High temperatures, large scale and low cost - How can a single-tank molten salt storage system be scaled up from a thermal stress perspective?

Kurzfassung

Molten salt thermal energy storage (TES) is a cost-effective option for grid-connected storage in both concentrating solar power (CSP) plants and retrofitted thermal power plants in a multimegawatt scale. While the state of the art consists of two tanks at two constant temperature levels, future systems may be built with a single tank being subjected to a transient temperature profile. It is predicted that costs and space requirements can be reduced in this way. However, one point that has not yet received attention in such forecasts is the structural-mechanical design of large-scale molten salt single-tank storages at temperatures of 560 °C. Also, it is not known how increasing the operating temperature to e.g. 620 °C would affect the design. The challenge of the single-tank is the presence of a temperature profile that leads to different thermal expansion of the tank shell along its height. For large tanks this difference is in the range of centimeters ultimately resulting in bending moments. To our knowledge, this study addresses bending stresses of large-sized high-temperature tanks with thermal stratification for the first time. The modeling approach is applied to single-tank CSP TES systems as a sample application to evaluate tank size and wall thickness constraints. With the help of experimentally validated numerical methods, a clear technical limit is revealed, showing that a low thermocline thickness is not compatible with large tank diameters. It is shown that the temperature has a major influence on maximum possible tank size: if the operating temperature is raised from 560 °C to 620 °C, the permitted tank diameter is significantly reduced. A possible design window is identified. Results show that single tanks require a moderate increase of the wall thickness compared to the two-tank system with constant temperature profiles. The paper concludes with recommendations on how increased wall stresses can be addressed by an appropriate design.