a design tool for wood fired storage appliances

Ninth International IBPSA Conference
Montréal, Canada
August 15-18, 2005
A DESIGN TOOL FOR WOOD FIRED STORAGE APPLIANCES
Gerhard Zweifel1, Rolf Friedlin1, Christian K. Gaegauf2, Heinrich J. Huber1, Bendicht Schütz1
1
University of Applied Sciences of Central Switzerland,
Lucerne School of Engineering and Architecture,
CH-6048 Horw, Switzerland
2
Centre of Appropriate Technology,
CH-4438 Langenbruck, Switzerland
ABSTRACT
Tile coated wood fired appliances are designed to
store heat from the intermittently burning fire in their
massive structure and release it slowly to the room.
The design of these systems should assure thermal
comfort during the whole process and involves
dynamic simulation. A customized design tool for
these systems was developed. A model of the
appliance was created and implemented in existing
building simulation software. It was validated and
tuned against laboratory measurements. It can be
used in the context of a multi zone building including
aspects like inter zone air flows. Data input is done
remotely by a web based user interface. An input file
is automatically generated and the simulation run
launched on a server. The results include plots of the
thermal comfort parameters in the different rooms.
For a few real cases the results were compared to
field monitoring data.
The tool is restricted to Swiss conditions. An
international extension in the frame of a European
project is planned.
INTRODUCTION
Figure 1: Example of an appliance
The Systems
Goal
Houses with a low heating energy demand favor the
use of renewable energy sources. One possible
approach is to cover this demand by wood fired
appliances. These appliances are designed to store
the heat from the intermittently burning fire in their
massive structure made from fire brick or similar
materials, and release it slowly to the room.
Depending on the floor plan of the house, a room or
a group of rooms can be heated by one appliance.
With an addition of secondary heat release/storage
elements served by flue gas ducts, so called
“satellites”, also more complicated house plans can
be fully heated by such systems.
The goal of the project was to develop a customized
design tool for this type of systems.
The design of these systems is a challenging task due
to the different dynamic effects which are involved
in a combined way. Assurance of thermal comfort
during the whole process becomes crucial in the case
where there is no additional heating system.
In order to enable the designer to prove sufficient
comfort throughout the heating process, dynamic
simulation had to be involved. The following steps
had to be addressed in the project:
•
Creation of a model of the appliance.
•
Validation of the model against laboratory
measurements and tuning.
•
Implementation in the existing building simulation software IDA-ICE, to be used in the context
of a multi zone building.
•
Assure consideration of aspects like inter zone
air flows. This involved the creation of a
horizontal opening model.
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•
Validation of whole building/appliance model
against field monitoring data.
•
Development of a user interface for the design
tool.
simplified way. The assumption for the time
dependency of the heat release is shown in figure 4.
APPLIANCE MODEL
Model Description
The model of the appliance was developed by using
the modeling feature of the IDA-ICE program, NMF
(Neutral Model Format). The model represents all the
processes shown in figure 2: convective and radiative
heat exchange from heat source to inner surfaces,
storage in fire brick parts, heat release through
massive surfaces and through fire door, flue gas
transport etc.
M: Anzahl angrenzende Räume
cKern
LambdaKern
RhoKern
cSch
LambdaSch
RhoSch
cMant
LambdaMant
PRauch
RhoMant
RauchAus
cpRauch := ƒ(cpRauch0, TRauch)
RhoRauch := ƒ(RhoRauch100, TRauchAbs)
hBrennSch := ƒ(hBrennSch0, VolBrenn, QBrenn)
TRauch
XRauch
HumRauch
QRauchAus
OberflTuer
TTuer
QTuer
Umgebung
QRauchTuer
QStrahlTuer
TUmg
TRauch
QStrahlUmg
RBack
StrahlungRaum
BrennInput
QBrenn
MRauch
TSchAu[j]
TSch2[j]
TSch1[j]
TSchIn[j]
TMantIn[j]
TMant[j]
TMantAu[j]
OberflMant[j]
Figure 3: Experimental appliance in calorimetric
chamber for measurements
Q0 := 0
50000
OberflGlas
QGlas
TGlas
QStrahlGlas
QRauchGlas
QStrahlKern
QKernRauch
FlGlas
FlTuer
MRauchInput
QMantAu[j]
QSch1[j]
QSch2[j]
QSch3[j]
QSpal[j]
TKernAu
QKern
TKern
hBrennGT
LambdaConv
EpsSch
EpsTuer
PRauchInput
EpsGlas
EpsStrahl
TRauch
XRauchInput
HumRauchInput
QRauchSch[j]
QStrahlSch[j]
TSchIn[i,j]
RauchInput
45000
QMantIn[j]
40000
DMant[j]
FlMant[j]
VolMant[j] :=
ƒ(DMant[j], FlMant[j])
35000
Heating power [W]
Azimut := 0
Elev := 0
DSpal[j]
RSpal[j] := ƒ(DSpal[j])
hSpal[j] :=
ƒ(RSpal[j], FSpal)
30000
25000
20000
15000
DSch[j]
FlSch[j]
VolSch[j] := ƒ(DSch[j], FlSch[j])
FlSchAu[j] := ƒ(FlaechFakAu[j], FlSch[j])
FlSchIn[j] := ƒ(FlaechFakIn[j], FlSch[j])
10000
5000
DKern
FlKern
FlKernNeu := ƒ(FlaechFakKern, FlKern)
VolKern := ƒ(DKern, FlKern)
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
Time [h]
Figure 4: Simplified representation of the wood
burning process
Figure 2: Graphic representation of the burning
chamber model
Validation
The model was validated against data from
laboratory experiments. The appliance used for the
experiment consisted of two different parts: the
burning chamber and a separate satellite.
