Three-dimensional transient numerical study on latent heat thermal storage for waste heat recovery from a low temperature gas flow

We propose a latent heat thermal storage (LHTS) system to recover the waste thermal energy from a low temperature gas flow. Paraffin wax is selected as the phase change materials (PCMs) for energy storage. To analyze the performance of LHTS during the heat recovery process, a numerical tool is developed. The time-dependent mathematical model is built in fully three-dimensional (3D) on the basis of the incompressible Navier-Stokes equations. The multi-physical fields including the laminar flow, heat transfer and phase change process are simulated simultaneously across the Air-Aluminum-PCM domains. Natural convection effect is considered and evaluated when the solid PCM melts to the liquid state. The heat transfer characteristics and mechanisms with phase change evolving are performed by the 3D simulations. Compared with the two-dimensional model, the 3D modeling provides more accurate phase change evolution in space. In a duration of 4-h waste heat recovery process, about 2239 kJ thermal energy is collected by the LHTS units, and 52.48% of them is stored in the latent heat form with only an 8 °C temperature rise. By comparing with the experimental results, a good agreement is observed on the evolution of temperature and heat transfer rate. Parametric studies on the flow rate and heater power input have also been conducted. When the flow rate increases by varying the fan speed from 25, 30 to 45 RPM under a fixed power input of 2 kW, the melting rate of PCMs becomes slower as the heat transfer rate decreases; while increasing the heater power under a fixed fan speed input can be helpful for the PCM melting. The presented 3D numerical model manages to predict the performance of the LHTS units for the waste heat recovery.