low cost photovoltaic roof tile

ELSEVIER Solar Energy Materials and Solar Cells 47 (1997) 325-337

Solar Energy Materials and Solar Cells

Low cost photovoltaic roof tile

S.R. W e n h a m a'*, S. Bowden a, M. Dickinson a, R. Largent a, N. Shaw a, C.B. Honsberg a, M.A. Green a, P. S m i t h b

"Photovoltaics Special Research Centre, The University of New South Wales, Sydney 2957, Australia b Dussek Campbell Pry. Ltd., Sydney, Australia

Abstract

Static concentrator designs that achieve concentration ratios approaching the theoretical limit have been developed and demonstrated. Concentration ratios in excess of 4 : 1 have been predicted although this reduces to 3.6 : 1 for the practical design used for photovoltaic roof tile applications. A new encapsulant based on " solid white oil" has been evaluated for use in the optical cavity, exhibiting excellent optical properties while simultaneously being low cost. Prototype roof tile efficiencies of approximately 15% (not independently confirmed) are well below the expected 17-18%, due primarily to optical losses associated with the rear reflector and poor rear surface performance for the Photovoltaic devices. Good progress has been made recently with regard to the latter, although further work is required to accommodate solar grade substrates with much shorter diffusion lengths, for the bi-facial cells in this application.

Keywords: Si; Concentrater; Encapsulation; Devices

1. Introduction

In recent years there has been a rapidly growing interest in the use of photovol ta ics in building applications and for residential roof tops. Numerous countries have initiated significant residential photovol ta ic roof top programs with a range of incentives and subsidies offered by respective governments. The interest in this area has been somewhat sparked by falling costs for photovol ta ic devices with the expectat ion by many that it is merely a mat ter of time until photovol ta ics can be used economical ly in many residential roof top applications without subsidy.

* Corresponding author. Tel.: + 61 2 9385 5171; e-mail: [email protected].

0927-0248/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S 0 9 2 7 - 0 2 4 8 ( 9 7 ) 0 0 0 5 9 - 7

326 S.R. Wenham et al./ Solar Energy Materials and Solar Cells 47 (1997) 325 337

The concept of the photovoltaic roof tile is an attractive one for photovoltaics since the displacement costs for new dwellings based on the cost savings through making roof tiles redundant are quite significant. In addition, static concentrators are an attractive option for photovoltaic roof tiles relative to the normal flat plate modules due to the different dimensional constraints, and in particular the increase in thickness for the former.

The benefits of concentrator systems for photovoltaic applications are well known and understood. Higher cell efficiencies (for direct sunlight) are achievable while significant cost savings can be achieved through the use of reduced device area in conjunction with large area low cost optics. High concentration systems, however, are unsuitable for residential roof top applications due to the need for the tracking of sunlight, lack of suitability in small systems, increased complexity (and reduced corresponding reliability), the difficulty in dissipating large amounts of heat from the photovoltaic devices and the inability to collect diffuse sunlight.

Static concentrators have been pursued for over a decade [1], to combine some of the benefits of tracking concentrator technology and flat plate one-sun technology. Static concentrator systems concentrate light onto the solar cells without the need for tracking the sun, therefore eliminating moving parts. While the levels of concentration are low compared to the tracking systems they are sufficient to make significant cost reductions compared to flat plate modules. The low levels of concentration place less demands on the required quality of the optics and alleviates the need for active cooling. However, many of the previous static concentrator designs are too bulky or impractical to fit within the dimensional constraints imposed by an ordinary roofing tile.

This paper discusses the present state of development of an innovative "slimline" static concentrator design [2] that is able to concentrate sunlight onto a solar cell without tracking the sun, while simultaneously fitting within the dimensional constraints of a roof tile. These static concentrator modules behave in a similar way to standard flat plate modules rather than the tracking concentrator designs. Based on a new concept for the light trapping, the static concentrator design can be imple- mented to suit virtually any roof tile shape, although for the purposes of this study, a tile design with flat top surface is used. Prototypes have been fabricated in preparation for proving 1 kWp of generating capability on a demonstration house. Unfortunately, this project has been postponed indefinitely due to major restructuring in the electricity industry in Australia. Fortuitously, this coincided with the establish- ment of a collaborative agreement with a major Japanese Company to commercialise the new photovoltaic roof tiles with static concentrators.

