Hydration fronts in packed particle beds of salt hydrates II: The role of temperature gradients and implications for heat storage

Thermo-Chemical Energy Storage (TCES) based on salt hydrate particles is promising for compact long term heat storage in the built environment. The hydration transition of salt hydrates is used, where water molecules from the gas phase are incorporated in the crystal lattice of the salt, resulting in heat output. The aim of this study was to develop an easy-to-use analytical model for the hydration process of a bed of millimetre sized salt particles inside a TCES reactor, which describes simultaneously the water vapour concentration distribution, the conversion profiles and temperature gradients in the bed. The model is an extension of a previously developed isothermal model for hydration fronts. The model describes advective transport of water vapour towards the hydration front, where the water vapour reacts with the salt particles. This reaction inside particles is diffusion limited. Analytical solutions have been obtained for the hydration front travelling through the particle bed: the front velocity and width. We show that temperature gradients significantly slow down the motion of the hydration front and reduce the width of the front. Front widths for salt hydrate particles under conditions relevant for TCES are of the order of tens of centimeters, which is comparable to the typical size of presently studied TCES reactors. Only when the hydration front is travelling inside the reactor, it will be discharged at a high constant power. Therefore, these reactors will have a short constant power phase (CPP), while most of the discharge happens in the so-called falling power phase (FPP), where the power is continuously dropping and the output temperature too. The model shows that the CPP phase can be prolonged by either increasing the reactor length, decreasing the flow rate or decreasing the particle size. The model enables engineers to do quick calculations for the first design of a TCES reactor.