Abstract
Thermochemical heat storage in salt hydrates is a promising method to improve the solar fraction in the built environment. The major concern at this stage is liquefaction followed by washing out of active material and agglomeration into large chunks of salt, thus deteriorating the diffusive properties of the porous salt hydrate structure. In this work, specific attention is given to the methods to stabilize a sample salt hydrate. Attempts have been made to stabilize calcium chloride by impregnation in expanded natural graphite and vermiculite, and by microencapsulation with ethyl cellulose. The effect of these stabilization methods on the performance of the material, such as kinetics and energy density, is investigated. Characterization of the materials is carried out with combined Thermo-Gravitational Analysis (TGA) and Differential Scanning Calorimetry (DSC) methods and microscopic observation, in order to evaluate the improvements on the basis of three subjects: reaction kinetics, heat storage density and stability. Within the boundary conditions for thermochemical energy storage as presented in this work, microencapsulated calcium chloride showed high multicyclic stability, compared with pure and impregnated materials, that liquefy upon hydration under the given conditions. Microencapsulated material remains stable over multiple cycles and at the same time shows the faster kinetics, but has a lower volumetric energy storage density.
Highlights
In Europe, energy consumption for domestic purposes accounts for almost 40% of the total energy demand [1]
∫ ∫ ΔH = tf [Differential Scanning Calorimetry (DSC) (t)−BL (t)]dt = tf Qḋ t ti ti Different samples are compared in terms of kinetics, energy density and stability
These criteria are investigated through the water adsorption and energy exchange of the materials during the de/re-hydration reaction, which are measured by the Thermo-Gravitational Analysis (TGA) and DSC methods
Summary
In Europe, energy consumption for domestic purposes accounts for almost 40% of the total energy demand [1]. As one of the most exploitable renewable energy sources, is available more than required in residential houses during summer, while the demand cannot be met during winter. A solution is to store excess of solar energy in summer by a so-called thermal battery which can be discharged to provide heat for the residential demand in winter [2]. In the sorption heat storage process, heat is stored into an endothermal dissociation reaction (charging), and at a later time, the energy can be retrieved from the reverse exothermal reaction (discharging). The energy stored in this way can be released during winter by introducing water vapor to the dehydrated material. For low temperature sorption heat storage, adsorption of water vapor on sorption materials [3] and hydration of salt hydrates [4,5] are frequently studied. It should be noted here that the physical phenomenon of fixation or capture of water vapor (sorbate) by sorbent is defined under the term sorption, the expressions “thermochemical” and “sorption” are used differently by authors [7]
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