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Tapping infrared radiation for solar energy harvesting

Posted: 21 Jan 2014     Print Version  Bookmark and Share

Keywords:solar energy  photovoltaic cell  infrared radiation 

A team of researchers from the Massachusetts Institute of Technology has formulated a method that is said to improve solar energy harvesting efficiency by using sunlight to heat a high-temperature material whose infrared radiation would then be collected by a conventional photovoltaic (PV) cell. This technique, say the researchers, could make it easier to store energy collected for later use.

In this case, adding the extra step improves performance, because it makes it possible to take advantage of wavelengths of light that ordinarily go to waste. The process is described in a paper written by graduate student Andrej Lenert, associate professor of mechanical engineering Evelyn Wang, physics professor Marin Soljacic, principal research scientist Ivan Celanovic, and three others.

A conventional silicon-based solar cell "doesn't take advantage of all the photons," Wang said. That's because converting the energy of a photon into electricity requires that the photon's energy level 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.

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Figure 1: A nanophotonic solar thermophotovoltaic device composed of an array of multi walled carbon nanotubes as the absorber, a 1D silicon/silicon dioxide photonic crystal as the emitter, and a 0.55eV PV cell.

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, as with a piece of iron that glows red hot, it emits light of a particular wavelength, which in this case 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 photovoltaic devices. That limit, called the Shockley-Queisser limit, imposes a cap of 33.7 percent on such efficiency, but Wang says that with TPV systems, "the efficiency would be significantly higher: it could ideally be over 80 percent."

There have been many practical obstacles to realizing that potential; previous experiments have been unable to produce a STPV device with efficiency of greater than one percent. But Lenert, Wang, and their team have already produced an initial test device with a measured efficiency of 3.2 percent, and they say with further work they expect to be able to reach 20 percent efficiency, enough, they say, for a commercially viable product.


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