Development and system performance evaluation of new thermochemical energy storage composite materials for direct photothermal conversion at low temperature

To improve the energy utilization capability under 120 °C, expanded graphite is selected as the matrix and combined with three promising hydrated salts (MgCl₂&MgSO₄, SrBr₂, LiOH) to prepare thermochemical composite adsorbents. The composites are evaluated at the material, reaction kinetics, photothermal conversion and practical application perspectives. The thermochemical energy storage densities of 5 %wt EG E-M and E-L can reach 1267.97 kJ/kg and 1114.69 kJ/kg at 120 °C. As the mass fraction of expanded graphite increases, the activation energies decrease, which is beneficial to the water adsorption and desorption. The composites have high solar absorption capabilities and can directly absorb solar energy and store it as chemical energy, reducing the system volume. 5 %wt EG E-S showed the best solar-chemical energy conversion efficiency, up to 76.99 %. In the simulation of practical applications, E-S can release heat energy stably for a long time, and 66.00 % of the chemical energy can be released and utilized for floor heating. Compared with existing studies, the energy storage densities, solar-chemical energy conversion efficiencies and system energy efficiencies of the proposed materials can reach a higher level.