Enhanced heat transfer in a phase change energy storage with helical tubes

Abstract

Solar collectors integrated with phase change materials (PCM) store heat energy for later use. However, the settling of PCM prolongs the melting duration in a vertical cylindrical container. Hence, selective methods are required to alleviate such solid PCM settling issues. A helical heat transfer tube (HTT) can improve melting by increasing the contact surface. The present research studies the charging and discharging of PCM using three helical coil HTT configurations: fully expanded, semi-compressed, and fully compressed. The fluid flow rate and inlet temperatures are 20 LPH and 80 °C, respectively. The simulation involves two-dimensional numerical modeling using Ansys Fluent software and the finite volume method with the enthalpy-porosity technique. The fully compressed HTT melt settled PCM inside the vertical cylindrical container faster. The experimental results show that using a fully compressed HTT reduced the melting time by 50 % compared with a fully expanded HTT of 160 min due to the effective melting of settled PCM. Its peak heat stored, energy efficiency, Nusselt, Stefan, and Rayleigh numbers are 356 kJ, 34.8 %, 75.34, 0.47, 2.8036 × 107, respectively. The compressed HTT is a practical design suggested for solar collectors and low-temperature heat recovery systems, which demand a fast heat storage rate.