Analysis of Molten Salt Thermal Energy Storage Tank Sizing and Preheating

Molten Salt Thermal energy storage (TES) is essential for concentrating solar power (CSP) plants, enhancing their capacity factor and dispatchability to ultimately reduce electricity costs. This technology is becoming more important for other technologies including nuclear power, thermal-electrical storage, and concentrating solar thermal (CST). While molten salt TES tanks have been successfully deployed in lower-temperature parabolic-trough plants, the current state-of-the-art central receiver systems operating at temperatures up to 565 degrees Celsius have experienced costly and frequent failures. These failures are associated with the immaturity of the technology and problems in tank design, fabrication, commissioning and, sometimes, aggressive operation. A primary contributing factor is the absence of a dedicated technical standard for high-temperature molten nitrate TES tanks; designers must currently rely on existing codes like API 650 and ASME Section II, which are insufficient because they fail to account for the unique demands of high temperatures, thermal cycling, and transient conditions. In March 2024, NLR released the Failure Analysis for Molten Salt Thermal Energy Tanks for In-Service CSP Plants report (NREL/TP-5700-89036) that summarized the failure mechanism and factors contributing to these failures. This report presented detailed information on the molten salt tank model formulation and validation and the employment of the model to evaluate the temperature, stress, and deformation distribution in a representative tank design for typical plant operation conditions. The report also presented a set recommendations for future work to improve tank's reliability including the consideration of new base materials and welding fillers, salt distribution ring optimization, feasibility analysis of post weld heat treatment, reevaluation and enhancement of welding procedure specification, and implementation of safety operation protocols. This report, part of the DOE SETO-funded project, agreement DE-EE0038475 - 'Failure analysis of molten salt tanks for in-service CSP plants,' investigates the efficacy of the new CSP industry hypothesis to use smaller TES tanks with thicker floors to reduce failure susceptibility. It was found that while residual stress and distortion from welding fabrication are independent of the tank diameter, a significant reduction in stress during operation was achieved with the smaller, thicker-floored design, thereby providing evidence to support this new CSP plant hypothesis. The preheating analysis results, in turn, are still in an early stage and require further research to provide an accurate quantification of the effect of this process on residual stress and distortion in the tank floor. Resolving failures in molten salt TES tanks is crucial not only for CSP, but also for other applications considering these TES technologies, including nuclear, geothermal, and direct heat thermal energy storage for industrial processes.