Hydration fronts in packed particle beds of salt hydrates: Implications for heat storage

Hydration of packed beds of salt hydrate particles underpins the working principle of low temperature thermochemical energy storage (TCES). Typically, the salt hydrate particles are millimeter sized. An isothermal model for packed bed hydration is formulated, and it is shown that for millimeter sized particles hydration can be described as an advection-reaction process. Traveling wave solutions have been obtained that describe a moving hydration front. The speed of the hydration front is about five orders of magnitude slower than the air velocity in the particle bed. The width of the hydration front is under relevant TCES conditions between 10 and 100 cm. Therefore, hydration fronts will only develop in meter-sized packed beds. A constant hydration rate (and power output) is related to the existence of a traveling hydration front. Therefore, constant hydration rates and power output can only be expected for meter sized TCES reactors. Finally, the influence of temperature gradients is analyzed for the case that the front width is smaller than the bed size. The temperature lift and power output are calculated. Future steps should involve a more detailed description of temperature gradients and a quantitative analysis of finite size effects.