideration. Furthermore, the effect of the heat capacity has only
a slight influence since the temperature changes outside the collector
insulation are low.
12. The temperature gradient in the glazing is neglected. The low thickness
and the moderate heat conductivity of the glazing limit the influence of
any heat conduction to a minimum.
13. The glazing is non-transparent for the wavelength of infrared radiation.
Thus, heat radiation is completely absorbed by the glazing material. This
assumption is valid for solar glass. The implementation of polymeric materials being partly transparent for infrared radiation requires an adjustment of the heat transfer network.
14. Since the impacts of the structured surface of the glazing are only marginal compared to the plain surfaces, the latter was not taken into consideration.
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4 Collector and System Modelling
15. The sky is a black body for infrared radiation.
16. The ambient temperature around the collector is homogeneous.
17. The effect of wind is considered homogenously over the glazing in xdirection.
18. Wind at the backside is neglected because of the small distance between backside insulation and roof as well as the small temperature difference between outer surface and ambient air.
19. Shading from the frame at the absorber is not taken into consideration.
20. Neither are effects from operation considered (dust, dirt, degradation,
moisture…) influencing the optical and thermal performance of the collector.
4.1.3 Model Build-up and Energy Balance
The developed numerical model of the collector is limited to the area of the risers.
Using the symmetry of the collector, the model can be described by only one
riser. This model of a riser is used for both halves of the double harp absorber.
Figure 4.7 shows the cross section of the riser or rather the collector model with
its capacities and its connecting energy fluxes. The ten-node model can be reduced by using the symmetry of the absorber plate to a seven-node model.
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4 Collector and System Modelling
Sg
Ambient
g
Glazing (g)
a1
a2
f
a3
a4
Absorber (a)
Fluid (f)
Casing / insulation (c)
c
Symmetry
Adiabatic
Ambient
Insu‐
Adiabatic
lation (edge)
Sa
Ambient
Figure 4.7: Cross section of the state-of-the-art collector model with nodes and energy fluxes
The equations (4.3) to (4.9) give an overview of the energy balance drawn in the
collector model describing the capacities of each node as well as the energy
fluxes between the nodes.
Glazing (g):
Absorber 1 (a1):
2
2
(4.3)
(4.4)
Absorber 2 (a2):
(4.5)
Absorber 3 (a3):
(4.6)
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4 Collector and System Modelling
Absorber 4 (a4):
(4.7)
Fluid (f):
(4.8)
∗
2
Casing (c):
2
2
(4.9)
The node a1 is the base of the fin and includes the riser pipe. The listed energy
fluxes connecting the nodes to each other are described in the following in detail.
4.1.4 Optical Properties
The optical properties of the collector comprise the behaviour of the absorber as
well as the glazing under irradiance. The occurring optical effects determining the
collector performance are reflection, absorption and transmission (Figure 4.8).
The effects depend on the irradiance and the incidence angle as well as on the
thickness, the refractive index and the extinction coefficient of the materials. The
refractive index
and the extinction coefficient
of the glazing material depend
on the wavelength of the incoming radiation. In the simulation model these properties are assumed to be constant for the irradiance. Duffie and Beckman (2006)
considered this assumption to be ideally suited for covers made of glass.
G
Air
G
ρG
Θ1
Air
ρG
Θ1
Glazing
Absorber
αG
Θ2
Air
αG
τG
Figure 4.8: Basic optical effects at glazing (left) and absorber (right)
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4 Collector and System Modelling
4.1.4.1 Refraction Factor of the Glazing
The incident unpolarised solar radiation is polarised at the boundary surface between air and glazing. The radiation is divided into a parallel and a vertical component with regard to the plane of incidence (Figure 4.9). The definition of the
polarised light is necessary for the calculation of the associated reflection factor
(Eicker 2003).
G
Parallel to the
plane of
incidence
Plane of
incidence
Vertical to the
plane of
incidence
Figure 4.9: Polarisation directions parallel and vertical to the plane of incidence
(according to Eicker 2003)
The reflection factor of the boundary surface of the glazing is defined as the ratio
between the reflected radiation
and the incident radiation . This ratio results
from the average reflection factor from the perpendicular and the parallel part
(equation (4.10)) (Duffie and Beckman 2006).
∥
(4.10)
2
The Fresnel formulae for smooth boundary surfaces between transparent materials with the refractive indices
angle
and the refraction angle
and
are calculated according to the incidence
as shown in Figure 4.10.
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4 Collector and System Modelling
Air
Θ1
Glazing
Boundary surface 1
Θ2
Θ2
Boundary surface 2
Air
Θ1
Figure 4.10: Relations between beam angle and refractive indices
(according to Eicker 2003; Duffie and Beckman 2006)
The reflection factors for vertical and parallel polarised radiation are shown in
equations (4.11) and (4.12).
∥
sin
sin
(4.11)
tan
tan
(4.12)
The refraction angle
changes with the incidence angle
two refraction indices
and
depending on the
. The relation is described by Snell’s law in equa-
tion (4.13).
sin
sin
Vertical incidence at the boundary surface is a special case. Both angles
(4.13)
and
are zero. Also, the vertical and parallel refraction factors are equal. Hence, the
refraction factor can be derived from equation (4.14).
0°
(4.14)
4.1.4.2 Absorption in the Glazing
The absorption of the radiation in the glazing caused by a partly transparent material is described by Bouguer’s law. The underlying, basic assumption is that the
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4 Collector and System Modelling
absorbed radiation
tion coefficient
is proportional to the incoming radiation
and the extinc-
of the material. This relation is shown in equation (4.15) (Duffie
and Beckman 2006).
(4.15)
The transmittance without reflection
is described by means of the factor of
radiation passing the material. This factor is the ratio between transmitted radiation
and incoming radiation
. This ratio is described by equation (4.16) de-
pending on the extinction coefficient
of the material and the path length of the
radiation.
(4.16)
cos
4.1.4.3 Transmission and Absorption Coefficient of the Glazing
Considering both reflection and absorption in a glazing, the transmission as well
as the absorption coefficient of the component can be derived. The boundary
surfaces of the glazing transmit and reflect the incoming beam. Thus, there is an
infinite number of transmissions, reflections and absorptions in the glazing (Duffie
and Beckman 2006). Figure 4.11 shows this effect in the glazing.
G
τa2 r(1-r)2
r
τa4 r3(1-r)2
Θ1
Glazing
1-r
τa2 r(1-r)
τa2 r2(1-r)
τa4 r3(1-r)
τa4 r4(1-r)
Θ2
τa(1-r)
τar(1-r)
τa3 r2(1-r)
τa(1-r)2
τa3 r3(1-r)
τa5 r4(1-r)
τa3r2(1-r)2
τa5r4(1-r)2
Figure 4.11: Transmission, reflection and absorption of a single glazing
(according to Duffie and Beckman 2006)
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4 Collector and System Modelling
The transmission coefficient
of the glazing is the average of the transmission
coefficients for both polarised directions (equation (4.17)). The infinite series of
beams caused by multiple reflections can be determined via ray tracing described
in equation (4.18) and (4.19).
∥
(4.17)
2
1
1
1
∥
1
(4.18)
1
∥
(4.19)
∥
The absorption coefficient
of the glazing is also calculated for both polarised
directions by an infinite series described in the equations (4.20) to (4.22).
∥
(4.20)
2
1
∥
1
1
1
1
1
∥
∥
1
1
(4.21)
(4.22)
4.1.4.4 Solar Energy Conversion in the Collector
Equation (4.23) describes the energy input at the absorber reduced by the effective transmission absorption product (τα)eff of glazing and absorber (Duffie and
Beckman 2006).
(4.23)
Simple approaches of (τα) only consider the light transmission through the glazing and the absorption of the absorber surface. In this model, however, a more
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4 Collector and System Modelling
detailed approach is applied as described by equation (4.24) (Duffie and Beckman 2006).
1
(4.24)
1
The multiple reflections between absorber and glazing are considered with the
non-ideal absorption coefficient α of the absorber and the reflection coefficient ρ
of the inner glazing surface. Figure 4.12 shows this effect in the schematic collector build-up.
G
Glazing with transmission coefficient τ
τ
τ(1-α)2 ρ2
τ(1-α) ρ
τ(1-α)
τα
τρ(1-α)2
τα(1-α)ρ
τα(1-α)2ρ2
Absorber with absorption coefficient α
Figure 4.12: Effective transmission absorption product (τα)eff of a single glazed collector
(according to Duffie and Beckman 2006)
The additional solar energy input in the glazing depending on absorption of the
glazing changes the temperature of this node and influences the collectors’ heat
losses (Duffie and Beckman 2006). The solar energy input in the glazing
is
shown in equation (4.25).
1
(4.25)
The optical effects for solar energy conversion in absorber and glazing depend
on the angle. Hence, these effects have to be considered for direct as well as for
diffuse irradiance (equation (4.26)).
(4.26)
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4 Collector and System Modelling
For the calculation of the optical properties for direct irradiance, the beam angle
at the inclined collector surface is used. The diffuse irradiance from the sky is
assumed to be isotropic. Depending on the collector slope , a certain fraction of
the sky appears for the glazing as well as for the absorber. Brandemuehl and
Beckman (1980) investigated the relation between the sky and the inclined glazing systems for diffuse irradiation. As a result, an approach for the calculation of
an equivalent angle depending on the collector slope was derived.
Equation (4.27) shows the equivalent angle of incidence for diffuse irradiance for
single-glazed collectors.
59.7°
0.1388
0.001497
(4.27)
4.1.5 Convection
Heat transfer by convection occurs at several layers in the collector. There is
forced convection in the absorber pipes caused by the pumped heat transfer fluid
and at the surface of the glazing caused by wind. Free convection occurs in the
air gaps in front of the absorber and behind the absorber as well as at the glazing
and at the backside of the collector. These heat transfer mechanisms are represented by equation (4.28) containing the convective heat transfer coefficient from
equation (4.29), that depends on the Nusselt number
length
and the characteristic
of the component as well as on the heat conductivity
of the fluid
(Stephan 2010).
(4.28)
(4.29)
The convective heat transfer of solid bodies cannot be defined by physical equations, because of its strong dependency on fluid properties, shape, dimension
and temperature difference. Therefore, the heat transfer was investigated in experiments. Empirical correlations were derived from the measurement results and
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4 Collector and System Modelling
were described by the dimensionless heat transfer coefficient
. This approach
defines the average convective heat transfer of the investigated set-up and is
valid for a certain range of fluid properties and dimensions. The average heat
transfer coefficient is usually referred to a uniform temperature of the body surface. Thus, average surface temperatures and average fluid properties are assumed in the constant temperature case. The constant heat flux case considering
a non-uniform surface temperature is not considered due to the very small error
in convective heat transfer of approximately ~0.1 % (Stephan 2010).
4.1.5.1 Forced Convection in the Fluid Channels
The forced convection in the absorber pipes is calculated according to the approach recommended by Gnielinski (1995). The characteristic length
diameter
is the pipe
in this case. The flow in the pipes was assumed to be constant along
the entire length without entrance effects. The temperature dependent properties
of the fluid were referenced to the fluid node in the centre of the pipe.
The hydrodynamically developed laminar flow for Reynolds numbers
below
2,300 and for a constant axial wall temperature is defined with a constant Nusselt
number of 3.657 (Mills 1999).
In the transition region between laminar and fully developed turbulent flow with
Reynolds numbers in the range of 2,300 and 10,000, the Nusselt number is calculated by the equations (4.30) to (4.35) for a constant axial wall temperature.
1
,
10
,
2,300
2,300
49.371
(4.30)
,
for
, ,
0
0.7
1
(4.31)
, ,
(4.32)
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4 Collector and System Modelling
1.615 2,300
, ,
, ,
1
2
22
2,300
0.0308
8 10
0.0308
12.7
8
,
1
(4.33)
(4.34)
1
(4.35)
1
The Nusselt number for fully developed turbulent flow by Gnielinski (1974) for
Reynolds numbers larger than 10,000 is described by equation (4.36). The friction factor
can be determined by equation (4.37).
8
1
12.7
1
(4.36)
1
8
1.8
1.5
(4.37)
These approaches are valid for a ratio between the pipe diameter
and the pipe
length less than or equal to unity as well as for Prandtl numbers
between 0.6
and 1,000.
4.1.5.2 Convection at the Glazing
The convection on the outer surface of the glazing is a combination of free and
forced convection. Based on experiments, Churchill (1977) developed an approach for an overlay of both convections as shown in equation (4.38).
,
128
,
(4.38)
4 Collector and System Modelling
The Nusselt number for forced convection with turbulent boundary layer is considered according to the equation (4.39) by Schlichting (1958).
.
0.037
,
1
2.443
.
(4.39)
1
Therefore, the air flow caused by the wind is assumed to occur along the collector
width of the glazing according to Figure 4.13.
Collector glazing
uwind
β
Figure 4.13: Wind at the inclined collector glazing
The average Reynolds number in equation (4.40) is calculated by the wind velocity
ity
, the characteristic length of the glazing
and the kinematic viscos-
. The equation is valid for Reynolds numbers between 5 x 105 and 1 x 107
as well as for Prandtl numbers between 0.6 and 2,000. Even during low temperatures and wind velocities, the Reynolds numbers are in the range of the validity.
Hence, laminar flow for forced convection is neglected.
∗
(4.40)
For free laminar convection at the glazing two cases are defined by Fujii and
Imura (1972). The most important case for thermal collector simulation is the heat
transfer upwards at the inclined glazing. During collector operation, the glazing
129
4 Collector and System Modelling
temperature turns out to be larger than the ambient temperature. Hence, the collector’s heat losses are driven by the upward flowing air. The second case with
lower glazing temperatures is only relevant for system simulations with fluid temperatures below the ambient air temperatures. Figure 4.14 shows these two
cases for the glazing.
Figure 4.14: Free convection at the glazing with heat transfer upwards (left)
and downwards (right)
The Nusselt number for free laminar convection at the glazing with heat transfer
upwards or downwards at vertical planes is described in equation (4.41) from
Churchill and Chu (1975) and is also valid for Rayleigh numbers between 1 x 10-1
and 1 x 1012. Using the correction factor
in equation (4.42) by Churchill and
Chu (1972) allows Prandtl numbers in the range between 0.001 and non-finite.
This correlation for the Nusselt numbers is also valid for inclined surfaces by substituting the Rayleigh number
for the adjusted Rayleigh number
according
to equation (4.43) (Fujii and Imura 1972). This adjustment considers the reduced
component of gravity at the inclined plane instead of the acceleration due to gravity . In this case the characteristic length
,
0.825
0.387
130
is the length of the glazing.
for
(4.41)
4 Collector and System Modelling
0.492
1
for
0.001
∞
sin
(4.42)
(4.43)
Boundary layer separation may occur for free convection at the glazing with heat
transfer upwards. Hence, there is a change from laminar to turbulent flow. The
laminar flow occurs for Rayleigh numbers below the critical Rayleigh number
depending on the collector slope
from equation (4.44). Figure 4.15 shows
the gradient of the upper limit of the critical Rayleigh number for the described
laminar convection.
