Cycle-level charging penalties arising from operating choices in a zeolite 13X thermochemical heat storage reactor

Thermochemical heat storage (TCHS) is a promising technology for long-duration thermal energy storage. However, TCHS reactors are often evaluated by discharging and charging as independent processes, even though the residual uptake field at the end of discharging sets the initial condition for regeneration. This study presents a cycle-consistent analysis of how discharging termination governs residual adsorption gradients, induces re-adsorption during charging, and degrades cycle-level performance in a zeolite 13X–water TCHS reactor. A lab-scale reactor was investigated using an experimentally validated three-dimensional transient model that captures cycle-coupled adsorption kinetics and residual uptake–dependent regeneration behavior in the bed. Discharging was terminated using three practical outlet-temperature thresholds, resulting in lower total uptake under earlier termination and leaving substantial unused adsorption capacity in the downstream region. During charging, the model revealed simultaneous upstream desorption and downstream adsorption at early times, and the effective cumulative desorbed mass exceeded the initially adsorbed water mass by up to 23.1%, confirming internal vapor recycling through re-adsorption and re-desorption. Energy classification further showed that 7.81% of the heat absorbed by the reactor during charging was expended to re-desorb water that had been temporarily re-adsorbed. This additional regeneration requirement explains a substantial portion of the gap between theoretical and actual cycle efficiencies and limits the benefit expected from simply reducing the initial uptake by earlier discharging termination. Therefore, robust efficiency gains require residual-state-aware termination criteria and reactor and flow-path designs that explicitly suppress re-adsorption.