2. Solar insolation and concentration

The limits to the ability to concentrate sunlight are determined by changes in the refractive index and the acceptance angle for incoming light. For instance, to accept light from all angles of incidence and without any change in the refractive index of the medium, it is impossible to concentrate sunlight. With the inclusion of a medium of

S.R. Wenham et al./Solar Energy Materials and Solar Cells 47 (1997) 325-337 327

refractive index n, the achievable concentration ratio is given by the "brightness theorem" as the square of the refractive index. This theoretical limit to the achievable level of a concentration is further increased proportionately to the factor by which the range of acceptance angles is reduced.

In tracking systems, the range of acceptance angles required for high performance, can be made very small (with the only penalty being the loss of diffuse light) with the theoretical limit to achievable concentration levels being very high. High-performance static concentrators, on the other hand, rely on the use of high refractive index transparent mediums in conjunction with relatively small reductions in the acceptance angle to give compromise between retaining the many benefits of flat plate modules and the economic and performance superiority of concentrating systems.

A key issue when designing static concentrators is the availability of suitable low cost, high transmission, good thermal conductivity material with the highest possible refractive index t o suit its use in the optical cavity of the static concentrator. Acrylic is a commonly used material for this purpose [1, 3], although has a poor thermal conductivity and can be difficult to work with in terms of injection moulding and surface metallisation. Unfortunately, few if any materials satisfy the lengthy list of requirements for use in the optical cavities while simultaneously having a refractive index above that of acrylic which is in the vicinity of 1.5. In the present work static concentrator designs have therefore been based on this value. The result of an evaluation of "solid white oil" as an alternative material has been carried out and is reported later in this paper.

The maximum value of concentration for a non-homogenous source is determined by the exact nature of the source, i.e., by not accepting light from the dimmest portions of the sky, the concentration ratio can be increased while still collecting most of the light. To map the inhomogeneities in the incident light, the total solar radiation in the plane of the module averaged over a year is calculated for each azimuth and elevation angle. These calculations result in a yearly average for each region of the sky. To constructively utilise inhomogeneities when designing a static concentrator, the directions of the sky with lowest yearly average solar radiation must be identified. The static concentrator is then designed to sacrifice response to angles from which there is low incident solar radiation, while maintaining near unity collection of light for angles at which there is high incident solar radiation.

The inhomogeneity of the incident solar radiation depends on both the site latitude and climatic conditions, thus requiring the incorporation of measured insolation data rather than relying solely on modelling the solar radiation based on ideal equations. Typical mean yearly (TMY) data gives the intensity of the direct, diffuse and global components of sunlight at each hour of the day for a typical year and is available for most locations. By determining the position of the sun throughout the year [-4] the direct component of the TMY data can be converted to a yearly solar angular distribution plot. The diffuse radiation is assumed to be uniformly distributed across the sky, as more complex models for the diffuse light are not needed since the data is averaged over the year. As the module is tilted, part of the module viewing angle will be ground-reflected light. While the level of ground reflection varies with vegetation a nominal albedo value of 0.2 is used.

328 S.R. Wenham et al. Solar Energ~v Materials' and Solar Cells 47 (1997) 325 337

The solar radiation resource for Sydney as seen by a module at tilt = latitude has been processed into the above form and has been reported by Bowden et al. [5]. By not accepting light from the dimmest portions of the sky an increase can be obtained in the concentration of light on the solar cells.

