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.
"The
long-term future of Photovoltaics is in many ways tied to thin-film development, as it
offers significant reductions in production costs and alleviates the solar-grade silicon
supply problem." This statement was printed in the
December 20, 1996 issue of "Solar Flare" published by Strategies Unlimited ,
speaks directly about the benefits of United Solar's new triple junction solar electric
modules. Uni-Solar modules use approximately 1\150th the amount of silicon that a
traditional single or multi-crystal solar module uses to generate the same wattage. This
is possible because the United Solar Photovoltaic cell is created by depositing a very
thin layer of sunlight converting silicon on a stainless steel back plate which replaces
the relatively thick, grown or cast, traditional solar cell with its active silicon layer,
the top one micron thick surface of that cell. Therefore, recent solar industry concerns
about quality, availability, and cost of solar- grade silicon have little or no impact on
United Solar's cost or ability to manufacture their economically priced thin film modules.
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