MIT researchers design ‘perfect’ solar absorber

New system aims to harness the full useful portion of the solar spectrum, no solar trackers required
October 1, 2014

This rendering shows a metallic dielectric photonic crystal that stores solar energy as heat (credit: Jeffrey Chou)

MIT researchers say they have developed a material that comes very close to the “ideal” for converting solar energy to heat (for conversion to electricity).

It should absorb virtually all wavelengths of light that reach Earth’s surface from the sun — but not much of the rest the longer-wavelength infrared portion of the solar spectrum, since that would increase the energy that is re-radiated by the material, and thus lost to the conversion process.

The material is a two-dimensional metallic dielectric photonic crystal, and has the additional benefits of absorbing sunlight from a wide range of angles and withstanding extremely high temperatures. It can also be made cheaply at large scales.

The creation of this material is described in a paper appearing this week in the journal Advanced Materials, co-authored by MIT postdoc Jeffrey Chou, professors Marin Soljacic, Nicholas Fang, Evelyn Wang, and Sang-Gook Kim, and five others.

Optimal absorption wavelengths

Measured absorption spectrum for the MIT photonic crystal with and without an anti-reflection coating (ARC) for 85% of photon energies from .7 electron-volts (1771 nm, or near-IR) to 5 electron-volts (248 nm, or ultraviolet) wavelengths. Yellow represents the solar spectrum received through the Earth’s atmosphere. (Credit: J. Chou et al./Advanced Materials)

The material works as part of a solar-thermophotovoltaic (STPV) device: the sunlight’s energy is first converted to heat, which then causes the material to glow, emitting light that can, in turn, be converted to an electric current.

Most of the sun’s energy reaches us within a specific band of wavelengths, Chou explains, ranging from the ultraviolet through visible light and into the near-infrared.

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

In addition, the absorption characteristics can be controlled with great precision: the material is made from a collection of nanocavities, and “you can tune the absorption just by changing the size of the nanocavities,” Chou says.

The material is also well matched to existing manufacturing technology. “This is the first-ever device of this kind that can be fabricated with a method based on current … techniques, which means it’s able to be manufactured on silicon wafer scales,” Chou says —- up to 12 inches on a side. Earlier lab demonstrations of similar systems could only produce devices a few centimeters on a side with expensive metal substrates, so were not suitable for scaling up to commercial production, he says.

To take maximum advantage of systems that concentrate sunlight using mirrors, the material must be capable of surviving unscathed under very high temperatures, Chou says. The new material has already demonstrated that it can endure a temperature of 1,000 degrees Celsius (1,832 degrees Fahrenheit) for a period of 24 hours without severe degradation.

And since the new material can absorb sunlight efficiently from a wide range of angles, Chou says, “we don’t really need solar trackers” — which would add greatly to the complexity and expense of a solar power system.

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

While the team has demonstrated working devices using a formulation that includes a relatively expensive metal, ruthenium, “we’re very flexible about materials,” Chou says. “In theory, you could use any metal that can survive these high temperatures.”

The group is now working to optimize the system with alternative metals. Chou expects the system could be developed into a commercially viable product within five years.

UPDATE Oct. 1: absorption spectrum figure added.