Solar hot water system using latent heat thermal energy storage
Creators: name: Seddegh Kiyaroudi, S
Publisher: name: University Of Tasmania
Year: 2023

description: Latent heat thermal energy storage, based on the absorption or release of heat when a storage material undergoes phase change, has received significant research attention due to its high storage density and small temperature variation. A comprehensive literature review found that a common issue raised is the lack of understanding of the heat transfer mechanism as phase change material (PCM) melts and solidifies during the charging and discharging processes, respectively. Thus, this project largely focuses on investigation of the heat transfer mechanism across a complete charging and discharging cycle. A proper heat transfer model was developed to study the thermal behaviour and heat transfer characteristics of PCMs during the phase change process. Experimental studies were also performed to visualise the formation of heat transfer mechanism at different stages in PCMs. It was found that the heat transfer mechanism during charging is different from that during discharging. The developed heat transfer model was first validated using experimental data available on literature. It was then used to investigate the heat transfer mechanism in a shell-and-tube latent heat energy storage system (LHTES). Results showed that the combined convection and conduction model can better describe the thermal behaviour of PCMs than a pure conduction model. This combined model was further used to study the effect of orientation of the LHTES on the system performance. The results showed that during the charging process for the horizontal orientation, convective heat transfer has a strong effect on melting of the upper part of the solid PCM and is less significant during melting of the lower half. However, in the vertical orientation, convective heat transfer maintains similar action during the entire charging process. In the discharging process, the thermal behaviour does not show any difference between horizontal and vertical systems. As different mechanism of heat transfer was found in vertical and horizontal systems, further visualized experimental study focused on the vertical conical and cylindrical LHTES systems which were followed by the numerical analyses. It was revealed that during the charging process, there existed a vertical convective circulation channel around the heat transfer fluid (HTF) pipe. The width of this vertical channel did not show significant change once it was formed. Thermal energy was transferred from the HTF to the liquid PCM and then carried upward via vertical convective circulation in the channel. This thermal energy was further transferred in the liquid PCM accumulated at the upper part of the storage system through horizontal convective circulation. Hence heat transfer was more effective at the upper part of the system and PCM melting front moves downward from the top to the bottom of the system. Further comparison study showed that the vertical conical system had better energy storage performance than the cylindrical system. This indicated that a vertical storage unit with geometry having large volume in the upper part and small volume in the lower part could have better energy storage performance. During the discharging process, it was revealed that the PCM solidifies around the HTF pipe and the solidification front moves outward. Due to its low thermal conductivity, the PCM behaves as an insulation material and the rate of heat transfer from the HTF to the PCM is reduced. The comparative results show that both cylindrical and conical systems have similar trend and it takes almost the same time to complete the discharging process. The study was then followed by an investigation of the effects of geometrical parameters on vertical cylindrical shell-and-tube LHTES systems. For this purpose, the temporal variations of four similar LHTES units with different HTF pipe diameters were measured and compared experimentally. The effect of different HTF inlet temperatures and flow rates were investigated. The results show that complete charging and discharging is highly dependent on the ratio of outside to the inside diameter as well as the HTF temperature. It is also concluded that there is no significant difference in the charging and discharging time by increasing the HTF flow rate as far as the HTF flow is turbulent. It was suggested that the selection of operating conditions and geometrical parameter dimensions depends upon the required heat transfer rate and the time within which the energy has to be stored and delivered. This analysis provides useful information on design and optimization of the shell-and-tube LHTES system.