Electricity from Photovoltaics
ELECTRICITY
FROM
PHOTOVOLTAICS
BY: SABRI M. SHERIF
2002-03
Department of Mechanical
Engineering
Faculty of Engineering
University of Strathclyde
1
THE COPYRIGHT
The copyright of this dissertation belongs to the author
under the terms of the United Kingdom Copyrights Acts as
qualified by University Strathclyde Regulation 3.49.
Due acknowledgement must always be made of the use of
any material contained in or derived from, this thesis.
2
ACKNOWLEDGEMENTS
I owe gratitude to Dr Craig Mclean who undertook the
supervision of this project. I would like to thank him
for his enthusiasm, his guidance and for sharing his
wide experience with me.
I would also like to thank MR Cameron Johnstone for
giving freely of his time for little reward and for his
guidance and instruction in doing the experiment and
with out him which this thesis would not have reached
completion.
I would like to thank Mr Eric Duncan for his cooperation and help doing the preparation of laboratory
items.
I would also like to thank all the postgraduate students
at Strathclyde University.
I would especially like to thank my wife and my children
for them support.
I would like to record my gratitude to Libya embassy
especially for Mr Younes Almahdi whose student
supervisor for financial support through out the course.
3
ABSTRACT
The aim of this project is to show how to produce the
electricity from photovoltaic solar cells and there are
experiment proofing that and overview of techniques and
principles of cell design. Although there is excellent
example where a lot of the concepts discussed in one
design the PERL design for high-efficiency Silicon Si
solar cells. These techniques and principles are the
result of intensive research over the last forty years
that has required the simultaneous understanding of
electrical and optical effects. The remaining challenge
is to find accost-effective way to apply these
principles to construct a low-cost solar cell with high
and stable efficiency.
Photovoltaic (PV) cells have social and commercial value
only when they are used in a system to provide a
service. This research has given a brief overview of the
technical and economic considerations that allow the
cells to provide such a service.
4
: CONTENTS:
ACKNOWLEDGEMENTS
ABSTRACT
CHAPTER 1
INTRODUTION
--------------------------------10
CHAPTER 2
PRINCIPLES OF CELL DESIGN --------------------12
2.1- Main cell types -------------------------13
2.2- optical design of cells -----------------14
2.2.1- Light trapping --------------------------------14
2.2.2- anti-reflective coatings: reduction of first
reflection -------------------------------------------17
2.3- Design and fabrication of the metal
contacts -------------------------------------17
2.3.1- Two-sided contact design --------------18
- Optimisation of the front contact pattern --18
- Screen printing ----------------------------20
- Plating/buried contact technology ----------21
- Transparent conductive oxides --------------23
5
2.3.2- One-sided contact designs -------------23
-
Cell structures with all contacts at
backside -------------------------------------24
-
Cell structures with all contacts at
frontside ------------------------------------25
CHAPTER 3
CYSTALLINE SILICON SOLAR CELLS ---------------21
3.1-Overview ---------------------------------28
3.2-Silicon cell development------------------31
3.3-Substrate production ------------------------39
3.3.1 - Standard process ------------------------40
3.3.2 - Multicrystalline silicon ingots -----------41
3.3.3 - Sheet and ribbon silicon -----------------42
3.4-Cell costs -------------------------------43
CHAPTER 4
PHOTOVOLTAIC MODULES, SYSTEM AND APPLICATIONS-46
4.1-photovoltaic modules ---------------------47
6
4.1.1- Electrical connection of the cells -----------49
4.1.2- Module structure -------------------------50
4.1.3- Variations in module design-----------------52
4.2-The photovoltaic array -------------------53
4.2.1- Electrical connection of modules -----------53
4.2.2- mounting structure -----------------------54
4.2.3- Tilt angle and orientation -----------------54
4.2.4- Sun-tracking/concentrator systems -----------55
4.2.5- Shading ---------------------------------56
4.3-The photovoltaic system ------------------57
4-3.1- System design ----------------------------57
4.3.2- System components ------------------------60
- The photovoltaic array ---------------------60
- Power conditioning -------------------------60
- Inverter ------------------------------------61
- Storage -------------------------------------62
- Load equipment -------------------------------63
- Cabling and switching equipment -----------------63
4.3.3- System sizing ----------------------------64
- Location information includes ------------------65
- Information on system purpose includes ----------65
7
4.3.4- System operation -------------------------65
4.3.5- Operating and maintenance -----------------70
4.3.6- Photovoltaic applications -----------------71
- Remote area power supplies (RAPS) ---------------72
- Building-integrated photovoltaic (BIPV) system ----75
4.4-Costs of PV components and system --------81
CHAPTER 5
(EXPERIMENT IN LAP OF STRATHCLYD UNIVERCITY) -86
CONCLUSION ----------------------------------109
REFERENCE -----------------------------------111
8
CHAPTER one
INTRODUTION
9
1-INTRODUTION
Due to the limited reserves of both fossil and nuclear
fuels, renewable resources will have to play major role
in the world’s future energy supply.
Among potential new energy sources are solar energy and
especially the Photovoltaic cells which generate
electric power when illuminated by sunlight or
artificial light. They are by far the most highly
developed of the man-made photoconversion devices. Born
of the space age in the 1950s, their earliest
terrestrial applications emerged in the 1970s and they
are now poised for significant market expansion in the
new millennium.
Photovoltaic technology has a number of advantages over
conventional methods of electricity generation. First,
solar energy is the world’s major renewable energy
resource. Photovoltaic power can be generated from the
sun any where in temperate or tropical locations and in
urban or rural environments. As a fuel-free distributed
resource, photovoltaics could, in long run, make a major
contribution to national energy security and carbon
10
dioxide abatement. Photovoltaic is uniquely scalable,
the only energy source that can supply power on a scale
of milliwatts to megawatts from an easily replicated
modular technology with excellent economies of scale in
manufacture. Typical crystalline silicon Photovoltaic
cell generates about 1.5 peak watts¹ (WP) of direct
current (DC) power, a typical Photovoltaic module about
50 WP and the world’s largest multimodule arrays
generate upward of a megawatt apiece. Photovoltaic cells
are made of thin films.
They contain small amounts of
non-toxic materials and when manufactured in volume,
have modest embedded energy. They posses no moving
parts, they don’t
need cooling water system and they
are silent in operation. Photovoltaic systems are easy
to use and long-live if properly maintained.
Photovoltaic has three drawbacks.
1- The intermittence and seasonality of sunlight.
2- Photovoltaic systems are very costly.
3- Photovoltaic is one faced by many emergent
technologies-ignorance.
11
CHAPTER two
PRINCIPLES OF CELL DESIGN
12
2-PRINCIPLES OF CELL DESIGN
2.1- Main cell types
First one single-junction homo-and heterostructure solar
cells where p-n homojunction is the most straightforward
realisation of solar cell and the dominance of Silicon
homojunction solar cells is a testimony to the success
of this approach. But the single junction cell will
never provide the highest conversion efficiency because
of the trade-off between current and voltage.
Second one multijunction system where a cell fabricated
in a semiconductor with a low band gap will provide a
larger current because of its good light absorption over
a broader spectral region. However it will never produce
high open-circuit voltages because the open-circuit
voltage can never exceed the band gap of the material.
In practice the open-circuit voltage is limited by the
large dark currents in a low-band-gap material.
Third one metal-semiconductor junctions (Schottky diode)
with rectifying properties would also be suitable for
the construction of a solar cell. But from the point of
13
view of ease of production this approach suffers from
limitations caused by the high dark currents flowing in
such structures as compared to the heterojunction
approach. In addition the predicted efficiency of solar
cells based on metal-semiconductor junctions will be
below 10% (see e.g.Hovel, 1975).
The last one is the metal-insulator-semiconductor (MIS)
solar cell which the efficiencies above 20% (see Metz,
1997) and efficiencies as high as 23% are predicted
(Kuhlmann, 1997) for a MIS-inversion layer cell.
2.2- optical design of cells
2.2.1- Light trapping
A lambertian surface is one that scatters light
uniformly in all [forward] directions and lambertian
light-trapping schemes are based on full internal
randomisation of the light ray direction by a lambertian
surface. Two possible implementations are shown in
(Fig.2.1 (a, b)).
14
Figure (2.1) (a) Schematic of a cell with a perfect lambertian rear
reflector and front surface with zero reflectance; (b) schematic of
a cell with a perfect specular rear reflector and a lambertian
front surface.
Light-trapping structures with lambertian surfaces
represent the limiting case for cells with an isotropic
response, i.e. cells whose response is independent of
the angle of incidence (Tiedje, 1984). Because of the
directionality of sunlight the use of surface structures
with dimensions larger than the wavelength of the light
can increase the cell absorptions above the limits of
the lambertian schemes.
Additional improvement in light-trapping behaviour is
obtained by structuring the backside of the cell. A very
15
efficient method is use of backside grooves
perpendicular to the frontside grooves as shown in
Fig.2.2a. In Fig.2.2b a recently proposed concept with
structuring of both sides is shown (Jorgensen et al.,
1997). This structure gives rise to a further
improvement of light-trapping properties.
Figure (2.2) (a) Artist’s view of a cell with light-trapping
structures at both front- and backside. The grooves at the
frontside are perpendicular to the grooves at the backside (after
Green, 1995, p.104, reproduced with permission from The Centre for
Photovoltaic Devices and Systems); (b) structure, proposed by
Jorgensen, 1997, reproduced with permission from IEEE.)
