New Solar Power Device Uses Heat To More Efficiently Capture Energy

By Ajit Jha on January 19, 2014 1:45 PM EST

solar power
MIT engineers created a solar power device (not pictured) that more efficiently captures the sun's various rays of light and turns it into energy. (Photo: Pure3D Visualisierungen, CC BY)

Massachusetts Institute of Technology (MIT) researchers have found a way to use solar energy that makes its harvest more efficient and stores it for later use as well. The strategy they found uses sunlight to heat a high-temperature material, which then transfers the infrared radiation it collects into a conventional photovoltaic (PV) cell.

The new process is described in the journal Nature Nanotechnology. The process is pretty simple, and was designed to access wavelengths of light that are otherwise wasted, leading to improved performance. The conventional silicon-based solar cell doesn't access all photons it's exposed to because silicon is not equipped to respond to every  wavelength of light, according to Evelyn Wang, associate professor of mechanical engineering at MIT.

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The team addressed this limitation by using novel materials, including carbon nanotubes and photonic crystals, to build a two-layer absorber-emitter device that is placed between the sunlight and the PV cell. The material is well equipped to collect broad-spectrum energy from sunlight while heating during the process. When it's heated to sufficient temperatures, it emits light of a particular wavelength that's matched to the energy level of the PV material, called a bandgap.

Although this basic concept is fairly known, applying it to actual materials has been relatively difficult. There is a theoretical limit on efficiently converting energy through semiconductor-based PV devices, called the Shockley-Queisser limit. It means that most systems only have about 33.7 percent efficiency. The engineers' solar thermophotovoltaic (STPV) system looks for a way around this limit. "The efficiency would be significantly higher - it could ideally be over 80 percent," Wang said in a press release.

Previous experiments have not been able to produce more than one percent energy efficiency with a STPV device. Wang and the team, however, claim that they have made an initial test device with over 3.2 percent efficiency, and that they expect to pull up to 20 percent efficiency with further work. This would be enough to make the product commercially viable. The two layer absorber-emitter material is the key component behind this improvement.

The researchers simulated sunlight, and found that peak efficiency came when the intensity of the light was equal to that of a focusing system concentrating sunlight to a factor of 750. This light was potent enough to heat the absorber-emitter to a temperature as high as 962 degrees Celsius.

This level of concentration is not as high as those obtained in earlier STPV studies that attained concentrated sunlight by a factor of several thousand. The MIT researchers, however, are confident that they can attain the same level of enhancement at lower sunlight concentrations after further optimization.

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