Stanford process aims to make solar cheap enough to compete with oil

Stanford University engineers believe an energy harvesting process called "photon enhanced thermionic emission" could make solar power production more than twice as efficient as existing methods and potentially cheap enough to compete with oil.

By Suzanne Deffree, Managing editor, news -- EDN, August 11, 2010

Stanford University engineers are claiming an energy harvesting process that could surpass the efficiency of existing photovoltaic and thermal conversion technologies.

The university's process simultaneously uses the light and heat of the sun to generate electricity and excels at high temperatures, unlike photovoltaic technology currently used in solar panels, which becomes less efficient as the temperature rises.

Stanford believes the process, called "photon enhanced thermionic emission," or PETE, could make solar power production more than twice as efficient as existing methods and potentially cheap enough to compete with oil.

"This is really a conceptual breakthrough, a new energy conversion process, not just a new material or a slightly different tweak," Nick Melosh, an assistant professor of materials science and engineering at the school and the leader of the research group, said in a statement. "It is actually something fundamentally different about how you can harvest energy."

The university reminded while high temperatures are necessary to power heat-based conversion systems, solar cell efficiency rapidly decreases at higher temperatures. According to the university, heat from unused sunlight and inefficiencies in silicon cells account for a loss of more than 50% of the initial solar energy reaching the cell. The Stanford team's work focused on wedding thermal and solar cell conversion technologies.

Melosh's group coated a piece of semiconducting material with a thin layer of the metal cesium, making the material 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 said. "In fact, it works better at higher temperatures. The higher the better."

Indeed, while most silicon solar cells are inert by the time the temperature reaches 100 C, the PETE system hits peak efficiency when it is well above 200 C, making it useful for large scale, desert-located solar farms.

The researchers used a gallium nitride semiconductor to test PETE as it is able to withstand a very high temperature range. With other materials, like gallium arsenide, the researchers estimate the process could reach up to 60% percent efficiency.

Stanford noted that the materials needed to build a device to make the PETE process work are inexpensive and easily available, making the power that comes from it affordable. The team said it would like to design the devices so they could be easily bolted on to existing systems, thereby making conversion relatively inexpensive.

"For each device, we are figuring something like a 6-inch wafer of actual material is all that is needed," Melosh said. "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 PETE process could really give the feasibility of solar power a big boost," he continued. "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."

A paper describing the tests the researchers conducted was published online August 1, in Nature Materials.

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