One of the most important weaknesses of PCMs ( Phase Change Material ) is their low thermal conductivity that affects their function in energy storage devices in terms of rate of thermal charging and discharging. As extensively studied before, application of extended surfaces such as fins is one of the methods that can address this problem. The thermal conductivity of metal fins is far higher than PCMs that help increase the heat transfer rate between the heat transfer fluid (HTF) and the PCM. In the present study, a triplex-tube, employing fin-enhanced phase change materials (PCMs), as a thermal energy storage (TES) system was studied numerically.
In order to develop a simplified mathematical model for simulating the solidification process, various
assumptions were used, including:
- transient and incompressible liquid phase flow
- negligible water temperature alteration and viscous dissipations
- no-slip condition for liquid at boundaries
- insignificant influence of temperature variation on PCM thermal features except liquid density in momentum equation calculated by Boussinesq Equation
- PCM and fins were at 366 K as initial condition
- negligible thermal resistance for junction of tube and fins and isolated outer surface of the external tube
Overall, the solidification enhancement can be attributed to the heat conduction through the copper fins that accelerates the heat transfer rate resulting in faster solidification. During the start of the solidification process, natural convection (happening in the PCM area) plays a major role. Afterwards, when the PCM is partially solidified, the main mechanism of heat transfer is conduction through the fins, tubes, and the solidified PCM layers that control the solidification process. This study showcase this phenomenon through modeling of an energy storage tube with and without fins.