Diffusive transport of water in magnesium chloride hydrates

Solar energy is one of the important pillar of renewable energy systems. The main challenge in exploitation of solar energy is intermittency on seasonal and daily basis. The solution of intermittency in solar based renewable energy sources is energy storage. Solar energy can be stored in batteries (direct conversion of solar energy into electricity) or in form of thermal energy. Further solar thermal energy can be stored in three forms; sensible heat, latent heat and thermo-chemical form. The energy storage density is higher in thermochemical form than sensible heat or latent heat. Thermochemical materials (TCM) offers a compact solar energy storage material. The cost of TCM material is 30 % of the total investment in the thermochemical heat storage system. One of the key aspect in designing the solar based heat storage system is selection of TCM materials [1]. Salt hydrates are one class of chemical compounds which absorbs solar energy and disintegrate into lower hydrate form or anhydrous form (charging cycle). Furthermore, it recombines with water vapor to form hydrate and dissipate energy(discharge cycle). The gas-solid interactions are essential for the behavior of water vapor in and outside the crystal structure. Experimentally it has been observed that storage capacity of magnesium chloride hydrates is keep on decreasing after continuous charging/discharging periods. The deterioration in storage capacity of salt hydrates is due to incomplete hydration and surface defects. The diffusive transport of water molecules plays an important role in dehydration rate as it may become rate determining step in some conditions. Water vapor pressure, temperature, surface transformation and surface defects/cracks influence considerably the dehydration rate. These factors affects the reaction rate and diffusion of water molecule inside solid hydrate. In present study we would like to understand the molecular perspective of water transport inside the salt hydrates under these various conditions. A new force field has been developed to describe the dehydration process of Magnesium chloride hydrates. Chemical reactivity has been incorporated using bond order dependent Reactive force field. Molecular dynamics (MD) simulations using reactive force field have been performed to explore the diffusivity of water in magnesium chloride hydrates. As the energy released from salt hydrates is dependent on the dehydration rate, and the dehydration rate is dependent on vapor transport inside the solid salt and heat released inside the structure, we investigate molecular water transport in the crystal structure of the proposed material. We perform MD simulations which mimic the vapor transport inside the magnesium salt hydrate under different temperature, vapor pressures conditions and also along the structural change during the dehydration reaction. The dependence of the diffusion coefficient on these various influencing factors is going to be analyzed and studied using velocity auto-correlation function (VAFs).