Overall evaluation of single- and multi-halide composites for multi-mode thermal-energy storage

Abstract

Sorption thermal-energy storage plays a critical role in addressing the mismatch between thermal-energy consumption and supply. However, the previous evaluations of sorption thermal-energy storage materials and cycles were mainly based on the equilibrium results of energy storage density. In this study, we develop evaluation models to provide an optimal working pair selection reference for single-stage sorption/resorption cycles under short- and long-term storage modes, and compare the thermal-energy storage density in the discharging stage, the effective discharging time, the temperature gradient, the exergy output during the discharge phase (ΔEdis), and the exergetic coefficient of performance (ECOP). By considering ΔEdis as a criterion, the best halides are determined to be NH4Cl and CaCl2 for single-stage sorption cycles at heat-source temperatures of 60–80 °C and 90–180 °C, respectively; however multi-halide can be a better choice when the temperature varies over optimal temperature range of single-halide, i.e., from 80 °C to 160 °C. Resorption is also analysed in this study. The results show that multi-halide is not suitable for resorption cycles because a part of the components will fail to sorb during the discharging stage. The effective reaction-temperature ranges of the resorption cycles are wider than those of the corresponding sorption cycles, whereas the performance values decrease mainly because of the larger required sorbent mass.