10
.
.
∗
°
.
(4.44)
10 9
Critical Rayleigh number [-]
10 8
10 7
10 6
10 5
10 4
10 3
10 2
0
15
30
45
60
Collector slope [°]
75
90
Figure 4.15: Upper limit of the critical Rayleigh number depending on the collector slope
Above the critical Rayleigh number, free turbulent convection at the glazing occurs. Fujii and Imura (1972) defined equation (4.45) accordingly.
131
4 Collector and System Modelling
,
0.56 ∗
∗ cos 90°
.
0.13 ∗ √
(4.45)
4.1.5.3 Convection at the Casing
Duffie and Beckmann (2006) regarded the heat transfer from the casing to the
ambient by radiation and convection as negligible because of the significant impact on the backside insulation and the resulting low temperature. In this model
the heat transfer coefficients from the backside to the ambient are calculated for
a hypothetical windless region between collector and roof by natural convection
and radiation. The effect of a chimney was also neglected. The heat transfer via
convection was assumed as free convection at an inclined plane. Figure 4.16
describes the two occurring cases for solar collectors.
Figure 4.16: Free convection at the casing with heat transfer downwards (left)
and upwards (right)
The most common case for the backside sheet is a heated plate transmitting heat
downwards without any separation of the boundary layer. Thus, the Nusselt number for laminar flow from the equations (4.41) to (4.43) serves as an exploration
for the free convection behind the collector. In this case, Lc-amb is the characteristic
length of the casing.
The special case of a cooled surface absorbing heat from the ambient air from
below is described by the equations (4.41) to (4.43) for laminar flow. Above the
critical Rayleigh number the separation of the boundary layer is expressed by
132
4 Collector and System Modelling
means of equation (4.44). Furthermore, the approach for turbulent flow from
equation (4.45) has to be used.
4.1.5.4 Convection Between Absorber and Glazing
Heat transfer in an enclosed air gap is driven by heat conduction as well as by
free convection of the circulating gas layer. This free convection between glazing
and absorber with the two possible directions of heat transfer is shown in Figure
4.17.
Adiabatic
Tg
Adiabatic
Tg
Tc
Tc
Ta
Ta
Qcond,a-g
Qcond,a-g
β
β
Adiabatic
Adiabatic
Figure 4.17: Free convection in the enclosed gas layer between absorber and glazing with heat
transfer upwards (left) and downwards (right)
Upwards heat transfer usually appears in collectors between absorber and glazing. Therefore, Matuska and Zmrhal (2009) derived a Nusselt correlation from
several correlations similar to Hollands et al. (1976) to verify the Nusselt number
approach for an increased range of collector slopes. Equation (4.46) describes
the Nusselt number for the air gap valid for slopes
teristic length
from 0° to 90°. The charac-
in that system is the distance between absorber and glazing.
The value of fβ in equation (4.47) constitutes an angle factor for the Nusselt number, that depends on the collector slope which was obtained as quadratic function
in the derived correlation. The approach is valid for 1 x 104 <
< 2 x 106 covering
the necessary operational range of solar-thermal collectors. The correlation, too,
showed good results independent of the aspect ratio between air gap length and
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4 Collector and System Modelling
air gap distance. Several approaches, however, are limited with regard to the
collector geometry.
.
0.1464
(4.46)
2.602
2.046
(4.47)
The approach for the Nusselt number with heat transfer downwards is derived
from Arnold et al. (1976). Therefore, equation (4.48) is used on the basis of the
calculated Nusselt number for heat transfer in vertical air layers ( =90°) derived
from equation (4.46) and (4.47). The validity of this approach is similar to the
approach of equation (4.46). However, the aspect ratio between air gap length
and air gap distance has to be larger than 20.
1
90°
1 sin 180°
(4.48)
4.1.5.5 Convection Between Absorber and Casing
The heat transfer in the enclosed air layer between absorber plate and casing is
equal to the Nusselt correlation for convection between absorber and glazing in
Chapter 4.1.5.4. The usual approach for the region behind the absorber is free
convection with heat transfer downwards expressed by equation (4.48).
4.1.6 Radiation
Heat transfer caused by radiation is based on the Stefan-Boltzmann law of a
black body radiator. The energy transfer is described with an emission coefficient
1 by Eicker (2003) by equation (4.49). The Stefan-Boltzmann constant
is
shown in equation (4.50).
(4.49)
5.67
134
(4.50)
4 Collector and System Modelling
Bodies above a temperature of 0 K emit radiating power. Radiating power from
other bodies placed in a room is also absorbed. The position of the bodies also
has great influence on the behaviour as described by their form factors Φ. The
form factor is the ratio between the radiation receiving surface and the entire
hemisphere. Figure 4.18 shows the heat exchange of the two surfaces.
A2
Q1
Q2
Φ12
A1
Figure 4.18: Heat fluxes via radiation between two elements (according to Eicker 2003)
The net radiation exchange
between two bodies takes the properties of the
bodies into account and defines the heat transfer. Therefore, the difference between the radiating powers reaching the other body is taken from equation (4.51).
, →
, →
Φ
Φ
(4.51)
The definition of the net radiation exchange between two bodies with an emission
coefficient smaller than unity is a more complex approach. Therefore, a multiple
of reflections have to be considered.
The radiation exchange in solar-thermal flat-plate collectors embraces only two
special cases. There is radiation exchange between two parallel plates e.g. between absorber and glazing on the one hand. On the other hand, radiation occurs
from a limited surface to the infinite sky hemisphere.
The heat transfer by radiation has to be determined according to equation (4.52).
Therefore, the heat transfer coefficients for radiation of the collector have to be
defined.
135
4 Collector and System Modelling
(4.52)
4.1.6.1 Radiation Between Two Parallel Plates
Due to the fact that the heat emitted is completely absorbed by the opposed area,
the form factors of the two surfaces are equal (Φ
Φ
1). It is assumed that
the effects of infinitely expanded parallel surfaces apply for heat transfer in solar
collectors. Multiple reflections have to be considered for grey emitters as well by
introducing their emission coefficients
into the approach. The heat transfer co-
efficient for radiation between two parallel plates is given in equation (4.53).
1
1
(4.53)
1
This radiation exchange is necessary for the set-up of glazing and absorber, absorber and casing as well as casing and roof.
4.1.6.2 Radiation Between Glazing and Sky
The radiation between glazing and cold sky is a special phenomenon of heat
transfer (Eicker 2003). The surface of glazing
sky hemisphere
is much smaller than the total
. Hence, radiation from the glazing is completely emitted to
the sky. Thus, the form factor of the glazing Φ
form factor from the sky Φ
is 1. In contrary to that, the
is almost zero since the proportion of the glazing
surface being seen from the sky hemisphere is very small. Therefore, the emitted
heat from the sky is neglected because of the infinite area of the sky’s half-space.
The heat transfer coefficient is shown in equation (4.54). The temperatures have
to be inserted in Kelvin because radiation refers to the absolute temperature of
the body or rather surface.
,
136
(4.54)
4 Collector and System Modelling
The temperature of the sky differs from the ambient air temperature (Duffie and
Beckman 2006). Approaches for the definition of the sky temperature are related
to the air temperature (Swinbank 1963), to the water vapour pressure
(Brunt 1932) or to the dew point temperature (Bliss 1961), for example.
4.1.7 Heat Conduction
Heat conduction is prevalent at glazing, absorber and casing. Equations (4.55)
and (4.56) describe the heat transfer that depends on the heat conductivity k and
the thickness or rather distance s of the component (Stephan 2010).
(4.55)
(4.56)
Vertical heat transfer through thin plates in the thermal network like glazing or
absorber can be neglected. In the model, the heat conduction is connecting the
heat transfer from convection and radiation to the nodes of the solid materials.
Hence, there are little influences on the overall heat transfer coefficients.
Relevant heat conduction occurs between the nodes of the absorber. The small
area of the heat transfer area and the distance between the nodes (Figure 4.19)
cause heat transfer behaviour with major influence on the collector performance.
Area
Distance
z
y
Qcond,a2-a3
x
Figure 4.19: Heat conduction between the absorber nodes
The second relevant heat conduction is seen in the collector insulation. These
heat losses occur at the side and backside insulation. In contrary to the massive
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4 Collector and System Modelling
solid materials such as metals, the heat conductivity cannot be regarded as constant. The cavernous structure of the material also enables heat transfer effects
of the enclosed air and radiation. Thus, the heat conductivity increases according
to the material temperature. For dry mineral wool Ochs et al. (2004) derived a
linear approach for the temperature depending calculation of the heat conductivity
according to equation (4.57). The influence on the heat conductivity of the
unknown humidity in the mineral wool is neglected. Moist mineral wool usually
has highly increased heat conductivity.
0.035
2.0
(4.57)
4.1.8 Edge Losses
The edge losses, which comprise the heat losses through the collector frame or
rather side, cannot be mathematically calculated in detail. Tabor (1958) describes
an approach considering the losses only connected to the absorber node. The
other nodes have an adiabatic behaviour against the edge. Equation (4.58) and
(4.59) show the heat transfer between absorber and ambient air as well as the
heat transfer coefficient considering only the heat conduction through the side
insulation.
(4.58)
(4.59)
4.1.9 Overall Heat Transfer Coefficients Between the Nodes
The heat transfer coefficients
of the collector connect the nodes and the ambi-
ent. Most of them are embedded in a network of the thermal resistances to calculate a heat flow rate
. The electrical analogy is used to generate the overall
heat transfer coefficients between the nodes based on the single thermal effects.
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4 Collector and System Modelling
Equations (4.60) to (4.62) demonstrate the analogy between electrical and thermal circuits (Mills 1999, Stephan 2010).
(4.60)
(4.61)
(4.62)
Hence, the thermal resistance is defined according to equation (4.63). This definition suggests expressing the heat transfer rate according to equation (4.64).
1
(4.63)
,
1
1
(4.64)
,
On that basis the thermal and the electrical circuit have the same build-up. Thus,
the build-up of series and parallel connections in circuits can be used to define
the heat transfer (Figure 4.20).
T1
Qn
T2
T3
Tn
...
Q
T1
Tn+1
Q
Q1
Q2
...
Qn
Q2
Q1
T2
Figure 4.20: Schemes of serial (left) and parallel (right) heat transfer
Resistances connected in series have the same heat flux through every resistance in the circuit (Figure 4.21). The resistances of the circuits can be added
to the total resistance.
139
4 Collector and System Modelling
Q
Rth,1
T1
Rth,2
T2
...
T3
Tn
Rth,n
Tn+1
Figure 4.21: Thermal circuit of a serial connected system
Equations (4.65) to (4.67) describe the total thermal resistance of a series connection.
⋯
,
,
(4.65)
,
⋯
,
,
,
,
(4.66)
,
⋯
(4.67)
,
A parallel connection of resistances is defined by the equal applied temperature
difference and the splitting of the total heat flux. (Figure 4.22).
Q1
Rth,1
Q2
Q
T1
Rth,2
...
T2
Qn
Rth,n
Figure 4.22: Thermal circuit of a parallel connected system
Equations (4.68) to (4.70) show the total thermal resistance of a parallel connection.
140
4 Collector and System Modelling
⋯
,
(4.68)
,
1
⋯
,
1
,
1
,
1
,
(4.69)
,
⋯
,
1
(4.70)
,
These schemes are used to generate overall resistances by combining single
resistances of serial and parallel connections according to Figure 4.7. The required overall heat transfer coefficients (related to the area) of the energy fluxes
from equations (4.3) to (4.9) are shown in the equations (4.71) to (4.75).
1
1
,
,
1
,
,
1
,
,
,
1
2
,
1
1
,
,
,
1
,
1
,
,
(4.72)
(4.73)
,
1
,
(4.71)
,
1
,
1
1
,
(4.74)
(4.75)
4.1.10 Implementation in the Simulation Tool
The collector physics is implemented in the simulation tool Matlab/Simulink by
means of a so-called “S-function (level 2)”. The S-function is a compiled source
code written in C. The S-function is equipped with connexions for data input from
fluid, weather and collector orientation. The relevant output data of the fluid at the
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4 Collector and System Modelling
collector outlet as well as the part temperatures are provided at the output of the
S-function. Figure 4.23 shows the Simulink block with embedded S-function (grey
block) in the simulation environment. The blue blocks write the simulation data in
the workspace of Matlab.
Figure 4.23: Build-up of the collector block with embedded S-function (grey) in Simulink
With regard to an improved clearness, the S-function block and the other relevant
blocks are put into a subsystem of the Simulink environment. Figure 4.24 displays
the masked subsystem of the embedded collector S-function. The connexions of
the block are the above mentioned inputs and the output of the fluid.
142
4 Collector and System Modelling
Figure 4.24: Mask of the collector block with embedded S-function of the collector in Simulink
The necessary data of the heat carrier and the air in the gaps to calculate the
heat transfers is obtained by a link to the Carlib Library within the source code.
The Carlib Library of the CARNOT toolbox contains the functions for the relevant
material properties — like heat conductivity
and kinematic viscosity .
4.1.11 Validation
The model was validated using the parameter set of a tested conventional flatplate collector. The validation of this model and physical effects is also adequate
for models with implemented overheating protection measures or adjusted buildup since the defined physical effects are varied and not exchanged.
The model was tested in various settings to investigate the accuracy of the results. According to the above-mentioned simplifying assumptions in the model,
there are, of course, imperfections in the build-up of the tested collectors. Hence,
the differences in the results have to be identified, justified and evaluated.
4.1.11.1
Comparison with an Existing Validated Model
In a first validation step, the developed dynamic multi-node model was compared
to the results of a one-dimensional multi-node model approach by
Hafner et al. (1999) based on the approach of equation (4.1). Therefore, the parameters derived from collector testing identified by Institut für Solartechnik SPF
(2005) and listed in Table 4.5 were implemented into the model. This validation
test is only able to compare the results of the collector efficiency of an already
143
4 Collector and System Modelling
validated parameter model approach with exact coverage. Therefore, two test
simulations were set up.
The first test comprised the simulation of the collector efficiency curve like the
practical test modus in a solar simulator according to DIN EN 12975-2 (2006).
Under constant ambient conditions (
2
885
27° ,
,
), a single collector was tested. The heat carrier, a water-glycol mixture
(33.3 %), has a flow rate of
0.08
and variable temperatures through
the collector.
The percentage difference of the collector efficiency describes the accuracy of
the model approach according to equation (4.76).
1 100 %
(4.76)
The values range between -1.0 % and +3.3 % for flow temperatures from 25°C to
90°C (Figure 4.25).
Parameter model
Dynamic model
Percentage difference
0.8
8
0.7
4
0.6
0
0.5
-4
0
0.01
0.02
0.03
0.04
0.05
(Tmean -Tamb )/G [m²K/W]
0.06
0.07
Figure 4.25: Comparison of the simulated efficiency curves of the parameter model
and the developed dynamic collector model
144
4 Collector and System Modelling
The differences are relatively low according to the assumptions in the model.
