Influence of Moisture on the Performance of the Thermal Energy Storage Systems

Thermal energy storage (TES) systems will play an important role in renewable energy grids by storing the excess energy during the daytime and releasing it back into the grid when the demand is high or during the night. The latent heat TES storage systems are promising as compared to the sensible heat TES systems due to their high energy density. MgCl₂ is a phase-changing material (PCM) that can be used as a latent heat storage candidate due to its high energy density and high melting point temperature (&gt;700 C). The PCM material is sometimes impregnated in the high thermal conductivity material foam to improve the overall heat transfer through the TES. In the TES facility, moisture infiltration is a problem, and it can cause alteration of the PCM material. The moisture can initiate the hydrolysis reactions and the chlorine released can corrode the storage metal containers. The byproducts of the corrosion will also mix with the PCM, eventually altering its composition and properties. MgCl₂ will absorb moisture because of its hygroscopic nature, and thus, its hydrous state will change, causing a change in the bulk property as well.<br/> <br/>In this work, we perform heat transfer simulations to analyze the influence of moisture on the thermal performance of the TES system. We first develop a mesoscale representative model of graphite foam impregnated with MgCl₂ of different hydrous states and derive its effective anisotropic thermal properties by using homogenization methods in the temperature ranges relevant to the TES operation. This model takes the single-phase properties of each component as an input. The derived properties are fed into the larger-scale heat transfer simulation on the TES system for evaluating the system’s performance. This system-level model provides the overall driving forces for degradation models on the lower scales, as well as quantifies the effect that moisture will have on the overall system performance of TES.