Using the light from the Sun to generate electricity is commonplace. So is generating electricity through heat, as in steam turbines. Combining solar light and solar heat to generate energy is an obvious juxtaposition, but until now undemonstrated as a feasible technology. That’s why the proof of concept testing on a concept called PETE (photon enhanced thermionic emission) performed by scientists at Stanford University (California, USA), and published in Nature Materials August 1 [Photon-enhanced thermionic emission for solar concentrator systems, payment or subscription required] sets up an interesting and potentially important new track for alternative sources of energy.
The key to the new approach is to use semiconductor material that with some modification can produce energy in the usual fashion of a solar cell, but also withstand temperatures high enough to be useful for generating thermal energy. Silicon is the familiar semiconductor used in most of today’s solar cells, but its tolerance for heat is quite low – about 100 degrees Celsius (the boiling point of water). Thermal heating requires at least 200 degrees Celsius, and higher is better. The original testing was done with gallium nitride as the semiconductor material, which was coated with a thin layer of cesium. The addition of the metal cesium adds the property of converting thermal energy to electricity.
In conventional solar cells, the silicon semiconductor is able to convert light to energy only in a limited portion of the spectrum. The rest of the spectrum of light is wasted, radiated in the form of heat. In the new technology, the heat energy is itself converted to electricity, which can in theory achieve 50 to 60 percent use of the available energy. This would be a huge increase over conventional solar cell efficiency. The current proof-of-concept test did not achieve that sort of energy conversion, but the researchers believe that using gallium arsenide, a common material used in electronics, will be much better. They are continuing to experiment with various semiconductor materials in hopes of finding the combinations that yield the most efficient combination of thermal and light conversion.
Even at lower efficiency rates the new technology has considerable promise. Because of its ability to use high heat, the PETE process lends itself for use in solar concentrators – the large parabolic mirror devices that are now used for some solar energy installations. These concentrators can run up to 800 degrees Celsius, and the Stanford scientists believe that their semiconductor material can be used to harvest a good portion of that concentrated light/heat, while easily installing and working with the existing thermal converter systems.
The goal, which is perhaps realistic if the PETE heat and light formula holds up to large-scale application, is to meet or beat the price of oil (as measured by contemporary prices). The closer solar energy comes to oil in cost, the more powerful the motivation to do more of it – which prompts economies of scale and lowers the cost even more.
“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.”
[Source: Stanford University]