Thermochemical Energy Storage with Salt Hydrates for Building Applications— Performance, Control and Comparative Analysis

Thermochemical materials (TCMs) store energy using reversible chemical reactions, and can efficiently shift thermal loads from high-demand to low-demand time periods, positioning the technology as an ideal solution for energy storage in buildings. For building applications the applicability of a thermal storage system lies in the temperature lift the system can deliver to air circulating in the building’s HVAC ducts. Here, we present three activities related to TCM energy storage systems targeting such building applications, using salt hydrate TCMs in an experimental test rig designed for precise measurement and control over the reversible hydration/dehydration reaction. First, we investigate how material properties, composite geometry, and operating conditions affect the thermochemical energy storage performance, particularly on the temperature lift, power output, and apparent energy density. Heat and mass transport through the system are impacted by reaction kinetics, temperature, relative humidity, flow rate, composite surface area, and diffusion depth. Secondly, we demonstrate how the input conditions during operation can be manipulated dynamically to ensure the resulting power output remains constant, which is valuable for practical building applications. Also, maintaining a constant power output allows benchmarking such TCM energy storage systems for direct comparison to other energy storage technologies. Rate capability curves and Ragone plots, traditionally used to characterize the relationship between energy and power in batteries (electrochemical energy storage systems), are created here in the thermal domain by discharging (extracting heat from) the TCM system at a constant rate and measuring the amount of thermal energy that can be accessed. Lastly, we develop analogous thermal rate capability and Ragone plots to describe the energy-power tradeoff in thermochemical energy storage systems. This framework enables straightforward comparison of system performance not only between thermochemical energy storage systems, but also with other storage systems like phase change materials and traditional electrochemical batteries.