Molecular simulations of salt hydrates for thermochemical energy storage
The transition from a fossil fuel-based economy towards a sustainable energy based economy is linked with a transition towards an intermittent energy supply based on wind and solar power. Therefore, a storage system is key in bridging the mismatch between this intermittent energy supply, and the daily/seasonal energy demand pattern. Thermochemical energy storage, based on endo- and exothermic chemical reactions, has promising characteristics to act as thermal storage. It allows one to store heat in a compact and efficient way, basically without heat losses over long time periods. Among others, the chloride-based salt hydrates CaCl2.nH2O (n=0,2,4,6) and MgCl2.nH2O (n=0,1,2,4,6) are promising materials to be used as thermochemical energy materials because of their availability, operating temperature, and potential storage density. The concept of storing heat in these salt hydrates goes according to the following reactions; when charging (endothermic reaction) the salt hydrates adsorb solar energy and disintegrates into a lower hydrate or anhydrous form, plus water vapor. When discharging (exothermic reaction) the dried salt recombines with water vapor, forming higher hydrates and heat is released. Nonetheless, many challenges related to the stability and kinetics, need to be solved, before these salts can be used as viable thermochemical energy storage materials. Undesired melting, overhydration, and hydrolysis can occur within the operating temperature of the thermal storage application. Furthermore, these salt are characterized by slow water diffusion through the crystals and low thermal conductivity. Cracks and pores in the salt crystals, formed by (de)hydration cycles, could decrease the thermal conductivity even further but also shows potential to promote the diffusion and thereby prevent overhydration and promote kinetics. Molecular dynamics enables us to track water molecules, stresses, and energy flows, within and around the salt crystals. This allows us to study the effects of cracks and pores on the diffusion, pressures, and thermal conductivity.