Improving the Efficiency of Linear Concentrator Receiver Systems V. A. Everett1, A. W. Blakers1, J.Cotsell1, J. Harvey1, R. van Scheppingen1, D. Walters1 1
Centre for Sustainable Energy Systems,
The Australian National University, Canberra, ACT 0200
[email protected] Ph: (02) 6125 8872 ABSTRACT The key objective of the APP Project is to resolve the remaining barriers to the widespread deployment of photovoltaic trough concentrator systems, which include: (1) The commercial difficulty of supply of high-performance, low cost concentrator solar cells suitable for parabolic trough concentrator systems; (2) The technical problem relating to performance issues caused by moving shadows and longitudinal flux irregularities on series-connected strings of solar cells; and (3) The integration and qualification of the system. The completed Project will also provide a solution to the technical, scientific, and engineering impediments associated with the deployment of photovoltaic linear concentrator systems. At a technical level, commercially available one-sun solar cell suitable for modification have been identified, and methods are being developed for reducing performance losses associated with dicing and handling. Modified one-sun solar cells operating at up to 10X concentration achieve better than 20% efficiency, with efficiencies for small cells dropping to around 19% at 30X. Work on improving the mounting techniques and methods for establishing modified electrical connections, and improving heat-sinking methods and materials has produced interesting results. A commercial partner in the US is developing a low-cost Fresnel mirror and tracking system for the improved Micro-ConcenTrator (MCT) receivers, as well as the structural enclosure for the MCT systems. A prototype hybrid PV-thermal MCT system, approximately 3.2 metres long by 1.2 metres wide and 0.3 metres high has been constructed. Several variations of this base system are easily realisable and optimisable for localised conditions. These variations extend to PV only passive-cooled applications, as well as thermal-only sources for applications ranging from domestic hot water, low-grade industrial heat, and solar air conditioning.
Keywords ۛ CPV, PV-thermal hybrid, Solar Photovoltaic, Solar Thermal
INTRODUCTION This Research Project is jointly funded by the Asia-Pacific Partnership on Clean Development and Climate, The Australian National University, and the ARC via a Linkage Project. The key objective of the APP Project is to resolve the remaining barriers to the widespread deployment of photovoltaic trough concentrator systems. The principal remaining barriers, listed above, include technical, design, and resource issues. Solar09, the 47th ANZSES Annual Conference 29 September-2 October 2009, Townsville, Queensland, Australia
V. A. Everett, A. W. Blakers, J.Cotsell, J. Harvey, R. van Scheppingen, D. Walter
The principal resource issue, the lack of suitable commercially-available concentrator cells, is being addressed by the modification of commercially-available one-sun solar cells. These cells are being adapted for use in the 20X to 30X concentration range. Significant reduction in cell cost per kWp output can be realised through low-cost modification of mass-produced silicon solar cells rather than higher concentration systems utilising expensive III-V cells. Operating in the low-to-medium concentration range reduces silicon material requirements by more than 95%, and also relaxes the requirements of mirrors, optics, and tracking systems necessary in high-concentration systems. This approach will also produce significant improvements in the expected Levelised Cost of Electricity (LCOE) produced, which is expected to be in the range of US$ 6¢ to 8¢ per kWhr. This will be partially realised by the use of 20X to 30X concentration with silicon PV cells rather than expensive III-V materials. The principal design issues relate to a commercially-acceptable concentrator concept with broad market penetration. For large-scale domestic and urban market penetration, a conventional ground- or roof-mounted trough system is unacceptable because of excessive support-structure weight loading, high wind loading – mainly from large mirror profiles, and aesthetic acceptance problems. In combination with performance considerations, these factors have driven the design concept towards a microconcentrator PV-thermal hybrid system. Enclosing the entire module, including the single-axis tracking mechanism and mirrors, under glass enables the use of very lightweight, inexpensive materials for first-surface reflectors, and inexpensive and lightweight tracking components that would otherwise require sealing or protection from the environment. Optimisation of the Fresnel mirror arrangement, sealed within the enclosure, will result in increased kWhrs produced per kWp installed over an extended system lifetime, along with minimising routine maintenance. Improved mean time between failure (MTBF) resulting from totally sealed and confined mirrors, tracking, and PV-T receivers, internal to the MCT system, mean fewer operating interventions and significantly