Heat transfer enhancement and melting behavior of phase change material in a direct-contact thermal energy storage container

Experimental study was conducted to investigate the heat transfer performance and melting behavior of phase change material (PCM) in a direct-contact thermal energy storage (TES) container. The PCM was erythritol and the melting point was 119 degrees C. The high temperature heat transfer oil (HTO), Thermia Heat Transfer Oil C, flowed into the PCM region from the bottom of the container. The results showed that the melting process of PCM could be divided into three stages, forming channel stage, melting stage and final stage. In the first stage, the HTO could not go through the solid PCM and the heat transfer was mainly thermal conduction. In the melting stage, the HTO flowed through the PCM region and convective heat transfer was dominated. The latent heat of phase change was stored in this stage. In the final stage, most of the solid PCM was liquefied and the temperature stratification in the PCM was almost disappeared under the strong disturbance generated by HTO. Sensible heat was stored in this stage. To make a quick flow channel in the first stage and accelerate the melting process, inlet tubes with plain fins was numerically discussed through computational fluid dynamic approach. The simulation utilized enthalpy-porosity formulation to solve the direct-contact melting process. The results indicate that plain fins significantly enhance the heat transfer performance and improved the phase change behavior of PCM due to the large surface contact area and natural convection. The flow channel was quickly built. The duration of forming channel stage was reduced from 71 min in the experiment to 28 min. However, the fins also impaired the buoyancy driven flow generated by the temperature difference. When the charging process entered the melting stage, the melting process was dominated by forced convection heat transfer and the solid PCM between two flow channels rapidly became liquid. The melted PCM removed from solid surface was brought into the liquid region and flowed along with the flow of HTO. The heat storage rate improved by up to around 270% compared to the experiment. When an auxiliary heat flux was added to the tube and fin, the heat transfer was further improved.