Impact of cycling on the performance of mm-sized salt hydrate particles

Abstract

Potassium carbonate is shown to be a promising salt for thermochemical heat storage. For a thermochemical reactor application, the salt hydrate is manufactured in mm-sized particles. It is known that salt hydrate particles undergo swelling and cracking during cyclic testing. Therefore, in this work the influence of cycling on structural and morphological evolution is investigated and the resulting impact on the hydration performance. It is found that the incremental volume increase during cycling is independent of the density at which a particle is produced. With lower starting relative density particles are found to be stable for more cycles compared to particles produced with high starting relative densities. Powder formation at the particle surface starts as soon as the particle density is close to values reported for percolation thresholds. The morphological changes during cycling result in formation of isolated pores and a highly tortuous pore system. As a result, the effective diffusion coefficient for cycled particles is lower compared to what is predicted for as produced particles with similar porosity resulting in lower power output than expected based on porosity. The results from this work help in understanding the reasons for swelling, cracking, powder formation and decreased performance with cycling, laying the foundation for mitigating these unwanted effects.