Optimization and design criterion of the shell-and-tube thermal energy storage with cascaded PCMs under the constraint of outlet threshold temperature

In the concentrating solar power generation (CSP), the latent heat thermal energy storage system (LHTES) is under the constraint of the outlet threshold temperatures, which caused lower effective utilization rate (U_ma) of the phase change material (PCM). The objective of the present work is to improve the performance of the shell-and-tube LHTES which is under the constraint of the outlet threshold temperatures in charging and discharging processes. In this study, a transient, two-dimensional, and axisymmetric model of a shell-and-tube LHTES is established. Based on the model prediction, first, the sensitivity analysis of the geometry parameters to the performance of the LHTES shows that with the increase of the specific surface area and the porosity, the U_ma monotonically increases, but the investment cost for LHTES per unit of stored heat (c_TES) has a minimum value. Then based on the influence of the number and filling thickness of cascaded PCMs, a design criterion of the cascaded PCMs distribution is proposed for the shell-and-tube LHTES constrained by the outlet threshold temperatures, and the effectiveness of this design criteria is verified. Based on the proposed design criterion, the U_ma of the optimal shell-and-tube LHTES with 5-cascaded PCMs can reach 77.6%, which is 3.1 times larger than that of a non-cascaded LHTES. Its c_TES is only about 2/5 of a non-cascaded LHTES. The results of optimizing the geometry parameters of the TES device and the distribution of cascaded PCMs can be beneficial to the shell-and-tube LHTES which is under the constraint of outlet threshold temperatures in the real utilization of CSP.