(a) Energy diagram of the PETE process. Photoexcitation increases the conduction-band population, leading to larger thermionic currents and enabling the device to harvest both photon and heat energy. (b) One possible implementation. Schwede et al. Click to enlarge.

Unlike photovoltaic technology currently used in solar panels—which becomes less efficient as the temperature rises—he new process excels at higher temperatures.

This is really a conceptual breakthrough, a new energy conversion process, not just a new material or a slightly different tweak. It is actually something fundamentally different about how you can harvest energy.

—Dr. Nick Melosh

Melosh is an assistant professor of materials science and engineering, and is senior author of a paper describing the tests the researchers conducted, published online 1 August in Nature Materials.

Most photovoltaic cells 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% of the initial solar energy reaching the cell.

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.

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

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 minimizing costs by meshing with existing technology.

Melosh calculates the PETE process can get to 50% efficiency or more under solar concentration, but if combined with a thermal conversion cycle, could theoretically reach 55 or even 60%—almost triple the efficiency of existing systems.

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—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, thereby helping to keep costs down.

The PETE process could really give the feasibility of solar power a big boost. 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.

—Nick Melosh

Resources

• Jared W. Schwede, Igor Bargatin, Daniel C. Riley, Brian E. Hardin, Samuel J. Rosenthal, Yun Sun, Felix Schmitt, Piero Pianetta, Roger T. Howe, Zhi-Xun Shen & Nicholas A. Melosh (2010) Photon-enhanced thermionic emission for solar concentrator systems. Nature Materials doi: 10.1038/nmat2814