Heat improves solar device efficiency

Print edition : February 07, 2014

RESEARCHERS from the Massachussetts Institute of Technology (MIT) have found a method with increased efficiency to harness solar energy. They have shown that first using sunlight to heat a high-temperature material, whose infrared radiation would then be collected by a conventional photovoltaic (PV) cell, could improve efficiency. This technique could also make it easier to store the energy for later use, the researchers say. The process is described in a paper published in the journal Nature Nanotechnology. It has been authored by Andrej Lenert, Evelyn Wang and others.

The extra step involved in the technique makes it possible to take advantage of wavelengths of light that ordinarily go to waste. A conventional silicon-based solar cell “doesn’t take advantage of all the photons”, Evelyn Wang explains. That is because converting the energy of a photon into electricity requires the photon’s energy level to match that of a characteristic of the PV material called a bandgap. Silicon’s bandgap responds to many wavelengths of light but misses many others. To address that limitation, the team inserted a two-layer absorber-emitter device—made of novel materials, including carbon nanotubes and photonic crystals—between the sunlight and the PV cell. This intermediate material collects energy from a broad spectrum of sunlight, heating up in the process. When it heats up, it emits light of a particular wavelength just as with a piece of iron that glows red hot. This wavelength is tuned to match the bandgap of the PV cell mounted nearby.

This basic concept has been explored for several years since in theory such solar thermophotovoltaic (STPV) systems could provide a way to circumvent a theoretical limit on the energy-conversion efficiency of semiconductor-based PV devices. That limit, called the Shockley-Queisser limit, imposes a cap of 33.7 per cent on such efficiency, but Evelyn Wang says that with TPV systems, “the efficiency would be significantly higher—it could ideally be over 80 per cent”.

But this potential could never be realised as there seemed to be practical difficulties in the approach; previous experiments were unable to produce a STPV device with efficiency of greater than 1 per cent. But Lenert, Evelyn Wang, and their team have already produced an initial test device with a measured efficiency of 3.2 per cent, and they say with further work they expect to be able to reach 20 per cent efficiency—enough, they say, for a commercially viable product.

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