Solar technologies are all measured and specified under standard test conditions. The conditions state that the solar panel be tested at 25°C and be subjected to 1000 W/m2 of light energy – closely approximating the power of the sun in broad daylight. This approach works well when you are comparing solar materials made from the same base technology. However, today there are a variety of solar technologies, and comparing only the specifications under standard test conditions gives you just part of the answer. In fact, in the real world, a solar panel rarely ever sees “standard” conditions. The National Renewable Energy Lab (NREL) recently performed a study of silicon vs. gallium arsenide over a period of 3 months in order to compare these two technologies in the real world.
In order to perform this test, two solar arrays were deployed and the power output and temperature of each was measured every hour, over the course of about 2 months from July 24th, 2013 to September 30, 2013, in two different locations. The purpose of the test was to understand how these two fundamentally different solar technologies compare in the real world.
We know that gallium arsenide solar material performs better under standard test conditions, as NREL had previously verified world record efficiency of Alta Device’s single junction solar cells at 28.8% and single junction modules at 24.1%. However, what we wanted to learn was how these two materials perform in the wild. In order to study the true real world effects, the results were normalized for efficiency, meaning that the efficiency advantage gallium arsenide has over silicon was eliminated.
The results were extremely interesting. What NREL has shown is that even if silicon had the same efficiency as gallium arsenide (which it certainly does not), gallium arsenide produces more total energy and runs 10°C degrees cooler than silicon. When you factor the efficiency advantage back in, the total energy collected on a hot roof with a gallium arsenide solar array is over 50% greater than a competing silicon-based solar panel.
One of the factors in this result is something called temperature coefficient. This is a measure of performance loss versus temperature relative to 25°C. Every solar technology has a temperature coefficient. Usually it’s negative – meaning that when temperature rises above 25°C, the solar material gets less efficient. The silicon solar panels used in the NREL testing cited here found the temperature coefficient for silicon to be -0.41% per degree Celsius and the temperature coefficient for gallium arsenide to be effectively 0 (no performance loss with respect to temperature).
Let’s demonstrate the difference using an example. Consider the roof of an automobile. A midsized sedan can easily support solar cells on a 3 foot by 3 foot area (or roughly one square meter). If we were to fill the area of this roof with silicon solar cells (at 20% efficiency), the roof would generate 200W (under standard test conditions). With gallium arsenide technology, the same surface area would generate at least 240W (under the same standard test conditions).
We know that car roofs absorb heat in the summer. At an ambient temperature of 40°C (104°F) and assuming the roof is 40°C hotter than that (80°C total), that silicon module gets de-rated to 156W due to it’s negative temperature coefficient. A gallium arsenide-based automobile roof solar array, on the other hand, will stay ~10°C cooler, and continue to produce 240W, a 54% improvement. Not only will more electricity get produced over the course of a day, the automobile roof stays cooler, reducing the overall load on air-conditioning the interior of the car, which in turn will result in more miles per gallon, or miles per charge in a vehicle. In addition, the auto manufacturer can receive off-cycle credits from the National Highway Traffic Safety Administration (NHTSA) toward continuously more stringent Corporate Average Fuel Economy (CAFE) standards for installing a solar roof. It’s a win-win situation all the way around.