Also, the simulated temperature range in the test covers a large operational range
of the collector to prove the use of the model.
The deviation is assumed to originate from the discretisation in the model. The
major influence in the test is the transition from laminar to turbulent flow at higher
values of the reduced temperature. The change in the model curve is remarkable
because of an averaged calculation of fully-developed flow conditions for the entire riser pipe. In contrast to that, the real collector shows a continuous development of flow conditions along the pipe length. There are also neglected entrance
effects increased by the vertical connection between header and riser.
Next to the analysis of efficiency, the models are compared to each other in an
annual simulation of a solar-thermal heating system for domestic hot water and
space heating. The build-up is equal to the system of the simulation analysis described in the following chapter. The collector surface of the parameter model
was adjusted due to the reduced absorber surface of the collector model because
of the missing header regions. The deviation of the solar yield is found to be
+2.9 %, similar to the results found in the comparison of efficiency. The deviation
originates from the fact that the major operational range of the collectors is at high
reduced temperatures. This is caused by both high flow and ambient air temperatures for hot water preparation in summer as well as moderate flow and low
ambient air temperatures in the heating period for space heating.
4.1.11.2
Comparison with Measurements
In a second step, the simulation results were compared to measurement data
from analysing the exposed field-testing collector. The test collector was already
equipped with sensors to measure the component temperatures. However, degradation of the absorber coating and humidity in the mineral wool can be named
as some of the reasons, resulting in the effect that the efficiency of the collector
is reduced in comparison to results from previous efficiency curves of a new collector in Chapter 4.1.11.1. Therefore, the emission coefficient of the absorber
145
4 Collector and System Modelling
coating and the heat conductivity of the mineral wool are adjusted to represent
the degradation of the components.
The first test is the comparison of the efficiency curves. Therefore, the conditions
of
the
test
1
stand
are
reproduced
). The heat carrier is water (
(
890
0.038
31° ,
,
). Figure 4.26 shows
the good coverage between measurement and simulation. According to equation (4.77), the percentage difference is only in the range between +0.4 % and
+3.1 %.
(4.77)
Percentage difference [%]
1 100 %
Collector efficiency [-]
Figure 4.26: Comparison of the measured and simulated efficiency of the collector
The second test also showed satisfying conformity in particular with regard to
individual component temperatures. During stagnation at insolation of 895 Wm-2
and an ambient air temperature of 33°C, the absorber temperature of model and
test (average of 9 measurement points) differ only by +1 K. Also the average
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4 Collector and System Modelling
temperatures (average of 3 measurement points) at glazing and backside vary
below +1 K (Table 4.6).
Table 4.6: Collector component temperatures during stagnation of a conventional
flat-plate collector in testing and simulation
Testing
Model
Difference
Glazing
71°C
71°C
±0K
Absorber
174°C
175°C
+1K
Backside (outside)
54°C
55°C
+1K
4.1.11.3
Evaluation of the Test Results
All in all, the model proves to be valid according to the test collector. Slight differences are identified as within the theoretical limits of the model. Thanks to the
assumptions that have been made, the model generates reliable results for flatplate collectors in due consideration of the limits of the model. Comparable model
approaches of state-of-the-art collectors show similar low uncertainties in simulation of efficiency or rather solar yield. The model from Koo (1999) showed percentage differences between -2 % and +3 % in comparison to experimental data
of the collector efficiency curve. Matuska and Zmrhal (2009) described a percentage difference of about 3–4 % in the model validation. Furthermore, Cadafalch (2009) identified differences below 2 %. Thus, there is confidence that the
model can be used for the investigation of entire systems to identify the solar yield
as well as for the analysis of the component temperatures in the collector.
4.2 System and Test Stand Modelling
For the investigation of the thermal loads on the collectors as well as of the system efficiencies two simulation cases are considered. A usual solar-thermal system represents the build-up to analyse the moderate thermal loads under operating conditions and the system efficiency over the year. A test stand for dry collectors without heat removal represents the maximum thermal collector loads as
a worst case scenario.
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4 Collector and System Modelling
4.2.1 System
The solar-thermal heating system in the simulation is typical for a single family
house in Central Europe and is described in Table 4.7.
Table 4.7: Description of the simulated solar-thermal heating system
Design data
System:
Building type
Single family house
Location
Würzburg (Germany)
Total system load
19,200
[kWh]
Space heating load
16,660
[kWh]
Domestic hot water load
2,540
[kWh]
Heat distribution system
Floor heating
Auxiliary heating
Oil heating boiler 15 kW
Buffer storage volume
897
[l]
Aperture area
14.6 (8 Collectors)
[m2]
Collector azimuth
0 (South)
[°]
Collector slope
45
[°]
Optical efficiency
0.798
[-]
Linear heat loss coefficient
3.34
[Wm-2K-1]
Quadratic heat loss coefficient
0.0075
[Wm-2K-2]
Heat capacity
9.5
[kJK-1]
Heat carrier
Water
Specific volume flow
40
Solar collectors:
[lm-2h-1]
The house requires 16,660 kWh per year for space heating and is equipped with
floor heating. Additional heating energy of 2,540 kWh per year is demanded for
hot water supply. The thermal energy is provided by an oil burner besides the
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4 Collector and System Modelling
solar-thermal energy supply. The solar-thermal system is a pressurized state-ofthe-art type. The eight collectors of the developed model are connected to buffer
storage with 897 l.
4.2.2 Test Stand
The test stand for collectors is similar to the collector exposition in the system
simulation. However, the collector is dry and not connected to a pressurized system. This represents the highest temperature loads on the collector components
over the test year as maximum reference.
Figure 4.27 (left) shows the build-up of the test stand in Simulink. The input blocks
feed the collector model with data from the fluid, the weather and the orientation
or rather the position of the collector.
Figure 4.27: Build-up of the test stand in Matlab/Simulink (left) and
parameter mask of the fluid block (right)
The fluid data for the model is provided by the block for constant fluid conditions
from the CARNOT library. Figure 4.27 (right) shows the mask for the definition of
the fluid at the collector inlet. The fluid data for the simulation of stagnation is set
to air at ambient pressure without any mass flow. The weather dataset of the test
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4 Collector and System Modelling
reference year of Würzburg is used for both, system simulation and test stand.
Also the orientation is similar to the system simulation.
150
5 Thermal Collector Analysis by Simulation
5 Thermal Collector Analysis by Simulation
The selected, promising overheating protection measures from Chapter 0 have
to be analysed in greater depth regarding their influence on system efficiency and
component temperatures. Therefore, the developed simulation model was used
to derive the necessary results by implementing the measures in collectors. The
approaches were investigated in annual simulations. On the basis of chosen temperature limits and a reference system the simulation results will be investigated
and compared.
5.1 Evaluation Basis
For comparison and evaluation of the simulation results two cases are of major
importance regarding polymeric collectors. On the one hand, reachable temperature reduction or rather limited maximum temperatures of the collector components have to be considered to allow cost-effective materials. On the other hand,
the system yield of the annual simulations is necessary in order to be able to
derive expectations about the heat generation costs of the systems.
5.1.1 Temperatures
Regarding thermal loads on components, two cases are of importance for the
polymeric material selection. Firstly, the maximum temperature has to be identified to consider the stability of the component as well as the degradation of the
material. This temperature has to be avoided or limited to short durations for components without mechanical loads. Secondly, the thermal loads over the defined
life time of the component are relevant to estimate degradation as well. Above
this temperature, depolymerisation and oxidation in the material occur or rather
occur at an advanced rate (Biron 2013). As a result, the material has decreased
mechanical properties like the impact strength. Therefore, three temperatures
from material tests are used for the thermal requirements of the components:
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5 Thermal Collector Analysis by Simulation
The
heat
deflection
temperature
(HDT)
according
to
the
standard
DIN EN ISO 75-1 (2013) describes a defined bend of a test specimen under a
test load. Hence, this temperature is connected to the stability and defines the
maximum temperature of components under mechanical loads. The HDT can be
derived from stagnation tests and from peak temperatures of annual simulations.
However, the focus of thermal investigation is on the durability of the materials
due to degradation caused by thermal loads. The stability has to be considered
in the developmental stage of the collector component design.
The short-term service temperature is defined as a temperature limit without appreciable damage of the material (Domininghaus et al. 2012; Biron 2013). Therefore, degradation and deformation is taken into account. The duration is usually
set ranging between a few minutes and several hours. This temperature will be
derived from annual simulations.
The long-term service temperature according to ISO 175 (2010) defines the temperature of a material specimen being resistant for 20,000 h of exposure in hot
air or 10,000 h of exposure in hot oil without losing more than 50 % of its mechanical properties like the tensile strength in comparison to the reference value measured at 23°C. Therefore, the annual simulations will be used to investigate preselected material requirements according to the long-term stability.
For the evaluation of the simulation results, the temperature boundaries of basic
polymeric material classes were chosen. The lowest material costs were
achieved by commodity plastics like polypropylene (PP). However, the temperature limits for their use are also low. Thus, this material represents the highest
requirements for temperature reduction. More durable materials are engineering
plastics. A relevant and cost-efficient example is polyamide 66 (PA 66). This example considers especially the use in absorbers. Therefore, the materials are
characterised for their durability against hot water. Table 5.1 shows the temperature limits of the materials being suitable for absorber, insulation and casing.
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5 Thermal Collector Analysis by Simulation
Table 5.1: Temperature limits of opaque materials for absorber, insulation and casing
regarding defined tests (Domininghaus et al. 2012)
Materials
Short-term service
temperature [°C]
Long-term service
temperature [°C]
140
100
~185
~100
(170–200)
(80–120)
Commodity plastics:
PP
Engineering plastics:
PA 66
5.1.2 Solar Yield
The yield of the solar-thermal system and the influence of the overheating protection measures have to be identified by comparison of the annual simulations.
The fractional energy savings
according to the standard DIN EN 12977-
2 (2012) are used to define the system efficiencies. Therefore, the energy consumption of the heater of a conventional heating system
and the
energy consumption of the auxiliary heater of the solar-thermal heating system
were taken into account whereas both heating systems have to supply the
annual total system load of the house
(Figure 5.1).
Qd
Qd
Qconventional
Heater
Storage
Qaux
Auxiliary
heater
Buffer
storage
Figure 5.1: Basic energy flow of a conventional heating system (left)
and a solar-thermal heating system (right)
The value of the fractional energy savings is the percentage of the energy being
saved by the solar-thermal system instead of using a conventional heating system being represented in equation (5.1).
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5 Thermal Collector Analysis by Simulation
100 %
The advantage of
(5.1)
is the consideration of the additional thermal losses of the
solar-thermal heating system mainly caused by the buffer storage.
Table 5.2 shows the description of the defined conventional heating systems as
a basis.
Table 5.2: Description of the simulated reference heating system
Design data
Building type
Single family house
Location
Würzburg (Germany)
Total system load
19,130
[kWh]
Space heating load
16,590
[kWh]
Domestic hot water load
2,540
[kWh]
Heat distribution system
Floor heating
Heater
Oil heating boiler 15 kW
Domestic hot water storage volume
130
[l]
Energy consumption of the heater
20,120
[kWh]
5.2 Reference Results
As a reference, the system described above was simulated with conventional flatplate collectors. Two scenarios were taken into account — conventional flat-plate
collectors with and without selective absorber coating. The scenario without selective absorber coating is used as a reference result for overheating protection
measures having a non-selective behaviour as well as a distinct concept to decrease component temperatures.
The system with highly-selective coated absorbers (optical characteristics:
0.95 and
154
0.04) reaches fractional energy savings
of 20.0 %. In the
5 Thermal Collector Analysis by Simulation
same system with a non-selective absorber coating (
0.95;
0.90), the frac-
tional energy savings decrease to 15.7 %. The higher radiation losses of the absorber cause a relative decrease of the solar yield by 21.5 % (Table 5.3).
Table 5.3: Fractional energy savings and relative decrease of the reference
systems in annual simulations
System
[%]
Relative decrease [%]
Reference system, selective
20.0
--
Reference system, non-selective
15.7
-21.5
However, the component temperatures, too, decrease as a result of the lowered
efficiency. The maximum absorber temperature declined from 223°C to 139°C.
The temperatures of the selective absorber exceed the limits of both material
classes whereas the non-selective absorber ranges between the short-term services temperatures of PP and PA66.
Figure 5.2 shows the histograms of the absorber temperatures of the two reference systems during operation and permanent stagnation over the period of one
year. The temperature levels from 100°C to 180°C represent the short-term temperature arrays for commodity as well as engineering plastics for orientation.
Figure 5.2: Histograms of the absorber temperatures with (black)
and without (grey) selective absorber coating over one year
Beside the temperature peak, also the duration of plate temperatures above 95°C
decreases considerably from 425 to 76 h for the collector being integrated into
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5 Thermal Collector Analysis by Simulation
the system. Especially the thermal loads of the dry collector were drastically lowered from 1,230 to 465 h.
5.3 Thermotropic Layers
Thermotropic layers at the collector front are able to lower the occurring temperatures of the collector components. In Resch et al. (2009) a number of layers with
fixed domains of thermoplastics were investigated in detail. Two layers with the
most promising properties were analysed in the system simulation to estimate
their potential for solar-thermal applications. The layer C-4-5 has a relatively high
hemispherical solar transmittance of about 87 % in the clear state. However, the
decrease to 75 % in the scattered state is low. In contrary to that, the second layer
disposes of lower hemispherical solar transmittance of about 78 % in the clear
state. With a decrease to 62 %, it has, however, a better switching performance.
Figure 5.3 shows the switching performance of the layers depending on the tem-
Hemispherical solar transmittance [%]
perature.
Figure 5.3: Temperature-dependent gradient of the transmittance of two thermotropic layers
for simulation (according to Resch et al. 2009)
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5 Thermal Collector Analysis by Simulation
5.3.1 Application at the Absorber Surface
Switching of the layers occur in the range between 70°C and 95°C. Thus, the
layer has the best switching performance being placed at the absorber surface.
The influence of the ambient temperature and the high trigger temperature are
disadvantageous for an application on the single glazing. Thus, the absorber temperature would exceed the proposed limits of the polymeric materials in this case.
The layer application on the absorbing surface inhibits selective absorber properties.
The collector systems being equipped with the thermotropic layers show a drastic
efficiency reduction in the annual simulations. The relative decrease of the fractional energy savings or rather the solar yield is between -35.0 % (C-4-5 / transmission: 87–75 %) and -42.0 % (C-6-7 / transmission: 78–62 %) compared to the
selective reference system, which is shown in Table 5.4. The additional reduction
of the transmittance of the collector front in clear state and the missing selective
absorber behaviour are the major reasons for that loss. But also the early switch
of the layer at temperature levels being relevant for the solar yield of the system
is disadvantageous. The reference systems are still operating with a maximum
efficiency at absorber temperatures between 80°C and 120°C whereas the protected systems decrease their efficiency due to the active overheating protection.
