Attenuation Coefficient of High Temperature Molten Salts: An Experimental Approach

Abstract

In order to make thermal solar power compete with the traditional sources of energy, the efficiency must increase and one way of doing it is by changing the operating fluid. Among the alternate fluids is the use of molten salts as a part of the process; either for thermal storage and later utilization for electrical production during the hours without sun or as a substitute of the operating fluid to provide higher temperatures resulting in better efficiency. The difficulty of using molten salts is the lack of physical properties in literature; such as viscosity, boiling point, vapor pressure and volumetric absorption of solar radiation, thus making the selection of a suitable salt a very difficult endeavor. As a part of the Multidisciplinary Research Initiative (MURI) of the Department of Energy in the project of High Operating Fluids, this work will focus on the optical properties of the molten salts (volumetric absorption). The objective of this Thesis is to design, build and test a device capable of measuring the light attenuation coefficient; which is directly related to volumetric absorption of solar radiation, as well as determine the attenuation coefficient of various eutectic systems for the ternary salt mixture of ZnCl2, NaCl and KCl. Based on the little existent literature, a device capable of measuring the attenuation coefficient was designed, built, validated and tested. This was done by projecting a stable beam of light simulating sun radiation through the molten salt sample and to a spectrometer with a wavelength range going from 400 nm to 1000 nm with operating temperatures going from 350oC to 600oC. This device is capable of controlling the thickness, from 1 to 60 mm, of the molten salt sample by a computer controlled linear stage with an accuracy of 0.1mm. Quartz was used as a container for the molten salts because of its high melting point and transparency. A ceramic heater was used as a heat source, which can heat up the sample to temperatures up to 1200 oC if necessary. Two validation tests for the device were done by measuring the light attenuation coefficient of clear water and extra virgin olive oil and then they were compared to the ones in literature. The eutectic systems were tested next; the results characterized the attenuation coefficient as a function of wavelength and temperature, something that no other experimental work has done before for this specific fluid. These values will help to determine an optimal operating fluid for high temperature thermal applications.