Optical properties of high temperature molten salt mixtures for volumetrically absorbing solar thermal receiver applications

Abstract

Molten salts are promising candidates for liquid volumetric absorbers in concentrated solar power systems. To characterize absorption and heat transfer performance in high temperature applications, their optical properties are required. Thus a method for experimentally determining the absorption coefficient of non-scattering high temperature semi-transparent liquids for large (∼1m-deep) direct absorption solar receiver applications was developed. It was used to measure the absorption coefficient in liquids over a broad spectral range and temperatures up to 800°C in a 40wt.% KNO3:60wt.% NaNO3 binary nitrate molten salt mixture (solar salt) and a 50wt.% KCl:50wt.% NaCl binary chloride molten salt mixture. The binary nitrate and binary chloride both demonstrated well distributed solar absorption (>95% absorption through 1m and 2m, respectively). At 400°C, the binary nitrate is optically thick in its re-emission spectrum and behaves as a blackbody radiator. The effects of thermal decomposition were also shown to have significant consequences on the overall performance of the binary nitrate mixture, transforming it into an opaque surface absorber following thermal degradation (>95% in <0.25m). The implications of these results for solar receiver design are discussed in terms of volumetric absorption, total effective emissivity, and capture efficiency. The measurement technique developed and results are relevant in a variety of high temperature applications including heat transfer systems, materials processing, pharmaceuticals, and food processing facilities.