Investigation of calcium hydroxide powder for thermochemical storage modified with nanostructured flow agents

Abstract Thermal energy storage is a key component for power supply based on concentrating solar power. A promising thermochemical storage material in terms of cost-efficiency, storage density and reaction temperatures is based on the chemical reaction of calcium oxide with water. Handling of the solid materials, however, has proven to be difficult due to their powdery state particularly in dynamic storage systems. It is therefore proposed in the presented work to modify calcium hydroxide powder with nanostructured flow agents to improve the powder's flowability. Therefore, five additives consisting of nanostructured silicon and/or aluminium oxide were mixed systematically with calcium hydroxide powder and the flowability of the mixtures was determined using a ring shear cell. Four additives caused an improved flowability of the powder with additive weight fractions of 6–12%. After thermochemical cycling of the mixtures, however, flowability of the mixtures was clearly decreased while that of the pure powder was increased. Analysis of the samples showed a correlation between growth in particle size and increased flowability. Additionally, formation of calcium silicate and/or aluminate phases led to a decrease in absolute heat release of up to 50% as measured by thermal flux calorimetry, although some of the side products seemingly add to the measured heat release by hydrating exothermally. Based on these results, stabilisation of a moderately increased particle size preferably without the addition of additives is recommended, as a sole prevention of agglomeration by means of flow agents does not result in a manageable storage material.