SIMPLE LOW CONCENTRATING MODULE DESIGN INCRESES SOLAR CELL OUTPUT 25%

Transcription

1 SIMPLE LOW CONCENTRATING MODULE DESIGN INCRESES SOLAR CELL OUTPUT 25% Daniel Simon 3D Solar, Inc N. Sheridan Rd. #1003 Chicago, IL ABSTRACT We present a simple low concentrating solar module design capable of boosting solar cell output by 25% (standard test conditions). The design has a theoretical concentration factor of ~1.4x and (unusually for CPV) does not require tracking, although the design benefits from tracking. Our design employs flat reflectors, depth of module, and simple geometric principles to boost output. Any solar cell technology can be used in the design. We present a simplified cost model useful for estimating the savings inherent in this design, which depends on relevant factors like the relative costs and efficiencies of the solar cells and reflectors used. We discuss test results from testing of the first generation prototype module. We also discuss design changes that were made in building a second generation prototype module, with the intention of improving the module's performance. Finally we discuss a closely related 2x concentrating module design which should boost cell output by 50% (2x design has not yet been built/tested). 1. INTRODUCTION Despite a variety of solar cell technologies--which produce solar modules with different cosmetic appearances--the vast majority of solar modules are constructed in the same 2-dimensional fashion. The solar cells are invariably wired together in rows and the rows are arranged in one plane, like tiles on the floor. The cells are covered by a sheet of glass for protection, and frequently laminated/encapsulated with EVA, or similar material to prevent water ingress. Many laminates, especially those constructed with silicon solar cells, are then framed with aluminu m extrusions to produce a finished module. We refer to this as a standard solar module, and historically over 85% of modules are constructed this way using silicon solar cells. In contrast 3D Sol ar Inc., has developed a (patent pending) solar module that uses flat reflectors and a simple 3D geometry that generates up to 25% more power per cell than the standard module produces. 2. 3D SOLAR MODULE GEOMETRY The basic 3D Solar module, sometimes called a V-module, is constructed with individual rows of solar cells (with the cells already strung together). Each row-of-cells is mounted in the module at a 45 degree angle to the cover, and is paired with a flat reflector placed opposite the row. Each row-of-cells and reflector forms a V-shape pair with a 90 degree angle between. The reflector also forms a 45 degree angle with respect to the module cover. Then the next row-of-cells is mounted at 45 degrees (parallel with the first) and paired with another reflector with the pattern repeated until the module is full. Viewed from the side, the cell, the reflector and the cover each form one side of a triangle. In this way each row-of-cells in the 3D Solar module is able to take advantage of the additional light reflected onto it by 1

2 the flat reflector. This additional light hitting each cell causes the cells to produce additional power. Assuming the sunlight falls normal to the cover of the 3D Solar module (see Fig. 1 below), the theoretical concentration factor is ~1.4 times. As shown, the cover is parallel with the hypotenuse of the triangle and admits the light which either falls directly onto the cells, or strikes the cells after a single reflection. A nice feature for PV is that the sunlight reaching the cells will be uniformly distributed. laminates were built using the same cells, stringing and laminating procedures, we felt this would allow an excellent side-by-side test of both module formats. The manufacturer used an older style half-cell in constructing the laminates (see Fig. 2 below). We used a low-iron glass typically used for solar applications because of the enhanced transmittance of sunlight for our module cover. We used plastic reflectors (reflectance ~88%) because the material was readily available, easy to work with and inexpensive in small quantities. Much higher quality reflectors (~94%) exist and are fairly easy to get, but not as easy to work with, and because of set-up & minimum lot charges these were not deemed affordable for the first prototype, we used a higher quality reflector in our second prototype. Using the plastic reflector reduced the maximum output by 6% (the reflector losses only effect the light that strikes it), whereas a 94% mirror would reduce maximum output by 3%. Stated another way, we could have squeezed up to 3% more power from the prototype simply by using a better reflector. Finally we found a local machine shop to bend/form a simple 5 sided aluminum box into which we mounted our laminates and reflectors (see Fig. 2 below). Fig. 1: Side view: cell, reflector and cover layout. A maximum theoretical power increase of ~40% can be achieved with this design. Real reflectors are not 100% reflective, and the glass cover cannot transmit all of the sunlight. These and other real world factors reduce the maximum per cell power increase. 3. 3D SOLAR MODULE PROTOTYPE 1.0 Fig. 2: Prototype with 4 laminates (right) and reflectors (left). After first proving to ourselves that the concept actually worked, by performing some simple backyard experimentation, we decided to build and test a prototype 3D Solar module. We found a custom solar panel manufacturer (in the US) willing to build a set of solar laminates that we could use to test the idea. This particular manufacturer had carved out a niche designing small portable modules for campers/hikers. In addition to the four, single row-of-cell laminates we used to make our prototype, we ordered a pair of single row-of-cell laminates arranged in the more standard 2D layout. Since all the The photo of the 3D Solar module can be a little difficult to interpret because you see both the cells and a reflection of the cells simultaneously. In Fig. 2 (see above) each of the laminated row-of-cells slants into the photo from right to left. But you also see a reflection of each laminate in the reflector (on the left), which slants into the photo from left to right. If you look along the bottom edge of the module you may be able to see the lines of the triangle shown in Fig. 1, except the right angle/corner opens out toward you, instead of up and to the left. 2