Table 5.4: Fractional energy savings and relative decrease of the reference systems
as well as of the systems with thermotropic absorber layers in annual simulations
System
[%]
Relative decrease [%]
Reference system, selective
20.0
--
Reference system, non-selective
15.7
-21.5
System with thermotropic layer C-4-5
13.0
-35.0
System with thermotropic layer C-6-7
11.6
-42.0
The higher transmittance of layer C-4-5 increases the system performance, but
also causes higher component temperatures due to the higher transmittance in
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5 Thermal Collector Analysis by Simulation
the scattered state. Thus, the absorber in the system is less than 3 h at temperature levels above 100°C and with a maximum value of 109°C. The absorber of
the dry stagnating collector is 139 h above this temperature level and reaches a
maximum temperature of 117°C. Figure 5.4 represents the durations of the ab-
Duration [h]
sorbers at the occurring temperatures over one year.
Figure 5.4: Histograms of the absorber temperatures without selective coating (grey) and with
thermotropic absorber layer C-4-5 (green) over one year
The layer C-6-7 has a lower transmittance in the clear state and a large switch to
the scattered state (Figure 5.5). This leads to a maximum temperature of only
Duration [h]
90°C in the system integrated collector and 104°C in the dry collector.
Figure 5.5: Histograms of the absorber temperatures without selective coating (grey) and with
thermotropic absorber layer C-6-7 (green) over one year
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5 Thermal Collector Analysis by Simulation
5.3.2 Application at the Glazing
For comparison, the layers were also placed at the glazing. The application at the
glazing causes an activation of the measure at higher absorber temperatures due
to the temperature gradient between plate and cover. The switch of the layer also
depends on the ambient air temperature near the glazing. Thus, the activation of
the measure is not connected strongly to the absorber temperature.
The collector systems being equipped with non-selective absorber and thermotropic layers at the glazing also show a drastic efficiency reduction in the annual
is between -35.0 % (C-4-5) and -42.0 %
simulations. The relative decrease of
(C-6-7) compared to the selective reference system (Table 5.5). The performance
of the system is equal to the approach with the layer application at the absorber.
Thus, the system efficiency is only limited by the disadvantageous optical properties of the application. The switching characteristics depending on the deposition do not have any influence on the system operation. Also the low maximum
glazing temperatures of 74°C or rather 72°C cause almost no change to the optical characteristics of the layer.
Table 5.5: Fractional energy savings and relative decrease of the reference systems
as well as of the systems with non-selective absorber and thermotropic layers on the
glazing in annual simulations
System
[%]
Relative decrease [%]
Reference system, selective
20.0
--
Reference system, non-selective
15.7
-21.5
System with thermotropic layer C-4-5
13.0
-35.0
System with thermotropic layer C-6-7
11.6
-42.0
At the absorber being connected to the solar-thermal system maximum temperatures of 121°C (C-4-5) or rather 108°C (C-6-7) occur. At the absorber in the dry
collector 124°C (C-4-5) or rather 119°C (C-6- 7) were reached.
In contrary to that, a set-up with selective absorber and thermotropic layer at the
glazing leads to the fact that the system efficiency can be improved. The reduced
159
5 Thermal Collector Analysis by Simulation
radiation from the absorber causes less thermal losses and a steeper temperature gradient between absorber and glazing. The low maximum glazing temperatures of 69°C or rather 66°C are also too low for an activation of the measure.
This leads to increased system efficiencies because the operational range of the
system and the switch of the layers do not overlap. The systems have a relative
decrease of -10.5 % (C-4-5) and -17.5 % (C-6-7) according to the reference with
selective absorber coating (Table 5.6).
Table 5.6: Fractional energy savings and relative decrease of the reference systems
as well as of the systems with selective absorber and thermotropic layers on the
glazing in annual simulations
System
[%]
Relative decrease [%]
Reference system, selective
20.0
--
Reference system, non-selective
15.7
-21.5
System with thermotropic layer C-4-5
17.9
-10.5
System with thermotropic layer C-6-7
16.5
-17.5
For both, system operation and dry stagnation, the maximum absorber temperatures of 200°C (C-4-5) and 188°C (C-6-7) are extremely high.
5.3.3 Summary
The system performance of collectors with non-selective absorber properties and
thermotropic layers is independent from the place of application of the measure.
The lower component temperatures of the collector with thermotropic absorber
favour this set-up.
The collector with selective absorber and thermotropic glazing shows a good system performance. However, the reduced heat transfer leads to glazing temperatures below the trigger point of the layer. Hence, the layer will not be activated
even at absorber temperatures of 200°C. Therefore, the expensive combination
of selective coating and thermotropic layer is not useful.
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5 Thermal Collector Analysis by Simulation
Higher transmittance of layer C-4-5 leads to a better system performance
whereas absorber temperatures also stay below critical levels. Thus, layer C-4-5
provides preferable collector characteristics. However, the major temperature reduction is caused by the low optical efficiencies of the approaches.
5.4 Micro-structured Layer
For the investigation of micro-structured layers for overheating protection, the
tested and simulated layers from Walze (2005) were implemented. The metalized
asymmetric CPC structure according to Figure 5.6 was placed at the glazing of
the collector model. This structure deflects a major part of the solar radiation at
low zenith angles occurring particularly during summer (incidence angle at the
collector ranging between -60° and 0°). Thus, the hemispheric transmission is
reduced at these conditions. For simulation, the behaviour of the tested asymmetric CPC from Figure 5.6 (right) was used in combination with a selective absorber.
Figure 5.6: Moulding of a negative asymmetric CPC structure (left) and simulation of the
transmission of metalized asymmetric CPC structures depending on the incidence angle
(right; according to Walze 2005)
5.4.1 Standard Configuration
The annual simulation showed that the measure is only marginally influencing the
system operation. The relative decrease according to the reference system is
only -15.0 % (Table 5.7).
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5 Thermal Collector Analysis by Simulation
Table 5.7: Fractional energy savings and relative decrease of the reference systems
as well as of the system with micro-structured layer on the glazing in annual simulations
System
Relative decrease [%]
[%]
Reference system, selective
20.0
--
Reference system, non-selective
15.7
-21.5
System with micro-structured layer
17.0
-15.0
The temperature loads on the absorbers were considerably reduced by the CPCs
(Figure 5.7). Duration at temperatures above 95°C was reduced from 425 h to
9 h in the system operation test and from 1,230 h to 500 h in the stagnation test.
Maximum temperatures were decreased to 133°C / 164°C for system operation / dry stagnation.
Figure 5.7: Histograms of the absorber temperatures with selective coating (black)
and with micro-structured layer (green) over one year
5.4.2 Combination with Service Water Cooling
The remaining critical thermal loads at the absorber being protected by the CPC
can be erased by implementing a service water cooling unit into the solar-thermal
system.
This
measure
combination
was
already
investigated
in
Reiter et al. (2011) with the same simulation system, same overheating protection
measure and a similar collector model. The water tap was connected to the return
of the collector array. A sensor at the collector flow provides the signal for the
controller for the valve of the water tap. The control strategy was set to stay below
a flow temperature of 90°C. The water consumption of the annual simulation in
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5 Thermal Collector Analysis by Simulation
the system was quite low with 2.5 m³. However, the collector array in permanent
stagnation required about 58 m³ to keep the flow temperature below 90°C.
5.4.3 Variation of the Collector Position
The dependency on the solar incidence requires a further investigation of various
collector positions to evaluate a variable installation. Therefore, the collector
slope
was changed to 30° as well as 60° and the collector azimuth was changed
towards South-East (-45°) and South-West (45°) direction.
Table 5.8 shows the major influence of the position on the collector behaviour.
The fractional energy savings of the system with micro-structured layer are between 11.6 % and 17.9 %, whereas the corresponding reference results are between 17.1 % and 20.0 %. Beside the rather varying fractional energy savings,
especially, the maximum absorber temperatures occur in a wide range between
126°C and 184°C. Thus, the overheating protection function cannot ensure a reliable temperature decrease in dependency on the collector orientation.
Table 5.8: Fractional energy savings and maximum absorber temperature of the system with micro-structured layer on the glazing in annual simulations depending on the collector position
System with micro-structured layer
Collector slope
Collector azimuth
30°
South-East
15.7
178
30°
South
17.9
184
30°
South-West
17.3
183
45°
South-East
14.5
158
45°
South
17.0
164
45°
South-West
16.5
170
60°
South-East
11.6
126
60°
South
14.1
132
60°
South-West
13.8
149
[%]
,
[°C]
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5 Thermal Collector Analysis by Simulation
5.4.4 Summary
The micro-structured layer shows a great reduction of the thermal loads whereas
the decrease of the system efficiency is relatively low. However, this measure
cannot prevent high peak temperatures during stagnation. On the one hand, an
additional service water cooling can reduce the temperatures. On the other hand,
the economic and ecological disadvantages outweigh the advantages when taking the water consumption into consideration. Another disadvantage is of major
importance towards which direction the collector is oriented. As opposed to this,
the results of this measure depend to a great amount on the location with regard
to the altitudes of the sun. This is why the development of further layers covering
the conditions of the possible collector positions and the sales areas is required.
The disadvantageous complexity of product and installation has to be considered
for the evaluation regarding production as well as error rates in installation.
5.5 Thermochromic Layer with Switchable Emission Coefficient
For the analysis of thermochromic layers the investigated sample from
Huot et al. (2008) was chosen. The described absorber layer has an emission
coefficient
ranging from 0.05 to 0.35 and a high absorption coefficient
of
0.973. The switch of the layer occurs in the range between 60°C and 68°C. Additionally, there is a switching hysteresis from 57°C to 52°C to the original state.
The fractional energy savings of the equipped system were sensitive to the solar
yield (Table 5.9). The triggered overheating protection has less influence on the
system although the switch is within a range of potential collector operation.
Table 5.9: Fractional energy savings and relative decrease of the reference systems
as well as of the system with thermochromic absorber layer in annual simulations
System
[%]
Relative decrease [%]
Reference system, selective
20.0
--
Reference system, non-selective
15.7
-21.5
System with thermochromic layer
19.9
-0.5
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5 Thermal Collector Analysis by Simulation
The overheating protection function is limited. As expected, the absorber temperature is 172°C between the selective and the non-selective reference results because of the relatively low range of the emission coefficient . Figure 5.8 represents the thermal absorber loads. The total duration when the temperature levels
were 100°C and more, is 280 h for the absorber in the system. With 840 h, the
dry absorber increases threefold.
Figure 5.8: Histograms of the absorber temperatures with selective coating (black) and with
thermochromic absorber layer (green) over one year
An additional service water cooling in the solar-thermal system with thermochromic absorber layer according to Chapter 5.4.2 was also simulated in
Reiter et al. (2011). The water consumption was quite high. The operating system
needed about 23 m³ to keep the absorber temperatures below 90°C. The system
in permanent stagnation wasted about 84 m³ of service water.
5.6 Adjustment of the Collector Efficiency
Beside active measures, the adjustment of the collector efficiency was also tested
in order to figure out the influence of reduced expenditure on manufacturing and
simple collector designs. Therefore, the thermal behaviour of the absorber and
backside was varied and analysed.
5.6.1 Reduction of the Absorber Emissivity
The first approach with an increased absorber emissivity is the reference collector
with non-selective absorber coating being already investigated in Chapter 5.2.
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5 Thermal Collector Analysis by Simulation
The TISS paint investigated in Orel et al. 2009 was simulated as a compromise
between the two extremes. In the simulation model the absorption coefficient
and the emission coefficient
of the absorber surface were set to 0.91 and 0.34
(Color n.d.).
According to the surface properties of the absorber, the fractional energy savings
of this system are between the selective and non-selective reference values (Table 5.10).
Table 5.10: Fractional energy savings and relative decrease of the reference systems
as well as of the system with reduced emission coefficients in annual simulations
System
[%]
Relative decrease [%]
Reference, selective (
0.05)
20.0
--
Variation, TISS paint (
0.34)
17.7
-11.5
15.7
-21.5
Variation, non-selective (
0.90)
The maximum absorber temperature of the annual simulations is 167°C. Beside
the relatively high temperatures, long durations above 95°C with 210 h (operation) and 770 h (stagnation) can be achieved (Figure 5.9).
Figure 5.9: Histograms of the absorber temperatures with selective coating (black)
and with TISS paint coating (purple) over one year
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5 Thermal Collector Analysis by Simulation
5.6.2 Reduction of the Backside Insulation
The reduced backside insulation was simulated by implementing less or rather
no mineral wool whereas the distances inside the collector remained constant.
This leads to comparable convective heat transfer conditions and decreased
heights of the collector. The backside insulation of the reference collector with
45 mm was reduced step by step by 15 mm. A collector without any insulation
was one extreme within the parameter variation. The two extremes of the variation was collector without any insulation. Only a thin plate was implemented as a
barrier for convection as well as radiation losses. The objective of this variation
is the identification of the optimal insulation thickness with respect to efficiency
as well as thermal loads.
The comparison of the fractional energy savings of the different collectors showed
less reaction on changing the insulation thickness (Table 5.11).
The good insulation properties of the mineral wool considerably reduced the heat
transfer through the component. As a result, the inner surface temperature of the
insulation is close to the absorber temperature. The small temperature gradient
keeps the heat transfer of the absorber on a similar level and fewer differences
between collector variations occur. Only the collector without any backside insulation shows extremely increased heat losses due to its high temperature gradient
between non-insulated backside sheet and ambient air.
Table 5.11: Fractional energy savings and relative decrease of the reference systems
as well as of the system with reduced backside insulation thicknesses in annual simulations
System
[%]
Relative decrease [%]
Reference system, selective
20.0
--
Variation 30 mm, selective
19.7
-1.5
Variation 15 mm, selective
19.2
-4.0
Variation 0 mm, selective
13.4
-34.5
The collector with 30 mm backside insulation continues to run almost 400 h at
temperature levels below 100°C. Thermal loads in the dry collector occur even
167
5 Thermal Collector Analysis by Simulation
for 1,170 h. The maximum absorber temperature is 215°C in both simulations.
The collector has almost the same thermal loads like the selective reference (Figure 5.10).
Figure 5.10: Histograms of the absorber temperatures with selective coating (black)
and with 30 mm backside insulation (purple) over one year
Also histograms of the annual simulations with 15 mm backside insulation in Figure 5.11 show similar durations at these levels (350 h / 1,050 h) and a maximum
temperature of 202°C. Thus, there is still almost no difference to the reference
results.
Figure 5.11: Histograms of the absorber temperatures with selective coating (black)
and with 15 mm backside insulation (purple) over one year
Moderate thermal loads with 28 h appear at collectors without mineral wool alone
in the range of the short-term service temperature of commodity plastics and a
peak value of 122°C (Figure 5.12). In contrary to that, at temperatures between
95°C and 127°C, the dry collector continues to run for 250 h.
168
5 Thermal Collector Analysis by Simulation
Figure 5.12: Histograms of the absorber temperatures with selective coating (black)
and without backside insulation (purple) over one year
5.7 Evaluation of the Approaches
For the evaluation of the annual simulation results the fractional energy savings
were confronted to the thermal loads. This proceeding combines the two
major thermal properties for polymeric collectors to determine an overall rating.