16
2.2.2- anti-reflective coatings: reduction of
first reflection
The reduction of the front-surface reflectance is
achieved by the use of so-called anti-reflective
coatings (ARC). The ARC layer which is interposed
between the photovoltaic cell material and the
surrounding environment acts as a quarter-wavelength
impedance matching element between the characteristic
impedance of the environment and the photovoltaic
material.
2.3- Design and fabrication of the metal
contacts
The section will address the design of the metallization
scheme. First one the design of the most widespread twosided contacting scheme. The second one is one-sided
contacting scheme.
17
2.3.1- Two-sided contact design
Optimisation of the front contact pattern
Most designs of the cells are based on two-sided
contacting scheme, a schematic illustration of which is
shown in Fig.2.3a and b. Fig.2.3b shows a cross-section
of the two-dimensional contacting scheme. Although the
carrier flow inside the cell is mostly vertical, the
flow in the upper (emitter) layer of the cell is in the
horizontal direction towards the collecting fingers.
Figure (2.3) (a) Typical front contact lay-out. S is the finger
spacing and Wf is the finger width. In the simple case, the finger
width is constant; (b) cross section of cell showing the vertical
carrier flow in the base of the cell and the horizontal majority
carrier flow in the emitter.
18
The main design parameters are those relating to the
finger grid at the frontside of the cell: the finger
spacing S and the finger width Wf.
Detailed
optimisations based on the real two-dimensional carrier
flow inside silicon (Si) high-efficiency n+-p Si
homojunction cell can be found in the work of Aberle,
1994.
When the carrier flux jb towards the junction is
homogeneously distributed and the upper layer is
FKDUDFWHULVHGE\WKHVKHHWUHVLVWDQFH
sheet,
the
resistive losses in the upper layer per unit length of
finger are given by
0œ
S/2
I2 (x) dR = 0œS/2 j²b b2 x2
= j2b S3
sheet
dx
sheet/8
This value compared with the maximum power delivered by
the cell:
Pmp = Vmp Imp S/2
19
Where Vmp and Imp are the current and voltage generated
at the maximum power point.
Then the relative loss defined as the ratio of the
contact loss to the maximum power is given by
5HODWLYHORVV sheet
Imp S2/4Vmax
This equation shows the importance of the finger spacing
and the sheet resistance of the upper layer. Achieving
low values for the sheet resistance is obviously a
promising way to reduce the resistive losses. Decreasing
this by going to deeper emitters or thicker transparent
conductive layers will increase other losses.
Screen printing
This technique widely is used for industrial nonconcentrating silicon Si solar cells and the main
advantage of screen printing is simplicity and costeffectiveness. Drawbacks which are often cited are the
relatively large line width, the high contact resistance
due to oxide precipitation from the glass frit
(dispersed glass particles), and the low (broad and
flat) aspect ratio of the metal lines, which results in
large shadowing losses. As a result, efficiencies
20
between 17% and 17.5%, both on large-area
monocrystalline (Nijs et al., 1996) and industrial-type
multicrystalline (Shirasawa et al., 1994) Si cells have
been reported.
Plating/buried contact technology
Plating is technique whereby a metal, dissolved in an
aqueous solution, is deposited on a substrate. This can
be realised by passing a direct current through the
solution (electroplating), but in the context of Si
solar cells the most widely used technique is
electroless plating. Here the plating is performed by
chemical deposition under such circumstances that the
metal deposition occurs selectively in regions where the
Si surface is in direct contact with the solution.
The low aspect ratio of the contacts is one of the
drawbacks of the screen-printing technique. A solution
to this problem is the so-called ‘buried contact’
technology, whereby the metal is electrolessly plated
into deep grooves, obtained by laser scribing or
mechanical cutting. A cross section of the final cell is
shown in Fig 2.4.
21
Figure (2.4) Schematic cross section of buried-contact structure.
The first versions of this technology made use of an
oxide layer in between the grooves to avoid metal
deposition (see e.g. Wenham, 1993). Subsequently this
oxide layer was replaced by a nitride layer since this
withstands high temperature steps and also provides
better anti-reflection properties. The buried-contact
approach is inherently a selective emitter approach,
because two different diffusions are carried out during
the process. There is a shallow diffusion in between the
grooves, but a very deep diffusion within the grooves.
Together with the low shadowing losses resulting from
the high aspect ratio of the fingers, this technology
22
has produced efficiencies between 16% and 18% on monoand multicrystalline large-area Si solar cells.
Transparent conductive oxides
When the sheet resistance of the emitter is high the use
of a finger contact pattern at the frontside of the cell
is excluded. Under such circumstances a high-conductance
layer covering the front surface is necessary. The
transparent conductive oxides (TCOs) are a special class
of materials that exhibit good electrical conductive
high optical transparency and a high band gap. This
makes them especially useful as n-type ‘window layers’
or transport layers on top of the actual window layer.
2.3.2- One-sided contact designs
One-sided contact designs are of very strong
interest. An obvious advantage is the elimination of
shadowing losses when all the contacts are at the
backside. This is important in cells for concentrating
systems where shadowing losses are more important.
Additionally one-sided contact designs are particularly
attractive at the level of module production. Monolithic
23
integration of cells on a large insulating substrate
could be an important factor in cost reduction because
the series interconnection between the front contacts of
a cell to the backside of the next cell in a
conventional module forms an important part of module
production costs.
Cell structures with all contacts at backside
We can distinguish two versions of his cell type, the
point-contact and interdigitated designs shown in (Fig.
2.5 (a, b)). The main difference between the two is the
reduced backside area with a high doping and
metallization in the point-contact design which achieves
higher open-circuit voltages because of reduced
recombination at the surfaces.
24
Figure (2.5) (a) backside point-contacted solar; (b) backside
interdigitated solar cell.
Cell structures with all contacts at frontside
Although the advantage of no shadowing loss is lost if
all the contacts are at the frontside, several groups
like ( Hebling 1995) are working on this concept in the
context of developing a thin-film crystalline Si solar
cell technology on an insulating substrate see (Fig
2.6).
25
Figure (2.6) Frontside-contacted interdigitated cell (after
hebling, 1995).
26
CHAPTER three
CYSTALLINE SILICON SOLAR
CELLS
27
3-CYSTALLINE SILICON SOLAR CELLS
3.1-Overview
The majority of solar cells fabricated to date have
been based on silicon in monocrystalline or largegrained polycrystalline form.
There are two main reasons for this. The first reason is
that silicon is an elemental semiconductor with good
stability and a well-balanced set of electronic,
physical and chemical properties, the same set of
strengths that have made silicon the preferred material
for microelectronics. The second reason why silicon
cells have been so dominant is that the success of
silicon in microelectronics has created an enormous
industry where the economies of scale directly benefit
the presently smaller photovoltaics industry.
Most silicon cells have been fabricated using thin
wafers cut from large cylindrical monocrystalline ingots
prepared by the exacting Czochralski (CZ) crystal growth
process and doped to one part per million with boron
during ingot growth.
28
To produce a cell, these boron-doped starting wafers
generally have phosphorus diffused at high temperatures
a fraction of a micron into the surface to form the p-n
junction required. Each cell is typically 10-15 cm
either in diameter or along either side if square or
rectangular.
Cells are sold interconnected and packaged into a
weatherproof, glass-faced package known as a module, as
in (Fig. 3.1.) since each cell gives a maximum output of
about 0.6 volts in sunlight. This meaning we get over 20
voltages in one module because in each module there are
36 cells soldered together in series.
The efficiency of the cells in the module would
typically lie in the 12-16% range, less than half the
fundamental ‘detailed-balance’ limits of 33% for silicon
(Tiedje 1984). Module efficiency is slightly lower than
that of the constituent cells due to the area lost by
frames and gaps between cells, with module efficiency
generally lying in the 10-13% range. Over the last few
years, commercial cells and modules of significantly
higher performance have been available in multi-megawatt
quantities using a more advanced cell processing
29
technology. This technology produces cells of 17-18%
efficiency and module efficiency in the 14-15% range.
glass
EVA
Cells
EVA
rear cover
Figure (3.1) Exploded view of a standard silicon photovoltaic
module. These layers laminated together under pressure at a
temperature around 140-150 C where the transparent EVA (ethylene
vinyl acetate) softens and binds the different layers together on
cooling. Source: Green and Hansen (1998).
30
3.2-Silicon cell development
The development of silicon photovoltaics is
inextricably intertwined with the development of the
general silicon electronics field and the subsequent
founding of the microelectronics industry. The rapid
increase in interest in the properties of doped silicon
led directly to the development of point contact and
junction transistors and junction transistors and
ultimately to integrate circuits.
The improvements of the early 1970s came about primarily
by improving the ability of the cell to collect carriers
generated by the incoming photons. Since cells now
appeared to be performing to close to their full
potential in this area, it seemed that any further
improvement in silicon cell performance would have to
result from improved open-circuit voltage. (Brandhorst
and Bbernatowicz, 1980).