lower maintenance costs compared with competitive systems. At a technical level, the three main considerations under development are optimising receiver performance to cope with variable flux distribution and moving partial shading, developing an economically practical by-pass diode arrangement, and establishing a robust and reliable thermal management solution. Improved system performance due to higher module voltage, with module output voltages in the range 350V to 480V, and operating over lower current ranges, typically around 1A, will result in lower parasitic resistance wiring-related losses and more efficient integration with low-cost inverters. In addition, moderate concentrations in the range 20X to 30X also results in considerably simpler thermal management. Thermal management is further simplified through the use of small-area cells, which limit the build up of stress caused by the differential expansion of various materials in the receiver. Moreover, with active cooling arrangements, the PV cells operate at a lower temperature and perform better than traditional flat-plate modules when operating in hot ambient climates such as the South West United States, Northern Australia, and Southern Asia. THE CASE FOR MICRO- CONCENTRATORS Energy payback time (EPBT) is the time required for a concentrator photovoltaic (CPV) system to deliver the amount of energy that was required for its manufacture, transport and installation. It is a powerful metric that captures both the upstream costs and the use-phase capabilities of a photovoltaic concentrator system (Minassians et al., 2006). Whole-of-system energy pay-back times between 0.7 and 1.3 years are typical of Solar09, the 47th ANZSES Annual Conference 29 September-2 October 2009, Townsville, Queensland, Australia
2
V. A. Everett, A. W. Blakers, J.Cotsell, J. Harvey, R. van Scheppingen, D. Walter
efficient, mid-range concentrator systems. Additionally, CPV systems use much less silicon per MW output capability than conventional systems, replacing expensive silicon solar cells with cheap mirrors and low cost tracking. PV-thermal hybrid systems EPBT can be significantly reduced because over-all system efficiencies can easily exceed 60%. The ANU micro-concentrator (MCT) concept can provide all the competitive advantages of CHAPS-style systems in a greatly expanded domestic and urban market by utilizing low-cost ultra light-weight Fresnel mirrors and small silicon cells to produce a narrow hybrid receiver, all securely confined in a low-profile, light-weight, fully-enclosed PV-thermal modular system. Along with EPBT, a key metric for assessing CPV performance is the cost per unit power, $/W. More than two-thirds of a conventional PV module manufacturing cost is related to the silicon material and cell processing. Concentrator technology based on medium concentration using silicon cells can provide a significant reduction in this cost, often more than 95%, but this cost reduction comes at the price of mechanical tracking. A better metric of the true cost of electricity production is to define the cost in terms of $/kWhr over the system lifetime. For concentrator systems, this will involve an accurate assessment of the quality and magnitude of the solar resource in a particular location, since concentrators cannot capture diffuse radiation. Over 40% of the world’s energy consumption is accounted for by building operations (International Energy Agency, 2008). The requirements for heating, ventilation, and air-conditioning (HVAC) are by far the major contributors to the energy budget of a building. More and more companies now understand the need for holistically addressing the energy needs of the built environment. The ideal solution is distributed generation, which reduces the load on the grid introduced by centralized generation, and the provision of thermal energy for heating and cooling at or on the building itself. A modular MCT hybrid PV-thermal system, with fully autonomous, internal single-axis tracking packaged in a low-profile, lightweight, and environmentally sealed housing with no external moving parts, mounted with standard racking hardware, requiring little or no cleaning, and which is designed for unattended operation provides this solution. The MCT module is a full combined heat and power (CHP) solution, which will offer greater than 70 percent combined power and heat capture of incoming direct solar radiation. Each MCT module can simultaneously produce 2kW of solar thermal energy and 500 Watts (peak) of electrical power from mono-crystalline silicon receivers operating in the range of 20 to 30 suns concentration. Solar concentration is provided by single-axis tracking, high performance linear Fresnel optics and best-of-class 97% front-surface reflectors. This will supply a least-cost CHP solution capable of meeting the power, process heat, and air-conditioning needs of large building owners as well as domestic residences. By applying well-known, stable, and highly reliable mono-crystalline silicon-based PV technology in combination with modest solar concentration, the MCT module has a very low cost $/kWh. In addition, the MCT module’s aperture efficiency is better than thin-film and, with internal single-axis Fresnel tracking, it has a 40 percent higher capacity factor than conventional fixed-mount flat-plate modules. The use of “one sun” commercially-available solar cells in a 20-sun application provides an immediate and substantial cost advantage by dramatically lowering silicon consumption and manufacturing costs. Moreover, application of these innovations enables greater manufacturing capacity while maintaining all the quality advantages of using silicon PV Solar09, the 47th ANZSES Annual Conference 29 September-2 October 2009, Townsville, Queensland, Australia
