Spectral selectivity of high-temperature solar absorbers.

Numerical calculations of the thermal emissivity (TH) and normal incidence solar absorptivity alpha(s) of model spectrally selective solar absorbers at high temperatures are reported. The model absorbers consist of Drude metal substrates coated with layers that are successively made better and better approximations to a selective solar absorber. We initially calculate the emissivity of the bare metal substrates (M)(TH) as a function of temperature. We then coat the metal substrate with a homogeneous dielectric layer of index n(L) and find that (TH) of the coated metal increases monotonically with n(L) from in(TH) at that temperature. The dielectric layer is then replaced by a selectively absorbing layer with the optimum physically realizable spectral absorptivity, and maximum values of alpha(s), and minimum values of (TH) are calculated as functions of operating temperature and layer thickness. Finally, we replace the homogeneous selective layer with one having a complex refractive index graded linearly through the thickness of the film. It is found that, compared with homogeneous films of the same thickness, the graded films typically have a higher alpha(s), and a lower (TH). For films thick enough to be useful absorber surfaces, however, the improvements in alpha(s), and (TH) are small.