The evaluation was conducted with an adjusted rating or rather strength diagram
according to guideline VDI 2225 (1998). In this case, the possible reduction of
thermal loads is the technical rating and the influence on the fractional energy
savings is the economic rating.
Therefore, the thermal loads were derived from the histograms of the absorber
temperatures. The histogram classes (temperature levels) above 100°C were
multiplied with the corresponding durations. The sum of these values represents
the thermal loads of the absorber. The thermal loads were normalised to the value
of the selective reference collector in dry stagnation. As this case poses the worst
technical solution, the technical rating is set to 0. The approach of the system
with thermotropic layer C-6-7 is the best technical solution due to the lowest thermal loads. Thus, this approach represents the highest possible technical rating
(rating value: 1).
The fractional energy savings were normalised to the highest reachable solar
yield which is referred to the system with selective reference collectors (economic
169
5 Thermal Collector Analysis by Simulation
rating value: 1). The lowest economic rating value is defined by the result of the
system with thermotropic layer C-6-7 and is set to 0.
Figure 5.13 shows the normalised results of all simulated overheating approaches in a rating diagram. System operation and dry stagnation spans a range
of possible absorber loads depending on the system function. The ideal case of
an approach would be located in the upper right corner of the rating diagram.
Ideal
1.0
Economic rating
0.8
0.6
Reference, selective
Thermotropic layer C-4-5
Thermotropic layer C-6-7
0.4
Microstructured layer
Thermochromic layer
TISS paint
Reference, non-selective
0.2
30 mm backside insulation
15 mm backside insulation
0 mm backside insulation
DRY STAGNATION
0
0
0.2
0.4
0.6
Technical rating
0.8
1.0
Figure 5.13: Rating diagram of the overheating protection measures
The maximum temperature is also important information to identify the most suitable solution. Table 5.12 lists overheating protection approaches by their highest
absorber being reached in the annual simulations.
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5 Thermal Collector Analysis by Simulation
Table 5.12: Sorted list of maximum absorber temperatures occurring in the annual simulations
Overheating protection approach
[°C]
1.
Thermotropic layer C-6-7
104
2.
Thermotropic layer C-4-5
117
3.
0 mm backside insulation
127
4.
Reference, non-selective
139
5.
Micro-structured layer
164
6.
TISS paint
167
7.
Thermochromic layer
172
8.
15 mm backside insulation
202
9.
30 mm backside insulation
215
10.
Reference, selective
223
These two data pools are sufficient to figure out the most promising approaches
for the implementation in polymeric collectors. The overheating protection
measures in combination with service water cooling would be the best solution
regarding thermal loads and peak temperatures. The system with thermochromic
absorber layer and service water cooling represents the most efficient system
variation, but the enormous water consumption of these approaches prevents the
solar-thermal heating systems of being put into practise.
The two versions of thermotropic absorber layers show excellent overheating protection for polymeric materials even during dry stagnation. However, in large part,
this reduction is caused by the missing selective surface behaviour and the reduced transmission of the collector front. Thus, these disadvantages are nonjustifiable for any extra costs of this measure such as layer material and application.
The micro-structured layer has a great potential in a functioning solar-thermal
system. The missing thermal intrinsic safety and the major dependency on collector position and location eliminate the use of this layer for polymeric collectors.
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5 Thermal Collector Analysis by Simulation
The thermochromic absorber layer enables an ideal solar yield of the system. The
disadvantage of the layer is the limited overheating protection, especially for the
requirements of polymers. Thus, the layers being already developed or rather
investigated show an insufficient potential for cost-effective collectors. Beside the
thermal behaviour, the application on polymeric components represents another
hurdle for a cost-effective solution.
The absorbers with permanently reduced selective surface properties show a
great reduction of thermal loads as well as peak temperatures. The reduction of
the solar yield is viable only if the cost savings by omitting an absorber layer or
using a paint coating are considered.
The comparison shows good results regarding efficiency for selective absorbers
with reduced backside insulation. However, the thermal loads are too high for a
cost-effective material choice. Only the simulation without any insulation shows
enough reduced thermal loads, but also a low system efficiency.
Taking the results for the simulations as well as the identified properties of the
measures into account, the two approaches for efficiency reduction by omitting
production steps have the best potential for realising a fail-safe overheating protection approach without any hurdles in production as well as in durability or degradation. The combination of both approaches will balance the conflict between
temperature and efficiency by means of low collector manufacturing costs. Therefore, the design approaches of polymeric collectors have to be analysed in simulations to derive the optimum collector properties.
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6 Development and Analysis of Polymeric Collector Designs
6 Development and Analysis of Polymeric Collector
Designs
Next to the overheating protection investigation, the polymeric collector design
concepts has to be developed on the basis of the acquired knowledge from literature review, theoretical framework and simulation.
By means of the boundaries of suitable polymeric production processes and suitable materials, collector concepts have to be derived. An adjusted simulation
model is necessary to predict component temperatures and solar yield to define
the optimal parameters of the geometries. This is also the relevant data for a final
economic evaluation polymeric collectors against state-of-the-art collectors on
system level.
For development of polymeric collector concepts the focus is on thermoplastics
and the associated manufacturing processes. This combination greatly fulfils the
required low component costs and high output rates. In contrary, thermosets have
a limited potential due to their higher material costs as well as their manufacturing
processes mostly being suited for small and medium output (Biron 2013). They
are also not suitable for further manufacturing steps like thermoforming or welding because of their special molecular structure being resistant against melting.
Additionally, this structure is also the reason that thermosets are not reusable as
virgin matter.
6.1 Overview of Polymeric Production Techniques
Polymeric production techniques are divided in main groups according to standard DIN 8580 (2003). Figure 6.1 lists the relevant main groups — primary shaping, moulding, machining and joining — suitable for making collector component
geometries.
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6 Development and Analysis of Polymeric Collector Designs
Primary shaping
Forming
Manufacturing processes
Machining
Joining
Figure 6.1: Overview on the main groups of relevant manufacturing
processes for collector components
Primary shaping comprises the manufacturing techniques producing components
from a liquid or pliable raw material. The most important processes for thermoplastics are injection moulding, extrusion and blow moulding. Thermoforming and
twin sheet thermoforming are the relevant processing techniques of forming.
These techniques and their use for collector components are described in the
following.
Machining processes like milling are used for secondary processing of components manufactured by primary shaping or forming. The joining technique adhesive bonding is necessary for fixing components like absorber and glazing. Furthermore, welding is used for producing hollow components from two shapes.
6.1.1 Injection Moulding
Injection moulding is a technique for discontinuous processing of polymeric materials. By means of the following properties, injection moulding is perfectly suited
for mass production:

Direct path from raw material to finished component

Total automation of the process

Almost none finishing necessary

High output rates
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6 Development and Analysis of Polymeric Collector Designs
Injection moulding is suitable for small components like gear wheels as well as
for large components like waste containers. The cycle times vary between a few
seconds and 10³ seconds. By means of the high mould and press costs is this
process normally efficient for mass production (Biron 2013; Michaeli 2006).
The injection moulding cycle is described in Figure 6.2.
Figure 6.2: Injection moulding cycle (according to Michaeli 2006)
Injection moulding processes the most available polymeric materials and is applied in almost all industrial sectors. The automotive sector produces fenders and
dash panels made of polyolefins. PC and PMMA is used for transparent components like headlights and taillights. Toys and home appliances are made from PS
and ABS. PA and other engineering plastics appear in expensive components in
the mechanical as well as electrical engineering sector (Michaeli 2006).
This polymer processing allows the realization of the most collector component
geometries. Flat hollow components like the absorber are difficult to manufacture.
However, two absorber components with fluid channels can be fixed together to
reproduce the component with cavity. Glazing, casing and frame components are
flat geometries and ideal for low cycle times. Also, insulation by mixing gas in the
shot (thermoplastic foam injection moulding) is realisable. Thus, the component
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6 Development and Analysis of Polymeric Collector Designs
is lightweight and has a low heat conductance. Figure 6.3 represents the capabilities for the collector component production.
Absorber parts
Glazing
Injection moulding
Insulation
Trough / frame parts
Figure 6.3: Capabilities of components manufactured by injection moulding
6.1.2 Extrusion
Extrusion is a continuous processing method for two-dimensional component geometries. Melted raw material is pressed by a screw through a die and is formed
to a profile of unlimited length. The material as granulate or powder is melted
within the barrel by a heating and frictional heat caused by the extrusion screw.
A polymer extrusion screw consists of three zones:

Feed zone:
Supplies the extruder with raw material

Melting zone:
Melting and compression of the material by the extrusion screw

Metering zone:
Homogenisation of the melted material and pressure build-up
Figure 6.4 shows these zones in a schematic build-up of an extruder.
Finally, the melted material is passing the feed pipe unit and the die. The extruded
material has to be cooled down by a water bath and cooling rolls (Kaiser 2007;
Michaeli 2006).
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6 Development and Analysis of Polymeric Collector Designs
Figure 6.4: Schematic build-up of a plastic extruder (according to Kaiser 2007)
The output of the extrusion process varies between kgh-1 and tonsh-1. Hence,
extrusion is suitable for mass production depending on the die and the machinery
size. The profile size of pipes, for example, reaches until 2,000 mm diameter and
40 mm wall thickness. The extrusion process is used for compounding, profile
extrusion, sheet extrusion, jacketing, film extrusion and coating (Michaeli 2006).
The materials for processing requires a sufficient melt viscosity to keep the shape
between die and cooling unit. Typical materials are PVC, PP, PE, PC and PMMA
(Johannaber 2004).
The possible extruded components for collectors are sheets for glazing and housing, profiles for frame components and multi-wall sheets for absorber, glazing and
housing (Figure 6.5).
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6 Development and Analysis of Polymeric Collector Designs
Absorber parts
Glazing
Extrusion
Insulation
Frame parts
Trough + insulation
Figure 6.5: Capabilities of components manufactured by extrusion
The channels within the multi-wall sheets can be used for absorber channels as
well as for insulation in glazing and housing by means of air enclosures. Such
multi-wall sheets require additional components to close the profiles. By mixing a
gas into the melted polymer the extruded material expands. The expanded polymer or rather the foam is ideal for insulation and also for a casing with integrated
insulation.
6.1.3 Blow Moulding
Most hollow components are produced by blow moulding or rather extrusion blow
moulding. The products embrace packaging items like bottles or canisters as well
as technical components like hard shell cases. Blow moulding enables product
sizes from packing for the pharmaceutical industry up to fuel oil tanks with
13,000 l volume (Michaeli 2006).
The blow moulding process begins with the vertical extrusion of a parison. In the
second step close the moulds and through the blow pin enters compressed air.
The parison is blown against the mould and hardens. Afterwards, the moulds
open and eject the component. Figure 6.6 represents the blow moulding cycle.
Commonly used materials are commodity plastics like PE, PP and PVC. Also
technical polymers like PC, PA and polyphenylenether (PPE) are suitable.
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6 Development and Analysis of Polymeric Collector Designs
Figure 6.6: Blow moulding cycle (CustomPartNet 2008)
The high production rate and the low labour costs due to the high automation are
advantageous. However, blow moulding has a poor control of wall thicknesses
and geometrical details (Biron 2013).
Blow moulding is suitable for the production of the absorber with its fluid channels
and connectors. Such components are already produced as pool absorbers. Using the enclosed air layer for the reduction of the convective losses, a doublewalled glazing or casing can be realised. This air layer may replace the insulation
in the housing. Also a second production step by filling the air gap with polyurethane foam for insulation is possible. Figure 6.7 represents the components which
can be produced by blow moulding.
Absorber
Blow moulding
Glazing
Trough + insulation
Figure 6.7: Capabilities of components manufactured by blow moulding
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6 Development and Analysis of Polymeric Collector Designs
6.1.4 Thermoforming
Thermoforming is a production process of the main group of forming and requires
a semi-finished component as input. Commonly, this component is an extruded
plate or film. The thermoforming process is suitable for the production of moulded
components without any cavities. The products range from yogurt cups in the
packaging industry with an output until 100,000 units per hour to basins for pools
or garden ponds with an output until 1.5 units per hour. The possible component
size is quite large with 8,000 x 4,000 x 1,500 mm for pool basins for example
(Michaeli 2006).
The thermoforming process begins with clamping the sheet or film into a holding
device. A heating softens the material via convection or radiation. The softened
sheet is stretched and pressed into a mould. After cooling down and hardening,
the component is ejected from the mould. The thermoforming cycle is described
in Figure 6.8 by means of the vacuum forming process.
Figure 6.8: Vacuum forming cycle (DUROtherm Kunststoffverarbeitung GmbH (n.d.))
Three types of thermoforming – vacuum forming, pressure forming and mechanical forming – are differentiated by the method of moulding the material in the tool.
Beside vacuum and air pressure, also a core plug can force the softened material
into the mould. Most thermoplastics can be processed by thermoforming. Commonly used materials are PE, PP, PVC, PC, PMMA and PS (Kaiser 2007).
Thermoforming is distinguished by low mould costs, short cycle times, details with
high quality and thin wall thicknesses for efficient material effort. However, the
degree of automation is relatively low and trimming of the material being clamped
in the holding device is required. Thus, there are an additional material effort and
a post-processing by milling necessary. Also undercuts in the component geometry are difficult to realise.
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Thermoforming is suitable for the production of absorber components by forming
two shapes with the fluid channels. Afterwards the two components have to be
fixed together to manufacture the complete absorber. Also the glazing can be
adjusted to higher stability by curving. The most suitable component for thermoforming is the casing (Figure 6.9). Thermoformed covers and casings are still
on the market (Chapter 1.2.3).
Absorber parts
Thermoforming
Glazing
Trough
Figure 6.9: Capabilities of components manufactured by thermoforming
An improvement of thermoforming is twin sheet thermoforming. This processing
consists of two thermoforming processes in parallel to manufacture hollow components. The thermoforming cycle is similar and has an additional step where the
already moulded components are pressed together. The components being still
at forming temperature are welded at the contact areas (Figure 6.10).
Figure 6.10: Twin sheet thermoforming cycle (DUROtherm Kunststoffverarbeitung GmbH n.d.)
The hollow components being suitable for twin sheet thermoforming are absorbers, double glazing and casings with enclosed air layer (Figure 6.11).
Absorber
Twin sheet thermoforming
Glazing
Trough + insulation
Figure 6.11: Capabilities of components manufactured by twin sheet thermoforming
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6 Development and Analysis of Polymeric Collector Designs
6.2 Polymeric Collector Designs
On the basis of the described manufacturing processes new design concepts for
collectors were derived and compared regarding their costs for a conventional
collector size of about 2,000 x 1,000 mm. The chosen materials for the investigation are described in the following.
6.2.1 Non-transparent Materials
The polymeric materials for absorber, casing and insulation of the collector designs embrace the basic types of commodity plastics. Depending on the component, various requirements to the materials have to be fulfilled. The absorber and
the areas with direct contact need the highest thermal resistance. Thus, the shortterm as well as long-term services temperatures have to be increased. In cases
of pressurised absorbers the mechanical properties have to be increased. Especially at high temperatures the stability of the component should be enabled.
