Thermochemical energy storage using a calcium oxide/calcium hydroxide/water (CaO/Ca(OH)₂/H₂O) reaction system is a promising technology for thermal energy storage at high‐temperatures (400°C‐600°C). The purpose of this study is to develop a practical composite material by enhancing heat transfer through the reaction bed and mitigating problems of pure CaO/Ca(OH)₂ materials, such as formation of centimeter‐scale agglomerates and change in the bulk volume during repetitive reactions. The present study focused on a novel composite material using a silicon carbide/silicon (SiC/Si) foam. In the literature, performance of thermochemical energy storage of the composite using the ceramic foam remains unclear. In this study, we evaluated the performance of the composite material (~63 g) by a 100‐W‐scale packed bed reactor. The volumetric heat output rate (for the first 5 minutes, under maximum hydration pressure) of the composite was 1.3 kW L‐bed⁻¹, which was 1.4 times higher than that previously reported for a bed of pure Ca(OH)₂ pellets. The composite material, in which the CaO/Ca(OH)₂ samples were subdivided inside pores with a mean diameter of 400 μm, retained high reactivity during cycle reactions, and maintained its bulk volume. Therefore, the composite material developed in this study shows promising application for high‐temperature thermochemical energy storage.

Thermochemical energy storage (TCES) using reversible gas‐solid reactions is a promising technology owing to the high energy density and capability of long‐term storage. TCES using a calcium oxide/calcium hydroxide/water (CaO/Ca(OH)₂/H₂O) reaction system has attracted considerable attention because of the low material cost and environmental friendliness of the reactants. Previous experimental studies have focused on the TCES performances of packed beds of calcium oxide/calcium hydroxide powder. However, pelleted materials can be more favorable than powdered materials from a practical viewpoint. The aim of this study is to evaluate the performance of TCES of a packed bed of calcium oxide/calcium hydroxide pellets. In this work, a total of 60 g of calcium hydroxide pellets (diameter of 1.9 mm and length of 2‐10 mm) was evaluated using a 100‐W scale packed bed reactor. The heat storage density of the bed was 1.0 MJ L‐bed⁻¹, and an average heat output rate of 0.71 kW L‐bed⁻¹ was observed for the first 10 minutes under a hydration pressure of 84.6 kPa. The stability of the reaction conversion of the bed was demonstrated during 17 cycles of experiments.

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.