The key to creating a material that is more efficient for converting solar energy to heat is to obtain the perfect absorption of the material. It should absorb virtually all wavelengths of light that reach Earth’s surface from the sun, but not much of the rest of the spectrum, since that would increase the energy that is reradiated by the material, and thus lost to the conversion process.
Now researchers at MIT claim they have accomplished the development of a material that comes very close to the “ideal” for solar absorption. 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. Perhaps most importantly, the material can also be made cheaply in large quantities.
The creation of this material is described in a paper that appeared in a current edition of 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.
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.
Some of the members of the team had previously worked on an earlier STPV device that took the form of hollow cavities, explains Chou, of MIT’s Department of Mechanical Engineering, who is the paper’s lead author. “They were empty, there was air inside,” he says. “No one had tried putting a dielectric material inside, so we tried that and saw some interesting properties.”
When harnessing solar energy, “you want to trap it and keep it there,” Chou says; getting just the right spectrum of both absorption and emission is essential to efficient STPV performance.
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.
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