3 4. SIMPLIFIED COST MODEL The theoretical design savings one can expect from using the 3D Solar module depends primarily on the ratio of reflector cost to solar cell cost. We are engaging in a light arbitrage between the reflectors and cells, so this design only makes sense if reflectors are much cheaper than cells. The design uses ~2.5 units of reflector to replace 1 unit of cells. In practice a cell/reflector cost ratio of at least 6 (preferably 8 or 10) is helpful to ensure savings using the 3D Solar design. Each square foot of 3D Solar panel contains 0.7 sq. ft. of cells and 0.7 sq. ft. of reflectors. Each sq. ft. receives ~100W of incoming sunlight. TABLE 1: 3D SOLAR MODULE DESIGN SAVINGS 1 square meter = ~10 square feet = 1000 W of incoming sunlight Cell cost per W $ 0.50 Reflector cost/sq.ft. $ 1.00 Per square foot of roof Design # Watts Total cost cost/w Savings Perfect mirrors Standard: 15% efficient cells 15 $ 7.50 $ D panel: 15% efficient cells 15 $ 5.95 $ % ^cost/w includes cell and mirror cost only Key Assumption: 1) Solar panels cost $1/W; solar cells are 50% the cost of a panel 2) Perfect mirror=100% reflective; save 5% less if 90% reflective Table 1 (above) shows a 21% design savings for the V- panel defined as the cost of 0.7 sq. ft. of cells plus 0.7 sq. ft. of reflector and constructed with 15% efficient solar cells and using perfect mirrors vs. a standard module with 15% cells. Using a realistic/cheap 90% reflective mirror would decrease the design savings to ~16%. Design savings will increase ~0.5% for each 1% increase in cell efficiency (i.e. using 17% efficient cells would yield 22% design savings). The cell and reflector are not the only costs involved in building a 3D Solar module, but even a fraction of the design savings would represent a significant savings for module manufacturers. 5. RESULTS OF TESTING PROTOTYPE We selected TUV-PTL in Tempe, Arizona to perform a baseline comparative test of the 3D Solar module (containing 4 laminates) vs. a standard 2D module (containing 2 laminates). We decided on TUV-PTL because the (PTL) laboratory has a long history of testing solar modules in the US, and TUV has an excellent reputation in Europe the largest market for solar modules in recent years. Basically we had high confidence in their ability to test solar modules. TUV-PTL first performed a baseline STC (standard test conditions) test of both modules and found the 3D Solar module produced 25.4% more power per cell, than the 2D module. There were twice as many cells in the 3D Solar module so TUV-PTL actually measured ~2.51 times as much power from the prototype as from the 2D control module. Later TUV-PTL performed three additional side-by-side tests at different airmass values to check the performance of the modules with angle. Their results showed the 3D Solar module produced 30.5% (AM3) morning, 21.4% (AM1.5) before midday and 23.2% (AM3) afternoon more power per cell. They reported irradiance (and ambient temperature) values for all measurements broadly in line for both modules with one exception. The irradiance value for the second AM3 test of the 3D Solar module was recorded as being 100W/m^2 less than for the corresponding measurement on the 2D control panel, so the final number could understate the relative 3D Solar module performance. That said the irradiance value for both/all AM3 measurements was low (under 500W/m^2) AM3 indicates a sun fairly low in the sky! Rather than complain about one reading, we simply point out that adding up all four of the per cell power readings (25.4, 30.5, 21.4, & 23.2) and dividing by four yields an average of ~25% more power per cell. 6. DESIGN IMPROVEMENTS: PROTOTYPE 2.0 More extensive testing of the first prototype by TUV-PTL in the Arizona desert demonstrated a tendency of the 3D Solar module/laminates (inside an aluminum box with a glass cover) to reach a much higher temperature than the 2D module/laminates. This was such an obvious problem that we built the 5 sided box with a few holes in the sides in the hope that natural convection would prevent overheating. The extreme summertime desert conditions of Tempe, Arizona, led to excessive heat gain within the enclosed panel which negated the extra power produced when the testing protocol allowed for a temperature rise above STC. This extra heat is not helpful for straight PV applications, but could be acceptable in thermal or hybrid modules. We decided to build a second prototype module similar to the first, except we built it with an aluminum frame that left the backside completely open to the air. We expect that this open back prototype will operate at a lower temperature than the 3

