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Research at the Massachusetts Institute of Technology (MIT) could pave the way for solar cells able to generate electricity to match demand, by absorbing solar energy in the form of heat.
A conventional silicon solar cell doesn’t capture the entire spectrum of light, because the semiconductor material’s ‘bandgap’ tends not match photons in the infrared range, and thus misses out on their energy. To address this, the MIT team has created a two-layer absorber-emitter made of carbon nanotubes (CNTs) and photonic crystals that sit between the sun’s rays and the photovoltaic cell, absorbing solar energy in the form of heat first, and then light. This results in a three-fold conversion efficiency increase. The system could lay the groundwork for on-demand solar PV generation, as the heat captured could be stored to generate electricity once the sun has set.
While it is by no means the first solar thermophotovoltaic (STPV) solution, previous experiments have only produced devices with conversion efficiencies of around 1%, compared to the 3.2% efficiency measured in the MIT device. Work is now underway to scale up the absorber-emitter device from a 1 cm sq area to 10 cm sq, which could increase efficiencies to 20% due to the larger active area and reduction in parasitic heat loss.
“One element that makes our approach interesting is that it may be easier and lower cost to store thermal energy than electrical energy”, says Evelyn Wang, an Associate Professor within MIT’s department of mechanical engineering, who worked on the research. This would require phase-change materials or chemical means to store the heat at high temperatures. “The thermal storage would allow the emitter to get to the desired temperatures, so that the energy of the thermal emission can match the band gap of the PV cell, and generate efficient electricity later on”, Wang explains.
To produce the absorber-emitter STPV device, a photonic crystal layer is made by depositing thin alternating layers of silicon and silicon dioxide. A layer made from multi-walled CNTs is grown by chemical vapour deposition on the other side of the substrate. When facing sunlight, the CNTs absorb it and turn its energy into heat. As the photonic layer gets hot it ‘glows’ with light at a wavelength tuned to match the bandgap of the adjacent PV cell. Most of the energy collected by the absorber is therefore turned into electricity.
Fatima Toor, a solar research analyst at Lux Research, thinks STPV technology is needed in the long term to achieve a low levelised cost of electricity ($/kWh) for solar energy. Electricity generation could occur at a competitive cost during the entire 24 hours instead of just sunlight hours. “STPV incorporates low cost conventional PV with additional material components to enhance the efficiency of PV cells, resulting in overall cost reductions”, she says. – Sara Ver-Bruggen