LARGE-AREA CONCENTRATORS David Faiman Department of Solar Energy and Environmental Physics Jacob Blaustein Institute for Desert Research Ben Gurion University of the Negev Sede Boqer Campus, 84990 Israel E-Mail: faiman@bgumail.bgu.ac.il 1. INTRODUCTION In last year s workshop I discussed the economics to be expected from ultra high efficiency PV cells [1]. In particular, I drew attention to the potential virtues of exposing comparatively small PV modules to highly concentrated sunlight from a very large solar dish. The underlying motivation behind concentrator photovoltaics (CPV) is to sufficiently reduce the amount of PV material per generated watt, that the bulk of the generating cost shifts from the expensive cells, where it is at present, to the lower-cost balance of system. In the process of enlarging upon this idea, I pointed out a number of additional important advantages, the most significant being: (1) it is important to use very large solar concentrators because they maximize the output power per amount of complexity; (2) the path to ultra high efficiency cells leads to an almost proportional reduction in electricity costs; (3) progress towards ultra high efficiencies can be made without the obstacle of needing to produce large-area cells. In the present paper I d like to review a number of distinct ways in which different groups are approaching the matter of large area concentrators. To this end it is necessary to realize that concentrator cells fall into two natural classes, as regards their use in a CPV system, both of which may find ultimate application as parts of large-area concentrators. One class constitutes the dense array concept, in which an entire module of cells is exposed to concentrated sunlight from a single optical concentrator. The other class involves the use of cells that are each mounted at the focus of its own optical system. 2. LARGE-AREA CONCENTRATORS FOR DENSE ARRAY CELL MODULES As the name suggests, cells fabricated for use as dense arrays must have two special geometrical features, neither of which is a trivial problem to solve. Both features arise from the need to utilize the maximum amount of incident flux in order to help minimize optical losses. These features are: (1) the need to maximize the active area of these relatively small cells; (2) the need for rear contacts so as not to cause undesirable shading losses by the attachment of conducting fingers to the front surface of such small cells. Neither of these features is necessary if the cells are to be used as individual units because, regarding the first feature, there will generally be plenty of room around the cell for inactive cell material and, regarding the second, it is possible to design the optics for a single cell so as to compensate for any shaded PV material.
Dense array CPV cells were first demonstrated by SunPower Corp. [2] and more recently, as part of a relatively large solar dish system, the SS20 (Fig.1), manufactured by the Australian company Solar Systems Pty. The SS20 system is powered by a 130 m 2 parabolic dish and can generate approximately 20 kw of AC power. Figure 1: The Solar Systems Pty SS20 dish-illuminated, 20 kw, dense array, CPV system The most important advantages of dense array systems are: (1) Such systems are capable of producing additional, thermal, energy that is pumped and delivered ready for use - owing to the need for active cell cooling. Active cooling is sometimes regarded as an undesirable additional complication to an already complicated system, but it does offer a potential additional source of revenue, thus improving the economics and overall energy efficiency of the system.
(2) The cell module is a distinct, small, and potentially low-cost part of a much larger unit. As such, it can be replaced if economic circumstances should encourage such action. This could come about if cells of significantly higher efficiency were to become available part way during the expected lifetime of the system, if accelerated cell degradation were to occur, or if accidental module destruction were to take place. 3. LARGE-AREA CONCENTRATORS FOR MODULES OF INDIVIDUAL CELLS The use of individual cells, each with its own concentrator optics, was the way in which relatively low-concentration CPV systems were first built. For example, in the early 1980s, Martin Marietta Corp erected 80 units; each rated at 2.8 kw, at Sky Harbor Airport, AZ [3] and an even larger 350 kw project in Saudi Arabia [4]. But no further large scale CPV projects were undertaken until relatively recently. Now, Amonix Corp. has taken up the gauntlet, also using individual cells but under high concentration (260X) and in quite large units (Fig.2). They manufacture sealed flat modules of 5 kw rating, 5 of which can be mounted on a single 2-axis tracker to form a rectangular array with total aperture area 182 m 2. Figure 2: The Amonix Corp. 25 kw, Fresnel lens-illuminated, individual-cell, CPV system The most important advantages of individual-cell CPV systems are:
(1) Most of the optics can be controlled at the time of manufacture so that fewer items need to be accurately aligned in the field. (2) No failsafe provisions need be included to prevent concentrated flux from causing damage in the event of erroneous sun-tracking. (3) The modules are sealed, flat panels, rendering cleaning a comparatively easy task. (4) The cells are passively cooled. Thus there are no fluids to handle, and no failsafe provisions need be incorporated to prevent damage caused by loss-of-coolant events. 4. REPORTED PERFORMANCE FIGURES 4.1 Dish-illuminated dense array CPV system (Solar Systems Pty.) Although no performance data have yet been published for Solar Systems large 130 m 2 solar dish (Fig.1), they have reported some impressive results [5] for a smaller 20 m 2 dish that illuminates a 24 cm x 24 cm dense array containing 384 rectangular cells manufactured by SunPower Corp. This particular dish is one of an array of 14 such units which Solar Systems Pty. have erected in White Cliffs, NSW, Australia, using recycled dishes which had been previously used to power a solar-thermal system [6]. Table 1 lists the measured performance figures for dish No.2 in the White Cliffs array, Table 2 lists the various parasitic power losses, and Table 3 lists the resulting derived system parameters. The measurements were reportedly made on April 5, 2001 at 10:40 am [5]. Table 1: Measured performance figures of Dish No. 2 in the White Cliffs array [5] Dish area 19.75 m 2 Receiver area 0.0576 m 2 Direct normal irradiance 872 Wm -2 (± 3%) Ambient temperature 19.7 o C (± 1.0 o C) Average module temperature 27.4 o C (± 5% sic) Water flow rate 33.44 lpm (± 5.0 %) T out T in (water) 3.9 o C (± 0.1 o C) DC power output 3,448 W (± 2.0 %) Table 2: Parasitic power loss per dish in the White Cliffs array [5] Water pumping 86 W (±1.5%) Control electronics 30 W (± 5 W) Elevation-tracking motor 3.52 W (± 2%) Azimuth-tracking motor 1.28 W (± 2%) Total 120.8 W (± 10 W)
Table 3: Derived performance parameters for Dish No. 2 in the White Cliffs array [5] Incident power on dish 17,222 W (± 3%) Incident power on receiver 14,552 W (± 5%) Thermal power delivered 9,099 W (± 5%) Electrical power delivered 3,448 W (± 2%) Dish optical efficiency 84.4 % (± 5%) Module efficiency 24.7 % (± 5%) Cell efficiency 24.0 % (± 5%) Receiver efficiency 23.7 % (± 5%) η DC (excl. parasitics) 20.0 % (± 5%) η DC (incl. parasitics) 19.3 % (± 5%) Table 3a: Additional calculated/simulated parameters needed for Table 3 [5] Total receiver reflectance 13.78 % Inc. power density on receiver 252,600W m -2 Cell U-factor 22,160 W m -2 K -1 Average cell temperature 38.52 o C (± 2.0 o C) A number of details from Tables 1-3 are worthy of note. First, as pointed out by the authors of ref [5], the specific field conditions under which these measurements were made are quite similar to the PVUSA PTC standard test conditions of 850 Wm -2 normal direct irradiance (NDI) and 20 o C ambient temperature. As such, a system DC efficiency of 20% or more is quite remarkable. Secondly, also pointed out by the authors of ref [5], the module actually has a (slightly) higher efficiency than the bare cells owing to the presence of an AR coated protective cover glass. Finally, a total parasitic electrical loss of only 3.5% is very encouraging, given that the bulk of this loss comes from water pumping an act that produces an additional 9,099 W of free thermal power which could, in principle be used. This would give the system a potential total useful energy efficiency of over 70%. 4.2 Individual cell CPV module system (Amonix Corp.) Amonix s latest CPV module, their so-called 5 kw megamodule, is a single flat plate of approximate dimensions 13.8 m x 3.2 m (chosen so as to render them stackable on a standard road delivery truck) which is pre-assembled in the factory. The front face of each megamodule is tiled with 48 parquets of Fresnel lenses, each parquet consisting of a 4 x 6 matrix of 260 X lenses. Behind each parquet of Fresnel lenses is a receiver plate of 24 series-connected CPV cells of area 1 cm 2, each rigidly mounted within its own secondary reflector (for redistributing stray flux). Each receiver plate has a nominal rating of 120 W (16 A at 7.5 VDC). Five of these megamodules are designed to be mounted on a single 2-axis tracker to form a 25 kw unit (Fig. 2) which powers a 480 VAC grid-connected 3-phase inverter. Although no performance details have yet been published for Amonix s 25 kw units, they have reported [7] some figures from an earlier 20 kw unit that was installed at the Arizona Public Service s STAR facility. Fig.3 has been re-drawn from the corresponding figure in ref