The successful approach has been the simplification of
the ingot growth processes by using cruder directional
solidification or casting approaches to produce
multicrystalline ingots (Ferazza, 1996). These
31
multicrystalline approaches involve basically a
reversion to the earlier ingot-forming approaches for
crystal rectifiers, techniques predating the
microelectronics explosion. In this time the
multicrystalline silicon cells accounted for about 30%
of the total market for photovoltaic product. Another
major area of developmental emphasis has been to reduce
the thickness of the silicon wafer by slicing it more
thinly.
The University of New South Wales developed microgroove
PESC (passivated emitter solar cell) (FIG.3.2a) which
was the first silicon cell to exceed 20% energy
conversion efficiency in 1985. Other groups have used
the same basic as roach to produce cells of similar
efficiency, with commercial quantities produced for
solar car racing and space. The approach is
characterised by the use of a thin thermally grown oxide
to ‘passivate’ (reduce the electronic activity of) the
top surface of the junction diffusion (he emitter of the
cell), combined with the use of a shallow, high sheet
resistivity phoshorus diffusion for this emitter.
Another is the use of photolithography to produce
relatively small contact area to this emitter region by
defining openings in the ‘passivated oxide’.
32
Cells of a similar quality to the first 20% efficient
PESC have also found heir way into high-volume
terrestrial cell manufacture through the laser-grooved
buried-contact cell of (FIG.3.2b). The buried-contact
approach now produces the highest performance
terrestrial cells that are produced in any appreciable
volume; with efficiency in the 17-18% range routinely
obtained using Standard low-cost commercial silicon
wafers.
(a)
33
(b)
Figure (3.2) (a) The microgrooved passivated emitter solar cell
(PESC cell) of 1985, the first silicon cell to exceed 20%
efficiency; (b) buried-contact solar cell. Source: Green (1995).
The next improvement in silicon cell design came in the
use of oxide passivation along both the front and rear
surfaces, as first demonstrated in the rear point
contact solar cell developed by Stanford University. As
shown in (FIG.3.3), this cell has an unusual design in
hat both positive and negative contacts are made at the
rear surface of the cell. The rear point contact cell
demonstrated 22% efficiency in 1988 and has since been
commercialised, finding use in photovoltaic systems
34
which rely on concentrated sunlight and for high valueadded applications such as solar car racing and highaltitude aircraft flights (Verlinden et al.,1997).
Figure (3.3)
Rear point contact solar cell which
demonstrated 22% efficiency in 1988 (cell rear shown
uppermost). Source: Green (1995).
The next improvement in silicon cell efficiency came,
again at University of New South Wales, by combining the
earlier developments in the PESC sequence with the front
and rear oxide passivation first demonstrated in the
rear point contact cell. This is possible in a number of
ways as shown in (FIG.3.4). In the PERC (passivated
emitter and rear cell) of (FIG.3.4a), the first to be
successfully demonstrated, rear contact is made to the
35
silicon substrate through holes in the rear passivating
oxide. This approach works reasonably well provided the
substrate is sufficiently heavily doped for contact
resistance between the metal and substrate. He PERC is
often suggested as a relatively low cost way for making
silicon cells above 20% efficiency, since it is the
simplest of the approaches of (FIG.3.4).
(a)
36
(b)
Figure (3.4) (a) The passivated emitter and rear cell (PERC cell);
(b) The passivated emitter, rear locally diffused cell (PERL cell)
which took efficiency above 24% in the early 1990s. Source: Green
and Hansen (1998).
HOW WILL CELL DESIGN EVOVLE IN THE FUTURE?
It’s provided by (Fig.3.5), which shows the calculated
intrinsic energy conversion efficiency bounds on singlejunction silicon solar cells. In ‘lambertian’ light
trapping schemes, the light direction within the cell is
randomised allowing path-length enhancements to be
quite. The best laboratory cells have demonstrated close
37
to 85% of the achievable efficiency, according to this
(Fig.3.5).
Figure (3.5) Limiting efficiency of a silicon solar cell as a
function of cell thickness with and without lambertian light
trapping (global AM1.5 spectrum,
100 mW cm-2, 25 C). source: Green (1995).
As opposed to the case of laboratory devices, most
manufacturers of commercial cells would be very pleased
to be producing consistently cells of half the limiting
efficiency of (Fig.3.5). Some of the difference between
laboratory and commercial cell performance is due to
poorer quality of silicon substrate material.
38
It seems that eventually it should be feasible to
produce low-cost commercial silicon cells of efficiency
above 20% with such improved cell designs by paying
attention to the passivation of both front and ear
surfaces, by thinning the cells to reduce bulk
recombination and by modifying the crystal growth
processes to withstand high-temperature processing
without loss of electronic quality.
An interesting result highlighted by (Fig.3.5), is the
way that light trapping allows high performance, in
SULQFLSOHIURPVLOLFRQFHOOVWKDWDUH PWKLFN7KLV
provides a justification for expecting very high
performance, eventually, from the thin.
3.3-Substrate production
a-
Standard process
b-
Multicrystalline silicon ingots
c-
Sheet and ribbon silicon
39
3.3.1-standard process
Silicon solar cell technology is capable of
benefiting directly from the economies of scale of the
silicon microelectronics industry, and also the capable
of using scrap material from this industry because he
requirements for material quality in photovoltaics are
less demanding than in the more general microelectronics
field. Accordingly, given the present size relativities,
most silicon cells are made from a standard silicon
source material originally intended for
microelectronics. Over the last ten years, the size
relativities have not changed enormously, since both
industries have been steadily growing. Explosive growth
in the photovoltaics industry, such as that stimulated
by urban residential rooftop applications of
photovoltaics in 1997, will increasingly upset this
delicate balance.
To produce ingots from the pure silicon feedstock, a
modification of the Czochralski (CZ) process which
produces ‘tricrystalline’ silicon which has been used
for photovoltaics (Endros et al., 1997). Another
alternative to the standard Czochalski process for
40
producing crystalline ingots is the floatzone (FZ)
process. This (FZ) process as commercially implemented
has capable of accepting feedstocks only in the form of
high quality cylindrical rods. That makes it unsuitable
for using low-cost off-grade material. However the
casting and directional solidification processes used to
produce multicrystalline silicon.
3.3.2- Multicrystalline silicon ingots
In 1998, about 30% of the world’s photovoltaic
production was based on multicrystalline silicon wafers.
Many companies have developed commercial processes for
producing the precursor multicrystalline silicon ingots
(Ferrazza, 1996). There are many way to produce the
multicrystalline silicon ingots and the advantages over
the Czochralski process are lower capital costs, higher
throughput and a higher tolerance to poor feedstock
quality.
The multicrystalline wafers are capable of producing
cells of about 80% of the performance of a
monocrystalline cell fabricated on a Czochralski wafer.
41
3.3.3- Sheet and ribbon silicon
The most advanced sheet or ribbon approach is based on
the edge-defined film-fed growth (EFG) technique of
(Fig.3.6). As originally developed in the early 1970s
this involved the pulling of a thin sheet of silicon
ribbon from a strip of molten silicon formed by
capillary action at the top of a graphite dye (Fig.3.6).
Commercial cells made from edge-defined film-fed growth
(EFG) material have been available sporadically since
the early 1980s with a large 25MW/yr facility recently
announced.
Figure (3.6) Edge-defined, film-fed growth (EFG) method. Source:
Green and Hansen (1998).
42
A somewhat elated approach is the string ribbon
approach. In this the molten silicon is trapped between
two graphite strings that are drawn from the melt. This
relaxes the requirement on thermal control. This
approach is under development by Evergreen Solar (Janoch
et al., 1997; Wallace et al., 1997).
Another approach, developed in the 1980s, relied on
direct casting of silicon wafers using a centrifugal
casting approach to overcome surface tension problems
within the closely spaced faces of a horizontally
aligned graphite mould (Maede and Hide, 1987).
3.4- Cell costs
The cost of the cells is important and there were many
studies of silicon cell production using different basic
assumptions. Probably the most recent is one conducted
under the auspices of the European Union Photovoltaic
Program (Bruton et al., 1997).
The key assumptions of the study were manufacturing
volume of 500 MWp of solar cells per annum and the
43
availability of silicon source material at about (US$25
per Kg). A number of different technologies were
compared in table 3.1.
Table 3.1 summary of published results of a European Commission
study of manufacturing costs for 500 MWp per year factory
ID
wafera process Cell
Estimated Key variable
efficiencyb cost
(ECU/Wp)c
study
(present)
1
DS
SP
15%(12.6-
0.91
Wafer
1.25
Wafer/process
1.15
Process
14.8%)
2
CZ
SP
16%(13.915.6%)
3
CZ
LGBC
18%(16.517.5%)
4
CZ
MIS/A
17%(N/A)
1.28
Process/module
5
CZ
MIS/B
17%(12.2%)
1.34
Module
6
CZ
PERL
20%(N/A)
1.78
Process
7
EFG
SP
14.4%(12%)
0.71
Wafer
Where DS directional solidification; CZ Czochralski growth; EFG
edge-defined film-fed growth; SP screen-printed; LGBC laser
grooved, buried-contact; MIS/A metal-insulator-semiconductor; MIS/B
as for MIS/A but with resin-fill packaging; PERL passivated
emitter, rear locally diffused (less appropriate acronym LBSF used
44
in study). b The cell efficiencies assumed in the study in some
cases differ appreciably from present average production values,
deduced by the present author from manufacturers’ data sheets or
the results from large field installations.
