Battle River Solar Products   

Comparison between Unisolar Seimens and Astropower modules

This test was set up and reported by Unisolar

June 20-21, 1998

The purpose of the Solfest '98 Shoot-Out was to publicly demonstrate the practical (i.e. real world) electrical performance characteristics of the United Solar Uni-Solar 64 (or US64), Siemens SP75, and AstroPower AP7105 modules. Solfest is an annual event held in Hopland, California, which showcases environmentally friendly alternative living. Displays, inspirational speakers, demonstrations, and children's events range from worm farms and hemp based economies to utility-tied photovoltaics. An incredibly diverse cross-section of environmentally concerned visitors descends onto this event giving, an ideal climate for the Shoot-Out.

Two consecutive days (10 a.m. to 5 p.m. local time) of head to head "real time" evaluation was accomplished (and recorded) and, while not complete, does allow for a significant amount of extrapolation to other sets of conditions. More Shoot-Outs are planned to validate relative performance under varied conditions.

METHODOLOGY: They selected four sample panels of each module type. It is important to note that the manufacturers had nothing to do with the selection. All modules were sent to Sandia National Labs for characterization* and the US64's were left outdoors at Sandia Labs for light soaking for a period of approximately 130 days. All modules were configured for the Solfest '98 Shoot-Out in Hopland.

At Solfest '98, the modules were deployed side by side, at a fixed southward tilt of 10.5 degrees and hooked up via a charge regulator to a 12 VDC battery bank. An adjustable load was hooked up via a load controller to discharge the batteries as desired. Each module's current and the array controller's input voltage were logged every minute. The load current, insolation, and ambient temperature were also logged for reference. A laptop computer was interfaced with the data logger during the entire demonstration to display the minute by minute performance. A representative from each manufacturer was on site, and had ample opportunity during the two days to question and/or refute any of the test equipment. None was noted. Upon completion of the demonstration, all modules were sent back to Sandia for a post Shoot-Out characterization.

Attached are graphical representations of the results of the Solfest '98 Shoot-Out. Figures 1 and 3 show actual 4-module average current vs. time of day for the two days of the event. Also shown on these figures are the specific conditions imposed on the modules for each day, as well as the average total energy measured in amp-hrs for each module type. In summary, the average 64-watt rated Triple Junction Silicon Uni-Solar module delivered from 7.9% to 12.8% more energy than the 75-watt rated crystalline modules over the course of the Shoot-Out. Figures 2 and 4 take the results a bit further, in that when adjusted for purchased watts, (delivered amp-hrs divided by rated watts) the average Uni-Solar module delivered from 26.4% to 32.2% more energy than the crystalline modules.

Why did the Uni-Solar 64 module perform so well in this Shoot-Out? The Triple Junction's advantage in real energy delivery is due principally to three interrelated operating characteristics of the US-64 ­ more favorable current and voltage characteristics (IV curve), more favorable temperature coefficients, and cooler operating temperature.

PV modules are rated and priced at the peak power point. However, typical battery charging applications impose operating voltages below the voltage needed to deliver peak power. The consequence is that while crystalline modules deliver relatively unchanging current levels over this operating range, the Uni-Solar module actually delivers increasing amounts as batteries are depleted.

Temperature coefficients, which are a consequence of the physics of the technology, define how PV modules respond to varying operating temperatures. In this test, each of the worst of the Uni-Solar 64s temperature coefficients is better than the best of all the crystalline products. This implies that keeping everything else constant, if ambient temperature is above that temperature which produces a module temperature of 25 C or 77 F, then the Uni-Solar will deliver more amp-hrs per purchased watt than crystalline modules. By the way, that ambient temperature is near 0 C (32 F).

Note that in the PV industry, knowledge of the temperature coefficients for a specific technology is critical to performance at the high temperature end of operating conditions. With that in mind, crystalline modules are designed to develop enough voltage to ensure the delivery of approximate rated current at the high, but occasionally attainable, temperatures. However, at more typical temperatures, an excess of voltage and hence power is available but unusable in typical battery charging applications. On the other hand, the Uni-Solar does not need as gross an excess of voltage at low temperatures to accommodate high temperatures, so fewer but bigger (i.e. more current) cells are hooked in series. The net result is a technology that delivers closer to rated wattage over more varied conditions.

The final operating characteristic responsible for the US-64's advantage stems from a previous test's observation that in general, the module operates cooler. Thick glass, a good thermal insulator, is used in traditional crystalline modules to protect the extremely fragile crystalline cells. Therefore most of the cooling is accomplished from the back. However, the Uni-Solar 64 using "unbreakable" cells, does not require the use of glass and hence allows additional cooling from the front.

EQUIPMENT:

 Benifits of Thin film.