Melting Characteristic-Based Study On Melting Heat Transfer Enhancement Of The Traditional Thermal Storage Units

The latent heat thermal energy storage is a technology that utilizes the phase transition of materials from one phase to another for thermal energy storage or release. Compared with the traditional sensible heat storage, the latent is more attractive due to its inherent high-energy storage density, isothermal characteristic, wide melting-temperature and many other advantages. Moreover, it can be widely used in many engineering fields, such as solar energy storage, waste heat recovery, peak load shifting, and electronic cooling. Therefore, it has become one of the most prospective ways of thermal energy storage. However, the latent heat storage technology suffers from a fatal weakness: low thermal conductivity of the phase change material the resulting slow charging and discharging rate. This leads to the heat transfer enhancement technique are needed for most of the latent heat thermal energy storage systems, and the melting heat transfer enhancement technique has become a hot issue in the field of both heat transfer and energy application at home and aboard.In this paper, the traditional rectangular and shell-and-tube thermal storage units are taken as examples to illustrate the condition where melting needs to be enhanced. The new melting heat transfer enhancement thinking that taking into account characteristics of the phase change problem itself and making full use of melting characteristic of PCMs is proposed. On basis of this, the wedge-shaped, curved unit and frustum-shaped thermal storage units are obtained. This not only enhances the melting heat transfer effectively, but also lays a foundation for the geometry design and performance optimization of latent heat storage systems. The main research contents are shown as follows:First, weakness in thermal performance of the traditional rectangular thermal storage unit has been found out by analyzing and summarizing melting and heat transfer characteristics of PCMs within this unit. Then, the thinking that unitizes melting characteristics of PCMs to enhance melting heat transfer is proposed.The wedge-shaped thermal storage unit is proposed for ―dead space‖ existed in the final melting stage of rectangular. Experimental and numerical studies have been conducted to analyze the effect of length ratio between top and bottom of the unit as well as enclosure aspect ratio on the temperature field, velocity field, melting front distribution and melting rate during charging.A curved unit of ―D‖ shape is proposed for thermal energy storage according to thermal behaviors of melting dominated by conduction-convection. The effective of this ―D‖ shape is validated by a comparison study between thermal performance of the traditional rectangular and new proposed curved unit. In addition, effect of the heating source temperature and PCM conductivity on melting is analyzed.The frustrum-shaped thermal storage unit is presented to enhance the coupled melting heat transfer between heat transfer fluid and PCM within the traditional shell-and-tube thermal storage unit composed by cylindricals. Then, effect of the multiple PCM technology on melting heat transfer enhancement for this novel thermal storage unit is analyzed.