WHERE (1ECU=US$1.2)
Source: Bruton et al., 1997.
From this table it is seen that the screen-printed cells
on ribbon (EFG) produces the lowest cost of 0.71 ECU/Wp
followed by the multicrystalline wafers at ECU 0.91/Wp.
The advantage of the ribbon stems almost entirely from
the fact that it doesn’t need to be sawn.
45
CHAPTER four
PHOTOVOLTAIC MODULES,
SYSTEM AND APPLICATIONS
46
4.1-photovoltaic modules
In order to provide useful power for application the
individual solar cells must be connected together to
give the appropriate current and voltage levels and they
must also be protected from damage by the environment in
which they operate. This electricity connected
environmentally protected unit is usually termed
a photovoltaic module although it can also be termed a
PV laminate when it is supplied without the frame.
Figure (4.1) (a) and (b) show typical module
constructions for crystalline silicon and thin film
silicon cells respectively.
Figure 4.1 (a)
47
Figure 4.1 (b)
Figure 4.1 a) Schematic of module construction for crystalline
silicon cells-exploded view showing the different layers which make
up the module; b) Schematic of module construction for thin film
cells.
Due to the difference in fabrication process module
designs for crystalline and thin film cells, whilst
following the same basic principles, differ
substantially in several aspects of module construction
and design. Indeed, it could be said that the thin film
cells are fabricated in modular for requiring only the
encapsulation step after completion of the deposition
processes. For simplicity the crystalline silicon solar
cell will be considered initially in each sub-section
48
since it is presently the most common cell type for
power applications. Variations introduced by the use of
thin film cells will then be identified.
4.1.1- Electrical connection of the cells
The electrical output of a single cell is dependent on
the design of the device and the semiconductor
material(s) chosen, but is usually insufficient for most
applications. In order to provide the appropriate
quantity of electrical power a number of cells must be
electrically connected. There are two basic connection
methods: series connection in which the top contact of
each cell is connected to the back contact of the next
cell in the sequence and parallel connection in which
all the top contacts are connected together as are all
the bottom contacts. In addition connected in a series
string to increase the voltage level and in parallel to
increase the current level. See figure (4.2)(a, b).
49
Figure 4.1 (a) Series connections of cells
Figure 4.1 (b) parallel connections of cells
4.1.2- Module structure
In modern crystalline silicon modules the front surface
is almost always composed of glass, toughened to provide
physical strength and with a low iron content to allow
50
transmission of short wavelengths in the solar spectrum.
The rear of the module can be made from a number of
materials. One of the most common is Tedlar (see
Fig.4.3), although other plastic materials can also be
used. If a level of transparency is required then it is
possible to use either a translucent Tedlar sheet or
more commonly a second sheet of glass. The glass-glass
structure is popular for architectural applications
especially for incorporation into a glazed façade or
roof.
Figure 4.3 Typical crystalline silicon module and cell (photograph
courtesy of BP Solarex).
51
The ideal module would also provide good heat transfer
in order to keep the cell temperature as low as
possible. However the encapsulant is required to provide
electrical isolation and physical protection, so a high
heat transfer coefficient is not always possible. The
operating temperature is also influenced by the exterior
materials of the module, with glass-glass structures
usually running at a higher temperature than the glassTedlar module under similar conditions.
4.1.3- Variations in module design
Module design varies according to the electrical output
required and the application of the PV system.
Considerable variation in size, shape, colour and cell
spacing has been introduced in recent years to
accommodate the consumer market especially where the
modules are incorporated directly into the product and
the building integration market where appearance is of
particular importance. It has also been possible to
design modules which have additional functions such as
the semi-transparent modules that can be used as shading
devices and to influence light patterns inside
buildings.
52
The choice of module structure and design is very
dependent on the application in question with output,
appearance, cost, compatibility with other components
and durability being the issues to consider.
4.2- The photovoltaic array
A PV array consists of a number of PV modules, mounted
in the same plane and electrically connected to give the
required electrical output for the application. The PV
array can be of any size from a few hundred watts to
hundreds of kilowatts, although the larger systems are
often divided into several electrically independent subarrays each feeding into own power conditioning system.
4.2.1- Electrical connection of modules
As with connection of cells to form modules a number of
modules can be connected in a series string to increase
the voltage level and in parallel to increase the
current level or in a combination of two. Matching of
interconnected modules in respect of their outputs can
53
maximise the efficiency of the array in the same way as
matching cell output maximises the module efficiency.
4.2.2- mounting structure
The main purpose of the mounting structure is to hold
the modules in the required position without undue
stress.
4.2.3- Tilt angle and orientation
The orientation of the module with respect to the
direction of the Sun determines the intensity of the
sunlight falling on the module surface. Two main
parameters are defined to describe this. The first is
the tilt angle which is the angle between the plane of
the module and the horizontal. The second one is the
azimuth angle which is the angle between the plane of
module and due south (or sometimes due north depending
on the definition used).
The optimum array orientation will depend on the
latitude of the site, the prevailing weather conditions
and the loads to be met. The optimum tilt angle is also
54
affected by the proportion of diffuse radiation in the
sunlight since diffuse light is only weakly directional.
Therefore for locations with a high proportion of
diffuse sunlight the effect of tilt angle is reduced.
The final aspect to consider when deciding on array
orientation is the incorporation in the support
structure. For building-integrated applications the
system orientation is also dictated by the nature of the
roof in which it is to be incorporated.
4.2.4- Sun-tracking/concentrator systems
Some arrays are designed to track the path of the sun.
This can account fully for the sun’s movements by
tracking in two axes or can account partially by
tracking only in one axis from east to west.
For a flat-plate array, single-axis tracking where the
array follows the east-west movement of the Sun has been
shown to increase the output by up to 30% for a location
with predominantly clears sky conditions. Two-axis
tracking where the array follows both the daily eastwest and north-south movement of the sun could provide a
55
further increase of about 20% (Lepley, 1990). It is
usually more economical to install a larger panel for
locations with less than about 3000 hours of direct
sunshine per annum.
For concentrator systems the system must track the Sun
to maintain the concentrated light falling on the cell.
The accuracy of tracking and hence the cost of the
tracking system increases as the concentration ratio
increases.
4.2.5- Shading
Shading of any part of the array will reduce its output
but this reduction will vary in magnitude depending on
the electrical configuration of the array.
Thus the reduction in output from shading of an array
can be significantly greater than the reduction in
illuminated area since it results from
- The loss of output from shaded cells and modules;
- The loss of output illuminated modules in any
severely shaded strings that cannot maintain
operating voltage;
56
- The loss of output from the remainder of the array
because the strings are not operating at their
individual maximum power points.
4.3- The photovoltaic system
The design of the photovoltaic system depends on the
task it must perform and the location and other site
conditions under which it must operate.
4-3.1- System design
There are two main system configurations- stand-alone
and grid-connected. In the stand-alone system
operates independently of any other power supply and
it usually supplies electricity to a dedicated load
or loads. It may include a storage facility (e.g.
battery bank) to allow electricity to be provided
during the night or at times of poor sunlight levels.
By contrast the grid-connected PV system operates in
parallel with the conventional electricity
distribution system. It can be used to feed
electricity into the grid distribution system or to
power loads which can also be fed from the grid.
57
It is also possible to add one or more alternative
power supplies (e.g. diesel generator, wind turbine)
to the system to meet some of the load requirements.
These systems are then known as ‘hybird’ systems.
This system can be used in both stand-alone and gridconnected applications but are more common in the
former because provided the power supplies have been
chosen to be complementary they allow reduction of
the storage requirement without increased loss of
load. See Figure (4.4) (a, b, c).
Figure 4.4 a) Schematic diagram of a stand-alone photovoltaic
system.
58
Figure 4.4 b) Schematic diagram of grid-connected photovoltaic
system.
Figure 4.4 c) Schematic diagram of hybrid system incorporating a
photovoltaic array and a motor generator (e.g. diesel or wind).
59
4.3.2- System components
The main system components are the photovoltaic array,
power conditioning and control equipment, storage
equipment (if required) and load equipment.
The photovoltaic array
The PV array is made up of the PV modules themselves and
the support structure required to position and protect
the modules.
Power conditioning
It is often advantageous to include some electrical
conditioning equipment to ensure that the system
operates under optimum conditions. In the case of the
array the highest output is obtained for operation at
the maximum power point. Since the voltage and current
at maximum power point vary with both insolation level
and temperature, it is usual to include control
equipment to follow the maximum power point of the
array, commonly known as the maximum power point Tracker
60
(MPPT). The MPPT is an electrical circuit which can
control the effective load resistance which the PV array
sees and thus control the point on the I-V
characteristic at which the system operates. There are a
number of ways in which the optimum operating point can
be found but an MPPT often operates by checking the
power levels on either side of the present operating
point at regular intervals and if a gain in power is
observed in one direction then the MPPT moves the
operating point in that direction until it reaches the
maximum value. For grid-connected systems the MPPT is
often incorporated into the inverter for ease of
operation, although it is possible to obtain the MPPT as
an independent unit.
Inverter
If the PV system needs to supply AC alternative current
loads then an inverter must be included to convert the
DC direct current output of the PV array to the AC
output required by the load. As with PV systems
inverters can be broadly divided into two types these
being stand-alone and grid-connected.