3
V. A. Everett, A. W. Blakers, J.Cotsell, J. Harvey, R. van Scheppingen, D. Walter
cells. Through the use of concentration and low materials usage, it is anticipated that the MCT system can achieve a cost target substantially below $2/watt. Because the MCT system also provides thermal energy in addition to its electrical output, the system can be used to meet a building’s reticulated hot water needs or to provide industries such as food and beverage processing, textile and paper manufacturing, and general process heat needs. The MCT module has combined rooftop efficiency far higher than PV or traditional solar-thermal systems individually. A ready market exists for systems able to provide both power and heat from a unit that is easily integrated on a building rooftop. As well as a comparatively low technology risk, another key advantage of the MCT system from a marketing perspective is the familiar form-factor of the module. Lowprofile, unobtrusive, and aesthetically-designed systems will achieve high acceptance levels and rapid market penetration. Multi-billion dollar market opportunities exist across food processing, low-grade heat industrial processing, the hospitality industry, and the domestic space. Tracking roof-top systems already exist, but without exception these competitor systems are large, complex, and cumbersome with externally moving parts which effectively preclude unattended operation. The present MCT modular system is designed to be highly manufacturable within existing factories using well-established technologies. The design lends itself to the use of a “contract manufacturing” approach, in which sub-assemblies are supplied by thirdparty vendors, with final assembly being conducted in the region or country of installation. This approach ensures quality is maintained, while significantly reducing transport costs. The use of contract manufacturing also minimizes initial capital requirements, and also enables flexibility and rapid leveraging of technological advances without requiring extensive manufacturing scale-up. RECEIVER DEVELOPMENT The particular constraints within the receiver development area relate to the characteristics of the selected solar cells. The internal cell design cannot be altered, so performance-related characteristics such as internal series resistance will limit the ultimate concentration ratio. The electrical features, such as electrodes, will determine the circuit topology and connection methods suitable for cell electrical interconnection. The granularity of the intrinsic cell structure and design will determine the minimum dimensions for “unit cells” and hence the dimensions of functional MCT cells, which will be integral multiples of these unit cells. Small area laboratory solar cells usually suffer significant efficiency loss after being detached from their host wafer due to recombination at the cut face (Guo et al., 2007) caused by disruption of the crystal lattice and an exposed, un-passivated surface. The damaged edge recombination rate is further increased when the pn-junction extends to the cell edge (McIntosh and Honsberg, 2000). For small area cells, particularly cells with a highly rectangular aspect ratio, the efficiency loss is significant because of the high cut-edge perimeter length to cell surface ratio. The edge-loss mechanism is manifested by reduced fill factor (FF) due to a high dark saturation current of ideality factor 2; while the short circuit current (Jsc) is lowered due to the loss of light-generated carriers near the cut surface (McIntosh, 2001). When cutting commercially available one sun solar cells into small concentrator cells, the above considerations are extremely important. If the cells were to be produced from scratch, many of the optimisation procedures could be included in the cell design and Solar09, the 47th ANZSES Annual Conference 29 September-2 October 2009, Townsville, Queensland, Australia
4
V. A. Everett, A. W. Blakers, J.Cotsell, J. Harvey, R. van Scheppingen, D. Walter
