Development and characteristics analysis of salt-hydrate based composite sorbent for low-grade thermochemical energy storage

Salt-hydrate based thermochemical energy storage is currently a momentous technique utilized for long-term energy storage due to the reversible gas-solid reaction under low-temperature. Among available salt candidates, LiOH·H₂O is a promising thermochemical material owing to its high heat storage density of 1400 kJ/kg and low charging temperature. The expanded graphite (EG) is selected as a host matrix owing to its excellent thermal conductivity and abundant microstructure, which can promote the heat and mass transfer. This work focuses on the thermochemical performances of the form-stable LiOH·H₂O/EG composite sorbents. Five samples were being synthesized with EG contents of 0, 5, 8, 12 and 15 wt%. These porous sorbents are characterized to understand the microstructure and thermophysical properties. Considering the comprehensive effect of thermal conductivity and storage density, as well as the adsorption kinetics, the 8 wt% EG-doped sample is the most favourable sorbent, which possesses the thermal conductivity of 6.92 W/(m K) and energy density of 1120 kJ/kg. The cyclability results also reveal the energy capacity of this composite maintains ∼90% of the original after ten consecutive heat charging (dehydration) and discharging (hydration), suggesting good stability. Additionally, the active energy of 2.58 × 10⁹ s⁻¹ and pre-exponential factor of 59.5 kJ/mol for the sorbent is derived. Finally, the thermal power of 123 W and thermal efficiency of 83.6% are achieved for the storage unit in simulation. All these results further confirmed the feasibility of the developed composite sorbent in low-grade heat storage.