Using the principles of ray tracing [6], a ray tracing program has been developed at the University of New South Wales specifically to model three-dimensional static concentrators [7]. The program incorporates the basic loss mechanisms of absorption in the optical material and reflection loss. The intensity of the ray is determined by the Fresnel Reflection Formulae at the interfaces of different refractive index materials. Based on Sydney Insolation data and the use of the tilted groove concept for static concentrator design [8], the exclusion of a small amount of light from the dullest portions of the sky (weighted throughout the entire year) has enabled the achievement of a design with predicted concentration ration of 5 : 1 while excluding only approximately 4% of the total incident radiation but with no allowance for reflection or absorption or scattering losses.

This is of interest from an academic viewpoint since the theoretical limit for a static concentrator ( with the same refractive index) is only 4.5 as determined by the brightness theorem when accepting all diffused light, increasing to theoretical maximum of only 5.1 when restricting the angular acceptance to eliminate 4% of the light from the dullest portions of the sky. This demonstrates the effectiveness of the developed geometrical schemes for accepting, trapping and steering light from specific directions while deliberately rejecting light from other directions.

For a location with little or no cloud, it is feasible to design to only accept light from directions from which direct sunlight is received. In this scenario, it is still possible to collect at least half of the diffuse light enabling approximately 95 % of all incident light to still be collected. The increased restriction in acceptance angle, however, results in the theoretical limit to achievable concentration going up to 5n 2 which corresponds to 11.25. No attempt at this stage has been made to develop a static concentrator to exploit this level of inhomogeneity in the light source.

Locations with a high level of diffuse radiation (i.e., a more homogeneous light source) are limited to maximum concentration ratios for static concentrator designs only marginally above 2n 2 due to the necessity to collect light from all possible incident angles. Unfortunately, it is probably impractical to develop static concentrator designs for the specifics of individual climatic conditions therefore necessitating a compromise design with good all round performance for a range of typical conditions.

In this work, the static concentrator design shown in Fig. l, requires bi-facial solar cells to give a theoretical concentration ratio slightly above 4: 1. The inclusion of absorption losses in this optical cavity and rear reflectors, and reflection from the top surface, reduces the predicted concentration ratio to 3.6 : 1. The tilted grooves on the rear surface have angles for the side faces of the rear grooves of 30 ° with the module preferably mounted so that the grooves run in an east-west direction. By taking advantage of the relatively small incident angle range for light in the north-south direction, the rear grooves are able to reflect light to the glass top surface so as to ensure total internal reflection for all direct light irrespective of the angle for c~. This

S.R. Wenham et al./Solar Energy Materials and Solar Cells 47 (1997) 325 337 329

incident sunlight , ~

/ / / total internal reflection glass top surface

/ / ,,' solar cell / / o

tilted grooves on rear surface acrylic with refractive index of 1.5

Fig. l. Static concentrator. Tilted rooves on the rear surface of the module run from the curved section to the edge.

facilitates values for ct as low as 20 °, significantly lower than required in similar designs without the use of the rear surface grooves. For the latter, ~ is required to be sufficiently large to cause total internal reflection at the glass/air interface for all light reflected from the rear surface. The grooves in theory can be made arbitrarily thin provided the dimensions exceed the wavelength of light concerned. Consequently, the grooves need not contribute to the module thickness. A more detailed treatment and explanation of the design including angular sensitivity and overall calculated module response is provided by Bowden et al. I-9, 5].

3. Photovoltaic roof tile fabrication

The following outlines the fabrication process used for a number of prototype roof tiles and as a starting point for the new commercialisation effort.

The roof tile consists of a glass cover sheet for consistency with standard PV module designs. The solar cells are initially laminated to this glass sheet prior to being integrated into the module design as shown in Fig. 2. The rear surface comprises a sheet of plastic that is initially stamped to form the rear surface structure shown prior to being chemically plated with a thin layer of silver. The optical medium is completed by appropriately locating the glass cover sheet and cells and then filling the cavity with liquid pottant, in this case the "solid white oil". The most important properties for the pottant include its optical transmission, thermal conductivity, thermal expansion coefficient, refractive index, density, resistance to moisture penetration and UV stability. Other important issues obviously include its cost and ease of use in the described application.