the cell fabrication process. However, this is not possible, so steps need to be taken to reduce the performance losses caused by the high cut-to-surface-area ratio and the series resistance losses caused by high cell currents. Cell Development The development of concentrator cells from commercially-available one-sun cells must proceed within well-understood boundaries imposed by the design and performance characteristics of the cells being modified in order to avoid un-necessarily constraining the extent of optimization that is possible with the modified cells. In particular, the major constraining considerations include the cut locations, the cutting methods, and the cut cell format. When selecting cutting locations within the cell, the first constraint is the electrode position. In order to maximize the voltage-building capability, the cells should be as narrow as practical in the direction of the receiver length. However, as the width of the cell decreases, the ratio of perimeter to surface area increases rapidly, reducing cell efficiency due to decreases in FF and Jsc. In order to minimize the effect of lateral flux non-uniformity, the cells must span the receiver width. This imposes strict requirements on longitudinal flux uniformity. Therefore, the cell format is constrained in both directions. Eff for Series 13, single pitch samples 18.00
17.00
Efficiency (%)
16.00
15.00
1311 1331 1351 1371
14.00
13.00
1321 1341 1361 1381
12.00
11.00 0.00
5.00
10.00
15.00 Concentration (Suns)
20.00
25.00
30.00
Figure 1. Cut cell efficiency as a function concentration for various cut locations. The plots in Figure 1 show cut cell efficiency as a function of concentration for eight different cut locations, relative to the cell electrodes. The eight selected cut locations represent the major possibilities for cut categories. Further refinements of cut locations are possible, but they do not appreciably affect the cell performance. The plot legend, in the form XXYZ can be interpreted by XX being the experiment series (in this case 13), with Y identifying the cut location (ranging from cuts 1 to 8), and Z identifying the cell pitch, or number of integral cell units in the test cell, in this case a single pitch. Clearly, there are three dominant features presented in this plot. Cut types 4, 1, and 6 show the worst reduction in efficiency, with initial rapid declines decreasing in rate as series resistance effects dominate at higher illumination intensities. Cut types 5 and Solar09, the 47th ANZSES Annual Conference 29 September-2 October 2009, Townsville, Queensland, Australia
5
V. A. Everett, A. W. Blakers, J.Cotsell, J. Harvey, R. van Scheppingen, D. Walter
eight show rapid increases in efficiency, as FF holds up and Voc increases with illumination intensity. The efficiency increases until series resistance effects dominate in the 10 to 15 sun rage whereupon the efficiency decreases almost linearly with increasing intensity due to series resistance. The efficiency of a solar cell incorporates the current and the voltage characteristics, as well as the relationship between the two as described by the fill factor. For this reason, it is instructive to examine FF as a function of cut location, particularly in relation to the effect on FF introduced by cutting through the pn-junction. Fill Factor for Series 13, single pitch samples
80.00
75.00
Fill Factor (%)
70.00
1311 1331 1351 1371
65.00
60.00
1321 1341 1361 1381
55.00
50.00
45.00 0.00
5.00
10.00
15.00 20.00 Concentration (Suns)
25.00
30.00
Figure 2. Cut cell Fill Factor as a function concentration for various cut locations. The plots in Figure 2 show cut cell Fill Factor as a function of concentration for eight different cut locations, relative to the cell electrodes. There are two dominant features presented in this plot. Cut types of all but 5 and 8 show initial rapid declines in Fill Factor as illumination intensity increases, while cut types 5 and 8 show the FF decreasing linearly due to series resistance associated with increases in the illumination intensity. The edge-loss mechanism manifested by reduced FF due to a high dark saturation current of ideality factor 2 is clearly greater where the cut surface traverses the pn-junction. Interestingly, this effect is significantly reduced as cell pitch increases due to the effect of the cut surface, regardless of its actual location but still dependent on cut type, playing an increasingly subsidiary role as the cell pitch increases. For example, Figure 3 shows how FF as a function of illumination intensity improves as cut cell pitch increases, for any given cut type. As the number of pitches in the cut cell increases; that is, the width of the cell increases by integral functional units, the cut perimeter to surface area ratio decreases. This results in the edge effects, whether due to carrier recombination, or dark saturation current, or ideality factor, having an increasingly smaller influence on the cell performance. The short circuit current of an ideal cell is, to a first approximation, linear with illumination intensity. However, for small area cells bounded by an un-passivated cut surface, the short circuit current (Jsc) is lowered due to the loss of light-generated Solar09, the 47th ANZSES Annual Conference 29 September-2 October 2009, Townsville, Queensland, Australia
6
V. A. Everett, A. W. Blakers, J.Cotsell, J. Harvey, R. van Scheppingen, D. Walter
carriers near the cut surface. The degree to which this affects cell performance depends on the cell design, the location of the cuts, particularly in relation to the pn-junction, and the perimeter length to cell surface area ratio. Fill Factor for Series 13, cut 1 samples
