Investigation on thermochemical heat charging and discharging behavior of Ca(OH)₂/CaO in a fixed bed reactor

Thermochemical heat storage (TCHS) technology plays a crucial role in the energy system, essential for maintaining the balance between energy supply and demand. The Ca(OH)₂/CaO system holds significant promise for TCHS thanks to its high energy storage density, cost-effectiveness, and minimal heat loss. However, further research is required to elucidate the coupling mechanisms of the various physicochemical processes involved and to optimize the overall system performance. This paper focuses on assessing and elucidating the multi-physics coupled transport rules during both the charging and discharging stages in a fixed-bed reactor based on the developed numerical model. Results manifest the bed temperature rapidly increases to around 950 K, then slows down as the endothermic reaction predominates, completing dehydration in 1,900 s. During discharging, hydration conversion advances in a “U”-shaped pattern from the vapor inlet and side wall to the outlet, driven by the high vapor content near the inlet and the lower temperature near the wall. Furthermore, a thorough sensitivity analysis of the main parameters such as thermal conductivity and porosity is conducted to determine how different operating conditions affect the charging and discharging processes. Finally, a new reactor structure is proposed, in which a gas-guide duct is added at the central axis of the reactor to improve mass transport and uniformly distributed finned heating tube bundles to enhance heat transfer. These two functions can shorten the charging period by 9 % and 16 % respectively. The results can aid in predicting thermochemical conversion behaviors and multi-physic coupling transport processes, while also providing a foundational reference for the design of TCHS systems.