Therefore, the material should have also a high heat deflection temperature. The
resistance of the absorber material against hot fluid mediums (water and glycol)
has to be high. Also the absorption of water should be low for the size accuracy
of the component. The insulation material has only requirements regarding the
temperature loads. Depending on the distance to the absorber, the temperatures
can reach the same level. The lowest thermal loads are at the casing. However,
the main hurdle for that material is the degradation by means of the ambient conditions. Thus, the UV-resistance is the most important property. In further development stages the material properties (mechanical, thermal or optical) can be
additionally improved by additives, fillers and fibres.
PE is a cheap, resistant and light polymer. PE is commonly used for containers
like fuel oil tanks or for piping for tap as well as sewerage water. The material is
suitable for all mentioned manufacturing techniques (Domininghaus et al. 2012).
There is a high resistance against many chemicals and hydrolysis. The low
weather resistance, especially against UV radiation, can be increased by UV-
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stabilisers or protective agents. Usually 2–3 % carbon black improves this resistance (Biron 2013). The low thermal properties limit the field of application to
the casing. The material data of PE is listed in Table 6.1.
Table 6.1: Properties of PE (Domininghaus et al. 2012; Kunststoffinformation n.d.)
PE
Density
0.95
[gcm-3]
Stress at yield
20–30
[MPa]
Short-term service temperature
90–120
[°C]
Long-term service temperature
70–80
[°C]
Material costs
1.23
[GBPkg-1]
PP is the most promising material for collector components. The material properties are similar to PE (Table 6.2). The thermal properties are increased. PP can
also be manufactured as foam. PP appears in bumpers, piping, insulations and
garden furniture, for example.
Table 6.2: Properties of PP (Domininghaus et al. 2012; Kunststoffinformation n.d.)
PP
Density
0.91
[gcm-3]
Stress at yield
30
[MPa]
Short-term service temperature
140
[°C]
Long-term service temperature
100
[°C]
Material costs
1.19
[GBPkg-1]
For increased mechanical and thermal properties glass fibres show a great potential as reinforcement agent in PP. The so called PP GF30 contains 30 % glass
fibres and has especially an improved long-term service temperature (Table 6.3).
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Table 6.3: Properties of PP GF30 (Domininghaus et al. 2012; Kunststoffinformation n.d.)
PP GF30
Density
1.14
[gcm-3]
Stress at yield
>33
[MPa]
Short-term service temperature
155
[°C]
Long-term service temperature
120
[°C]
Material costs
1.71
[GBPkg-1]
6.2.2 Glazing
The most suitable transparent materials for polymeric covers are PMMA and PC.
They are already used for windows in cars and planes, for green houses as well
as for sunbeds.
PMMA has a high light transmittance as well as a high resistance against weathering (Table 6.4). Furthermore, the material can be processed by all mentioned
techniques.
Table 6.4: Properties of PMMA (Domininghaus et al. 2012; Kunststoffinformation n.d.)
PMMA
Density
1.18
[gcm-3]
Stress at yield
70
[MPa]
Short-term service temperature
84–108
[°C]
Long-term service temperature
74–98
[°C]
Light transmittance
92
[%]
Material costs
2.54
[GBPkg-1]
PC is also a very transparent material and has an increased thermal behaviour
(Table 6.5). The sensitive behaviour against UV radiation is improved by suitable
anti-UV agents or coatings. Also PC can be used for the relevant manufacturing
techniques.
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Table 6.5: Properties of PC (Domininghaus et al. 2012; Kunststoffinformation n.d.)
PC
Density
1.20
[gcm-3]
Stress at yield
>55
[MPa]
Short-term service temperature
150
[°C]
Long-term service temperature
130
[°C]
Light transmittance
91
[%]
Material costs
2.84
[GBPkg-1]
Ruesch and Brunold (2008) and Ruesch et al. (2008) showed in a 20 year weathering test for glazing materials the best results for PMMA, PC and solar glass
regarding resistance against degradation and dirt.
The collector glazing has to withstand distributed loads from wind and snow. But
also the thickness should be minimal to enable low material costs, low weight and
high transmittance. Three possible geometries for glazing were considered on
the basis of available purchased components. The polymeric sheets are made
from PMMA and bases on the data from EVONIK (2011).
The highest transmittance for polymeric covers represents a single glazing. However, this shape has the lowest stability against deflection. For an overview, plate
thicknesses of 3 mm and 4 mm were chosen. A corrugated version of a single
glazing provide more stability, but has increased costs and weight due to the
higher material content. The stiffest and lightest glazing type is a double-walled
cover. Because of the additional boundary surfaces, the transmittance is lower.
However, the enclosed air layers reduce the thermal losses. The complex structure also increases the costs. Table 6.6 lists the costs and weights of the different
PMMA glazing types for a cover with 2 m² area.
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Table 6.6: Costs and weight of various PMMA glazing types (EVONIK 2011)
Type
Costs [GBP]
Weight [kg]
PMMA, planar, 3 mm
20.50
7.1
PMMA, planar, 4 mm
27.10
9.5
PMMA, corrugated, 3 mm
23.00
7.6
PMMA, double-walled
26.30
5.7
Apart from polymeric covers a state-of-the-art glazing made of solar glass also
shows great properties for polymeric collectors. Transmittance, stability and aging behaviour are advantageous. Jäger and Terschüren (2007) identified unit
costs of 158 GBP for a state-of-the-art collector. The share of the glazing is 13 %
according to Figure 1.5 (Hochreiter and Trinkl 2008; Treikauskas and Zörner
2005b). Thus, the costs of 20.70 GBP are the lowest of all mentioned glazing
types (Table 6.7). By using the stiffness of the cover sheet for the collector stability, the dimensions of the housing can be reduced.
Table 6.7: Costs and weight of a single glazing made of solar glass
Type
Costs [GBP]
Weight [kg]
Solar glass, planar, 3.2 mm
20.70
15.7
According to the better physical properties and the lower costs a state-of-the-art
glazing is the most interesting option for the following investigation of polymeric
collector concepts.
6.2.3 Cost Basis for Evaluation
The first economic comparison of the collector approaches is carried out according to the unit costs consisting of the costs for manufacturing and material. Table
6.8 lists the boundary conditions for the manufacturing costs of the collector designs in the concept stage. The production line is configured to an optimum annual capacity utilisation of the processing machine.
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Table 6.8: Boundary conditions for the calculation of the manufacturing costs
Cost category
Parameter
Production data
Days of production
250
[da-1]
Number of shifts
3
[-]
Hours per shift
8
[h]
Capacity utilisation
92
[%]
→ Production hours per year
5,520
[ha-1]
Imputed interest per year
7
[%]
Amortisation period
5
[a]
Labour costs
20.00
[GBPh-1]
Non-wage labour costs
70
[%]
Financing data
Staff data
6.2.4 Absorber Concepts
The disadvantageous low heat conductivity of polymers has to be compensated
by the absorber design to gain a collector efficiency factor ′ being comparable
metallic sheet-pipe absorbers. The main factor of ′, which can be influenced, is
the heat transfer via conduction
within the component shown in equa-
tion (6.1) and (6.2). The parameters being independent from the material are the
distance
and the area .
(6.1)
(6.2)
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6 Development and Analysis of Polymeric Collector Designs
Figure 6.12 shows the geometries of a sheet-pipe and the volumetric absorber
as well as the heat conduction within the component. The distance
for heat
conduction of the sheet-pipe absorber represents the average value.
A
s
l
z
y
Qcond
x
A
s
l
Qcond
Figure 6.12: Heat conduction in a sheet-pipe (above) and a volumetric absorber (below)
The extreme low heat conductivity of the material is balanced by a short distance
and a large area in the polymeric absorber design. Table 6.9 describes the differences in heat conduction between the two absorbers in an exemplary calculation
depending on the pipe length . As a result, the volumetric absorber shows a
3.8 times increased heat transfer from the light absorbing surface to the inner
surface of the fluid channel.
Table 6.9: Comparison of the heat conduction in a metallic
sheet-pipe and a polymeric volumetric absorber
Absorber type
[Wm-1K-1]
[m-1]
[m²m-1]
[WK-1m-1]
Sheet-pipe (Copper)
360
0.025-1
0.0002
2.9
Volumetric (PP)
0.22
0.002-1
0.1
11
0.0006
12.5
500
3.8
Factor of difference [-]
volumetric
sheet pipe
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6 Development and Analysis of Polymeric Collector Designs
A second aspect of this design is the increased area for convective heat transfer
between absorber and fluid. Thus, both design aspects leads to a comparable or
improved collector efficiency factor ′.
The necessary channel design to gain a homogenous flow behind the absorber
surface was investigated by Treikauskas (2009) especially for metal-based absorbers. Beside the harp and the serpentine design there are also complex channel structures. Reiter et al. (2013) identified a volumetric absorber design for polymeric collectors with parallel channels which shows a homogenous fluid flow
and a low pressure drop. However, the structure and the channel dimensions
have a secondary influence on the estimation of the unit costs.
The twin sheet thermoforming process enables the production of the absorber
with all relevant details for fluid channels and pipe connections. The whole absorber requires only a production step as well as a post-processing by milling.
Figure 6.13 shows an exemplary absorber design for twin sheet thermoforming.
A plane surface or rather an asymmetric design enables improved results for the
application of a selective paint.
Figure 6.13: Sectional view of a twin sheet thermoformed absorber
The 2 mm thick PP sheets for the absorber are considered as purchased components for 7.00 GBP. The costs for the purchased components base on the costs
for PMMA sheets being adjusted according to the specific material price. The
output of the production line (thermoforming and milling) is 80,000 units per year
and has a total investment of 560,000 GPB. This leads to manufacturing costs of
4.60 GBP according to the boundary conditions. The unit costs for such an absorber is estimated to 18.60 GPB (Table 6.10). Using sheets with 1 mm, the unit
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6 Development and Analysis of Polymeric Collector Designs
costs would be lowered to 9.70 GBP. However, the inherent rigidity is quite low
and there would be no resistance against fluid pressure.
Table 6.10: Unit costs of a twin sheet thermoformed absorber
Type of costs
Costs [GBP]
1 mm
2 mm
Material (purchased components)
7.00
14.00
Manufacturing
2.70
4.60
Sum
9.70
18.60
A cost-efficient alternative to this design is an absorber made of film material.
Therefore, two films or a film hose are welded according to the design of the
absorber structure. Such an absorber involves an extremely low material effort
and low manufacturing costs. Thus, expensive resistant materials can be chosen
without increasing the unit costs considerably. However, the component has to
be fixed in the housing because of the missing inherent rigidity. Also the flexing
material in a drain-back system or the pressure in a closed system represent a
hurdle for the approach for a life time of more than 15 years.
Another design version is a twin-wall sheet with enhanced headers. Therefore,
an extruded sheet has to be combined with injection moulded headers by adhesive bonding or welding. Figure 6.14 shows an example in serpentine design.
The unit costs for the components (twin-wall sheet and headers) made of PP with
1 mm wall thickness are estimated to 20.30 GBP in case for using the twin-wall
sheet as purchased component. The following process for combining the single
components causes a slight increase of the unit costs. The in-house production
of the twin-wall sheet can reduce the unit costs considerably.
By means of the total investment, the number of production steps, the component
properties and the unit costs poses the thermoformed absorber the best design
concept for the following evaluation.
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6 Development and Analysis of Polymeric Collector Designs
Figure 6.14: Sectional view of an extruded absorber with serpentine structure
6.2.5 Casing Concept
The simplest concept for a casing is the analogue of a state-of-the-art aluminium
casing (Figure 6.15). Using the stiffness and stability of the solar glass, the casing
can be thin dimensioned. Moulding a rib structure increases the stiffness of the
casing. Recesses in the component enable easy handling for installation. The
cover can be fixed at the housing by clipping or adhesive bonding.
Figure 6.15: Sectional view of a polymeric collector casing with exemplary clipped double glazing
For production, thermoforming as well as injection moulding is suitable. Due to
the low thermal loads at the housing, PE as well as PP can be chosen. The actual
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6 Development and Analysis of Polymeric Collector Designs
market situation favours the cheaper PP. The casing dimensions for the determination of the costs are 2,000 x 1,000 x 80 mm. A low thickness of the plate may
cause defective wall thicknesses at the shaped region. Therefore, plate thicknesses of 2 mm and 3 mm were chosen.
The thermoformed component reaches unit costs of 9.50 GBP or rather
13.80 GBP and an output of 160,000 / 107,000 units per year (Table 6.11). The
low manufacturing costs are caused by short cycle times and moderate investment costs of 480,000 GBP.
Table 6.11: Unit costs of a thermoformed casing
Type of costs
Costs [GBP]
2 mm
3 mm
Material
7.00
10.50
Manufacturing
2.50
3.30
Sum
9.50
13.80
The injection moulded component has an output of 210,000 / 160,000 units per
year and reaches unit costs between 6.50 GBP and 10.70 GBP (Table 6.12). The
large dimensions of the casing requires a machine with high closing force. Thus,
the investment costs of about 1.35 million GBP are quite higher than for a thermoforming machine and increase the manufacturing costs. The costs are not
scalable to the casing thickness because of constant sprue and non-linear cycle
times.
Table 6.12: Unit costs of an injection moulded casing
Type of costs
192
Costs [GBP]
1 mm
2 mm
Material
3.30
6.50
Manufacturing
3.20
4.20
Sum
6.50
10.70
6 Development and Analysis of Polymeric Collector Designs
Finally, the costs of the casing vary between 6.50 and 13.80 GBP depending on
the requirements regarding investment costs and stability.
The housing is suitable for any kind of insulation like mineral wool or polymer
foam. The substitution of a fraction of the insulation by an air layer reduces the
material effort whereas the thermal performance remains on a similar level (Figure 6.16).
β
Figure 6.16: Cross section of the collector with casing and insulation
6.2.6 Double-Walled Casing Concept
The concept of a double-walled casing focuses on the advantage of combining
casing and insulation in a single component (Figure 6.17). Therefore, the insulation is replaced by an air layer. This leads to low component and assembly costs.
Furthermore, the collector is insensitive to moisture. Formed ribs increase the
stiffness of the casing.
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6 Development and Analysis of Polymeric Collector Designs
Figure 6.17: Sectional view of a double-walled casing with exemplary fixed double glazing
The position of the absorber has to be distanced to the inner sheet of the casing
to create a second air layer. The inner sheet functions as a barrier to reduce the
convection (Figure 6.18).
β
Figure 6.18: Cross section of the collector with double-walled casing
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6 Development and Analysis of Polymeric Collector Designs
The design is suitable for twin sheet thermoforming as well as for blow moulding.
The dimensions are similar to the simple casing concept.
Twin sheet thermoforming offers the option for combining two different sheets.
