The problem that solar photovoltaic devices always run up against is one of efficiency. PV devices are only able to receive energy from a certain range of wavelengths of light, meaning that the energy in the other wavelengths of the light is wasted. A new technique developed by MIT researchers looks likely to improve efficiency by using the sunlight to first heat a special material which then begins to produce infrared radiation with a much smaller wavelength range. The PV device then converts the radiation into electricity, meaning that far more of the energy from the original sunlight is converted. The process is known as thermphotovoltaics (TPV).

The team of researchers include graduate student Andrej Lenert, associate professor of mechanical engineering Evelyn Wang, physics professor Marin Solja?i?, principal research scientists Ivan Celanovi?, and three others.

Wang explained that a conventional silicon-based PV solar cell cannot use the energy from all the photons that hit its surface. PV materials can only convert a photon’s energy into electricity if the photon’s energy level is within a certain bandgap (the range to which the PV material is receptive). Silicon can absorb the energy from a large bandgap, but there is still a lot of energy contained in the light that goes to waste.

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Due to this problem with PV materials there is a theoretical limit to the energy-conversion efficiency achievable; known as the Shockley-Queisser limit, which is set at 33.7 percent. By including an extra step more of the energy in the light can be used, with Wang claiming that the conversion efficiency of a TPV system “would be significantly higher — it could ideally be over 80 percent.”

The TPV system uses a two-layer absorber-emitter device made of carbon nanotubes and photonic crystals. A focussing system concentrates sunlight by a factor of 750 onto the carbon nanotubes which absorb the energy from a broad spectrum of sunlight in a very efficient manner and convert it into heat (920°C). The nanotubes are bonded to a layer of photonic crystal that, when heated, glows and emits radiation of a specific range. This radiation will then hit a standard PV device with a bandgap calibrated to the range of radiation emitted by the crystal.

The system effectively works like a funnel, channelling a larger amount of energy from the light down onto the PV device to be turned into electricity.

Despite the potential to create incredibly efficient systems, previous attempts by other teams to create TPV devices have generally been unable to record any efficiency over one percent. However the this MIT team have generated excitement by producing a conversion efficiency of 3.2%, and claiming that with further work they will be soon able to reach 20 percent efficiency.

Zhuomin Zhang, a professor of mechanical engineering at the Georgia Institute of Technology stated that “this work is a breakthrough in solar thermophotovoltaics, which in principle may achieve higher efficiency than conventional solar cells because STPV can take advantage of the whole solar spectrum. … This achievement paves the way for rapidly boosting the STPV efficiency.”

By. Charles Kennedy of Oilprice.com