Experimental and numerical investigation of a rectangular finned-tube latent heat storage unit for Carnot battery

Latent heat storage technology has a great potential in Carnot battery systems, and its thermal performance directly affects the system performance. In this paper, a rectangular phase change heat storage unit with finned tube structure is designed and manufactured, and polyethylene wax/expanded graphite is adopted as the composite phase change material. The thermophysical properties of the polyethylene wax are characterized by experiments first, and then the heat transfer performance of the unit is investigated through varying inlet mass flow rates and inlet temperature on the test rig. Meanwhile, the detailed evolution of the phase change process is visualized and obtained by corresponding numerical simulations. Furthermore, the effects and selection principle of fin number, thickness, and material on the melting process are clarified by numerical simulations. Results shows that the influence of the inlet temperature is more significant than the mass flow rate on the melting/ solidification time, however, changing the inlet temperature and mass flow rate has little effect on melting/ solidifying the last 10 % of PCM. Heat conduction dominated the heat transfer process due to the high viscosity of polyethylene wax, inducing little stratification of the temperature distribution in the regions bounded by fins. The regions close to the corners and sidewalls are the primary factors delaying melting/solidification rate. Increasing fin number and thickness can effectively decrease the melting/solidification time at the expense of the heat storage capacity. Compared to selecting only high thermal conductivity fin materials, adopting fins with high thermal conductivity, low density, and low specific heat is a better approach for achieving higher heat storage capacity and shorter melting time. The present study explores candidate materials for medium-low temperature latent heat storage and potential applications of traditional finned-tube heat exchangers in medium-low temperature latent heat storage.