MIT device uses heat store to crank up solar panel output

US researchers have found a way to use heat to improve the efficiency of solar panels.

Photovoltaic (PV) cells typically produce less electricity the hotter they get, but the engineers from Massachusetts Institute of Technology (MIT) have developed a solar-powered heat store to produce infrared radiation that can then be collected by the cells.

The researchers say the device, heated by wavelengths of the sun’s light that aren’t captured by conventional PV cells, effectively turning them into wavelengths that can be captured, could also make it easier to store solar energy for later use.

Previous attempts at such solar thermophotovoltaic (STPV) systems have operated at efficiencies of under one per cent but the MIT team claim their device already has an efficiency of 3.2 per cent and that they expect to increase this to 20 per cent – enough to make it commercially viable.

Conventional solar PV cells have a theoretical efficiency limit of 33.7 per cent. With the STPV system ‘the efficiency would be significantly higher — it could ideally be over 80 per cent’, said researcher Evelyn Wang in a statement.

The new system uses a two-layer absorber-emitter device made of novel materials including carbon nanotubes and photonic crystals, placed over the PV cells.

To achieve the desired efficiency, the outer sunlight-facing layer of the device features an array of multiwalled carbon nanotubes that efficiently absorbs the light’s energy and turns it to heat.

This layer is bonded to a layer of a photonic crystal that is engineered to covert the heat back into light at wavelengths that can mostly be capture by the adjacent PV cells.

The researchers used simulated sunlight to test the device and found its peak efficiency came when its intensity was equivalent to a focusing system that concentrates sunlight by a factor of 750, heating the absorber-emitter to a temperature of 962ºC.

This level of concentration is already much lower than in previous STPV systems, which concentrated sunlight by a factor of several thousand. But the MIT researchers say that further optimisation should produce enhancement at even lower sunlight concentrations.

One way to improve efficiency should be to increase the size of the device: the larger it is, the lower its surface area relative to its volume, which means it loses heat less quickly.

A paper on the research written by Wang, physics professor Marin Soljačić, principal research scientist Ivan Celanović and graduate student Andrej Lenert, was published this week in the journal Nature Nanotechnology.