Advanced diffusion methods for H2O in salt hydrates

A thermochemical heat storage system based on salt hydrates can fulfill a crucial link in a renewable energy society by bridging the gaps between the intermittent heat supply and demand. However, slow diffusion of water through the salt hydrates limit the power output of the storage. Simultaneously, slow water diffusion gives rise to local over-hydrated spots. Recent studies show that cracks and pores in the salt crystal increase the (de)hydration rate. In this sense, we analyzed the diffusion behavior of water in crystalline CaCl2.2H(2)O and around crystal imperfections, like cracks and pores, through reactive force fields molecular dynamics. A Smoluchowski diffusion equation-based method was used to perform this diffusion study. This method enabled the possibility to study a local diffusion behavior instead of the often used global diffusion methods. We found that the diffusion of H2O is extremely low in the crystalline regions of the CaCl2.2H(2)O (< 10(-10) m(2)/s), which corresponds with literature for similar salts. However, in cracks, pores, and amorphous regions of the salt, the diffusion is significantly higher (>= 10(-9) m(2)/s). Thereby, this enhanced local diffusivity explains why imperfections of the crystalline salt hydrate can improve its characteristics for thermochemical heat storage applications.

Automated summary

Through simulation and analysis of the diffusion behavior of water in crystalline CaCl2.2H(2)O, it was found that crystal imperfections such as cracks and pores can significantly enhance the diffusion performance of salt hydrates, thereby improving their characteristics for thermochemical heat storage applications.