Researchers at Stanford University have developed a process to simultaneously convert solar heat and light directly into usable electricity. Recent tests proved that the concept, known as photon enhanced thermionic emission, or PETE, was in fact, reality. The effect of PETE on the efficiency of solar power generating systems is unprecedented.
Conventional photovoltaic (PV) systems use a semiconductor, typically silicon, to convert incident light directly into electric current. Today’s market-ready PV systems peak at about 20 percent conversion efficiency (the percentage of sunlight striking a solar cell that is converted to usable electricity), with the remaining energy lost as waste heat. Moreover, the efficiency of these systems decreases as temperature increases.
Meanwhile, heat-based solar energy systems, or solar thermal electric generators, rely on very high temperatures to create steam to run an electricity-generating turbine. This opposition has prevented anyone from successfully marrying the two technologies… until now.
With PETE, Stanford researchers have found a way to redefine the relationship between solar thermal and photovoltaic processes. By converting both heat and light into electricity, the PETE process actually increases in efficiency as temperature rises, and researchers believe that PETE could reach up to 60-percent conversion efficiency– more than triple the average solar electric product on the market today.
PETE is different from other solar innovations in that it is not an incremental improvement on an existing technology. It is an entirely new process. The Stanford research team, led by Nick Melosh, assistant professor of materials science and engineering, found that when they coated a semiconductor material (gallium nitride for the initial tests), with the metal cesium, the material developed the ability to generate electricity from both heat and light.
“This is really a conceptual breakthrough, a new energy conversion process, not just a new material or a slightly different tweak,” said Melosh. “It is actually something fundamentally different about how you can harvest energy.”
The materials needed to build the device are cheap, readily available and sufficient in small amounts for each PETE system. Furthermore, Stanford scientists are initially aiming for the device to be easily integrated into existing solar thermal power plants, where farms of parabolic dishes focus sunlight onto black tubes full of fluid. These farms already utilize a thermal conversion process to create electricity. PETE, researchers say, would integrate nicely into that scenario.
“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,” Melosh said. “And then we would dump the waste heat to their existing thermal conversion systems. So the PETE process has two really big benefits in energy production over normal technology.”
He added that a PETE device could reach up to 50 percent efficiency under solar concentration but could get even higher (up to 60 percent) when used in tandem with a thermal process.
The initial testing proved their technology worked, but efficiency was well below that stated goal of 50 to 60 percent, which researchers have calculated should be PETE’s high end. Much of that has to do with the semiconductor material. They used gallium nitride in the tests because it was the only material that appeared able to handle the high temperature. With the right material, possibly gallium arsenide, efficiency should spike.
The Stanford team’s findings were published on August 1st in Nature Materials.