Researchers from Massachusetts Institute of Technology are developing and perfecting a new way of harvesting solar energy, which has potential to increase the efficiency of solar electricity by more than a factor of three.

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Traditional solar panels utilize photovoltaic (PV) cells which are primarily composed of the semiconductor silicon. Silicon is an ideal PV material, possessing many special qualities which the harvesting of solar energy requires.

However, silicon is not perfect – because of a property called ‘bandgap’, which must correspond to the energy levels of sunlight’s photons in order to produce electricity, silicon is unable to use many wavelengths of sunlight.

Because of this limitation, silicon has a cap on its possible efficiency called the Shockley-Queisser limit, which sets the amount of energy that silicon PV cells can harvest from sunlight at a maximum of 33.7%

However, researchers from MIT are developing and perfecting a new way of harvesting solar energy which could cheat this limit, and increase the amount of usable energy by more than a factor of three.

Associate professor of mechanical engineering Evelyn Wang, who helped to pen a paper on the process published this week in Nature Nanotechnology, said of the process, “the efficiency would be significantly higher — it could ideally be over 80 percent.”

The solution to silicon’s limit is simple: between the sunlight and the silicon cells, add a third element that converts the unusable wavelengths of light into usable ones.

In this case, the converter comes in the form of a double-layered absorber-emitter, composed of specials materials like carbon nanotubes, and photonic crystals.

The elements are heated up by a concentration of sunlight. Material in the absorber-emitter which has been finely engineered to emit just the right wavelength of light is activated, and in turn stimulates the actual solar cells which begin the conversion of light into electricity.

This marriage of thermal energy and PV cells, both of which have enjoyed a long career separate from each other, is known as thermophotovoltaic (TPV) energy conversion.

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The researchers at MIT haven’t come near to the 80% efficiency described by Wang – however, experiments with the technology in laboratory conditions have proved to be a breakthrough. Until now, nobody has managed to produce TPV energy at more than 1% efficiency.

However, the team at MIT were able to achieve 3.2% with their initial device, and expect to go on to 20% in the near future. This brings the technology closer to its capacity of revolutionizing solar electricity.

Professor of mechanical engineering at the Georgia Institute of Technology Zhuomin Zhang lauds the project, saying “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.”

Source: MIT