White oil is a hydrocarbon oil, based on petroleum refined with sulphuric acid. The nature of the refining process makes the white oil very resistant to acid attack. The white oil is solidified through mechanical mixing with an appropriate rubber

330 S.R. Wenham et al./Solar Energy Materials and Solar Cells 47 (1997) 325 337

glass cover sheet

Rigid reflective backing plate

Fig. 2. Module can be manufactured by laminating cells lo the cover plate and using the pottant material as the optical medium to fill the cavity tormed by the reflective backing plate.

compound at elevated temperatures. Evaluation of the material regarding its suitability as a pottant for the optical cavity of the static concentrator, was carried out using acrylic (polymethyl methacrylate or PMMA) as the standard for basis of comparison [10].

Key optical properties include the refractive index and the absorption for wavelengths of light below 1.1 ~tm. Acrylic and solid white oil are virtually identical in both respects with the only significant difference in transmission properties occurring for wavelengths of light longer than those of interest to silicon solar cells. Early samples of solid white oil had superior short wavelength transmission although the addition of UV inhibitors to solve degradation problems during accelerated testing with UV exposure, severely reduced the short wavelength response (below 400 nm). Overall absorption losses in a 6 mm thick sample were negligible, with the integrated absorption losses from 400 to 1100 nm being very similar for solid white oil, acrylic and low iron glass.

UV stability tests for a range of acrylics have returned varied results. Similarly, solid white oil requires appropriate UV stabilisers for satisfactory performance. For these, tests equivalent to eight years exposure in the field, have revealed no noticeable deterioration in transmission properties.

Poor thermal conductivity is the main disadvantage for acrylic as a constituent for static concentrators, being almost an order of magnitude worse than glass. In comparison, solid white oil was determined by the manufacturer to have a thermal conductivity of 1.7 W/re°C, almost a factor of two better than glass. Similarly, whereas the high thermal expansion coefficient for acrylic presents a problem when considering enshrouding the silicon wafer in the material, the same problem does not occur with the solid white oil due to its "rubber-like" properties. Melting point on the other hand is a problem for the solid white oil with early samples melting at an unacceptably low temperature of 65°C. This parameter appears to be controllable based on the type


and amount of rubber constituents although corresponding implications for transmission properties, etc., are yet to be determined.

Important physical properties include the density and resistance to moisture penetration. For the former, solid white oil has a particularly low value of 0.85 kg/1 at room temperature, significantly below that for acrylic and a factor of three lower than glass. This is important when considering use for roofing materials. With regard to moisture penetration, no absolute values are available although submersion tests indicated that solid white oil is marginally more resistant to moisture penetration than acrylic but poorer than glass.

Finally, solid white oil being a thermoplastic is easy to work with, melting and solidifying in a reversible transition at a temperature determined by the rubber additives. It is low cost, expected to retail at about $2/Kg making its contribution to the roof tile costs relatively insignificant. Further accelerated testing however is necessary.

4. Bi-facial cell and photovoltaic roof tile results

When designing static concentrators, the use of bi-facial solar cells facilitates an approximate doubling in the achievable concentration ratios. Of key importance is the achievement of high efficiencies for light entering the rear of the device as well as for the front. This is quite challenging for a conventional single junction device using material quality and dimensions similar to those currently being used commercially. There has yet to be a suitable bi-facial cell sold commercially, and although it has been previously argued that the buried contact solar cell (BCSC) is well suited for this application [7], there is still no commercially available product to satisfy this requirement.

The three essential requirements for a bi-facial solar cell are low shading for the front and rear surfaces, the achievement of diffusion lengths substantially larger than the furthest distances required for the charge to diffuse to the junction, and the achievement of well passivated front and rear surfaces. Most commercially available solar cells fail to satisfy all three of these requirements including the commercially available BCSC from BP Solar. Two appropriate approaches for satisfying the requirements include the use of thinned substrates with well passivated front and rear surfaces, while the other is to retain a thicker substrate but make use of collecting junctions on both the front and rear to maximise the collection probabilities for carriers generated at both surfaces. The disadvantage of the latter approach is the increased complexity of processing linked to the requirement for contacting at leat three separate layers. Fig. 3 shows a schematic of this structure when fabricated using a BCSC approach. This device structure has been successfully developed and demonstrated at the University of New South Wales although with degraded performance resulting from shunting between the n + and p + intersecting layers at the rear surface.

