Comparison of key performance indicators of sorbent materials for thermal energy storage with an economic focus

To expand the application scope of renewable energy sources, it is essential to further develop storage systems aimed at compensating the discrepancy in time between an energy-generation surplus and energy-demand peak. To this end, sorption thermal energy storage (STES) has recently gained interest for long-term (or seasonal) thermal energy storage. In the STES process, charging and discharging are typically based on reversible reactions between a solid sorbent and fluid (sorbate) that form the working pair. Sorbates are typically low-cost and environmentally safe fluids (e.g., water); however, the lack of robust and low-cost sorbent materials still creates a technological bottleneck for the long-term storage of thermal energy and, more generally, for sorption-based heat transformation. This study provides a general review of the existing sorbent–sorbate pairs, which consist of four main classes: liquids, solids, chemicals, and composite materials, with a special focus on their current costs. The results are presented in the form of several charts, which provide a comprehensive overview of sorbent materials in terms of their energy storage density (MJ/m3), energy storage capacity (kJ/kg), and desorption temperature (i.e., charging temperature). In addition, novel charts are provided for a less explored parameter: the specific cost of current sorbents (expressed in kWh/€, which is the inverse of the storage capacity cost (SCC−1). SCC−1 is a crucial figure of merit for a given sorbent because it affects the sorbent's real potential for widespread future applications.