Molten Salt Spectroscopy for Quantification of Radiative Absorption in Novel Metal Chloride Enhanced Thermal Storage Media

Abstract

This study describes the development and characterization of novel hightemperature thermal storage media, based on inclusion of transition metal chlorides in the potassium–sodium chloride eutectic system, (K–Na)Cl (melting temperature of 657 آ°C, latent heat of 278 J/g). At the melting temperature of (K–Na)Cl, infrared (IR) radiation can play a major role in the overall heat transfer process—90% of spectral blackbody radiation falls in the range of 2–13 خ¼m. The authors propose inclusion of small amounts (less than 0.2 wt.%) of IRactive transition metal chlorides to increase radiative absorption and thereby enhance heat transfer rates. A new IRreflectance apparatus was developed to allow for determination of the spectral absorption coefficient of the newly formulated phasechange materials (PCMs) in the molten state. The apparatus consisted of an alumina crucible coated at the bottom with a reflective (platinum) or absorptive (graphite) surface, a heated ceramic crucibleholder, and a combination of zinc sulfide (ZnS) and zinc selenide (ZnSe) windows for containment of the salt and allowance of inert purge gas flow. Using this apparatus, IR spectra were obtained for various transition metal chloride additives in (K–Na)Cl and improved IR activity, and radiative transfer properties were quantified. Further, thermophysical properties relevant to thermal energy storage (i.e., melting temperature and latent heat) are measured for the pure and additiveenhanced thermal storage media.