80.00
1311 1312 1313 1314
75.00
Fill Factor (%)
70.00
65.00
60.00
55.00
50.00
45.00 0.00
5.00
10.00
15.00 20.00 Concentration (Suns)
25.00
30.00
Figure 3. Cut cell FF as a function concentration for cell pitch 1 to 4; cut type 1. Figure 4 illustrates how the different cut locations affect cell performance on single pitch, dicing saw-cut cells. Since these sample cells were prepared from a single largearea solar cell using identical cutting and interconnection methods, the variation in performance is a single function of cut location. Isc for Series 13, single pitch samples
0.00 0.00
5.00
10.00
15.00
20.00
25.00
30.00
Isc (A)
-0.50
-1.00
-1.50
1311 1321 1331 1341 1351 1361 1371 1381 Concentration (Suns)
Figure 4. Cut cell short circuit current as a function concentration and cut locations. Solar09, the 47th ANZSES Annual Conference 29 September-2 October 2009, Townsville, Queensland, Australia
7
V. A. Everett, A. W. Blakers, J.Cotsell, J. Harvey, R. van Scheppingen, D. Walter
21.00
20.00
1381 Efficiency (%)
19.00
1382 1383
18.00
1384 17.00
16.00
15.00 0.00
5.00
10.00
15.00
20.00
25.00
30.00
Concentration (Suns)
Figure 5. Cut cell short circuit current as a function concentration and cut locations. By comparing cell efficiency alone, it is possible to bundle together all of the cell cut methods, cut locations, and interconnection process effects, effectively optimising for all these factors. The results shown in Figure 5, for cut location #8 and cell pitches 1 through 4, illustrate the combined effects of all these considerations. This indicates there is little to be gained from increasing the cell pitch beyond three, which corresponds to a cell width of approximately 6.3 mm. Doing so only increases the current and reduces the voltage-building capability of the receiver. The headline result of this work is that commercially available one-sun solar cells can be modified to operate at around 30 suns with small-area cut cell efficiency around 19%. The unit cost, in terms of projected $/kWhr of these small-area modified cells is far lower than that of small-scale production of purpose-designed concentrator cells. Thermal management The single most common failure point in concentrator systems is in the thermal management area. Receiver design and operation places significant demands on thermal transfer materials performance and durability. Bond-line stress, adhesion, differential expansion, and high thermal conductivity are difficult problems to resolve individually and in isolation; and it is even more challenging to provide an integrated solution. A large group of materials is under test to determine material properties such as adhesion and thermal conductivity characteristics. These materials are also under test within prototype receivers operating at 30 suns. A selection of these candidate materials will undergo accelerated lifetime testing to determine durability and compatibility with constituent receiver materials. Interim results for a selection of materials exposed to 100 ºC are provided in Figure 6. The thermal conductivity reported here is for the “thermal system” rather than the intrinsic thermal conductivity of the interface material. Solar09, the 47th ANZSES Annual Conference 29 September-2 October 2009, Townsville, Queensland, Australia
8
V. A. Everett, A. W. Blakers, J.Cotsell, J. Harvey, R. van Scheppingen, D. Walter
Lifetime Thermal conductivity 2.3
Thermal conductivity (W/mK)
2.1 1.9 1.7 1.5 1.3 1.1 0.9 0.7 0.5 0
50
100
Days
150
200
250
Figure 6: Thermal conductivity of bonded systems using four adhesive materials. The thermal contact resistance of the interface between receiver materials is an often over-looked, but potentially important aspect of receiver thermal design. For thin layers of material, the thermal contact resistance can exceed, sometimes to a significant extent, the thermal resistance of the film. Thermal contact resistance is a function both of the materials and the physical and mechanical properties of the two surfaces. Where filled or composite materials perform the dual roles of adhesion and thermal transfer, this aspect becomes critically important. Experimental work is under way to determine thermal contact resistance with the aim of minimising the overall system thermal resistance. SYSTEM DEVELOPMENT The balance of the MCT system is being developed by an industry partner in collaboration with ANU. The industry partner has significant experience in the solar thermal area, and in particular, experience with the development and installation of Fresnel mirror-based solar thermal facilities. Mirrors and Tracking A super light-weight Fresnel mirror system is under development. The mirrors and tracking designs take full advantage of the protection of a fully-sealed enclosure, allowing light-weight mirrors that do not need to withstand wind loading, highperformance front-surface reflectors that are not exposed to the weather and do not require cleaning, and fully balanced, lightweight moving parts that allow very low power accurate tracking.
