Sensible energy storage options for concentrating solar power plants operating above 600 °C

Abstract

To lower the cost of electricity produced, advanced high-efficiency power cycles operating at temperatures above 600 °C (such as the supercritical CO2 Brayton cycle) are presently being developed for use in both nuclear and concentrating solar power (CSP) plants. Incorporating thermal energy storage into CSP plants allows renewable energy to be generated while simultaneously providing reliability and stability to the grid. Sensible energy storage using molten nitrate salts is used in the majority of CSP plants. However, nitrate salts decompose at around 600 °C, hence an alternative storage medium is required to support the development of next generation high-efficiency CSP plants. Because of practical experience with molten salt storage in the two-tank configuration in industry, continuing to use fluid media is an attractive option, although thermal storage is also possible with other types of storage media (e.g. solids and phase change materials). This paper critically reviews options for energy storage in fluids that are stable over 600 °C. The focus is on three alternative molten salts — fluorides, chlorides and carbonates — which are assessed based on their thermophysical properties and cost. A brief review of liquid metal and molten glass storage options is included for completeness. Corrosion of containment materials is an important consideration in the choice of storage media, because if exotic materials are required, the cost of the storage tanks can dominate the overall storage cost. Therefore, this paper includes a summary of the main corrosion issues relating to containment of the more promising storage fluids considered herein, identifying possible tank materials and corrosion mitigation options.