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Staying Cool in the Sun

Alta Devices builds highly efficient solar material. We’ve set world records for single junction cells at 28.8% conversion efficiency and dual junction cells at 30.8% efficiency under one sun (non-concentrated light).  This means that over a quarter of the light that lands on an Alta solar cell is converted into electricity.  However, efficiency measurements are made in a controlled environment.  We wanted to understand our true outdoor performance and asked the National Renewable Energy Laboratory (NREL) to help us.

The biggest factor that can impact true performance is temperature. When you put something out in the sun, it gets HOT. Solar panels can run up to 40°C hotter than the ambient temperature.  And when the air temperature is high (like in Phoenix or Las Vegas) you start to deal with extreme heat.  Silicon solar modules don’t like heat.  They lose 4.0% of their performance for every 10°C of excess heat (over 25°C) due to a poor temperature coefficient.  Therefore, when the sun is the brightest, and it’s 40°C outside (104°F), a silicon module will be operating at 80°C (176°F) and be generating 22% less power than its rated performance.

On the other hand, Alta Device’s solar material has much better thermal performance. In a real world experiment, NREL found that an Alta Devices solar module lost only 0.8% of performance per 10°C over 25°C, and was almost entirely offset by a different positive effect — changes in the sun’s spectrum due to water vapor in the air.  If we go back to our theoretical hot day in Phoenix, while the silicon module loses 22% of its performance, the Alta module continues to operate nearly at its full rated performance number.

NREL saw another interesting effect.  Not only did the Alta technology harvest more energy in these hot environments, it stayed cooler.  This is because our material does a better job at converting solar energy into electricity instead of wasted heat. NREL found that the silicon module operated up to 10°C hotter in the same environment as Alta’s module.  You can download the paper here: http://www.nrel.gov/docs/fy13osti/57902.pdf

Significant for Roof-Integrated Applications

So what does all this really mean?  We, at Alta, are excited about putting solar in places that are truly portable: directly onto planes, cars, consumer devices, and in people’s hands.  Consider the roof of an automobile.  A midsized sedan can easily support solar cells on a 3 ft by 3 ft area (or one square meter).  If we were to fill the area of this roof with high performance silicon solar cells (at 20% efficient), the roof would generate 200W (in a lab environment).  With Alta’s mobile power technology, the same area would generate 240W (in the same lab environment).  We know that car roofs get HOT in the summer.  At an ambient temperature of 40°C (104°F) and assuming the roof is 40°C hotter than that, that silicon module gets de-rated to 156W. An Alta-based roof, 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 the day, the automobile roof stays cooler reducing the overall load on air-conditioning the interior of the car.  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 all the way around.

In summary, we now have real-world evidence that Alta’s technology is more efficient, stays cooler, and generates more energy throughout the day when exposed to high heat.  This finding is significant for most outdoor solar installations and particularly beneficial for roof-embedded applications.  We believe this will change the way solar is used.