Thermochemical energy storage (TCS) based on reversible gas-solid reactions has great potential to be part of a future diverse energy system. Not only do these storages work at a wide range of temperatures at potentially high energy storage densities, they also offer unique features due to the controllability of the reaction temperature according to their thermodynamic equilibrium.

Modelling and simulation of TCS are being intensively investigated. This is quite challenging as the solid bulks undergo significant mechanical and chemical changes due to the thermochemical cycling, which represents the charging and discharging of the thermal storage. These changes are very difficult to model and parametrize. The structural changes in the reacting solids, however, lead to significant changes in the heat and mass transfer properties of the storage material, which are particularly problematic in applications requiring high performance. As a result, the design of suitable reactors for TCS is challenging, as the actual properties of the reacted storage material cannot be predicted based on the current state of knowledge.

In this contribution, two TCS systems with water vapor as gaseous reaction partner are considered. The reaction between SrBr₂ monohydrate and anhydrate is currently investigated as chemical heat pump with reaction temperatures of 180-280 °C, while the reaction between Ca(OH)₂ and CaO is mainly considered for applications such as seasonal storage or power plant integration (reaction temperature 450-600 °C). Both reaction systems have been demonstrated in kW-reactors – and both systems have significant limitations at technical scale due to structural changes of the involved solid bulks [1,2].

We present recent work on µCT-imaging of TCS materials as fresh bulks as well as after thermochemical cycling. Based on these data, effective transport parameters are determined. The results are compared to experimental data of larger bulks.

References

[1] Stengler, J. et al.; International Journal of Heat and Mass Transfer, 2021, 167, 120797.

[2] Gollsch, M., Linder, M.; Journal of Energy Storage, 2023, 73, 0108790.