Therefore, the casing can consist of a cost-effective shell material and heat resistant sheet near the absorber. Also the inner sheet can be configured with lower
plate thickness due to the reduced requirements for stability. The calculation is
similar to the absorber concept. Due to the different sheet thicknesses and cycle
times result unit costs of 18.60 GBP and an output of 80,000 units per year for a
component with 2 mm wall thickness (Table 6.13). The version with a 2 mm interlayer and a 3 mm shell causes unit costs of 23.20 GPB and has an output of
64,000 units per year.
Table 6.13: Unit costs of a thermoformed double-walled casing
Type of costs
Costs [GBP]
2 / 2 mm
2 / 3 mm
Material
14.10
17.60
Manufacturing
4.50
5.60
Sum
18.60
23.20
Depending on the wall thickness, the blow moulded casing has unit costs of
17.80 GBP or rather 24.60 GBP and an output of 105,000 / 90,000 units per year.
Especially the high investment costs of 1.35 million GBP causes the increased
manufacturing costs.
Table 6.14: Unit costs of a blow moulded double-walled casing
Type of costs
Costs [GBP]
2 mm
3 mm
Material
11.80
17.60
Manufacturing
6.00
7.00
Sum
17.80
24.60
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6 Development and Analysis of Polymeric Collector Designs
The unit costs of the double-walled casing differ between 17.80 GBP and
24.60 GBP whereas the thermoformed casing (2 / 3 mm) the best balance between economic and technical advantages represents.
6.2.7 Evaluation of the Unit Costs for the Collector
The first economic evaluation of the polymeric collector concepts was conducted
by comparing the unit costs against state-of-the-art collectors. The unit costs of
158 GBP for a state-of-the-art collector with aluminium casing from Jäger and
Terschüren (2007) are the reference value for the evaluation. These costs were
referenced to the single items according to the breakdown of the unit costs of flatplate collectors from Figure 1.5 (Hochreiter and Trinkl 2008; Treikauskas and
Zörner 2005b).
Table 6.15: Typical breakdown of the unit costs of the reference collector
Component
Share [%]
Costs [GBP]
Absorber
46
72.70
Housing
20
31.60
Glazing
13
20.50
Miscellaneous
12
19.00
Insulation
9
14.20
Sum
100
158
The costs for assembling, sealing and adhesive were summed up to 19.00 GBP
(12 %) as miscellaneous. The mineral wool insulation (~50 mm) is estimated to
14.20 GBP (9 %). The purchased component appears also in the casing concept.
Therefore, two chases were chosen for comparison. A 10 mm insulation in combination with a 20 mm air layer and a single 30 mm insulation were taken into
account. The proportional costs are 2.80 GBP and 8.50 GPB.
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6 Development and Analysis of Polymeric Collector Designs
Table 6.16 lists the compositions of the collector concepts for the evaluation of
the unit costs. Therefore, the cost-effective and the expensive components were
combined to illustrate the range of the concept costs.
Table 6.16: Composition of collector concepts for the evaluation
Concept
Absorber
Glazing
Housing
Twin sheet, 1 mm
Solar glass
Injection moulded, 1 mm
+ mineral wool
Twin sheet, 2 mm
Solar glass
Thermoformed, 3 mm
+ mineral wool
Twin sheet, 1 mm
Solar glass
Blow moulded, 2 mm
Twin sheet, 2 mm
Solar glass
Blow moulded, 3 mm
Casing concepts:
Double-walled casing concepts:
The comparison of the concepts in Figure 6.19 illustrates a possible cost benefit
of more than 50 %.
200
Unit costs [GBP]
Absorber
Glazing
150
Housing + insulation
Miscellaneous
100
50
0
Reference
Casing
concept
10 mm + air
Casing
concept
30 mm
Double-walled
casing concept
Figure 6.19: Comparison of the unit costs of the polymeric collector concepts
197
6 Development and Analysis of Polymeric Collector Designs
The concepts are designed to low weight and costs which leads to a low stability.
Depending on the requirements, the concepts represent the lowest possible dimensioning. The missing selective coating reduces the costs as well as the efficiency. Thus, the influence of the concepts on the system yield has to be investigated. Furthermore, the limited thermal properties of the cost-effective material
has to be analysed.
Table 6.17 lists the unit costs of the collector components and shows the decrease according to the reference collector.
Table 6.17: Composition and decrease of the unit costs of the collector concepts
Concept
Absorber
[GBP]
Glazing
[GBP]
Housing +
insulation
[GBP]
Miscellaneous
[GBP]
Decrease
of unit
costs [%]
Reference
72.70
20.50
45.80
19.00
--
Low costs
9.70
20.50
9.30
19.00
-63.0
High costs
18.60
20.50
16.60
19.00
-52.5
Low costs
9.70
20.50
15.00
19.00
-59.5
High costs
18.60
20.50
22.30
19.00
-49.0
Low costs
9.70
20.50
17.80
19.00
-57.5
High costs
18.60
20.50
24.60
19.00
-47.5
Casing concepts
10 mm + air:
Casing concepts
30 mm:
Double-walled casing
concepts:
Annotation: The costs of the item “miscellaneous” is set to a constant value. However, the included assembly costs should be lower for the double-walled casing
concept because of the missing production step of inlaying the insulation.
198
6 Development and Analysis of Polymeric Collector Designs
6.3 Thermal Analysis
The reduced thermal efficiency of the design concepts and the influence on the
system level has to be analysed via simulation in order to evaluate the benefit
from solar-thermal systems with cost-effective collectors. Therefore, the validated
collector model was used for the prediction of the collector and system behaviour.
Additionally, a test stand for stagnation according to DIN EN 12975-2 (2006) was
built to identify the maximum temperatures under standardised conditions (Table
6.18). This simulation set-up provides a second reference of maximum thermal
loads under defined conditions being comparable to collector measurements.
Table 6.18: Test conditions for stagnation according to DIN EN 12975-2 (2006)
Parameter
1,000
[Wm-2]
30
[°C]
1
[ms-1]
45
[°]
The dataset of the weather is provided by source blocks of Simulink with constant
values. Figure 6.20 shows the subsystem of the weather block in Simulink for the
stagnation test.
Figure 6.20: Build-up of the subsystem of the weather block for the stagnation test in Simulink
199
6 Development and Analysis of Polymeric Collector Designs
6.3.1 Adjustment of the Reference Results
The collector being used for measurement and simulation showed up an extraordinary collector efficiency curve. Thus, a moderate efficiency curve of a flat-plate
collector was chosen in the annual system simulation for not overestimating the
difference between state-of-the-art and polymeric collectors. Table 6.19 lists the
differences between the two reference collectors. The collector area is equal in
all cases.
Table 6.19: Reference results from simulation of two different state-of-the-art flat-plate collectors
Model type
Relative decrease
Collector curve parameters
[Wm-2K-1]
[-]
[Wm-2K-2]
[%]
[%]
Previous reference:
Dynamic
~0.798
~3.34
~0.0075
20.0
--
Curve-based
0.798
3.34
0.0075
19.4
-3.0
0.771
3.68
0.0127
17.9
-10.5
New reference:
Curve-based
The pressure loads were neglected in the system simulations for being able to
compare two identical systems without any changes beside the collectors.
6.3.2 Adjustment of the Collector Model
The validated model of the state-of-the-art collector from Chapter 4.1 has to be
adjusted to the design concepts of polymer collectors. The most parameters in
the new model which have to be changed, are material data and component distances. However, due to the new absorber design, the collector model has to be
changed. Figure 6.21 shows the basis of a polymeric collector model.
200
6 Development and Analysis of Polymeric Collector Designs
Sa
Sg
Ambient
Insu‐
Adiabatic
lation (edge)
Glazing (g)
a1
f
Absorber (a)
Fluid (f)
a2
Casing / insulation (c)
Adiabatic
Ambient
g
Symmetry
c
Ambient
Figure 6.21: Cross section of the polymeric collector model with nodes and energy fluxes
The equations (6.3) to (6.7) give an overview on the energy balance drawn in the
collector model describing the capacities of each node as well as the energy
fluxes between the nodes.
Glazing (g):
(6.3)
Absorber 1 (a1):
(6.4)
Fluid (f):
∗
(6.5)
Absorber 2 (a2):
(6.6)
Casing (c):
(6.7)
201
6 Development and Analysis of Polymeric Collector Designs
The heat transfer of the forced convection in the absorber channels was calculated according to the approach from Chapter 0 for forced convection in ducts.
The new geometry of the absorber channels consisting of two parallel plates requires an adjustment of the equation. Therefore, the doubled gap height instead
of the commonly used pipe diameter represents the characteristic length
and
the transition from laminar to turbulent flow begins at a Reynolds number of 2,800
instead of 2,300 (Mills 1999, Stephan 2010). The hydrodynamically developed
laminar flow for Reynolds numbers below 2,800 is set to the constant Nusselt
number 5.385 (Mills 1999). The approach for the Nusselt number was used for
convective heat transfer coefficients between absorber front (a1) and fluid as well
as between fluid and absorber back (a2). The fins between the absorber sheets
were neglected because of their low influence.
6.3.3 Casing Concept
The casing concept with 10 mm mineral wool insulation and air enclosure has a
decrease of the fractional energy savings of 24.5 % in comparison to the reference system. The concept with 30 mm insulation shows a relative decrease of
only 18.5 % (Table 6.20).
Table 6.20: Fractional energy savings and efficiency decrease of the reference system
as well as of the systems with polymeric collectors in annual simulations
System
[%]
Relative decrease [%]
Reference system
17.9
--
System with polymeric collectors (10 mm + air)
13.5
-24.5
System with polymeric collectors (30 mm)
14.6
-18.5
The collector concept with 10 mm insulation and air layer reaches a maximum
absorber temperature of 117°C in the annual system simulation. The dry collector
in stagnation has its climax at 126°C.
Figure 6.22 shows the occurrences of thermal loads on the absorber depending
on duration and temperature level over one year. The data is prepared to display
202
6 Development and Analysis of Polymeric Collector Designs
the duration when a temperature level is exceeded and the number of occurrences of this effect. Due to the strong dependency on the sun, the increase of
the absorber temperature is similar to the gradient of the solar irradiance
(cf. Chapter 2.2.1). The gradient of the peaks in the diagram (yellow dashed line)
from more than 360 minutes above 60°C to states with short durations at maximum temperatures illustrates the thermal behaviour of the collector on clear days.
The thermal absorber loads within the pattern at moderate temperatures and
shorter durations occur on cloudy days in the summer period as well as on clear
days in winter.
Analysing the data with regard to the material properties of the absorber material,
the collector behaviour can be classified as non-critical. The most loads do not
exceed 100°C, the long-term service temperature of PP. Only a few situations
25
20
15
10
5
435
375
Occurrences [-]
appear in the short-term service temperature array (up to 2 h and 140°C).
0
60
80
315
255
Du
rat 195
ion
135
[mi
n]
75
120
15
140
°C
100
re [
u
t
a
per
m
e
T
]
Figure 6.22: Occurrences of thermal loads depending on duration and temperature level on a
polymeric absorber with 10 mm mineral wool insulation and air enclosure during
stagnation over one year
203
6 Development and Analysis of Polymeric Collector Designs
The concept with 30 mm mineral wool reaches maximum values of 130°C in operation and 133°C in permanent stagnation. The thermal loads on the absorber
are slightly increased in the second concept (Figure 6.23). Thus, there is a more
distinctive pattern at higher temperature levels. However, the thermal loads do
25
20
15
10
5
435
375
Occurrences [-]
not considerably exceed the recommended boundaries of PP.
0
60
80
315
255
Du
rat 195
ion
135
[mi
n]
75
120
15
140
100
[ °C
ure
t
a
per
Tem
]
Figure 6.23: Occurrences of thermal loads depending on duration and temperature level on a
polymeric absorber with 30 mm mineral wool insulation during stagnation over one year
In the additional stagnation test according to DIN EN 12975-2 (2006) remain the
simulation results also within the boundaries of the proposed material (Table
6.21).
Table 6.21: Stagnation temperatures according to DIN EN 12975-2 (2006) of the polymeric
collector concepts with mineral wool insulation
Stagnation temperatures [°C]
Absorber
Glazing
Insulation
(inside)
Casing
(outside)
Casing concept (10 mm + air)
123
74
91
49
Casing concept (30 mm)
131
78
120
43
204
6 Development and Analysis of Polymeric Collector Designs
6.3.4 Double-Walled Casing Concept
The air gap distances between absorber and inner casing wall as well as between
inner and outer casing wall were set to a similar value. Simulations with various
air gap parameters between 10 and 30 mm show almost no influence on the
thermal performance. The maximum temperatures are identical and the discrepancy in the fractional energy savings is approximately 1 %.
The fractional energy savings in Table 6.22 are quite lower according to the reference system. The decrease of 27.5 % is mainly due to the poor collector efficiency at the necessary temperature levels being required from the system. However, the maximum absorber temperature of 110°C is optimal for the absorber
material.
Table 6.22: Fractional energy savings and efficiency decrease of the reference system
as well as of the system with polymeric collectors with double-walled casing
in annual simulations
System
[%]
Relative decrease [%]
Reference system
17.9
--
System with polymeric collectors
13.0
-27.5
Also the absorber temperatures of the dry collector in stagnation reach a maximum of only 122°C. The thermal loads in Figure 6.24 are moderate and cover
with the requirements from the short-term and long term services temperatures
of PP. Furthermore, the diagram shows that the collector is also able to reach
temperature levels between 60–80°C for quite long durations, however, the peak
values of thermal loads are reduced due to the lower collector efficiency. On
many clear days, the gradient of the absorber temperature reaches only maximum temperatures below 90°C. Thus, there are many peaks in the range between 60–90°C and 0–210 minutes.
205
25
20
15
10
5
435
375
Occurrences [-]
6 Development and Analysis of Polymeric Collector Designs
0
60
80
315
255
Du
rat 195
ion
135
[mi
n]
75
120
15
140
°C
100
re [
u
t
a
per
m
e
T
]
Figure 6.24: Occurrences of thermal loads depending on duration and temperature level on a
polymeric absorber with double-walled housing during stagnation over one year
The results of the stagnation test in Table 6.23 also show component temperatures being suitable for PP. Especially the casing temperatures are quite low.
Table 6.23: Stagnation temperatures according to DIN EN 12975-2 (2006) of the polymeric collector concept with double-walled casing
Stagnation temperatures [°C]
Absorber
Glazing
Casing
(inside)
Casing
(outside)
Double-walled casing concept
119
72
80
52
6.3.5 Optional Efficiency Increase
The emission coefficients of the absorber backside and the casings (
0.95)
cause a relatively high heat transfer coefficient for radiation and increase the
backside losses. Applying a mirror for infrared radiation can lower the heat transfer coefficient. The simplest way is the implementation of an aluminium film (
0.04) at the surface. Equation (6.8) demonstrates that the reduction of the heat
206
6 Development and Analysis of Polymeric Collector Designs
transfer coefficient is independent from deposition of the film (at hot or cold surface).
1
1
(6.8)
1
Table 6.24 shows the influence of aluminium films on the heat transfer coefficient
for radiation. The decrease of
for constant surface temperatures has its op-
timum with one aluminium sheet when the material effort is also considered.
