Thermal performance of a solar latent heat storage unit using rectangular slabs of phase change material for domestic water heating purposes

Abstract

In this paper, the thermal performance of a rectangular latent heat storage unit (LHSU) coupled with a flat-plate solar collector was investigated numerically. The storage unit consists of a number of vertically oriented slabs of phase change material (PCM) exchanging heat with water acting as heat transfer fluid (HTF). During the day (charging mode), the water heated by the solar collector goes into the LHSU and transfers heat to the solid PCM which melts and hence stores latent thermal energy. The stored thermal energy is later transferred to the cold water during the night (discharging process) to produce useful hot water. The heat transfer process was modeled by developing a numerical model based on the finite volume approach and the conservation equations of mass, momentum, and energy. The developed numerical model was validated by comparing the simulation results, obtained by a self-developed code, with the experimental, numerical and theoretical results published in the literature. The numerical calculations were conducted for three commercial phase change materials having different melting points to find the optimum design of the LHSU for the meteorological conditions of a representative day of the month of July in Marrakesh city, Morocco. The design optimization study aims to determine the number of PCM slabs, water mass flow rate circulating in the solar collector and total mass of PCM that maximize the latent storage efficiency. The thermal performance of the LHSU and the flow characteristics were investigated during both charging and discharging processes. The results show that the amount of latent heat stored in the optimum design of the storage unit during the charging process is about 19.3 MJ, 16.54 MJ, and 12.79 MJ for RT42, RT50, and RT60, respectively. The results also indicate that depending on the mass flow rate of HTF, the water outlet temperature during the discharging process varies within the temperature ranges 43.6 °C-24 °C, 51.7 °C-24 °C, and 62.86 °C-24 °C for RT42, RT50, and RT60, respectively.