Authors / Presenters
Lee A. Weinstein 1; Vazrik Chiloyan 1; Svetlana V. Boriskina 1; Gang Chen 1
1, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Solar thermal systems, or systems which convert sunlight to heat, commonly use spectrally selective surfaces in order to achieve high efficiency. Spectrally selective surfaces are designed to be absorptive in the solar spectrum, but reflective (and therefore non-emitting) in the blackbody spectrum of their intended operating temperature. Recently, attempts to improve the performance of spectrally selective surfaces have included using a variety of nano-scale features such as pyramids or inverse-pyramid geometries. Ideal selective surfaces should have selectivity change as abruptly as possible, switching sharply from absorbing to reflecting at a cutoff wavelength. However, real selective surfaces fall far short of such an ideal structure, with the switch from absorbing to reflecting occurring over a band of wavelengths. Since the response of a material to light must be causal, the Kramers-Kronig relations provide constraints on the real and imaginary parts of a material's index of refraction (or equivalently its susceptibility), precluding an emittance profile with a perfect step function. In this work, we establish the fundamental limit on the abruptness of spectral selectivity change, starting from the Kramers-Kronig relations. This limit sets the ultimate level of performance that fabricated spectrally selective surfaces should aspire to.
This material is based on work supported as part of the Solid State Solar-Thermal Energy Conversion Center (S3 TEC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award DE-SC0001299/DE-FG02-09ER46577.