Integrated chemisorption cycles for ultra-low grade heat recovery and thermo-electric energy storage and exploitation

Highlights: • Integrated chemisorption cycle for simultaneous electric and thermal energy storage. • Recover ultra-low grade heat (30–100 °C) with the aid of the compression process. • Thermal efficiency and exergy efficiency is 47–100% and 62–93%, respectively. • Upgrade the low-grade heat by between 10 and 80 °C depending on adsorbent pairs used. • The round-trip efficiency of electric energy storage could achieve 100% or higher. - Abstract: The integrated chemisorption technology driven by ultra-low grade heat for simultaneous electrical power and thermal energy storage has been investigated in this work. A resorption cycle employing low and high temperature salts, with the coupling between manganese chloride (MnCl₂), calcium chloride (CaCl₂) and sodium bromide (NaBr), was evaluated and compared in the proposed integrated system for a heat source temperature of between 30 °C and 100 °C. During the energy charging process, working fluid compression was introduced to convert mechanical power or electricity into chemical energy so that it could be stored during the adsorption process at the same time as utilising low temperature heat energy. During the energy discharging process, mechanical power could be generated via the expansion of the desorbed high pressure working vapour from the low temperature salt reactor, there is also a by-product potential of the cooling energy which can be extracted from the cold expansion exhaust. In addition to the power generation and the potential cold energy, upgraded heat could also be provided by the exothermic adsorption process in the high temperature salt reactor. The performance of this integrated cycle in terms of energy and exergy efficiency and energy density have been discussed. With the help of ultra-low grade heat, a 100% round-trip electricity storage efficiency of the proposed system has been found to be achievable using CaCl₂–NaBr and MnCl₂–CaCl₂ pairs when the heat source temperature was higher than 50 °C and 60 °C, respectively. Also a temperature lift of the heat by 15–33 °C and 22–68 °C respectively was possible using these two adsorbent pairs.