Investigations of nano coated calcium hydroxide cycled in a thermochemical heat storage

• Coating of fine-grained material for thermochemical heat storage is investigated in pilot scale.
• A prevention of agglomeration effects due to addition of nanoparticles has been observed.
• Different coating conditions lead to varied effects.
• Mechanical stress has a great impact to the appearance of agglomeration.
• Side reactions of nanoparticles with host particles stabilize coating structure at the expense of a capacity loss.

Thermochemical heat storage systems are a promising new technology for concentrated solar power plants and can contribute to improve the efficiency of industrial processes Neveu et al. (2013) [21]. However, for example for the reaction system calcium oxide/calcium hydroxide (CaO/Ca(OH)2), the good availability at low cost is accompanied by poor powder properties that demand complex reactor solutions. During thermochemical cycling agglomeration effects occur and originate inhomogeneity resulting in permanent changes of bed characteristics especially related to the heat and mass transport. One approach in order to stabilize the material is to coat the reacting material with nanoparticles in order to minimize attractive forces leading to less agglomeration. But, high temperatures, change of volume and surface configuration, permeance for reaction gas, side reactions and mechanical stresses within the storage represent challenges for nanoparticles. Therefore, in this work, Aerosil® as additive for thermochemical storage is investigated during cycling in an indirect operating pilot-scale thermochemical reactor with regard to side reactions, stability on the surface and various coating configurations. It is shown that the reaction bed properties can be highly improved depending on the modality of the insertion process whereas occurring side reactions lead to a stabilization of the surface structure at the expense of a capacity loss of the thermochemical reactor.