It’s not a new idea to improve upon traditional solar cells by first converting light into heat, then reemitting the energy at specific wavelengths optimally tuned to the requirements of the solar cell, but this method has suffered from low efficiencies. However, new research at MIT using nanoscale materials finally shows how thermophotovoltaics could become competitive with their traditional cousins, and grant benefits such as storing solar energy in the form of heat to postpone conversion into electricity.
The concept of a thermalphotovoltaic system in theory overcomes the limitations of two techniques for using solar energy. Traditional solar cells only respond to certain wavelengths of light, the so-called bandgap limit, but using solar energy simply to generate heat, as with a solar heat engine, limits production to utility-scale plants.
However, a hybrid of the two techniques yields not only more efficient energy production by overcoming the bandgap of the solar cell, but the ability to store and port energy by storing heat. However, past experiments have only yielded efficiencies of one percent, where traditional solar cells can reach efficiencies of 20 percent.
By using nanomaterials, the team at MIT developed a thermalphotovoltaic system which overcomes the earlier low efficiencies and hints at higher efficiencies in the future by simply scaling up their system.
In their design, a multiwalled carbon nanotube layer absorbs energy from the sun and converts it to heat, and a one-dimensional silicon-based photonic crystal is heated and emits that energy at a wavelength of light tuned to the bandgap of the solar cell – just as iron when heated glows in a red wavelength.
They reported a 3.2 percent efficiency with these new materials, which is certainly low compared to a traditional photovoltaic cell. But by just scaling up their existing materials, the team expects to see efficiencies of 20 percent, which becomes comparable with traditional systems. For example, the absorber-emitter layer they used was only 1 cm (0.39 in) in diameter, and at such small sizes is more prone to heat loss than a larger surface would be because of a higher surface-area-to-volume ratio. A larger absorber-emitter would retain more heat to be emitted to the solar cell, rather than radiate it into the air.
The Shockley-Quiesser bandgap limit is 33.7 percent for a traditional single-layer solar cell, with silicon systems theoretically having a 29 percent efficiency, though in practice performing much lower.
With improvements in thermalphotovoltaic systems, solar power generation would benefit in multiple ways. Heat energy could be stored during the day to port solar power to remote places or generate electricity at night, while existing solar cell components can instantly become more efficient with the help of one of these thermal devices.
In 2011 we saw a battery-like prototype from the same lab at MIT, which used photovoltaic cells with a heat source to generate efficient electricity in the absence of the sun.
The research was recently published in the journal Nature Nanotechnology.
In the video below, the MIT research team explains the process and expectations of their research.
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