Performance gap between thermochemical energy storage systems based on salt hydrates and materials

Thermochemical energy storage (TCES) systems using salt hydrates have great applicable potential to store solar energy for space heating/cooling. However, because of different test conditions, various salt hydrates, and variable-sized TCES systems, there is still no information on the performance gap between TCES systems and materials of salt hydrates. This review focuses on the open and closed TCES systems based on salt hydrates, including types of reactors, charging temperatures, energy storage densities and costs. The reactors of open TCES systems are summarized including multi-layer packed beds, integrated reactor with air channels, and moving beds. Different types of highly efficient heat exchangers to enhance heat transfer are summarized in closed TCES systems. The volumetric energy storage densities at system level are lower than half of the values at material level, while the released energy costs at system level are nearly twice those of salt hydrates. If the system's scale increases, research on alternative cheap and abundant salt hydrates and optimization of all the components in the TCES system should be done to increase the thermal energy storage density and decrease the cost. It is expected that this study will help readers to understand the TCES systems based on salt hydrates comprehensively, and provide ideas to opmitize TCES systems from material, reactor, components and layout.