61
The stand-alone inverter is capable of operating
independently from a utility grid and uses an internal
frequency generator to obtain the correct output
frequency (50/60). By contrast the grid-connected
inverter must integrate smoothly with the electricity
supplied by the grid in terms of both voltage and
frequency.
When the inverter is grid-connected it must be ensured
that the system will not feed electricity back into the
grid when there is a fault on the grid distribution
system. The problem is known as islanding and safeguards
are required in order to provide protection for
equipment and personnel involved in the correction of
the fault.
Storage
For many PV system applications, particularly standalone, electrical power is also required from the system
during hours of darkness or periods of poor weather
conditions. In this case storage must be added to the
system.
62
Load equipment
The nature of the load equipment will determine the need
for and suitability of the power-conditioning equipment
and the capacity of both the PV system and the storage.
The first consideration is whether the load or loads use
DC or AC electricity. In the former case the loads can
be operated directly from the PV system or battery
storage whereas AC loads will require an inverter to be
included in the system.
Cabling and switching equipment
The array cabling ensures that the electricity generated
by the PV array is transferred efficiently to the load
and it is important to make sure that it is specified
correctly for the voltage and current levels which may
be experienced. Since many systems operate at low
voltages the cabling on the DC side of the system should
be as short as possible to minimise the voltage drop in
the wiring. Switches and fuses used in the system should
be rated for DC operating. In particular DC sparks can
63
be sustained for long periods leading to possible fire
risk if unsuitable components are used.
4.3.3- System sizing
It is important to determine the correct system size in
terms of both peak output and overall annual output in
order to ensure acceptable operation at minimum cost. If
the system is too large it will be more expensive than
necessary without increasing performance levels
substantially and therefore the system will be less
cost-effective than it could be. However if too small a
system is installed the availability of the system will
be low and he customer will be dissatisfied with the
equipment. Again the cost-effectiveness is reduced.
Although many of the same principles are included in the
sizing process the approach differs somewhat for standalone and grid-connected systems. The first step is to
gather the relevant information on he location and
purpose of the system.
64
Location information includes
- Latitude and longitude;
- Weather data-monthly average sunlight levels
ambient and maximum temperatures, rainfall, maximum
wind speeds, other extreme weather conditions;
- Constraints on system installation-tilt angle,
orientation, risk of shading
Information on system purpose includes
- Nature of load or loads;
- Likely load profile-daily, annual variation (if
any);
- Required reliability-ability to cope with loss of
load (for example clinic lighting required a higher
level of demand-many systems fail because they are
sized for an existing load, but demand increases
soon after provision of the PV supply.
If an autonomous system is required the PV system must
provide sufficient electricity to power the loads even
under the worst conditions. Thus system sizing is
usually carried out for the month that represents the
65
worst conditions in terms of the combination of high
load levels and low sunlight conditions.
For a given system design the average electrical output
in the sizing month can be calculated from the average
daily insolation level (usually expressed in KWh m-2)
taking into account he number of modules, their rated
efficiency, the efficiencies of all control and the
power conditioning equipment, the efficiency of any
storage system, mismatch losses, wiring losses and the
operating temperature. For an autonomous PV system, the
average daily electrical output should match or exceed
the average daily load. If his is not the case then the
PV array size must be increased.
The battery storage allows for variations in the load
level during the day and the provision of power at
night. The battery bank must be sized to accommodate the
average daily need for electricity which cannot be
directly supplied by the PV system and so that this
results in only a shallow discharge of the batteries.
Clearly the sizes of the PV array and battery bank are
linked and an increase in the size of one can often
allow a decrease in the size of the other. The size
66
operating is usually an iteration of the problem to find
the most cost-effective solution taking into account the
requirements and preferences of the user. Most companies
have their own computer programs for performing this
iteration and also use their experience to determine the
parameters which should be input for any given case. It
is also possible to purchase sizing software from
several companies.
Therefore most sizing packages are used to determine
potential output and to compare different options of
system location and design rather than optimising system
size as such. Not all sizing packages are suitable for
building-integrated applications because they do not
take account of the higher operating temperatures or the
shading levels which can be experienced. However more
complex system simulation programs taking these factors
into account have been developed in recent years (see
for example, Reise and Kovach, 1995).
4.3.4- System operation
The output of any PV system depends mainly on the
sunlight conditions but can also be affected by
67
temperature, shading and the accumulation of dirt on the
modules. The overall system performance is usually
represented by the efficiency which is defined as the
ratio of the electrical output (in KWh) to the sunlight
energy input (also in KWh) over the surface of the array
in the same period. In general this overall efficiency
results from several processes to which individual
efficiency values can be assigned, e.g. the conversion
of sunlight to DC electricity, the conversion of DC to
AC by the inverter.
The system yield is also a useful parameter. This
expresses the annual output (or that over another
defined period) as a function of the nominal rating of
the system and is in units of KWh/KWp. this allows
comparison of systems in different locations. However
since this parameter does not explicitly include the
sunlight level received over the period, account must be
taken of whether the level was above or below average if
the yield is to be used for a critical assessment of
system performance.
Another often-quoted parameter is the performance ratio
which is either given as a percentage or as a number
between zero and one. Essentially this parameter
68
expresses the performance of the system in comparison to
a lossless system of the same design and rating at the
same location. It provides a measure of the losses of
the system but because the sunlight level is included in
the calculation it becomes independent of sunlight
conditions. The performance ratio (PR) is calculated
from the following formula:
PR =
system output over period / (average daily
irradiance
period
x
x
array rating
x
number of days in
monitoring fraction)
Where all parameters are values for the same period, the
system output is in KWh, the average daily irradiance is
in KWh
m-2
and the array rating is in KWp. The monitoring
fraction is the fraction of the period considered for
which monitoring data are available and have been used
to determine the values of the other parameters. The
formula makes the assumption that average conditions are
experienced for the time when data are not collected and
so care must be taken with the use of PR values
calculated for monitoring fractions less than (0.9).
69
4.3.5- Operating and maintenance
Because of its lack of moving parts and simple
connections, a PV system generally requires little
maintenance. However it is necessary to ensure continued
access to sunlight by cleaning the panels at appropriate
intervals by refraining from building any structures
that could shade the panels and by cutting back any
branches or other vegetation hat could cover the system.
The electrical connections should also be checked at
regular intervals to eliminate any problems e.g.
corrosion, loose connections. If included in the system
the battery bank may need regular maintenance according
to the type chosen.
The requirement for cleaning is often overestimated by
those with little experience of PV systems. In most
cases it can be assumed that 3-5% of performance will be
lost if the system is only cleaned annually with up to
half of that loss being experienced within a few week of
cleaning. However the losses incurred and thus the
requirement applications operating under similar
condition. For example if there is the possibility of
dust or sandstorms causing accumulation on the modules
70
perhaps in a desert area installed in industrial areas
close to sources of airborne pollutants. For building
integrated systems on houses in many parts of Europe, it
may not actually be necessary to clean the systems since
the action of rainwater on the inclined panels removes
surface dust.
Most operational problems occur as a result of poor
maintenance of the BOS components (including loads and
batteries) or allowing the array to become obscured or
damaged. This latter problem indicates a lack of
understanding of the operation of system correctly. This
is also demonstrated by system failures arising from the
addition of loads that were not included in the original
system sizing. In this case the combination of the PV
and storage system cannot meet the increased demand and
there is a danger of damage to the batteries from deep
discharging.
4.3.6- Photovoltaic applications
Two particular examples will be discussed. First, remote
area power supplies (RAPS); second building-integrated
photovoltaic (BIPV) systems. These represent two of the
71
major markets for photovoltaics, both now and in the
future. They also provide examples of stand-alone and
grid-connected applications respectively.
Remote area power supplies (RAPS)
These systems supply electrical power to a wide variety
of loads remote from any utility distribution grid. The
systems range in size from a single module powering a
Solar Home System (SHS) to a few kilowatts of PV
supplying a local area grid network. The systems are
autonomous and so must include energy storage of some
sort to supply power in the absence of sunlight. The
economics of storage dictate that for larger systems and
for those where high reliability is paramount, some of
the energy storage will be in the form of fuel for an
internal combustion engine. In locations where the
seasonal availability of wind energy is complementary to
that of the solar irradiance, it is often cost-effective
to include a wind turbine in the hybird system.
In a small non critical system such as a small home
system (SHS) a PV module charges a battery during the
day and the power is used at night for a few high-
72
efficiency lights and a radio or small TV. A charge
controller ensures that the battery is not overcharged
or deep-discharged, to provide as long a battery
lifetime as possible. System sizing is simple using
estimates of average daily usage of the loads and in the
absence of 10 years of solar data in most locations,
estimates of solar irradiance and its variability. In
order to keep costs as low as possible a standard system
is sold to all users although richer households may
purchase a ‘2 module system’, i.e. double the standard
system. The reliability of the systems depends to a
large extent on the users observing the remaining
battery charge from indicator lights on the charge
controller and modifying their usage accordingly. A
longer than average period of low irradiance will result
in a loss of power to the loads but this is an
inconvenience to the users rather than a threat to life
or to the system.
