The key is to prevent the heat from escaping the thermoelectric material, something the MIT team achieved by using a photonic crystal: essentially, an array of precisely spaced microscopic holes in a top layer of the material.
If you put an ordinary, dark-colored, light- and heat-absorbing material in direct sunlight, “it can’t get much hotter than boiling water,” because the object will reradiate heat almost as fast as it absorbs it. But to generate power efficiently, you need much higher temperatures than that. By concentrating sunlight with parabolic mirrors or a large array of flat mirrors, it’s possible to get much higher temperatures — but at the expense of a much larger and more complex system.\
Nanoscale research letters - Tailoring photonic metamaterial resonances for thermal radiation
Selective solar absorbers generally have limited effectiveness in unconcentrated sunlight, because of reradiation losses over a broad range of wavelengths and angles. However, metamaterials offer the potential to limit radiation exchange to a proscribed range of angles and wavelengths, which has the potential to dramatically boost performance. After globally optimizing one particular class of such designs, we find thermal transfer efficiencies of 78% at temperatures over 1,000°C, with overall system energy conversion efficiencies of 37%, exceeding the Shockley-Quiesser efficiency limit of 31% for photovoltaic conversion under unconcentrated sunlight. This represents a 250% increase in efficiency and 94% decrease in selective emitter area compared to a standard, angular-insensitive selective absorber.
The next step in the research, Bermel says, is to test different materials in this configuration to find those that produce power most efficiently. With existing solar thermophotovoltaic systems, he says, “the highest efficiency [in converting solar energy to electricity] is 10 percent, but with this angular-selective approach, maybe it could be 35 to 36 percent.” That, in turn, is higher than the theoretical maximum that could ever be achieved by traditional photovoltaic solar cells.
It was found that although in principle solar thermophotovoltaic systems in unconcentrated sunlight can exceed efficiencies of 42%, achieving such performance requires suppression of emissivities to unreasonably low levels. Conventional materials with undesired emissivities of 0.05 display much lower efficiencies of 10.5%. However, most of the theoretically allowed performance can be restored by introducing angular selectivity of the assumed form in Equation 5, with up to 37% overall system efficiency. The system also acts as a thermal concentrator, with receiver areas 20 times larger than the emitter areas. Finally, we considered 2D arrays of nanoscale cylindrical holes in single crystal tungsten as a candidate metamaterial for angle-selective operation, and found the optimal design parameters to be a period of 800 nm, a radius of 380 nm, and a depth of 3.04 μm, with a thermal transfer efficiency of 75.0% in unconcentrated sunlight at 400 K.
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