Optical properties of high-temperature materials for direct absorption receivers

Abstract

Recent solar central receiver designs emphasize direct absorption receiver (DAR) concepts primarily because of their ability to absorb high flux densities. An attractive design utilizes molten salt as the transport/storage fluid, which is pumped to the receiver and allowed to flow over a high-temperature absorber surface. As the salt runs down the absorber surface in a thin film, concentrated solar flux heats the salt to 900°C, either directly (blackened fluid) or by convective heat transfer with the irradiated absorber (clear fluid). The feasibility of such a design depends on the optical efficiency of the absorber/fluid combination. The optical properties of candidate absorber materials and transport fluids are therefore required at appropriate elevated temperatures. Because such salts can be extremely reactive at high temperatures, it is important to measure optical properties as a function of exposure history of the salt/absorber. Optical characterization of a clear molten carbonate salt (Li 2CO 3Na 2CO 3K 2CO 3 ternary eutectic) and a high-temperature metal alloy (Inconel 600) has been carried out at elevated temperatures using a recently developed integrating sphere solar spectrometer. Reflectance measurements of oxidized Inconel 600 alone and covered with several thicknesses of molten salt have been made. Measurements were also made of the reflectance of Inconel 600 samples that had been exposed to molten salt under high cyclic temperatures.