Recent work has focused on improving the rear surface passivation to facilitate application to thinner substrates without performance loss. The approach has been to

332 S.R. Wenham et al./Solar Energy Materials and Solar Cells 47 (1997) 325 337

~ x i d e

oxide

Fig. 3. Bifacial buried contact solar cell with front and rear collecting junctions.

O" e

n o.

oxiae

Fig. 4. Bifacial BCSC with front collecting junction and rear floating junction for improved surface passivation.

use a floating junction at the rear, formed by lightly diffusing the surfaces with phosphorus prior to the application of the rear passivating dielectric and groove formation. As part of this work, test structures have been formed in which the floating junction has been used in conjunction with photolithographically defined metal contacts to achieve record open circuit voltages in excess of 720 mV. These voltages have been independently confirmed by Sandia National Laboratories [11], and although these test structures are not practical devices, they clearly demonstrate the suitability of using floating phosphorus diffused junctions at the rear surface to give extremely good effective rear surface passivation. When applying the rear floating junction approach to the bi-facial buried contact solar cell as shown in Fig. 4, the


" O [ L U . [ I L.

+8.0e-05 +2.1e-04 +5.7e-04

i

+1.5e-03

j- Afcm,,2

+4.0e-03

Fig. 5. 2-D modelling of device at short circuit with localised shunting of the rear floating junction that transforms the rear surface into one with effectively infinite rear surface recombination velocity. Arrows show electron current.

same shunting problem resulted between adjacent n + and p + regions at the rear as was described for the structure of Fig. 3. This shunting has been shown to be exacerbated by damage in the groove region formed by the laser scribing process. Recent work has shown that this problem can be solved by either etching away the damaged region in the grooves, or else reducing the doping concentration in the rear n + layer to increase the value for the shunt while simultaneously providing lateral isolation by restricting lateral flow of electrons in the rear n-type layer. The former approach is preferable for the device structure of Fig. 3 since good lateral conductivity in the rear n-type layer is necessary for conducting current to the rear n-type metal contact. The latter approach is preferable for a single collecting junction device for which the best possible rear surface passivation is required.

Fig. 5 shows the effect of localised shunting of the rear floating junction when applied to a bi-facial BCSC. Ideally, under short circuit conditions with a well passivated rear, surface, the minority carrier concentrations across the base continual- ly increase towards the rear surface, providing the concentration gradient necessary for minority carrier diffusion towards the front junction. If the shunt of the rear floating junction is bad enough, carriers from the lower half of the substrate travel towards the rear floating junction, where they are collected and conducted laterally before flowing through the shunted region. For front illumination as shown approximately 10% of the photogenerated current can be lost in this way, with the rear surface effectively having an infinite rear surface recombination velocity (irrespective of how well the actual surface is passivated by the overlying silicon dioxide layer).

334 S.R. Wenham et al./Solar Ener~,~v Materials and Solar Cells 47 (1997) 325 337

Table I Biracial BCSC's with rear floating junction with high sheet resistivity (2000 ~/square) for the n-type layer. Cells do not have textured or AR coated surface

Vow(my) J ~ ( m A / c m 2) FF (%) EFF

662 ~ 32" 80.7 a 17.1 ~ 668 32 81.9 17.7 670 32 81.5 17.6

"Measurements by Sandia National Laboratories.

Even under these conditions of bad shunting of the rear floating junction higher injection levels can enable sufficient numbers of carriers to diffuse from the front to the rear of the device to bias the shunt to an appropriate voltage that effectively allows the rear surface to be transformed into one with a low surface recombination velocity. Earlier generations of these devices suffered from this phenomenon which manifested itself as degraded fill factors due to the injection level dependent rear surface recombination velocity. High open circuit voltage could nevertheless be achieved due to the good effective rear surface passivation at these injection levels.

