Intrinsic kinetics in local modelling of thermochemical heat storage systems

Thermochemical heat storage is expected to play an important role in the ongoing transformation of the European energy system. The efficiency of thermochemical heat storage systems largely depends on that of the reactor, since it limits the amount of the heat stored per unit volume as well as the rate of the heat recovery. Thus, the optimization of the reactor design is of great importance and can be achieved via local numerical modelling, where intrinsic parameters of the materials are utilized. In the present study, we developed a procedure for the determination of the intrinsic kinetic parameters of ammonia sorption on metal halides and applied it to strontium chloride SrCl2 - ammonia NH3 working pair. A distinctive feature of the procedure is that the kinetic measurements were performed on a Sieverts type apparatus at constant temperature, which allowed resolving the common problem of heat-up time. Besides, the kinetic measurements were carried out using the optimal mass of SrCl2 - expanded natural graphite composite material (70 mg of SrCl2), ensuring that the chemical reaction rate is not constrained by the heat and mass transfer limitations. As a result, the intrinsic kinetic equations of NH3 sorption on SrCl2 ammines were derived for the first time and were demonstrated to predict the experimental data, from which they had been computed, over a wide pressure-temperature range. In addition, the obtained intrinsic kinetics, as well as the ones found in literature, were implemented in a three-dimensional numerical model computing the local temperature, pressure, and reaction advancement through the coupled equations of the heat transfer, fluid dynamics, and chemical reaction rate. The numerical results were compared with experimental data obtained on 466 mg of SrCl2 powder at various pressure-temperature conditions. In contrast to the literature kinetics, the simulation results with the intrinsic kinetic equations were found to be in a good agreement with the experimental data, demonstrating the importance of using intrinsic parameters in the local modelling of sorption reactions.