Model development and validation was an iterative
process. Several sets of experimental data with
increasing complexity allowed refining the model
step by step. Simplifications included replacement of
the wood fire by a gas burner and omitting the tile
shell.
For the more realistic cases where wood was really
burned, this burn down process, which in reality is a
complex dynamic process, had to be modeled in a
Figure 5 shows a good agreement between measured
and simulated temperatures in the burning chamber.
HORIZONTAL OPENING MODEL
Model Description
A considerable part of the heat released by the
appliances is distributed in the buildings by natural
convection. It is obvious that in multi storey houses
openings between the floors like open staircases have
a strong influence on this.
Although the used simulation software has an inter
zonal air flow model included, a proper model for
large horizontal openings to represent the elements
characterized above was missing.
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450
400
Temperature [°C]
350
300
250
200
150
100
50
0
18
24
30
36
42
48
54
60
66
72
Time [h]
Measured flue gas
Simulation flue gas
Measured fire brick inner surface
Simulation fire brick inner surface
Measured fire brick outer surface
Simulation fire brick outer surface
Figure 5: Comparison of simulated and measured temperatures for the stove
The model is based on the work of Blomqvist et al
(2004). The air flow through a horizontal opening in
a flat plate between two spaces with different
temperatures is described by equation 1.
m
m& = f St ⋅ k ⋅ An ⋅ [g ⋅ (ρ 2 − ρ1 )]
with
m&
f St
The results were within 20 % of deviation when
using the values shown in table 1.
(1)
QANTITY
air mass flow rate
stair factor
k
proportional factor
A
opening area
n
exponent for area correction
g
ρ2
gravitational acceleration
ρ1
air density lower zone
m
exponent for density difference
Table 1: Values used in equation 1 for conformity
f St
VALUE
1.3
k
n
m
0.5
0.5
0.5
air density upper zone
Validation
CFD simulations were used to evaluate this
approach. Air flow rates through the opening
resulting from a surface integral from the CFD
simulation were compared to the result from equation
1.
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Figure 6: CFD calculation of horizontal opening
with closed staircase
enhanced turbulence. Equation 1 is not valid for
this case.
•
For galleries, equation 1 is only valid when there
is an air gap in the balustrade as shown in
figure 8.
•
The room height has little influence on the flow
rate.
The model has restrictions in application:
Figure 7: Air flow distribution in horizontal opening
with closed staircase
•
The temperature in the lower zone must be
higher than in the upper zone.
•
The geometry of the opening is not taken into
account. It must be near quadratic. Narrow gaps
or cracks are not adequately represented.
•
The size of the openings was between 4 and
8 m2. Larger openings may be well represented,
but this is not proven.
•
Heat sources placed directly under the opening
largely increase the flow rate. This is not
represented.
IMPLEMENTATION AND VALIDATION
OF COMBINED BUILDING/APPLIANCE
MODEL
Model Description
The two models for the stove and the horizontal
opening described above were implemented in the
simulation program IDA ICE, where it is possible to
connect these to the existing building component
models.
Validation
In two houses equipped with appliances as examined
here, field monitoring data were taken. This allowed
evaluating the behavior of the whole building /
appliance model in real operation mode. One of the
houses had a simple stove a mechanical ventilation
system. Figure 9 shows the floor plan and section of
the other house, which is equipped with a stove on
the ground floor and a satellite in the upper floor.
Figure 8: Air flow distribution on gallery with open
balustrade
The following points were learned from the
validation:
•
•
The mass flow rate is increased by 30 % when a
closed stair is put under the opening as shown in
figures 6 and 7. The stair factor of 1.3 is valid for
these cases. Thus, a closed stair increases the
mass flow rate.
A stair with an open air gap between the steps,
however, decreases the mass flow rate due to
Figure 10 shows the comparison of measured and
simulated air temperatures in one of the bedrooms on
the upper floor. There are times with very good
agreement and times with larger deviations. The
reasons for this are some idealizations which had to
be made for the simulations, especially for the wood
burning process, which could not be modeled exactly
as in reality.
It has to be stated that the distribution of the heat in
the house is an important issue with these systems. In
the object shown here this is supported by the family
life with open doors and movements. In the other
case the mechanical system helps a lot in this respect.