Table 6.24: Influence of the emission coefficients on the heat transfer coefficient for radiation
1
1
Emission coefficients
of the surfaces
1
0.95
0.90
--
0.04
-95.5
0.02
-97.7
0.95&
0.04
0.04
Relative decrease [%]
1
Table 6.25 shows the relative increases of the fractional energy savings as well
as the stagnation temperatures for collector concepts equipped with one aluminium film.
Table 6.25: Influence of the aluminium film on the fractional energy savings and the stagnation
temperatures of the absorber according to DIN EN 12975-2 (2006)
Tstag [°C]
Concept
Increase of
Casing concept
+2.0
135
Double-walled casing concept
+7.0
129
[%]
The good insulation properties of the backside insulation in the casing concept
cause a small temperature difference between the surfaces and limit the heat
transfer via radiation. Thus, the influence of the aluminium film is relatively low in
this concept. The aluminium film in the double-walled casing concept shows with
a relative increase of 7.0 % an efficient measure to improve the limited thermal
207
6 Development and Analysis of Polymeric Collector Designs
performance. The absorber temperatures during stagnation are in both cases
slightly higher but stay below the short-term service temperatures of the material.
6.4 Economic Evaluation of the Systems with Polymeric Collector Approaches
The final economic evaluation of the polymeric collector approaches is based on
the total costs for consumers for the investigated solar-thermal system in Germany. Therefore, the average total costs for customers of three systems with a
similar configuration like the analysed reference system were considered. The
average total costs for customers of the system amount to 13,100 GBP (incl.
19 % VAT). Furthermore, the relative values of the average total costs were separated. According to the breakdown of Figure 6.25, the collectors’ share has a
percentage of 36 %.
Figure 6.25: Typical breakdown of the total costs for end-users of the reference system
The proposed reduction originating from the polymeric collector approaches has
to put in relation to the share of the total system costs. This percentage represents the cost benefit from polymeric collectors according to the total costs for
208
6 Development and Analysis of Polymeric Collector Designs
customers of the complete system. Table 6.26 shows the influence on the system
costs.
Table 6.26: Influences of the polymeric collector concepts on the system costs for customers
Influences on the system
costs for customers
Decrease of
collector
costs [%]
Decrease of
system
costs [%]
System
costs [GBP]
Reference:
--
--
13,100
Low costs
-63.0
-22.5
10,150
High costs
-52.5
-19.0
10,600
Low costs
-59.5
-21.5
10,300
High costs
-49.0
-17.5
10,800
Low costs
-57.5
-20.5
10,400
High costs
-47.5
-17.0
10,850
Casing concept 10 mm + air:
Casing concept 30 mm:
Double-walled casing concept:
The savings from the system costs have to balance the decrease of the solar
yield to gain an economic advantage with the alternative collector approaches.
Therefore, the additional costs for fuel oil over a defined life time of 20 years
(Verein Deutscher Ingenieure e.V. 2012) have to be considered. Based on the
German price development for fuel oil from 2000 to 2012 carried out by
BMWI (n.d.), two scenarios for the price development were taken into account
(Figure 6.26). The moderate scenario refers to a linear approximation of the fuel
oil price. The extreme scenario refers to a quadratic approximation. The two scenarios lead to an average price increase of about 3.7 % or rather 6.8 % per year
in the extrapolated period between 2013 and 2032, which is considered in the
economic evaluation.
209
Fuel oil price [GBP / 1,000 l]
6 Development and Analysis of Polymeric Collector Designs
Figure 6.26: Development of the fuel oil price
The annual energy consumption of the heater from the simulations was used as
a basis for determining the fuel oil consumption. Table 6.27 represents the fuel
oil consumption by means of the heating value of 10.85 kWhl-1 (IWO n.d.) as well
as the energy costs for 20 years based on the two price development scenarios.
Table 6.27: Energy costs of the reference system over 20 years
Energy costs of the reference system
(
17.9%)
Energy consumption
[kWh]
Fuel oil
consumption [l]
Energy
costs [GBP]
1 year
16,510
1,522
--
20 years – Linear price development
412,750
38,050
33,430
20 years – Quadratic price development
412,750
38,050
50,790
Table 6.28 represents the energy costs of the systems with polymeric collector
concepts over the period of 20 years by using the annual fuel oil consumption
from the simulations.
210
6 Development and Analysis of Polymeric Collector Designs
Table 6.28: Energy costs of the systems with polymeric collector concepts over 20 years
Energy costs over 20 years
Linear price
development [GBP]
Quadratic price
development [GBP]
Reference:
33,430
50,790
Casing concept 10 mm + air:
35,230
53,530
Casing concept 30 mm:
34,730
52,760
Double-walled casing concept:
35,480
53,900
The total costs for the customer for a solar-based heat supply comprise the sum
of the system costs and energy costs. The results were compared to the reference solar-thermal system to show the influence on the total costs for customers
by changing the collectors. Table 6.29 lists the cost savings from the systems
with polymeric collector concepts in the period from 2013 to 2032 with a linear
fuel oil price development. The economic benefit is slightly positive for most approaches in relation to the reference system.
Table 6.29: Influences of the polymeric collector concepts on the total costs for customers
with linear fuel price development over 20 years
Difference
in system
costs [GBP]
Difference
in energy
costs [GBP]
Difference
in total
costs [GBP]
Economic
benefit [%]
Low costs
-2,950
+1,800
-1,150
+2.5
High costs
-2,500
+1,800
-700
+1.5
Low costs
-2,800
+1,300
-1,500
+3.0
High costs
-2,300
+1,300
-1,000
+2.0
Low costs
-2,700
+2,050
-650
+1.5
High costs
-2,250
+2,050
-200
+0.5
Influences on the costs
for customers
Casing concept 10 mm + air:
Casing concept 30 mm:
Double-walled casing concept:
211
6 Development and Analysis of Polymeric Collector Designs
Contrary to that, Table 6.30 shows the results with a quadratic fuel oil price development. The increasing energy costs lead to slightly negative economic benefits.
Table 6.30: Influences of the polymeric collector concepts on the total costs for customers
with quadratic fuel price development over 20 years
Difference
in system
costs [GBP]
Difference
in energy
costs [GBP]
Difference
in total
costs [GBP]
Economic
benefit [%]
Low costs
-2,950
+2,740
-210
+0.5
High costs
-2,500
+2,740
+240
-0.5
Low costs
-2,800
+1,970
-830
+1.5
High costs
-2,300
+1,970
-330
+0.5
Low costs
-2,700
+3,110
+410
-0.5
High costs
-2,250
+3,110
+860
-1.5
Influences on the costs
for customers
Casing concept 10 mm + air:
Casing concept 30 mm:
Double-walled casing concept:
The economic impact from the polymeric collector concepts with reference to the
total costs for customers is very limited or even disadvantageous. Hence, the
substitution of state-of-the-art collectors can hardly reach an economic benefit in
the investigated system set-up.
Annotation:
This evaluation is based on the assumption that costs for the system — except
for the collectors — remain stable. However, there are slight differences in fluid
content, glycol degradation and operational costs. The maintenance costs were
also assumed to be similar. Furthermore, subsidies and financing were neglected
for this comparison.
All costs of the calculations are referenced to German prices in € and were converted to GBP with the exchange rate of 0.82 GBP€-1 (Bankenverband n.d.).
212
7 Conclusions and Outlook
7 Conclusions and Outlook
This research covers the development and the analysis of collector concepts
made of polymers with regard to a cost reduction of solar-thermal systems for
domestic hot water preparation and space heating. The main focus was on the
analysis and the reduction of the mostly unknown and high thermal loads on the
collector components which represent the major hurdle for the implementation of
polymers against the background of low material costs. Apart from that, the adaption of the collector component design to the boundaries of relevant plastics processing techniques for an efficient production and low unit costs was necessary
research work. The subsequent simulation analysis was used to check the thermal loads on the components and, hence, on the chosen polymeric materials.
Furthermore, the function of the collector concepts within the solar-thermal system was demonstrated via simulation. The final evaluation was based on the
comparison between the possible decrease of the system costs caused by the
polymeric collector concepts and the influence on the annual solar yield of the
system. The outcomes are summarised below.
7.1 Thermal Collector Loads
The high thermal loads on the components in collectors were identified by field
testing. State-of-the-art flat-plate collectors showed very high absorber temperatures in the period between spring and autumn. Furthermore, the occurrences or
rather the durations at temperature levels which are critical for cost-effective commodity plastics are relatively high. In contrast to these loads, the temperatures at
glazing, frame and backside are suitable for alternative material selections. Thus,
conventional collectors only show critical temperatures for polymeric materials at
the absorber and at sections with direct contact to the absorber. The increased
thermal loads cause an accelerated degradation of the polymers as well as a
decrease of the stability of the component.
213
7 Conclusions and Outlook
The high thermal loads identified in state-of-the-art collectors have to be reduced
by measures or approaches to enable the implementation of commodity plastics.
Therefore, viable concepts are to control or reduce temperatures by changing the
optical efficiency or by removing the thermal energy.
The relevant aspects from the technical point of view for the evaluation of the
identified concepts are reachable temperature reduction, life time and intrinsic
safety of the collector. Especially, the active measures with triggered protection — like devices for the reduction of the transmission, ventilation and heat dissipation — show a high temperature reduction. However, these approaches have
a limited life time or no intrinsic safety. Furthermore, the economic point of view
with regard to solar yield, additional unit costs and additional operational costs is
a main problem of most measures that are able to cause a high temperature
reduction. Finally, early development stages and problems with the application
on polymeric substrates can be named as further drawbacks of the concepts
identified.
The most promising concepts for temperature reduction are the active measures
that change the optical efficiency or the radiation losses. The thermotropic and
thermochromic layers are temperature controlled and independent from secondary influences. However, current thermotropic layers show a transparency too
low in the clear state for being used efficiently in the collector. The thermochromic
absorber layer with a switchable emission coefficient is the most appropriate
measure to enable high collector efficiencies. The small switching range, however, is a major drawback for an overheating protection of proposed polymeric
materials.
The micro-structured layer controlling the solar irradiance at the absorber by
means of the incidence angle is the most suitable passive approach. The layer
proves to have less influence on the solar yield and has a relative simple and
cost-effective set-up. However, the dependency on the incidence angle is problematic for a flexible application. In spite of using the measure in a correct way,
extreme temperatures may occur.
214
7 Conclusions and Outlook
The combination of the thermochromic or the micro-structured layer with temperature controlled service water cooling reaches a similar solar yield and an optimal
temperature reduction. However, due to its considerable water consumption, this
measure is not viable from an economic as well as ecological point of view.
Finally, the reduction of the collector efficiency by means of an accompanied cost
reduction represents the best balance between temperature reduction, solar yield
and unit costs. Saving the extra costs of a selective absorber layer by using a
black absorber material is the most relevant adjustment in collector design for
overheating protected polymer collectors. An additional decrease of the backside
insulation enables the best results for the alternative collector concepts.
7.2 Polymeric Collectors
The considered standard manufacturing techniques for polymeric materials prove
interesting options for new absorber as well as casing designs. Therefore, the
focus of the analysis was on a simple casing design similar to the state-of-the-art
housing as well as on a double-walled casing design. The most promising material for absorber and casing is PP, a material which greatly fulfils the requirements
of temperature resistance, ageing stability and costs. Furthermore, the material
can be processed by all relevant manufacturing techniques. Apart from that, polymeric covers only benefit from lower component weight in comparison to stateof-the-art glazing. The material costs and the ageing resistance are the major
drawbacks of transparent plastics. Furthermore, the stability of the solar glass
can be used for the stiffness of the collector as well as for the mounting components.
The relative savings of the unit costs were between 47.5 % and 63.0 % in comparison to state-of-the-art collectors. The simulation results prove the suitability
of the material selection regarding thermal loads and show decreases of the annual solar yield between 18.5 % and 27.5 % by substituting the collectors of the
system. Especially, the double-walled casing showed a considerable decrease in
215
7 Conclusions and Outlook
efficiency, although the design represents advantages in manufacturing and assembling. Since the collector costs only make up for a small percentage of the
total system costs, savings on the system level are relatively moderate. For an
assumed life time of 20 years, the lowered solar yield or rather the increased
energy costs produce almost the same total costs for system and heat supply.
Finally, the research has proven the feasibility of polymeric collectors for hot water preparation and space heating in central European climates, especially for the
German market. However, the economic benefit of alternative collectors in standard systems is negligible. As a result, further changes regarding performance and
costs are necessary for a considerable advantage of polymeric materials in solarthermal systems.
7.3 Recommendations for Further Investigations
Three further research fields are necessary for the development of cost-efficient
polymeric applications in solar-thermal systems. The improvement of current
overheating protection measures, the detailed investigation of the collector design and the adjustment of set-up of the solar-thermal system can be named as
the main objectives.
The overheating protection measures have to be improved with respect to their
limited performance as well as to their costs. Especially, the above-mentioned
problems of the thermotropic and thermochromic layers have to be solved to enable the useful polymeric material application in collectors. Furthermore, the manufacturing costs have to be very low to gain an economic advantage. Otherwise,
the limited cost potential of systems with polymeric collectors cannot be balanced
under the current conditions.
The collector designs have to be investigated especially regarding mechanical
loads. The components have to fulfil the standards for wind and snow loads.
Therefore, a detailed analysis of the developed components has to be conducted.
The main focus has to be on the loads during high collector temperatures
216
7 Conclusions and Outlook
whereas the stiffness is reduced. The necessary structures and material thicknesses will influence the material effort and the manufacturing costs. The resulting unit costs will change the economic benefit of the design approaches.
Another design aspect should be the structure of the fluid channels to gain an
optimal fluid distribution and, hence, an increased collector efficiency. The development of the volumetric design has to consider the geometrical boundaries from
the manufacturing process as well as the fluid-mechanical design parameters.
The adjustment of the system set-up is the most important field for further investigations. The decreased stability of polymeric materials at high temperatures and
the parallel increasing pressure in the solar circuit are not compatible with each
other. These pressure loads cause extreme stresses and deformations in particular for the necessary volumetric absorber design. This problem has to be solved
by a solar circuit with lower or no pressure at all. Thus, a drain-back system represents a viable option. According to such a system set-up, the implementation
of polymeric materials should be extended to the solar circuit aiming for a costeffective solar-thermal system with reduced costs for components, installation
and maintenance. The open system requires no air bleed valves, pressure relief
valves, diaphragm expansion vessels or a pressure resistant piping. Water as
heat carrier also reduces the total costs further. Thus, the adjusted system decreases the total costs and influences the economic benefit by a compromised
system solution. However, changing the set-up into a drain-back system causes
increased pump energy. The filling of the collectors at the start of the system
operation needs additional pump energy to lift the water column against the static
pressure. Thus, the influence of the pump energy has to be considered in the
economic evaluations as well. Therefore, the simulation model has to be extended regarding pressure drop in piping and electric power consumption of the
pumps.
217
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