New process that harnesses heat energy could double efficiency of solar cells

Photovoltaic solar cells convert light energy from the sun into electricity. Although significant strides have been made in increasing the efficiency of photovoltaic technology, they usually only result in incremental increases. Researchers at Stanford University have come up with a way that could more than double the efficiency of existing solar cell technology and potentially reduce the costs of solar energy production enough for it to compete with oil as an energy source. Instead of relying solely on photons, the new process, called “photon enhanced thermionic emission,” or PETE, simultaneously combines the light and heat of solar radiation to generate electricity.

Unlike photovoltaic technology currently used in solar panels – which becomes less efficient as the temperature rises – the new process excels at higher temperatures. The Stanford engineers who discovered it say the process promises to surpass the efficiency of existing photovoltaic and thermal conversion technologies. And the materials needed to build a device to make the process work are cheap and easily available, meaning the power that comes from it will be affordable.

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

Most photovoltaic cells, such as those used in rooftop solar panels, use the semiconducting material silicon to convert the energy from photons of light to electricity. But the cells can only use a portion of the light spectrum, with the rest just generating heat. This heat from unused sunlight and inefficiencies in the cells themselves account for a loss of more than 50 percent of the initial solar energy reaching the cell.

Capturing heat energy

The researchers knew that if this wasted heat energy could somehow be harvested, solar cells could be much more efficient. The problem has been that high temperatures are necessary to power heat-based conversion systems, yet solar cell efficiency rapidly decreases at higher temperatures. Until now, no one had come up with a way to wed thermal and solar cell conversion technologies.Melosh's group figured out that by coating a piece of semiconducting material with a thin layer of the metal cesium, it made 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."

While most silicon solar cells have been rendered inert by the time the temperature reaches 100 degrees Celsius, the PETE device doesn't hit peak efficiency until it is well over 200 C.

Solar farms

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 800 C. 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 minimize costs by meshing with existing technology."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," Melosh said. "So the PETE process has two really big benefits in energy production over normal technology."

Efficiency rates of up to 60 percent

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. Melosh calculates the PETE process can get to 50 percent efficiency or more under solar concentration, but if combined with a thermal conversion cycle, could reach 55 or even 60 percent – 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, thereby making conversion relatively inexpensive.

Proof of concept

The researchers used a gallium nitride semiconductor in the "proof of concept" tests. The efficiency they achieved in their testing was well below what they have calculated PETE's potential efficiency to be – which they had anticipated. But they used gallium nitride because it was the only material that had shown indications of being able to withstand the high temperature range they were interested in and still have the PETE process occur.With the right material – most likely a semiconductor such as gallium arsenide, which is used in a host of common household electronics – the actual efficiency of the process could reach up to the 50 or 60 percent the researchers have calculated. They 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 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 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 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 PETE process could really give the feasibility of solar power a big boost," Melosh said. "Even if we don't achieve perfect efficiency, let's say we give a 10 percent boost to the efficiency of solar conversion, going from 20 percent efficiency to 30 percent, that is still a 50 percent increase overall."

And the researchers say that is still a big enough increase that it could make solar energy competitive with oil.

Melosh is senior author of a paper describing the tests the Stanford University researchers conducted. It was published online in Nature Materials.