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The CaO/Ca(OH)2 system can be the basis for cost-efficient long-term energy storage, as the chemically stored energy is not affected by heat losses, and the raw material is cheap and abundantly available. While the hydration (thermal discharge) has already been addressed by several studies, for the dehydration (thermal charge) at low partial steam pressures, there is a lack of numerical studies validated at different conditions and operation modes. However, the operation at low steam pressures is important, as it decreases the dehydration temperature, which can enable the use of waste heat. Even if higher charging temperatures are available, for example by incorporating electrical energy, the reaction rate can be increased by lowering the steam pressure. At low pressures and temperatures, the limiting steps in a reactor might change compared to previous studies. In particular, the reaction kinetics might become limiting due to a decreased reaction rate at lower temperatures, or the reduced steam density at low pressures could result in high velocities, causing a gas transport limitation. Therefore, we conducted new measurements with a thermogravimetric analyzer only for the specific steam partial pressure range between 0.8 and 5.5 kPa. Based on these measurements, we derived a new mathematical fit for the reaction rate for the temperature range between 375 and 440 °C. Additionally, we performed experiments in an indirectly heated fixed bed reactor with two different operation modes in a pressure range between 2.8 and 4.8 kPa and set up a numerical model. The numerical results show that the model appropriately describes the reactor behavior and is validated within the measurement uncertainty. Moreover, our study revealed an important impact of the operation condition itself: the permeability of the reactive bulk is significantly increased if the dehydration is initiated by a rapid pressure reduction compared to an isobaric dehydration by a temperature increase. We conclude that the pressure reduction leads to structural changes in the bulk, such as channeling, which enhances the gas transport. This finding could reduce the complexity of future reactor designs. Finally, the presented model can assist the design of thermochemical reactors in the validated pressure and temperature range.

Concentrated solar power is capable of providing high-temperature process streams to different applications. One promising application is the high-temperature electrolysis process demanding steam and air above 800 °C. To overcome the intermittence of solar energy, energy storage is required. Currently, thermal energy at such temperatures can be stored predominately as sensible heat in packed beds. However, such storage suffers from a loss of usable storage capacity after several cycles. To improve such storage, a one-dimensional packed bed thermal energy storage model using air as a heat transfer medium is set up and used to investigate and quantify the benefit of the incorporation of different thermochemical materials from the class of perovskites. Perovskites undergo a non-stoichiometric reaction extension which offers the utilization of thermochemical heat over a larger temperature range. Three different perovskites were considered: SrFeO3, CaMnO3 and Ca0.8Sr0.2MnO3. In total, 15 vol% of sensible energy storage has been replaced by one perovskite and different positions of the reactive material are analyzed. The effect of reactive heat on storage performance and thermal degradation over 15 consecutive charging and discharging cycles is studied. Based on the selected variation and reactive material, storage capacity and useful energy capacity are increased. The partial replacement close to the cold inlet/outlet of the storage system can increase the overall storage capacity by 10.42%. To fully utilize the advantages of thermochemical material, suitable operation conditions and a fitting placement of the material are vital.

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