In recent work, the use of lightly diffused rear n-type layers with sheet resistivities in excess of 2000 ~ per square, has enable the shunted regions to be laterally isolated. Even under short circuit conditions with only front illumination, sufficient carriers are able to be collected by the rear floating junction to provide lateral isolation through voltage drops along the n-type layer. In these devices, the rear surface has already transformed into one with a very low effective surface recombination velocity even at the relatively low injection level corresponding to the maximum power point. Conse- quently, high fill factors are achieved while simultaneously retaining the high open circuit voltages. Examples of such devices are shown in Table 1, with the important parameters being the open circuit voltage and the fill factor in terms of indicating the ability to passivate the rear surface and eliminate the injection level dependent recombination mechanisms associated with shunting of the rear floating junction. The short circuit current densities are low as expected for devices without texturing or anti-reflection coatings.

The results in Table 1 correspond to float-zone substrates with minority carrier diffusion lengths substantially greater than the substrate thickness. These substrates are inappropriate for mass produced commercial products, requiring the developments to be adapted for use with low cost solar grade Czochralski substrates. The key difference for the later is that solar grade substrates are unlikely to have minority carrier diffusion lengths at the end of processing greater than about 250/am. To achieve good rear surface performance from the structure of Fig. 4, substrate thick- nesses must be reduced to approximately 150 ~m. This will clearly introduce some difficulties regarding device processing although the grooves need only be approximately 10-15 ~m deep for good performance provided interconnect bus-bars (metal strips) are used like on most conventional commercial solar cells. Strength tests on


wafers have shown that these types of grooves, although initially weakening the wafers immediately following scribing, have little or no impact on their strength following cleaning of the grooves to alleviate stress, with crystal planes determining fracture lines.

Based on the results for the cells of Table 1, the equivalent rear surface design when applied to a Czochralski substrate of only 150 ~tm thickness will actually lead to a substantial increase in performance even for light entering the front surface. The trend in photovoltaics is towards thinner substrates (regardless of the requirements for bi-facial solar cells) due to economic benefits. All present technologies need to address the issue of rear surface passivation since substrate thinning without it will lead to substantial loss in both current and voltage.

The final issue regarding the use of bi-facial solar cells in these static concentrator designs relates to resistive losses. Due to operation under concentration, resistive losses become significantly more important. One of the major strengths of BCSC approach is the ability to achieve extremely low resistive losses, with virtually no changes necessary in cell design to accommodate concentration ratios at least as high as 4.

The structure of Fig. 4 when optimised for the optical losses appears inherently capable of achieving efficiencies on float zone substrates of about 22% under standard test conditions increasing to about 23 % under 4 suns concentration. In comparison, it is anticipated that with solar grade Czochralski substrates these efficiencies will fall to about 20% under standard test conditions and about 21% efficiency for illumination under 4 suns concentration. Prototype photovoltaic roof tiles also have lower than anticipated performance due in part to the lower performance of the cells, but also due to larger than anticipated losses in the rear metal reflector. These latter losses are contributed to partly by absorption in the rear metal, but primarily due to scattering of the light by the rear reflector due to imperfections in the die used for stamping the rear surface pattern in to the rear plastic sheet. The net result has been that prototype module efficiencies have peaked at only about 15% (not independently confirmed) rather than the anticipated values of 17-18%. These issues are believed to be solvable as part of the commercialisation effort now taking place in conjunction with a major Japanese manufacturer.

5. Conclusions

The use of static concentrators in photovoltaic modules offer a compromise between high concentration systems that require tracking and one-sun flat plate modules. It appears that many of the economic benefits and increased performance offered by concentrating systems still apply to static concentrator photovoltaic modules, but without necessitating the complexities, reduced reliability and loss of diffuse light collection, that normally accompanies concentrator systems. In particular, the innovative "slim line" design presented in this paper is particularly well suited for incorporation into photovoltaic roof tiles due to the compatibility with the dimensional constraints imposed.

