A desirable feature of concentrated solar power system is to provide electricity in a dispatchable manner during cloud transients and non-daylight hours. A Dish-Stirling concentrating solar power prototype demonstration system was built to incorporate a thermal energy storage (TES) module containing a phase-change material between the solar thermal receiver and the Stirling engine. This paper presents the results of a material compatibility study conducted to determine the suitability of two different metal alloys for use in the construction of the TES module. Key requirements of the materials include strength and corrosion resistance at elevated temperatures, commercial availability, and manufacturability using common fabrication methods. The TES module contains a NaCl/NaF eutectic salt, at temperatures ranging from local ambient to 700°C, where the salt is slightly superheated above its melt temperature. Sample containers made from SS316L and Inconel 625 were fabricated and thoroughly cleaned for compatibility studies based on an extensive literature review. Both the containers and the salt constituents were subjected to a bake-out cycle to drive off moisture, and permit outgassing of contaminants. The containers were filled with salt in a controlled-atmosphere glove box. Filled containers were crimped and sealed by electron-beam welding. The finished samples were placed in a furnace, heated, and held at 750°C. One of each sample container material was removed from the furnace at both 100 and 2500 hours. The containers were cut open to analyze and evaluate the material surface and cross-section. After 100 hours, both SS316L and Inconel 625 exhibited a very small amount of corrosion. The stainless steel suffered a shallow inter-granular grain boundary attack, on the order of 1–2 mm in depth. The Inconel 625 surface formed an oxide complex, which is resistant to dissolution into the molten salt. After 2500 hours, the surface morphology for both materials was largely unchanged, with the corrosion process having switched from an initial localized pattern, to a more uniform pattern. The corrosion depth measured at 2500 hours remained near 1–2 mm, suggesting that the corrosion rate decelerated. Both materials showed promise for compatibility with the chosen salt.
Joint LLNL & SNL Material Compatibility Study Commences
