Numerical Analysis ofLatent Thermal Energy Storage in a Cavity

Abstract: Latent Thermal Energy Storage (LTES) has drawn attention because the technology is a simple and cost-efficient method to store large amounts of energy. Latent energy is either stored or released when the material inside the LTES undergoes phase-change. As LTES operates at a constant temperature it can be utilized in several fields such as waste heat management, building insulation, storage of solar energy and electronic cooling to name a few. An obstacle to widespread use of LTES is its low energy recharge and discharge rate due to the phase-change materials (PCM) thermophysical properties, namely thermal conductivity. PCMs such as fatty acids, salt hydrates and paraffins are potential materials for domestic application because of their melting temperature and are especially affected by low thermal conductivity.The objective is to numerically model a Latent Thermal Energy Storage and simulate the melting and solidification process with different boundary conditions, and afterwards analyze how it impacts natural convection, heat transfer rates, and the solid-liquid interface. Special attention will be given to natural convection as a change in its strength can have a large impact on heat transfer. Optimization and enhancing the rate of heat transfer is important as it improves LTES effectiveness.The geometry used in the numerical model is two-dimensional with 50 mm in width and 120 mm in height. The heat transfer surface area is the 120 mm wall. Four cases are examined; two of which are melting and two of solidification. The geometry is identical in all cases but placed in either a vertical or horizontal orientation.Transient simulations are performed using ANSYS Fluent which is a computational fluid dynamics software tool. The geometrical model used for ANSYS mimics the experimental setup that Kamkari and Shokouhmand (2014) built to analyze melting in a rectangular enclosure. This allows for a comparison between numerical data and experimental observations in one of the melting cases.The comparison between the numerical and experimental results show good agreement as the solid-liquid interface is nearly identical and the amount of liquid in the enclosure differs by less than 5 percent after two-hundred minutes. Natural convection is present in all cases to a varying degree, and the amount of phase-change correlates to its strength and duration. During melting convection is the main mode of heat transfer in both orientations, but in the vertical case the strength tapers off as time progresses. The horizontal orientation produces a natural convection for the entire duration of the simulation therefore leading to a higher melting rate.The solidification process entails conduction as the dominant mode of heat transfer. In the horizontal orientation there is no detectable natural convection. The vertical position shows convection in the early stages of solidification but disappears quickly. As a result there is a higher amount of solid material in the vertical orientation by the end of the simulation.