Solar09, the 47th ANZSES Annual Conference 29 September-2 October 2009, Townsville, Queensland, Australia
9
V. A. Everett, A. W. Blakers, J.Cotsell, J. Harvey, R. van Scheppingen, D. Walter
The Fresnel mirror concept allows several innovative approaches to resolving problems with mirror profile, focal line width and transverse flux distribution, and tracking accuracy requirements. For example, a parabolic cross-section is more difficult to manufacture than a cylindrical section, but the latter suffers from greater aberration.
Figure 7. Prototype single receiver Fresnel mirror array. The tracking accuracy and focal line width of edge mirrors is more demanding than central mirrors. However, careful management of the optical and tracking characteristics of individual mirrors allows a simplified, optimised solution that actually harnesses some of the difficulties to a performance and management advantage. Further development of the mirror and tracking system is in progress with prototype systems planned to go on-sun in the near future.
Figure 8. Engineering rendered image of a prototype micro-concentrator system Solar09, the 47th ANZSES Annual Conference 29 September-2 October 2009, Townsville, Queensland, Australia
10
V. A. Everett, A. W. Blakers, J.Cotsell, J. Harvey, R. van Scheppingen, D. Walter
System Enclosure A prototype micro-concentrator system is shown in Figure 8. The system is approximately 3.2 metres long by 1.2 metres wide and 0.3 metres high. This is a hybrid PV-thermal system, but several variations are possible, easily optimised for localised conditions, as well as thermal-only sources for applications ranging from domestic hot water, low-grade industrial heat, and solar air conditioning. CONCLUSION The MCT concept described in this paper has exciting commercial prospects. The development of a Commercialisation Plan and technology transfer to other partner countries for use in demonstration systems is being planned. Low cost modification of commercially available conventional solar cells has been demonstrated, with headline efficiencies of small area cells, around 1 cm2, in the 19% efficiency range. Thermal management is a critical aspect of concentrator systems. A large range of materials and assembly processes are being investigated for performance and durability. The balance of systems concepts have been demonstrated and optimisation development is under way. REFERENCES International Energy Agency. IEA Joint Project: “Towards Net Zero Energy Solar Buildings.” 2008. K. R. McIntosh, C. B. Honsberg, “The Influence of Edge Recombination on a Solar Cell’s J-V Curve,”, Proceedings of the 16th European Photovoltaic Solar Energy Conference, Glasgow, 2000. K. R. McIntosh, PhD Thesis, Centre for Photovoltaic Engineering, The University of New South Wales, Sydney, Australia, 2001. Jiun-Hua Guo, Jeffrey E. Cotter, Keith R. McIntosh, Kate Fisher, Florence W. Chen, Anahita Karpour, “Edge Passivation for Small Area, High Efficiency Solar Cells”, 22nd European Photovoltaic Solar Energy Conference, 3-7 September 2007, Milan, Italy. Artin Der Minassians, Rouin Farshchi, Jimmy Nelson, Corinne Reich-Weiser, Teresa Zhang, “Energy Payback Time of a SolFocus Gen1 Concentrator PV System”, Modern Technologies in the Context of a Growing Renewable Energy Market, Final Project Report, MSE-ER C226 - Photovoltaic Materials, December 7, 2006 BRIEF BIOGRAPHY OF PRESENTER Dr Vernie Everett has been at The Centre for Sustainable Energy Systems for eight years. His PhD and early research work was in the field of plasma physics. He is the author or co-author of more than 60 papers and articles, 11 patent applications, and the recipient or co-recipient of 11 major awards and prizes. As well as his work, he enjoys family, cycling adventures, camping, and reading.
Solar09, the 47th ANZSES Annual Conference 29 September-2 October 2009, Townsville, Queensland, Australia
11
