Numerical investigation and experimental validation of the thermal performance enhancement of a compact finned-tube heat exchanger for efficient latent heat thermal energy storage

Abstract

This paper presents a numerical study of heat transfer performance inside a compact finned-tube heat exchanger employed in latent heat storage (LHS). A three-dimensional mathematical model of the storage system was developed and validated based on experimental results of charging and discharging. The heat transfer problem during phase change was addressed by solving Navier–Stokes equations combined with enthalpy-porosity technique. Visualization of thermal process inside the storage unit, which cannot be fully realized using an experimental set-up, was carried out by exploiting simulation results. Due to the heat transfer fluid (HTF) circuit design, it was found that phase change occurs faster in the lower half of storage tank and especially in phase change material (PCM) neighboring HTF inlet. Stripes of PCM adjacent to tank walls where heat transport is slow were identified. The model was used to conduct parametric studies of the effects of the HTF inlet temperature, the flow rate, the number and the position of fins on charging and discharging performances. Concerning operation parameters, increasing HTF flow rate from 0.4 to 0.62 l/min reduces charging time by 31% and discharging time by 29%. Similarly, increasing temperature difference between HTF and PCM by 10 °C accelerates melting process by 30% and solidification process by 21%. Since charging is slower than discharging in the studied system, the influence of design parameters on charging was investigated. As a consequence, it was proven that increasing the number of fins reduces melting time and raises storage power. However, adding more than 13 complete fins to the studied device was shown to cause limited improvement. Moreover, positioning fins in the upper half of the heat exchanger has the best benefit on melting time. In addition, increasing fins thickness and changing their material from aluminum to copper was proven to reduce total melting time. Therefore, an improved design was proposed based on the previous conclusions and melting time was reduced by 9% compared to the initial configuration. Comparison of the improved compact finned-tube storage device with other LHS units found in literature showed that the non-dimensional storage time obtained with the novel configuration compares well with other well-established LHS configurations. The results of the present work provide operation and design assistance for LHS systems consisting of compact finned-tube heat exchangers intended to store heat such as solar thermal energy.