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Figure 9: Ground floor plan and section of example house with stove and satellite. The dots represent sensor
placements
Temperature [°C]
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
12
12
36
12
12
60
84
12
08
13
13
Measured air temperature 1
32
56
13
80
04
13
14
Time [h]
28
14
Measured air temperature 2
14
52
76
14
00
15
15
24
48
15
Simulation air temperature
Figure 10: Comparison of simulated and measured air temperatures in an upper floor bedroom
comfort conditions
appropriate time.
DESIGN TOOL
in
all
rooms
at
the
Goal
The final step of the project was to develop a
customized design tool for the dimensioning of these
appliances. It enables the designer to do several
steps:
•
Perform a heating load calculation of the house
•
Do the design of the stove and possibly of
satellites
•
Allocate the heat release surfaces to the different
rooms
•
Perform a dynamic calculation of the whole
setup under winter design conditions to assure
Description
The design tool is strongly linked to the general web
based design tool IDEAXP described in Zweifel et al
(2005).
The user interface is web based; no local installation
is needed for most of the tasks, with one exception
mentioned below.
The heating load calculation is performed according
to the relevant standard. The result, the total heating
load of the building and the distribution to the rooms,
is the base for the design of the appliance.
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Figure 11: Wood stove assistant window
For the detailed design of the stove and the satellites,
the specialists are customized to a special Austrian
design tool described in Baumgartner et al (1997). It
was decided to leave this tool as a stand alone
solution. An import interface was created to import
the results of this tool.
A special assistant added to IDEAXP (figure 11)
allows allocating the right file, to import the data and
to edit the data afterwards.
Figure 12: Comfort calculation result
When imported, the data are shown in the tree
structure of the program, and several input windows
enable the user to allocate heat release surfaces to
rooms and flue gas ducts to the surfaces etc.
When the whole input process is completed, the
comfort calculation can be launched, with this action,
an input file for the IDA ICE program is
automatically generated, and then the simulation run
is launched. The results include temperature profiles
during a winter design period for all rooms
(figure 12) and allow a comfort evaluation according
to the known theories of thermal comfort.
OPERATIONAL EXPERIENCE
The tool covers all the requirements of the wood
stove designers to do a complete design of their
systems and evaluate its behavior.
Due to the fact, that a detailed multi zone building
model is needed and that there are many different
effects with partly high calculation intensity
involved, the calculation time was initially pretty
long. In collaboration with the program supplier
team, the models could be cleaned up and speed was
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enhanced by a factor. In the meantime calculation
time is reduced to less than 30 min.
CONCLUSIONS
The simulation shows that the massive structure of
the appliance strongly influences the characteristic of
the heat release curve. The appropriate heat output
over time is crucial for the room comfort. It is
important to avoid an overshooting of the room
temperatures by peak heat outputs. An optimum
design of the appliance gives the biomass branch the
possibility to guaranty quality standards to their
customers, which helps disseminating wood heat
appliances due to a good reputation and feasible
costs. This is very important for the manufacturers to
be competitive on the market.
With the developed design tool the stove designer is
in the position to do the layout of an appliance
according to the energy requirement.
OUTLOOK
REFERENCES
Gaegauf C.K., Zumsteg H., Friedlin R., Huber H.J.,
Schütz B., Chiquet C. (2004) Komfortberechnungsprogramm für Holz-Speicheröfen. Final
report, 2004
Bösch D., Friedlin R. (2002) Berechnungswerkzeug
für Ganzhausheizungen mit Holz-Speicheröfen.
Diploma Thesis, HTA Luzern, 2002
Gaegauf C.K., Zumsteg H., Huber H.J., Friedlin R.,
Schütz B., Chiquet C. (2004) Computer Aided
Design Tool For Wood Heat Appliances To
Optimise Room Comfort. 2nd World Conference
on Biomass for Energy, Industry and Climate
Protection, Rome, 2004
Blomqvist C., Sandberg M. (2004) Air Movements
through Horizontal Openings in Buildings. A
Model Study. The International Journal of
Ventilation, Volume 3, Issue 1, June 2004
The design tool is restricted to Swiss or at least
meddle European conditions. It is available in
German language.
Baumgartner G., Hofbauer H., et.al. (1997) Bemessung von Kachelöfen, Schriftenreihe Nr. 1,
Österreichischer Kachelofenverband, Vienna,
Austria, 1997
It is planned to further develop the tool in the frame
of a European project, in order to extend its usability
to a wider range of countries.
Zweifel G., Gadola R., Rieder U., Schütz B.,
Wandeler A. (2005) A Web Based Design Tool,
Building Simulation Conference 2005, Montreal
ACKNOWLEDGMENTS
The work would not have been possible without the
funding from the Swiss Innovation Promotion
Agency (CTI) and the Swiss Federal Office of
Energy. The work was done in collaboration with
and with substantial contributions from the Swiss
Association of Masonry Stove and Tile Enterprises.
The Lucerne School of Engineering and Architecture
(HTAL) supported the conference contribution.
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