Thermodynamic study of MgSO4 – H2O system dehydration at low pressure in view of heat storage

Abstract

Study about magnesium sulfatewater vapor equilibrium proved to be very interesting especially on the use of dehydration-hydration reactions for the heat storage application in recent research. Heat is realized by hydration of lower hydrates as this reaction is exothermic. Therefore, reversible reaction, endothermic thermal dehydration of higher hydrates, is used for charging of system and in this state the energy can be stored over long time. Even if magnesium sulfate appears as promising candidate with high theoretical energy density of 2.8 GJ/m-3 , technological process is rather complicated. The main problem that thermodynamic and kinetic data are poorly understood to present. In these study salt hydrates equilibrium of magnesium sulfate was investigated by new approach. It makes possible to understand the dehydration reaction of MgSO 4 •6H 2 O for heat storage application. Dehydration reaction under various water vapor pressures and temperatures were investigated by thermogravimetric analysis. The result showed that water content in the solid phase is a function of temperature for given water vapor pressure. So, we can conclude that this magnesium sulfatewater vapor system is bivariant and some hydrates appear as the non-stoichiometric hydrates.

Morphological and Structural Evaluation of Hydration/Dehydration Stages of MgSO4 Filled Composite Silicone Foam for Thermal Energy Storage Applications

“…The quite slow kinetics could be due to the transition from the amorphous phase (stable in dehydrated conditions, reported in XRD patterns in Figure 3) to the crystalline phase (salt hydrate), which takes place during the hydration process. Nevertheless, the hydration/dehydration of magnesium sulfate hexahydrate depends on both temperature and water vapor pressure showing that the equilibrium between water vapor and the salt is bivariant [11].…”

“…Several combinations of salt hydrates and matrices for thermal energy storage exist. One of the salts that have been extensively evaluated in the scientific literature is MgSO 4 •7H 2 O, whose theoretical energy storage density is 2.8 GJ/m 3 [11] and can be used under both ambient pressure and sub-atmospheric pressure [12], thus being suitable for the application in open cycles or closed cycles. The salt can be efficiently regenerated also at temperatures < 150 • C, compatible with the integration in low-grade heat recovery systems or solar systems in buildings [13].…”

Comprehensive Thermodynamic Study of the Calcium Sulfate–Water Vapor System. Part 1: Experimental Measurements and Phase Equilibria

“…That is, the crystalline water content of the compounds could vary continuously with temperature and water vapor pressure without any change in their crystalline system (e.g., this effect was recently observed for MgSO % hydrates.) 20 Discussions about the accuracy of this interpretation of the CaSO % -H + O system is found in several works as, for instance, the reviews presented by Posnjak 21 or Ramsdell and Partridge 22 . We can also find the works from Gardet et al 23 and Soustelle et al 24 that reported calcium sulfate hydrates of the form CaSO % ⋅ εH + O for which the water content would vary in the interval 0 ≤ ε ≤ 0.67 depending on two intensive parameters: temperature and water vapor pressure.…”

Comprehensive Thermodynamic Study of the Calcium Sulfate–Water Vapor System. Part 2: Physical Modeling of Adsorption Phenomena

“…When the shift from ideal behavior is not large, it is reasonable to consider the approximation of regular solutions. Hence, the activity coefficients of each component of the surface solution can be expressed by the second-order Margules activity model as follows 4,8,9 ln γ G = B(T)x GH , = B(T)θ ,…”