DYNAMIC INVESTIGATIONS OF THE SLOW RECRYSTALLIZATION OF INCONGRUENTLY MELTING GLAUBER SALT

The practical use of Glauber salt (sodium sulfate decahydrate) as a heat storage material severely suffers from stratification which impedes recrystallization during the cooling period. One strategy to solve this problem is to reduce the height (volume) of the storage cell. A water diffusion model for stratified Glauber salt is proposed which allows to calculate the maximum storage cell height allowing complete recrystallization. The crucial kinetic parameter of the model is the water diffusion coefficient in the hydrate layer. From DTA experiments its average value is found to be 5.1 . 10(-8) cm2 sec-1. For a complete recrystallization within an unloading period of, e.g., 5 days the model predicts a maximum storage cell height of about 1.3 mm at a discharge temperature of 20-degrees-C. The admissable height increases with the square root of the unloading period. The theoretical predictions have been verified for hydrate crystals embedded in a cold-setting vinyl ester resin.