Assessment of critical tank design aspects for high temperature large-scale molten salt storage

Abstract

Solar thermal power plants use large atmospheric steel tanks filled with molten salt operating at temperatures up to 560°C for thermal energy storage. The ever increasing demand for higher storage capacities and lower costs is the reason why the scalability of these tanks is of great interest for industry and research. State of the art two tank storage systems consist of at least one hot tank and one cold tank. Only little information from literature is available on the design of such tanks and there is no standard according to which such a high-temperature tank with molten salt is designed. To resolve this issue a literature overview on the different design aspects such as material strength, thermal stresses, static and cyclic stresses, gas handling, corrosion as well as thermal losses is given in this study. The focus will be on the tank shell thickness, insulation concepts and foundation. The question of optimum tank dimensions will be assessed. The advancement of the two-tank storage system is characterized by two general directions. One direction is to improve the commercial two tank system. Aspects of interest are the optimum number and size, as well as an increase of the maximum temperature of the storage system. Higher temperatures have an influence mainly on material strength and thermal losses. Another direction is to decrease the total costs by installing only one tank. In the single tank concept there is a hot zone at the top and a cold zone at the bottom of the tank. Some design aspects such as thermal stresses become more severe than in the two tank concept due to temperature gradients in the tank walls. The technical boundaries from a tank design point of view will be highlighted for both development directions and a comparison of the conceptual tank design for a two tank and a single tank system will be presented. The outcome of this work provides an overall contribution to a better understanding of the molten salt thermal energy storage design and its potential for improvements in terms of scalability, temperature increase and single tank design.