Research on dimensionless structure size of coil heat exchanger in heat discharging process of molten salt single tank heat storage system

There has been an increasing attention on single-tank thermal energy storage, attracted by the low cost, high efficiency and compactness in the operation. However, the most researches on single-tank thermal energy storage focused on the way of heat discharging and the arrangement of heat exchangers. Few studies on the dimensionless size of heat exchanger, which is important for the design of single-tank thermal energy storage, were carried out. In order to obtain higher heat exchange efficiency and design criteria of coil heat exchanger, the optimal dimensionless size of heat exchanger is studied by numerical simulation. The results show that both the dimensionless pitch and the dimensionless pipe diameter of the coil heat exchanger have a great influence on the heat discharging performance when the air is used as the heating medium and the molten salt is used as the heat storage medium. With the change of dimensionless pitch, the heat discharging performance and the flow field of molten salt are affected. When the dimensionless pitch L/d = 4, the heat discharging power and heat release are the highest. At this time, the temperature and flow field of molten salt are evenly distributed, and no temperature stratification and vortex are formed. When the dimensionless pipe diameter is changed on the premise of the optimal dimensionless pitch, the heat discharging power increases first and then decreases with the increase of dimensionless pipe diameter. When the dimensionless pipe diameter d/a = 0.25, the molten salt flow rate in the single tank is relatively large and no vortex hindering heat transfer, and the heat discharging performance is the best. Therefore, under the joint action of molten salt velocity and vortex, the optimal dimensionless pitch and pipe diameter appear. The research results provide a theoretical basis and an example for the design of single tank energy storage system.