Novel solar device to use heat to enhance efficiency

Researchers from Massachusetts Institute of Technology (MIT), used sunlight to heat a high-temperature material whose infrared radiation would then be collected by a conventional photo-voltaic cell.

A conventional silicon-based solar cell does not take advantage of all the photons. “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 photovoltaic (PV) material called a bandgap. Silicon's bandgap responds to many wavelengths of light, but misses many others,” said Evelyn Wang, associate professor of mechanical engineering at MIT.

To address this, 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, said the study published in the journal Nature Nanotechnology.

Adding the extra step improves performance, because it makes it possible to take advantage of wavelengths of light that ordinarily go to waste.

“The design of the two-layer absorber-emitter material is key to this improvement,” said physics professor Marin Soljacic.

According to MIT researchers, after further optimisation, it should be possible to get the same kind of enhancement at even lower sunlight concentrations, making the systems easier to operate.

Such a system combines the advantages of solar photovoltaic systems, which turn sunlight directly into electricity, and solar thermal systems, which can have an advantage for delayed use because heat can be more easily stored than electricity, added Ivan Celanovic, principal research scientist.

The new solar thermo-photovoltaic systems could provide efficiency because of their broadband absorption of sunlight, scalability and compactness and ease of energy storage, the study said.