Abstract:

The use of MgSO₄∙7H₂O (Epsomite) as a potential thermochemical based thermal energy storage material has attracted considerable attention in recent decades. This material has a high theoretical energy density with abundant availability and low costs. Despite considerable efforts, little progress has been made in achieving the high energy density, largely due to insufficient understanding of the reaction mechanisms and unfavourable dehydration/hydration kinetics. A systematic study on the hydration/dehydration kinetics and cyclability of MgSO₄∙7H₂O has therefore been conducted in this study. The results show conclusively that the dehydration process depends on the heating rate with the 5 ℃/min being the optimal, leading to seven steps of MgSO₄∙7H₂O dehydration with the dehydration heat very close to the theoretical value. The reaction kinetic studies show that the rate of hydration is ~50% lower than that of dehydration. The thermal cycling tests of MgSO₄∙7H₂O under the conditions of this study (small sample size) suggest a good cyclability, with the hydration rate increasing with increasing cycling number up to ~10 cycles where level-off occurs. This is consistent with scanning electron microscopic analyses, which shows the formation of cracks and channels on the salt hydrate particles, facilitating mass transfer and the enhanced kinetics.

Key words: Thermochemical energy storage, thermal analysis, thermal cycling, MgSO₄ hydration/dehydration kinetics