An innovative intermediate-based sorption thermal energy storage (STES) concept for power-to-heating/cooling purposes in buildings: From experimental dynamics to operational examination

This study presents an innovative sorption thermal energy storage (STES) system for building power-to-heat and cooling applications, utilizing liquid refrigerant distribution and sorption reactors’ direct evaporation/condensation, eliminating the need for complex/expensive vacuum valves as in conventional sorption technologies. The system employs an impregnated CaCl2@SG_25 composite sorbent, with a lab-scale examination of its isotherms and kinetics. A dynamic model was developed to analyze the STES system’s charging and discharging modes across seasons, focusing on design factors (i.e., sorption kinetics) and boundary conditions (i.e., charging power and condensation/evaporation inlet temperatures). Key operational metrics are examined including sorption characteristics, state of charge (SOC), and energy storage density (ESD). The study highlights how SOC and idle times affect discharging performance, round-trip efficiency (RTE), and self-discharge rates (SDR). The energy ratio of auxiliary loads (i.e., dry cooler/dry heater/heat pump) to charging energy is indicated. The results imply that the sorption kinetics appreciably affect the system dynamics at higher charging powers; however, its effect is limited at lower charging powers. Meanwhile, the STES system’s dynamics improve with increased charging power and reduced condensation/evaporation inlet temperatures, achieving ESD of 199 kWh/m3. Furthermore, increasing SOC enhances the system’s charging and discharging performance. Operating at a lower SOC of 20 % significantly reduces discharging ESD by 72.7 %. Unlike the limited effect of idle time in summer, longer idle times increase heat leakage, with SDR of 10.9 % within a month, leading to RTE of 104 % compared to 93 % for direct discharging.