3 August 2010


Stanford combines thermal and PV processes using cesium-coated GaN

Researchers at Stanford University have devised a process that can simultaneously use the light and heat of solar radiation to generate electricity in a way that could double the efficiency of existing solar cell technology, potentially reducing the costs of solar energy production enough for it to compete with oil as an energy source, it is claimed (published online in Nature Materials on 1 August 2010, doi:10.1038/nmat2814).

Unlike existing photovoltaic technology used in solar panels (which becomes less efficient as the temperature rises) the new ‘photon-enhanced thermionic emission’ (PETE) process excels at higher temperatures, promising to surpass the efficiency of existing photovoltaic and thermal conversion technologies.

“This is really a conceptual breakthrough, a new energy conversion process, not just a new material or a slightly different tweak,” says assistant professor of materials science and engineering Nick Melosh, who led the research group. Also, the materials needed to build a device to make the process work are cheap and easily available. “We showed this physical mechanism does exist,” Melosh adds.

Most photovoltaic cells that use silicon to convert energy from photons into electricity can only use a portion of the light spectrum, with the rest just generating heat. This heat, plus inefficiencies in the cells themselves, account for a loss of more than 50% of the initial solar energy reaching the cell. Harvesting this wasted heat energy could boost solar cell efficiency. However, heat-based conversion systems require high temperatures, which decreases solar cell efficiency rapidly.

Until now, no one had devised a way to combine thermal and solar cell conversion technologies, but Melosh’s group figured out that coating semiconducting material with a thin layer of the metal cesium made it able to use both light and heat to generate electricity.

“What we’ve demonstrated is a new physical process that is not based on standard photovoltaic mechanisms, but can give you a photovoltaic-like response at very high temperatures,” Melosh says. “In fact, it works better at higher temperatures. The higher the better,” he adds. While most silicon solar cells are rendered inert before the temperature reaches 100ºC, the PETE device doesn't reach peak efficiency until it is well over 200ºC.

Because PETE performs best at temperatures well in excess of what a rooftop solar panel would reach, the devices will work best in solar concentrators such as parabolic dishes, which can get as hot as 800ºC. Dishes are used in large solar farms (similar to those proposed for the Mojave Desert in southern California) and usually include a thermal conversion mechanism as part of their design, which offers another opportunity for PETE to help generate electricity, as well as minimizing costs by meshing with existing technology, the researchers say.

“The light would come in and hit our PETE device first, where we would take advantage of both the incident light and the heat that it produces, and then we would dump the waste heat to their existing thermal conversion systems,” says Melosh. “So the PETE process has two really big benefits in energy production over normal technology.”

Photovoltaic systems never get hot enough for their waste heat to be useful in thermal energy conversion, but the high temperatures at which PETE performs are perfect for generating usable high-temperature waste heat, say the researchers. Melosh calculates that the PETE process can achieve 50% efficiency or more under solar concentration but, if combined with a thermal conversion cycle, could reach 55% or even 60% - almost triple the efficiency of existing systems.

The team would like to design the devices so they could be easily bolted on to existing systems, making conversion relatively inexpensive.

The researchers used gallium nitride (GaN) in the ‘proof of concept’ tests. The efficiency achieved was well below the calculated potential efficiency for PETE, which they had anticipated. But they used GaN because it was the only material that had shown indications of being able to withstand the high temperature range and still manifest the PETE process.

With the right material (most likely gallium arsenide) the actual efficiency of the process could reach the calculated 50–60%. The researchers are already exploring other materials that might work.

Another advantage of the PETE system is that, by using it in solar concentrators, the amount of semiconductor material that is needed for a device is quite small. “For each device, we are figuring something like a 6-inch wafer of actual material is all that is needed,” Melosh says. “So the material cost in this is not really an issue for us, unlike the way it is for large solar panels of silicon.” The cost of materials has been one of the limiting factors in the development of the solar power industry, so reducing the amount of investment capital needed to build a solar farm is a big advance, the researchers reckon.

“The PETE process could really give the feasibility of solar power a big boost,” Melosh says. “Even if we don't achieve perfect efficiency — let’s say we give a 10% boost to the efficiency of solar conversion, going from 20% efficiency to 30% — that is still a 50% increase overall.” That is still a big enough increase that it could make solar energy competitive with oil, it is reckoned.

The research was largely funded by the Global Climate and Energy Project at Stanford and the Stanford Institute for Materials and Energy Science (a joint venture between Stanford and SLAC National Accelerator Laboratory), with additional support from the US Department of Energy and the Defense Advanced Research Projects Agency (DARPA).

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