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Nanomaterial thermophotovoltaic system increases efficiency and portability of solar power


January 21, 2014

A closeup of the tiny thermal PV system, in which a carbon nanotube layer absorbs solar en...

A closeup of the tiny thermal PV system, in which a carbon nanotube layer absorbs solar energy and heats the single layer of photonic crystals, which re-emits energy back at a optimal wavelength for the solar cell

Image Gallery (2 images)

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.

The thermalphotovoltaic device glows while converting solar energy to heat and re-emitting...
The thermalphotovoltaic device glows while converting solar energy to heat and re-emitting light to the solar cell

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.

Source: MIT

About the Author
Heidi Hoopes Heidi measures her life with the motley things she's done in the name of scientific exploration. While formally educated in biology and chemistry, informally she learns from adventures and hobbies with her family. Her simple pleasures in life are finding turtles while jogging and obsessively winnowing through her genetic data.   All articles by Heidi Hoopes

"A larger absorber-emitter would retain more heat to be emitted to the solar cell, rather than radiate it into the air."

Given that the unit absorbs infrared energy and re-emits it in another more desirable frequency, why does there have to be any exposure to air?

ie, if you create a spherical chamber in a solid block of iron,

then paint the chamber walls with said new material which would begin to radiate inwards towards a solar collector in the center of the chamber

21st January, 2014 @ 07:56 pm PST

so these crystals in combination with solar cells are more effective than peltier elements?

22nd January, 2014 @ 12:30 am PST

"[...] and a one-dimensional silicon-based photonic crystal [...]", something wrong here? One-dimensional refers to a mathematical line of zero width, I believe, which is something which doesn't actually exist in the real universe. Maybe you meant two-dimensional, a sheet? But even that is a bit dodgy really because even when a sheet is just one atom thick, it's a whole atom thick, and that's a lot bigger than zero. Best would be simply to stick to the old one-atom-thick idea really?

22nd January, 2014 @ 12:34 pm PST
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