336 S.R. Wenham et al./Solar Energy Materials and Solar Cells" 47 (1997) 325-337

A new encapsulant based on solid white oil/rubber has been evaluated for use as the optical transmitting medium for static concentrators. It appears to satisfy all the important requirements such as optical transmission properties, low cost, thermal conductivity, refractive index and ease of use. A new collaborative program jointly between the University of New South Wales and a major Japanese manufacturer has been initiated to commercialise these photovoltaic roof tiles.

The previous difficulties experienced in achieving high conversion efficiencies for light entering both front and rear surfaces of bi-facial cells have been largely solved. For suitability in the described application, bi-facial cells must have high collection probabilities for carriers generated throughout the entire device, and particularly at the front and rear surfaces where most light is absorbed. A double junction device is particularly well suited to this profile of generation throughout the substrate, although recent developments with the use of floating junctions for passivating surfaces, appears to open up the opportunity for using conventional single junction devices of reduced thickness to still achieve near unity collection probabilities for carriers generated throughout the device. Previous problems manifesting themselves as degraded fill factors in floating junction devices, have been identified as being contributed to by localised shunting of the floating junction. A number of remedies have been identified, with the preferred option being to use a much lighter phosphorus diffusion of the rear surface to simultaneously improve the value of the shunt resistor across the floating junction while facilitating lateral isolation of localised shunting regions by restricting the lateral flow of majority carriers in the rear n-type layer.

Prototype photovoltaic roof tiles have demonstrated unconfirmed efficiencies in the vicinity of 15% although still well below the performance levels ultimately anticipated approaching 20%.

Acknowledgements

This work has been directly supported by the Australian Research Council and the Energy Research and Development Council. The many valuable contributions from members of the Photovoltaics Special Research Centre at the University of New South Wales are acknowledged.

The Photovoltaics Special Research Centre is supported by the Australian Re- search Council and Pacific Power.

References

Il l J. Parada, J.C. Minano, J.L. Silva, Construction and measurement of PV module with static concentration, 10th European Photovoltaic Solar Energy Conf., 1991, p. 975.

[2] S.R. Wenham, M.A. Green, Improved optical design for photovoltaic cell encapsulation, Australian Patent 652291 (10th January, ! 993), International (PCT) Application PCT/AU92/00536 (7th October, 1992).


I-3] S.R. Wenham, S. Bowden, M. Dickinson, R. Largent, D. Jordan, C.B. Honsberg, M.A. Green, Prototype photovoltaic roof tiles, 25th IEEE Photovoltaic Specialists Conf., Washington, DC, May 1996.

1-4] S.R. Wenham, M.A. Green, M. Watt, Applied Photovoltaics, Bridge Printery, Sydney, 1994. 1-5] S. Bowden, S.R. Wenham, M.A. Green, Prog. Photovoltaics 3 (6) (1995). 1-6] A.S. Glassner, Academic Press, 1989. [7] S. Bowden, A high efficiency photovoltaic Rooftile, PhD. Thesis, The University of NSW, April 1996. 1-8] S.R. Wenham, P. Campbell, S. Bowden, M.A. Green, Improved optical design for photovoltaic cells

and modules, Conf. Rec. IEEE Photovoltaic Specialists Conf., Las Vegas, October 1991, p. 105. 1-9] S. Bowden, S.R. Wenham, M.A. Green, 12th European Photovoltaic Solar Energy Conf., April 1994.

1-10] N. Shaw, The development of a high efficiency PV roof tile module, Bachelor of Engineering Undergraduate Thesis, The University of NSW, June 1996.

1-11] S.R. Wenham, S.J. Robinson, X. Dai, J. Zhao, A. Wang, Y.H. Tang, A. Ebong, C.B. Honsberg, M.A. Green, Rear surface effects in High efficiency silicon solar cells, Conf. Rec. 24th IEEE Photovoltaic Specialists Conf., Hawaii, December 1994.

low cost photovoltaic roof tile

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