Microscopic analysis of the agglomeration behaviour of Ca(OH)2/CaO as thermochemical storage material for heat storage

Abstract

To cope with the numerous renewable energy production systems such as concentrated solar power (CSP), heat storage technologies are being improved in the direction of a sustainable energy system. Thermochemical energy storage in the form of reversible gas-solid reactions is worth considering thermal energy storage system at high temperatures. Energy is stored in the reversible reaction as reaction enthalpy which can be recovered from, the system at any chosen time as long as the gassolid reactant components are separated. Due to high availability, low cost, high reversibility, and cycling stability; Ca(OH)2 ⇌ CaO + H2O gas-solid reaction is taken into consideration. Previous works describe that on nanometer to micrometer scale, the agglomeration of the Ca(OH)2 powder does a negative impact on reversibility of the reaction system although cycling stability is confirmed for the system. Presence of H2O induces the lump formation which hinders the conversion over the repetitive cycles of dehydration and rehydration of Ca(OH)2 powder causing instability in the system. The agglomeration phenomenon incorporated with the reaction was proved by particle size distribution with the increasing number of cycles. Moreover, degradation of the granules was also observed. The presented work aims to quantify the size progression behavior of the individual agglomerates of the scale µm to mm, over the cycles under different vapor pressure dehydration conditions. The outcome suggested that different vapor pressure dehydration conditions did not have similar effects on the agglomerates. To confirm the cycling stability, conversion at each cycle experiment was confirmed with a thermogravimetric analysis of the material. With increasing vapor pressure dehydration, conversion was likely to be decreased, and with that, the behavior of the agglomerates was hypothesized to be a size enlargement. Findings indicate that for low vapor pressure dehydration, an agglomerate of any size undergoes a shrinkage and vice versa in the case of high vapor pressure dehydration where size enlargement is observed as hypothesized. For the intermittent vapor pressure dehydration system, it was mixed findings, and with more cycle experiments the trend of the size progression can be more precise.