Some autonomous systems are part of safety-critical
networks for instance in aircraft navigation aids or
telecommunication systems. In these cases it is
permitted to lose power to the loads only one day in 100
years and the system design must guarantee this very low
loss-of-load probability (LOLP). Even if there were
73
long-run, accurate solar data for the site, it must be
remembered that the stochastic variability of solar
irradiance is such that past data are only an average
predictor for the future and once in 10 year events are
not predictable (Lorenzo and Narvarte, 2000). It is
always possible to oversize the PV array and battery to
give such a LOLP in an average 10 year period, at a high
cost but even then there is no guarantee that a 1-in-100
year low or worse will not occur in the first year of
operation. The cost-effective solution is to include
additional charging from a small internal combustion
engine, usually a diesel, with a fuel store large enough
to need refilling only on visits to the site for
periodic maintenance of the electronic systems. The PV
array and battery system are sized so that the engine is
run at full power for about 1 hour/day, to keep it in
good condition.
The PV/diesel hybird system is used in many parts of the
world as an alternative to grid extension. In Australia
farms and small communities in the outback are supplied
with a RAPS system in a standard container unit. All
parts are transported in the container which on location
becomes the base for the system. The PV array is mounted
on the roof with the diesel engine, batteries and all
74
power conditioning and controls mounted inside the
container. The daytime load is supplied by the PV
system, with the diesel engine as a back-up charger for
the supply of night-time loads. The diesel engine is run
at full power for at least one hour per day, to maintain
it in good condition without excessive use of fuel. The
fuel tank is sized so to need refilling only at long
intervals so reducing the transport cost of the fuel.
Remote area power supplies make use of the fact that
sunlight is freely distributed to all sites however
remote (at least in the Sunbelt). The challenge in
system design is to match the power output to the load
as far as possible and maintain a very high availability
for safety-critical systems whist keeping costs as low
as possible. Storage is essential for any system that
has a night-time load and while battery storage remains
expensive it will be cheaper for system over 500Wp or so
to include a diesel engine.
Building-integrated photovoltaic (BIPV) system
One of the fastest growing sectors of the photovoltaic
market is the building integrated photovoltaic system.
75
This is an ideal application for the use of
photovoltaics in an urban environment and takes
advantage of the distributed nature of sunlight and of
the electrical load. The benefits of the BIPV system can
be summarised as follows:
- in common with other PV systems and most renewable
energy technologies, it has a lower environmental
impact than production of electricity from
conventional fuels;
- the electricity is generated at the point of use so
reducing the impact and cost of distribution;
- there is a possibility of offsetting some of the
cost of the PV array by the amount which would have
been paid for the building material it has
replaced;
- the system does not require additional land area
since building surfaces are used to accommodate the
array.
The PV modules can be integrated in several different
ways for example to replace roofing tiles, in place of
façade material or as sunshades. (Figure 4.5) shows an
example of façade integration but there are many
different ways of including the PV array in the building
design.
76
Figure 4.5 Example of façade integration of photovoltaics. The
photograph shows the 40 kWp PV façade on Northumberland Building at
the University of Northumbria. The PV array is integrated into the
rainscreen overcladding. This system was installed in 1994 and is
one of the early examples of façade integration (photograph
courtesy of University of Northumbria).
77
The principle of the technical system design is similar
to that for other PV applications but there are some
additional aspects to be taken into account. It may be
designed to match the general load profile or to provide
higher output levels when, for instance, air
conditioning is required but it does not need to be an
autonomous system since most of the buildings also have
a grid supply.
However the area available for the BIPV array may be
constrained by building design, shading from surrounding
structures or owner preference. Thus the system size is
often dictated by the nature of the building rather than
its electrical loads. The visual aspect of the system is
also important and this often affects the choice of
module type, location and detailed integration method.
Finally he system design must take into account ease of
installation, maintenance and operation and compliance
with building regulations.
The heat at the rear of the modules can also be used in
some cases. Even in the most efficient modules, only
about 15% of the light falling on the module is turned
into electricity and whilst a few percent is reflected,
78
the rest is absorbed as heat. This results in a module
operating temperature that can be 25-50 k above ambient
temperature. Reducing the operating temperature by
removing some of the heat is advantageous in terms of
increasing system efficiency and a double benefit can be
obtained if the heat is useful for another purpose.
Because of the rather large area of the module and the
relatively modest temperature differential between the
module and ambient temperatures, it is not usually cost
effective to use forced air or fluid flow to extract the
heat unless three is a direct use for that heated air or
fluid. However, the heat can be used to assist natural
ventilation within the building by taking in cold air at
the bottom of the building. As this air is heated behind
the PV façade, it rises and pulls in more cold air to
replace it. Examples of such ventilation systems include
the Doxford Solar Office in the UK (Lloyd Jones et al.,
1998) and the Mataro library in Spain (Lloret et al.,
1997).
Even for a system where no use is made of the heat, care
must be taken to ensure that the PV array operating
temperature remains at an acceptable level. For most
stand-alone systems there is free air movement around
79
the array and so some cooling is effected. This is not
the case for a BIPV system which forms pat of the
building fabric. The design must include adequate
ventilation around the modules if significant losses in
efficiency are to be avoided.
Most BIPV systems are grid-connected with the
conventional electricity supply meeting any shortfall
between the BIPV electrical output and the building
demand. The system must conform to safety regulations
for connection. There is a wide range of tariffs offered
for this electricity, ranging from the replacement
generation cost to several times the normal electricity
rate where a scheme to promote BIPV exists (more
information in Haas, 1998).
Several countries (e.g. Germany, the Netherlands and
USA) have major promotion schemes for BIPV stimulated by
environmental concerns over global warming and
pollution.
80
4.4-Costs of PV components and system
The generation of electricity from PV systems is unlike
that of other systems in that the cost of generation is
only weakly dependent on the size of the system. This is
a result of the modularity of PV systems and such
differences as do exist at present arise mainly from
sales installation and maintenance costs rather than
hardware costs. These costs will fall as the throughput
of PV systems in the supply, installation and
maintenance chains increases with increased sales.
The cost of manufacturing a PV module consists of the
material, labour, capital and energy costs. The purchase
price of a module is of course higher since it must also
include marketing and sales costs, the profits to
manufacturer and supplier and the costs of management
and other overheads. The price of materials falls as
they are purchased in tonnes rather than kilograms,
whilst large-scale production uses machinery rather than
labour so that the labour costs/unit also falls. The
capital cost of equipment to make 1 million modules per
year is much less than 10 times the cost of equipment to
make 100000 per year, the equipment would occupy much
81
less than 10 times the space and it would use much less
than 10 times the energy. It is also the case that large
companies can borrow money more cheaply than small ones
so the capital repayments/unit of borrowing become
smaller as the PV industry grows, further reducing the
capital costs of manufacture.
The cost of a PV system is the sum of the costs of the
hardware and the costs of transport, system design,
installation and maintenance. The price paid by a
customer also includes the mark-up of the wholesaler and
retailer in many instances and often must included taxes
and duties.
The non-hardware costs also have benefits of scale. The
unit cost of transport is lower for a container load
than for a small number of modules or systems. Spreading
design costs over large numbers of systems reduces the
cost to each system, whilst the installation and
maintenance of many systems/year in one locality reduces
the cost per system. The increasing market for PV system
will therefore lead to a reduction in all of the system
costs.
82
One of the most interesting applications for PV is on
buildings where building integrated PV (BIPV) systems
can effectively result in no additional cost. When PV
modules are integrated into the structure of a building
they have a dual function. They act as a building
element, replacing a conventional roof or façade as well
as being a generator of electricity. On houses the BIPV
system replaces roof tiles which are of relatively low
cost. On commercial office buildings, however, the BIPV
system replaces the cladding elements that ensure both
the weather-tightness of the building and its physical
appearance. Conventional cladding systems vary widely in
cost but for luxury cladding such as polished stone, the
cost can be over £1000/m2. Where a BIPV system replaces
such cladding, the cost of the building is lower with PV
than with the polished stone and the owner of the
building gets electricity generation at no additional
costs.
Property developers use expensive cladding for prestige
and companies buy or occupy such buildings to enhance
their public image. With the increase in ‘green’
awareness, a BIPV façade on a building can make a very
significant public statement for the owners and
occupiers of the building and the image value can
83
justify its classification as a luxury cladding. As the
cost of PV modules falls then BIPV systems can replace
cheaper conventional cladding at zero additional cost
and the market for BIPV will expand greatly. The cost of
electricity generated by a BIPV system is greatly
influenced by the avoided cost of the conventional
cladding that is replaced by the PV. Table 4.1 shows the
cost of electricity from PV costing £2/Wp about US$3/Wp
for a range of cladding under the assumptions specified.
It is clear from Table 4.1 that PV laminates costing
£2/Wp and replacing conventional cladding costing
£300/m2 or more can generate electricity at a cost below
the retail price from a utility. The electricity is a
free by-product if the PV replaces cladding costing
£350/m2 or more. A modest insolation level reasonable
for UK facades was chosen to demonstrate that the
economic use of BIPV is not only possible for regions
with high sunlight levels. Competitive electricity costs
would be reached at higher module and/or Balance of
System (BOS) costs for locations with higher sunlight
levels.
