Operation strategies for gas solid reactions in thermal energy storage systems

Thermochemical energy storages are so far considered and operated as passive systems, where the mass flow rate and the temperature of the heat transfer fluid determine the resulting thermal power. However, the basic storage principle is a gas solid reaction that can in general be looked at from two different perspectives: Not only by the thermochemical energy storage perspective, but by a gas storage perspective. The latter is based on a rather active gas flow-controlled strategy, that is e.g. very common for gaseous hydrogen storage in solid state reactions. In this manuscript, this gas flow-controlled strategy is transferred to thermochemical energy storage applications and its impact on flexibility of storage operation is discussed. First, using analytical expressions, an equation characterizing the thermal power limit of a plate type reactor is deduced and the two operation regimes are described. Second, using simulations, the behavior of a thermochemical energy storage that is controlled not only by the heat transfer fluid, but also by the released gas flow rate is analyzed. It is shown that the control strategy can lead to significantly different profiles for temperature and pressure inside of the reactor. Especially the axially dominated profiles for the passive, step-response like, systems differ from rather radial temperature and conversion profiles for the gas-controlled, active, operation. These findings are crucial in the context of partload operations and open new possibilities for the design and control of thermal energy storage systems based on thermochemical gas solid reactions.