MIT’s Quest for the Perfect Solar Absorber

Thursday, October 9th, 2014
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Researchers from MIT have developed a material which could be closest thing yet invented to a “perfect” solar absorber, while also possessing a range of other key advantages that make it ideal for commercial deployment.

Jeffrey Chou

Jeffrey Chou

According to Jeffrey Chou, a postdoc from MIT’s Department of Mechanical Engineering, the theoretical ideal for a solar absorber is a material which is capable of capturing that precise band of wavelengths of light through which the sun’s energy is conveyed, while also excluding the remainder of the spectrum, the absorption of which would increase re-radiation levels and conversion loss.

“It’s a very specific window that you want to absorb in,” said Chou. “We built this structure, and found that it had a very good absorption spectrum, just what we wanted.”

The material developed by Chou and his colleagues is a two-dimensional metallic dielectricphotonic crystal, which operates as part of a solar-thermophotovoltaic (STPV) device.

Upon exposure to solar radiation the material converts a broad range of wavelengths of light into heat, which causes the material to glow. The light emitted by the material as it glows can in turn be converted into electricity.

The device builds upon an earlier STPV device that contained hollow cavities, which were altered by Chou and his team to enhance the material’s conversion properties.

“They were empty, there was air inside,” said Chou. “No one had tried putting a dielectric material inside, so we tried that and saw some interesting properties.”

These tiny nano-cavities are the key to the heightened energy harvesting of the material, and can be adjusted in size to fine-tune its absorption.

In addition to absorbing the most beneficial part of the spectrum for energy conversion purposes, the material possesses a range of other advantages that make it ideal for solar energy harvesting.

It absorbs sunlight from a broad range of angles, dispensing with the need for costly and complicated solar trackers, while its ability to withstand temperatures as high as 1,000 degrees Celsius for prolonged periods means that it can be used with systems that concentrate sunlight by means of mirrors.

While prototypes of the device have been made using ruthenium, which comes at a comparatively high cost, Chou says it is extremely flexible when it comes to the materials it employs.

“We’re very flexible about materials,” Chou said. “In theory, you could use any metal that can survive these high temperatures.”

Most importantly of all from a commercial perspective, the material is highly suited to manufacturing techniques that are already widely employed  by the solar industry.

“This is the first ever device of this kind that can be fabricated with a method based on current techniques, which means it is able to be manufactured on silicon wafer scales,” said Chou.

According to Chou, up to 12 inches of the material can be produced on the side of a silicon wafer scale, as opposed to other systems which can be manufactured to be just a few centimetres in size using expensive metal substrates, rendering them unsuitable for large-scale commercial production.

“This is the first device that is able to do all these things at the same time,” said Chou. “It has all these ideal properties.”

Chou has co-authored a paper on the material with colleagues Marin Soljacic, Nicholas Fang, Evelyn Wang and Sang-Gook Kim. The paper has been published in the journal Advanced Materials, and the team hopes to see their discovery become a commercially viable product within five years time.

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