84
Laminate
Cladding
Net PV
System
Electricity
cost £/m2
cost £/m2
cost
cost Net
cost
Laminate-
PV+BOS
pence/KWh
Cladding
£/Wp
£/m2
£/Wp
280
100
180
1.3
1.8
27
280
150
130
0.9
1.4
22
280
200
80
0.6
1.1
18
280
250
30
0.2
0.7
10
280
300
-20 -0.14
0.36
5
280
350
-70
0
0
-0.5
Table 4.1 the cost of electricity generated by a BIPV system for a
range of cladding costs. Assumption PV laminates: efficiency 14%
cost£2/Wp; BOS costs
£0.5/Wp; insolation 700 KWh m-2yr-1; discount
rate 8%; lifetime 30 years.
85
CHAPTER 5
(EXPERIMENT IN LABORATORY
OF STRATHCLYDE
UNIVERSITY)
86
(EXPERIMENT IN LAPORATORY OF STRATHCLYD
UNIVERCITY)
This chapter presents and discusses the experiments
performed inside the laboratory. The system consisted
of:
- solar simulator calibration
- Monocrystalline photovoltaic module
- instruments to measure the temperature
- instrument to measure the current and voltage
-solar simulator calibration
In order to calibrate the PV module, the calibration of
the solar simulator was required. The Strathclyde
University solar simulator consists of 50 tungsten
halogen’s lamps, type PRA 38, manufactured by Thorn
Lighting LTD; see figure 5.1. Irradiance varies from
point to point. The framework for the lamps is held by
threaded ropes and pulleys which are fixed to the
87
ceiling of the simulator room. The distance between the
lamps and the test area can be varied by lowering or
raising the frame.
The distance between the solar simulator calibration and
the PV module is more or equal one metre in order to
obtain uniformity of illumination.
Figure 5.1 University solar simulator consists of 50 tungsten
halogen’s lamps, type PRA 38, manufactured by Thorn Lighting LTD
88
Monocrystalline photovoltaic module
The PV module was disconnected from the battery and
placed under the solar simulator. The irradiance was
varied by varying the electrical voltage to the solar
simulator lamps and output (voltage and current) of the
module. See figure (5.2)
Figure (5.2) monocrystalline photovoltaic module in lap. Of
Strathclyde University module type: BP 585 F
Normal peak power (Pmax) —-- 85.00 watt
Peak power voltage (Vmp) --- 18.00 V0lt
Peak power current (Imp) --- 4.72 Ampere
Short circuit current (Isc) --- 5.00 Ampere
Open circuit voltage (Voc) --- 22.03 volt
Minimum power (Pmin) --- 80.00 watt
Power specifications measured at standard test conditions,
insolation of 1000w/m2, am 1.5, 25 °c cell temperature.
Module certified to CEC Specification 503.
89
From this information it found the fill factor (F) was
found as:
F= Vmp * Imp / Isc * Voc
F = 0.7713
Power P = Isc * Voc * F
Instruments to measure open circuit voltage and
short cicuit current
There after, the open circuit voltage and short circuit
current could be measured. See figure (5.3).
Figure (5.3) Digital Multimeter
90
Instruments to measure temperature and output
voltage current
The temperature at the rear surface of the cell was
measured by placing a type K thermocouple (copper
constantan) with an attached electrical thermometer. See
figure (5.4).
Figure (5.4) Thermometer. The range (-50
accuracy :( ±1
°c to 1300 °c) and the
°c) for the range (-50°c to 1000°c).
91
Experimental work
An important performance characteristic of PV systems is
the relationship between power output and the cell
temperature. The objective of the experiments was to
quantify this relationship by plotting power output
against the temperature of the photovoltaic module. This
was done by irradiating the test panel and measuring its
temperature over a series of time steps as the
temperature of the cell increased towards the
equilibrium value.
The work was carried out in the solar simulator located
within the Mechanical Engineering Laboratories at
Strathclyde University. The solar simulator and panel
used are as described previously.
Two levels of irradiation were used, 500 W/m2, 600 W/m2,
and 700 W/m2 and reading were taken at 1 minute
intervals.
92
Experiment (1), irradiance 500 W/m2:
The solar simulator was run for 50 minutes and readings
were recorded at 1 minute intervals
Temperatures recorded
– Air temperature = 20 °c
-
Module temperature
-
Different in temperature (¨7 PRGXOHWHPSDLU
temperature
Electrical power recorded
– Voc (open circuit voltage)
- Isc (short circuit current)
93
Experiment (2), irradiance 600 W/m2:
The solar simulator was run for 50 minutes and readings
were recorded at 1 minute intervals
Temperatures recorded
– Air temperature = 20 °c
-
Module temperature
-
Different in temperature (¨7 PRGXOHWHPSDLU
temperature
Electrical power recorded
– Voc (open circuit voltage)
- Isc (short circuit current)
94
Experiment (3), irradiance 700 W/m2:
The solar simulator was run for 50 minutes and readings
were recorded at 1 minute intervals
Temperatures recorded
– Air temperature = 20 °c
-
Module temperature
-
Different in temperature (¨7 PRGXOHWHPSDLU
temperature
Electrical power recorded
– Voc (open circuit voltage)
- Isc (short circuit current)
95
Results
The results are given in tabular form in Tables 1, 2 and
3.
The results are given in graphical form in figures 5.5,
5.6 and 5.7.
96
voltage(volt) current (A)
Temp (Ts)
¨7
power (W)
20
0.57
22
2
8.7894
19.82
0.58
24
4
8.8631076
19.74
0.58
26
6
8.8273332
19.6
0.59
28
8
8.915844
19.49
0.59
30
10
8.8658061
19.4
0.59
32
12
8.824866
19.19
0.6
34
14
8.877294
19.07
0.6
36
16
8.821782
18.91
0.6
38
18
8.747766
18.77
0.6
40
20
8.683002
18.64
0.6
41
21
8.622864
18.57
0.6
43
23
8.590482
18.46
0.6
44
24
8.539596
18.38
0.61
45
25
8.6442978
18.3
0.61
46
26
8.606673
18.24
0.61
47
27
8.5784544
18.19
0.61
47
27
8.5549389
18.14
0.61
48
28
8.5314234
18.08
0.61
49
29
8.5032048
18.04
0.61
50
30
8.4843924
18.01
0.61
50
30
8.4702831
17.98
0.61
50
30
8.4561738
17.95
0.61
51
31
8.4420645
17.91
0.61
51
31
8.4232521
17.87
0.61
52
32
8.4044397
17.84
0.61
52
32
8.3903304
17.8
0.61
52
32
8.371518
17.77
0.61
53
33
8.3574087
17.75
0.61
53
33
8.3480025
17.72
0.61
53
33
8.3338932
17.7
0.61
53
33
8.324487
17.67
0.61
54
34
8.3103777
17.65
0.61
54
34
8.3009715
17.64
0.61
54
34
8.2962684
17.62
0.61
55
35
8.2868622
17.61
0.62
55
35
8.4179322
17.6
0.62
55
35
8.413152
17.58
0.62
55
35
8.4035916
17.57
0.62
55
35
8.3988114
17.55
0.62
56
36
8.389251
17.54
0.62
56
36
8.3844708
17.53
0.62
56
36
8.3796906
17.52
0.62
56
36
8.3749104
17.51
0.62
56
36
8.3701302
17.5
0.62
56
36
8.36535
17.49
0.62
56
36
8.3605698
17.49
0.62
56
36
8.3605698
17.48
0.62
56
36
8.3557896
17.48
0.62
57
37
8.3557896
Table (1) for the input irradiation 500 w and air temp. 20 °c
97
voltage (volt) current (A)
Temp (Ts)
¨7
power (w)
20.3
1
22
2
15.6513
20
1.01
24
4
15.5742
19.9
1.01
26
6
15.49633
19.75
1.01
28
8
15.37952
19.58
1.01
30
10
15.24714
19.33
1.02
32
12
15.2015
19.24
1.02
34
14
15.13072
19.04
1.03
36
16
15.12024
18.92
1.03
38
18
15.02494
18.75
1.03
40
20
14.88994
18.64
1.03
42
22
14.80258
18.52
1.04
43
23
14.85008
18.43
1.04
44
24
14.77791
18.34
1.04
46
26
14.70575
18.27
1.04
47
27
14.64962
18.18
1.04
48
28
14.57745
18.13
1.04
48
28
14.53736
18.08
1.05
49
29
14.63666
18.02
1.05
50
30
14.58809
17.96
1.05
50
30
14.53952
17.93
1.05
51
31
14.51523
17.89
1.05
51
31
14.48285
17.84
1.05
52
32
14.44237
17.8
1.05
53
33
14.40999
17.77
1.05
53
33
14.3857
17.75
1.05
54
34
14.36951
17.73
1.05
54
34
14.35332
17.71
1.06
54
34
14.47367
17.68
1.06
54
34
14.44916
17.66
1.06
55
35
14.43281
17.64
1.06
55
35
14.41647
17.61
1.06
55
35
14.39195
17.59
1.06
55
35
14.3756
17.59
1.06
55
35
14.3756
17.57
1.06
56
36
14.35926
17.56
1.06
56
36
14.35109
17.54
1.06
56
36
14.33474
17.53
1.06
56
36
14.32657
17.51
1.06
57
37
14.31022
17.5
1.06
57
37
14.30205
17.48
1.06
57
37
14.2857
17.47
1.06
58
38
14.27753
17.46
1.06
58
38
14.26936
17.45
1.07
58
38
14.39573
17.44
1.07
58
38
14.38748
17.43
1.07
58
38
14.37923
17.42
1.07
58
38
14.37098
17.41
1.07
