Design, Fabrication and Testing of an Apparatus for Material Compatibility Testing in Nitrate Salts at Temperatures Up to 700°C

Thermal energy storage is one of the key differentiators between Concentrating Solar Power (CSP) and other renewable energy technologies. Molten salt is an effective and affordable method of storing thermal energy. Current salt storage systems charge at temperatures between 390°C and 585°C (oil filled parabolic trough systems to molten salt towers). It is highly desirable to increase the operating temperature of salt storage systems in order to increase the efficiency of the power cycle and to permit the use of alternative, high-temperature cycles. However, higher salt temperatures cause increased reactivity and thus increased corrosion rates in many materials. In order to utilize molten salt at higher temperature, it is necessary to test and understand these corrosion interactions at elevated temperature. A corrosion test system has been designed and built for evaluating molten salt/material interactions to 700°C. The primary components of this system are several salt containment vessels that are constructed of 6^″ dia. × 24^″ long stainless steel, aluminum diffusion treated pipes with flat plate welded to one end and a flanged lid on the other. The vessels are designed to operate with a charge of 10 kg of molten salt and accommodate a “sample tree” on which corrosion test coupons may be suspended. The salt vessels are heated and insulated on the bottom half, roughly to the salt fill level, and cooled on the top half to protect the flange gasket and feedthrough ports. The samples trees have a stainless plate that reduces radiative heat transfer from the molten salt to the lid. Finite element analysis was performed to determine the pipe length and heating and cooling requirements to maintain molten salt at 700°C while limiting the lid gasket to 300°C or less. The vessels are designed to have an oxygen atmosphere in the ullage region to mitigate nitrate decomposition. Oxygen systems for operation at 700°C require careful design including the sizing, routing, cleanliness, and material selection of components in order to reduce risk of fire. Additionally, the system is designed to run at 1–2 psig which requires specialized low pressure / high temperature components. In this paper we present the design of the molten salt corrosion test system including details related to the containment vessels, oxygen handling system, and control software along with a discussion of the safety considerations necessary for these high temperature, high oxygen partial pressure tests.