Abstract
Thermal energy storage through encapsulation of phase change materials is critical for the efficient usage of renewable energy. In this paper, a model is proposed to include thermal and pressure induced density changes in a confined phase change material. To the best of the authors’ knowledge, the local energy balance at the interface and the total mass balance that couple thermal and pressure induced density changes during melting of confined phase change materials are proposed for the first time. The proposed model is solved for a KNO3 salt and several values of the elastic constant in order to probe the phase transition in the whole pressure domain. The behavior of the thermodynamic variables is investigated for other kinds of salts used in high temperature thermal energy storage applications and in the high pressure regime. In all the examples shown, and when the phase change process takes place close to the isochoric regime, the effects of temperature dependent densities are found to be enhanced by pressure induced density changes. However, close to the isobaric regime, the effects of temperature dependent densities are almost negligible. Finally, the thermal energy stored during the melting process is obtained. Close to the isochoric regime, and depending on the types of boundary conditions, the sensible heat stored is found to be enhanced by thermal expansion, while the latent heat absorbed is significantly reduced.
Highlights
The use of phase change materials (PCMs) for thermal energy storage in concentrating solar power (CSP) plants represents an extremely appealing application for the generation of clean thermoelectric energy.1,2 The types of PCMs used in CSP plants consist of salts with high fusion points, which constitute high temperature phase change materials (HTPCMs), where thermal energy is stored.2 The extra heat captured from the concentrated solar radiation is transported to the HTPCM by a heat transfer fluid (HTF)
This paper only deals with the thermodynamics of the melting process in HTPCMs
Of the types of boundary conditions, major contributions to the relevant thermodynamic variables resulted from the thermo-elastic coupling close to the isochoric regime
Summary
The use of phase change materials (PCMs) for thermal energy storage in concentrating solar power (CSP) plants represents an extremely appealing application for the generation of clean thermoelectric energy. The types of PCMs used in CSP plants consist of salts with high fusion points, which constitute high temperature phase change materials (HTPCMs), where thermal energy is stored. The extra heat captured from the concentrated solar radiation is transported to the HTPCM by a heat transfer fluid (HTF). The thermo-mechanical models developed are focused on determining the effects of pressure on shell cracking and latent heat storage capacity, the PCM performance upon charging/discharging cycles, effects of fluid velocity on the local and total mass balance, cold storage performance, Micro-encapsulated phase change material (MPCM) slurry performance in photovoltaic/thermal modules, numerical model validation through the experimental acquisition of melting front data, etc. Close to the isochoric regime, the thermal expansion of each phase is strongly coupled to the density changes induced by pressure increments In this regime, it is found that temperature dependent densities have a significant impact on the behavior of the thermodynamic variables for each of the materials considered. When the melting process takes place at constant volume, it is found that depending on the types of boundary conditions, sensible heat is enhanced by thermal expansion, while latent heat is reduced
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