| 31 July 2012
Heat from the sun is typically seen as detrimental to a solar cell. Over time, devices degrade and become less efficient. But research at the Michigan Technological University (MTU) on Michigan’s Upper Peninsula (US) now indicates that purposefully heating up an amorphous silicon solar cell once a day for one hour could actually increase its performance in a hybrid solar thermal PV application.
“This research has shown a viable method of reducing the cost of both solar electricity and solar thermal energy in tandem,” says Joshua M. Pearce, PhD, an associate professor in MTU’s Department of Materials Science and Engineering. His studies, he projects, “open up an entirely new application of amorphous silicon.” Highly economical photovoltaic-thermal (PV/T) hybrid systems would become feasible. “We need both solar electricity and solar heating in most parts of the industrialised world. Already, we are running into ‘roof real estate’ issues in places like Germany where PV has taken off,” Pearce says.
Devising amorphous silicon in a PVT system, the MTU researchers solved two symbiotic problems: “Normally, PVT are made to operate at low temperatures to cool crystal silicon solar cells to improve their efficiency. But this makes the PVT terrible at capturing heat when compared to stand-alone, high-efficiency solar thermal collectors,” Pearce explains. Amorphous silicon is less efficient in PV applications, albeit using far less silicon, making it cheaper and better for the environment. Manufacturers are making amorphous silicon cells very thin to lessen the impact of the heat on the cell. Unfortunately, light escapes through these very thin cells.
Pearce’s team tested amorphous silicon solar cells at PVT operating temperatures, which improved the theoretical performance of the thermal side of the PVT. They further set out to prove that making amorphous silicon solar cells thicker would improve efficiency as long as the system operated at higher temperatures. “Most strikingly, we showed that if you ‘bake’ the amorphous silicon once a day at high temperatures, you ‘heal it’ and increase the solar electric output by more than 10%,” Pearce enthuses. “When you heat up amorphous silicon, you can reverse the Staebler-Wronski effect (light-induced degradation) by annealing out defect states.” The cells could be “baked” at about 100ºC for an hour after peak sunlight by not running the solar thermal system and letting the modules heat up. “Think of the thermal energy as providing the ability for broken bonds to reform or heal in the amorphous network that were previously broken by light-induced degradation,” Pearce explains the spike annealing process.
The tests were performed on lab cells. Pearce says, “The next step is to scale up and demonstrate the same effect on full modules and arrays in outdoor conditions.”
This method, however, would not work for crystalline silicon photovoltaics, according to Pearce.
In the image, each saw tooth represents a "bake" cycle.
Written by Sandra Henderson, Research Editor, Solar Novus Today