58
38
14.36273
17.41
1.07
59
39
14.36273
Table (2) for the input irradiation 600 w and air temp. 20 °c
98
current (A)
Temp(Ts)
¨7
20.5
1.31
23
20.3
1.31
26
20.1
1.31
30
19.82
1.32
33
19.56
1.32
36
19.26
1.33
39
19.05
1.33
42
18.81
1.34
45
18.63
1.35
47
18.42
1.35
49
18.29
1.35
51
18.12
1.36
52
18.03
1.36
54
17.91
1.36
56
17.82
1.37
57
17.73
1.37
58
17.66
1.38
59
17.58
1.38
60
17.51
1.38
61
17.46
1.39
62
17.42
1.39
63
17.39
1.39
63
17.35
1.39
63
17.33
1.4
64
17.31
1.4
64
17.28
1.4
65
17.24
1.4
65
17.2
1.4
66
17.18
1.41
66
17.15
1.41
66
17.13
1.41
67
17.11
1.41
67
17.08
1.41
67
17.07
1.41
68
17.04
1.41
68
17.03
1.41
68
17.02
1.41
68
17.01
1.41
68
16.99
1.41
68
16.97
1.41
69
16.95
1.41
69
16.94
1.41
69
16.93
1.41
69
16.91
1.41
69
16.9
1.41
70
16.89
1.42
70
16.88
1.42
70
16.87
1.42
70
16.86
1.42
70
16.85
1.42
70
Table (3) for the input irradiation
Voc(volt)
99
power(w)
3
20.70521
6
20.5032
10
20.3012
13
20.17121
16
19.9066
19
19.74978
22
19.53444
25
19.43336
27
19.39104
29
19.17246
31
19.03715
32
18.99991
34
18.90554
36
18.77971
37
18.82273
38
18.72767
39
18.78989
40
18.70477
41
18.63029
42
18.71171
43
18.66884
43
18.63669
43
18.59382
44
18.706
44
18.68441
45
18.65203
45
18.60886
46
18.56568
46
18.67655
46
18.64394
47
18.62219
47
18.60045
47
18.56784
48
18.55697
48
18.52435
48
18.51348
48
18.50261
48
18.49174
48
18.47
49
18.44826
49
18.42651
49
18.41564
49
18.40477
49
18.38303
50
18.37216
50
18.49151
50
18.48056
50
18.46961
50
18.45867
50
18.44772
700 w and air temp. 20 °c
output (voltage, current, power) with respect to
increase of dT
voltage(volt)
40
35
current (A)
30
scale
25
20
dt ©
15
10
5
power (W) =
voltage*curre
nt* fill factor
0
1
6
11
16
21Time
26(t)
31
36
41
46
power output with respect to increase of dT
40
35
dt ©
30
scale
25
20
15
10
power (W) =
voltage*current*
fill factor
5
0
1
6
11
16
21
26
Time (t)
31
36
41
46
current (A) with respect to time (t)
1
6
11
16
21
26
31
scale
1
36
41
46
current (A)
0.1
Time (t)
100
output voltage w ith r e s pe ct to s ur face te m pe r atur e
60
te
m
p
e
ra
tu
re(T
)
50
40
30
20
10
0
17
17.5
18
18.5
19
19.5
output volta ge (volt)
20
20.5
y=mx + c
P
9977 output current with respect to surface temperature
60
tem
perature( T)
50
40
30
20
10
0
0.56
0.57
0.58
0.59
0.6
0.61
0.62
0.63
output curre nt (A)
P ,,77 output power with respect to surface temperature
9
8.9
output pow
er (W
)
8.8
8.7
8.6
8.5
8.4
8.3
8.2
0
10
20
30
40
surfa ce te mpera ture (T)
m = (P2-P1)/ (T2-T1) =-0.01239
Figure (5.5) the result for first experiment
101
50
60
output (voltage, current, power) with respect to increase
of dT
45
voltage
(volt)
40
35
current (A)
scale
30
25
20
dT
15
10
5
power (w)
0
1
6
11
16
21
26
Time (t)
31
36
41
46
power output with respect to increase of dT
45
40
35
scale
30
25
dT
20
power
(w)
15
10
5
0
1
6
11
16
21
26
Time (t)
31
36
41
46
current (A) with respect to time (t)
current
10
current (A)
1
1
6
11
16
21
26
Time (t)
31
102
36
41
46
output oltage with respect to surface temperature
70
tem
perature(T)
60
50
40
30
20
10
0
17
17.5
18
18.5
19
19.5
20
20.5
voltage (volt)
P
9977 output current with respect to surface temperature
70
tem
perature(T)
60
50
40
30
20
10
0
0.99
1
1.01
1.02
1.03
1.04
1.05
1.06
1.07
1.08
current (A)
P ,,77 output power with respect to surface temperature
15.8
15.6
power (W
)
15.4
15.2
15
14.8
14.6
14.4
14.2
0
10
20
30
40
tempe rature (T)
m = (P2-P1)/ (T2-T1) = -0.0348
Figure (5.6) the result for second experiment
103
50
60
70
output (voltage,current, power) with respect to increase
in dT
60
dT( C)
50
Voc(volt)
scale
40
30
Isc(A)
20
10
power(w)
0
1
6
11
16
21
26
Time(t)
31
36
41
46
power output with respect to increase in dT
60
50
dT( C)
30
20
10
power(w)
0
1
6
11
16
21
26
Time (t)
31
36
41
46
current with respect to time
10
current
scale
40
Isc(A)
1
1
6
11
16
21
26
Time(t)
104
31
36
41
46
output voltage with respect to surface temperature
80
70
tem
peratue(T)
60
50
40
30
20
10
0
0
5
10
15
20
25
output volta ge (volt)
P
9977 output current with respect to surface temperature
80
70
tem
perature(T)
60
50
40
30
20
10
0
1.3
1.32
1.34
1.36
1.38
1.4
1.42
1.44
output curre nt (A)
P ,,77 output power with respect to surface temperature
21
output power(W
)
20.5
20
19.5
19
18.5
18
0
10
20
30
40
50
surface te mpera ture (T)
m = (P2-P1)/ (T2-T1) = -0.048
Figure (5.7) the result for third experiment
105
60
70
80
G iradiance with respect to dT
700
50
5
600
3
39
G irra d ia n c e
4
2
500
37
1
0
100
200
300
400
Temp dT
dT G
Figure (5.8) maximum ¨7SHU*LUUDGLDQFH
106
500
600
700
800
Discussion
The figures showed the output of current, voltage, power
and temperature of the cell surface in respect to the
time.
According to the figure (5.5) over the 50 minutes the
voltages showed gradual decreased while. The current
remained stable but the temperature of the cell surface
showed dramatically increased within first 20 minutes,
followed by a slight increase thereafter the power
output showed a slight decline over the 50 minutes from
1st to 50th minutes. This means that an increase in the
cell surface temperature does not have a significant
effect on the power output.
The figure (5.6) and figure (5.7) showed that the result
of the second experiment was approximately similar to
the result of the first experiment, except that the
increasing input irradiation has affected on the power
output.
107
Over the test period the power output fell from 8.7 W to
8.3 W for 1st experiment, 2nd experiment from 15.6 W to
14.3 W and for 3rd experiment from 20.7 W to 18.4 W
which represents more reduction in 3rd experiment.
There is relationship between ¨7WRLUUDGLDQFH*
For Libya G
§
700 W/m2
Therefore max ¨7DW:P2
Module temperature in Libya =
= Ambient air temp + max ¨7 °C
Specify module power output at 700 W/m2 and 95 °C.
Therefore the power output for 85Wp module operating in
Libya.
G § 700 W/m2
T module = 95 °C
PG = (700/1000) * 85 = 59.5 w
M = 0.048 w/°C
Temperature effect = 95 – 25 = 70 °C
Power loss with temperature = 70 * 0.048 = 3.36 w
Then the power for module operating in Libya is 56.1 W
about 66% of rated power.
108
CONCLUSION
A PV module is an electricity generator and requires
additional equipment if it is to provide a useful
service.
A PV is in the midst of benign cycles where increased
sales lead to larger scale production, which leads to
lower costs which leads to increased sales. The targets
for low-cost production can be met almost entirely by
this increasing scale of production, which follows from
increased sales. Technological improvements in the solar
cells are an additional bonus, although much remains to
be done in bringing laboratory-scale performance to
commercial production.
Photovoltaics have the potential to become a major
electricity generation technology in the next few
decades. It will fulfil this potential only if it is
recognised that technical success with cells or modules
109
is a necessary but not sufficient criterion for
commercial success. It is the PV system that provides
the services for which users will pay and these must be
designed and implemented to the same level of quality
and performance as the modules themselves.
The experiments showed that the power output exhibited a
slight decrease as the ¨7YDOXHLQFUHDVHG7KDWPHDQV
there was significant effect of the increasing in the
surface temperature on the power output.
There is relationship between temperature ¨7WR
irradiance G. Therefore in Libya (my country) in which
the air temperature reaches 45 °c in summer would result
in a maximum surface temperature 95 °c . This increase in
temperature together with the lower intensities of
irradiance experienced in practice would result in the
module tested delivering 66% of its rated power when
used in Libya.
110
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