Characterisation and sorption behaviour of LiOH-LiCl@EG composite sorbents for thermochemical energy storage with controllable thermal upgradeability

Considering the eminent benefits of high energy storage density (ESD) and long-term energy storage ability with ignorable heat losses, thermochemical energy storage (TCES) in salt hydrates is a potential technology to bridge the gap between supply and demand for renewables in domestic heating. The development of thermochemical material is currently the primary concern. In this work, the composite sorbents consisting of expanded graphite (EG) and varying mass ratios of LiOH and LiCl are synthesised and characterised, and the thermochemical behaviours such as sorption kinetics and sorption isotherms are also investigated. The results suggest the salts are uniformly dispersed in the EG matrix in the form of hierarchical micro-nano scale particles, and the salt contents of samples are over 60 wt%, which in favour of the enhancements of vapour sorption property and ESD. By regulating the mass ratio of LiOH and LiCl, the composite sorbents (LiOC@EG) can achieve thermal upgrade with different temperature requirements, i.e., 35–45℃ for space heating and 45–55℃ for domestic hot water (DHW) production, accompanied by high volumetric ESD (over 200 kWh/m³) benefiting from the reaction enthalpy. Besides that, the effects of regenerative temperature and vapour pressure on the water uptake and ESD of the sorbent are revealed. The cyclability results indicate that more than 95% and 96% of the original ESDs are retained after 20 dehydration-hydration cycles for the samples of LiO2C1@EG and LiO3C1@EG, suggesting good stability of the composites. The developed composite sorbents provide new insights into the fields of long-term energy storage and heat upgrade with high energy density.