4 fully enclosed one. For the second prototype we purchased 1 meter long laminates using (modern) 6 in. x 6 in. cells wired together from a Chinese supplier. We also used thin aluminum reflectors with about 94% reflectivity which should perform better than the plastic ones. We have also attempted to thermally connect the back of the laminates to the aluminum (extrusion) frame, so the frame will also act as a heat sink. Finally we built our prototype with a cover that could be opened. (See Figs. 3 and 4 below.) The idea is to build a module that can be serviced in the field (i.e. we designed the module so each laminate could be removed and replaced). This is not a standard feature of solar modules, but due to the specific 3D module geometry, and the fact that we use individual rows-of-cells, it seems like a simple modification that could make the 3D Solar module a more useful product. We also ordered an extra laminate so we can again test the prototype module and a 2D laminate side-by-side. We have sent the new and improved prototype 3D Solar module to a solar module testing laboratory for testing. We hope to get test data from this new prototype module soon. We think these pictures (see Fig. 3 and Fig. 4 above) of the second prototype capture the internal arrangement of cell (left) and reflector (right) within the panel, more clearly than Fig X CONCENTRATING DESIGN 3D Solar has begun developing a related solar module design capable of a theoretical 2-sun concentration factor. Just as the basic 3D Solar module replaces a standard 2D module with a series of single row-of-cell laminates and reflector pairs below the horizontal plane of the module, the iterated V replaces each laminate with a series of single cell and reflector pairings below the plane on the solar laminates. (See Fig. 5 below) This iterated design again uses only flat reflectors (but some are cell size and the others run the length of the module) in order to place more light on each cell and thereby boost the amount of power each cell produces. Based on the actual testing of the basic (1.4x) 3D Solar design, we expect this iterated design to produce ~50% more power per cell at STC. Fig. 3 Second prototype with open cover cells on the left. Fig. 5 Drawing of the iterated module cells are cross-hatched. Fig. 4 Close up of a cell and reflector pairing--cover down. The iterated module will require tracking in order to produce power consistently throughout the day. The basic 3D Solar module is able to self-track over 45 degrees of solar angle, so if one performs a fixed mount with the V-axis running E-W, the module will passively track the sun. Still there is some self- 4

5 shading from the module frame (as currently constructed) which could be eliminated by single axis tracking. Unfortunately the higher concentration of the iterated V design requires tracking for optimal performance all day or at some point the sun will wander off the solar cell no matter how you set it up. Alternately one could install the iterated module in a location where it will be in shade part of the day such as on the east (or west) side of a building just make sure during installation that the cells have an unobstructed view of the sun. 3D Solar has demonstrated a simple low-concentrating solar module prototype which is capable of producing 25% more power per cell under standard test conditions. We had our module tested by a respected module testing facility and have reported the results. We are working to improve upon these results by using a higher quality reflector and through design improvements, like using an open back design (with an extruded aluminum frame to address a thermal management issue) that bring the module more in line with typical module manufacturing practices. Finally we disclose a second relate module design with a higher concentration ratio, and we expect even greater power production per cell. 5

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