[7], which depicts a sample of 10-minute averaged data, collected under a variety of operating conditions. The data points which are given in the original figure present a 5-10% scatter about the straight line shown in Fig.3, with maximum output in the vicinity of 20 kw at a DNI of 900 Wm -2 and minimum output around 5.5 kw at a DNI level of 300 Wm -2. 20 Data from AMONIX 20 kw IHCPV system at Arizona Public Service (redrawn from [7]) Output [kw] 15 10 5 300 400 500 600 700 800 900 Figure 3: Graph of DC output power vs. DNI, for an AMONIX 20 kw array at APS (re-drawn from Ref [7]: Actual data points scatter about the straight line by 5-10%) From the data in ref [7]: DNI [W/sq.m] Gross module area 159 m 2 Net aperture area 127.5 m 2 DC Output @ DNI 850 Wm-2 18.7 kw (from Fig. 3) Net module efficiency 17.3 % Gross module efficiency 13.8 % Because ref [7] does not present any operating temperature data, the above efficiency estimates are not necessarily representative of PTC operating conditions. According to the authors of ref [7], their system has 18.5% efficiency at PVUSA operating conditions. 5. POSSIBLE FUTURE DIRECTIONS Thus far, we have discussed two approaches to large-area CPV systems that have actually been tested. But, naturally, there are also other approaches in the offing, albeit less developed. In particular, another approach to large-area CPV systems is, perhaps
paradoxically, to think in terms of extremely small ultrahigh concentration subunits. The latter could be mass-produced for multiple mounting on a large tracker. The motivation here is that it is easier to approach optical perfection for small mirrors than it is for large. The resulting concentrators, operating at 1000 X or more, would thus require only extremely small CPV cells in order to produce relatively large amounts of power, and the cells could be actively cooled. A number of examples of this approach have appeared in the recent literature. A European group [8] propose employing nonimaging optics within a dielectric medium in order to achieve illumination in excess of 1000 suns, within an extremely low-profile lenscell combination. In their design a GaAs cell is actually embedded in a specially formed PMMA (polymethylmethacrylate) lens. A large array of such lens-cell units would then constitute a flat plate CPV module. Some of these authors are also collaborating with a US group [9] who are working on another flat plate concept that will employ more than 1000 Si cells on a single wafer. The realization of this idea also calls for the use of appropriate non-imaging optics, in order to produce a parquet of lenses that will each direct its 300X concentrated sunlight to the appropriate cell on the wafer. Yet another intriguing idea comes from some of my colleagues at Sede Boqer, who have developed high-precision mini-dishes and fiber optics in order to provide beams of ultrahigh intensity sunlight as an alternative to laser surgery. This group suggests using fiber optics to extract the energy from a large array of such mini-dishes and direct it to some convenient location where it can then be converted to electric power by, for example, CPV cells [10]. 6. CONCLUSIONS In this review we have discussed two different approaches to large-area CPV systems that have actually reached the commercialization stage, and we have mentioned a few additional approaches that may also ultimately yield promising results. Of the two system types that have been realized, one company has taken the individual cell approach and placed considerable effort into the logistics of mass-production, ease of delivery to the site, and rapid on-site construction. These are all vitally important considerations if CPV (indeed PV!) is to be able to provide power at the GW scale as do conventional fossil-fueled power plants. The other company has chosen to take advantage of the dense array concept so as to be able to provide additional thermal energy to the user, and also, to be able to replace the cell array should this become economically desirable at some point in time after the system has been installed. At the present time it is too early to judge which of these directions is preferable, particularly since large-area CPV is only now starting to become a reality. What makes this field particularly exciting is that rapid progress towards the achievement of ultra high efficiency cells can be expected at the small cell scale and we are talking about a technology that only requires small cells in order to achieve high system efficiencies.
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