Experimental and numerical study on the performance of a new high-temperature packed-bed thermal energy storage system with macroencapsulation of molten salt phase change material

In this paper, a new high-temperature packed-bed thermal energy storage system (PBTES) with macro-encapsulation of molten salt phase change material has been established. A new phase change material (PCM) capsule is designed and constructed with the macro-encapsulated molten salt as its PCM. The ternary carbonate Li2CO3-K2CO3-Na2CO3 (32-35-33 wt%) has been selected as the optimal option of PCMs. The melting point is 395.1 °C and the energy storage density is 174.7 kJ·kg−1. Moreover, the thermal performances such as the temperature evolution of heat transfer fluid and that of PCM capsule, average charging/discharging rate and overall heat storage efficiency are investigated in detail. Finally, a mathematical model is presented complementarily to simulate the thermal performance of PBTES. The results are concluded as follows. (1) The temperature variation of the heat transfer fluid and that of the capsule in the PBTES are obtained. There is a certain temperature difference between the capsule and the heat transfer fluid. Moreover, the convection heat transfer resistances of the capsule and the heat transfer fluid are the main influential factors in the heat exchange process. It can be improved by the inlet temperature and mass flow rate of the fluid. (2) The improvement of inlet temperature and mass flow rate in PBTES can increase its charging and discharging efficiencies. The overall efficiency of the system can be increased to 86.1% from 77.4% through increasing inlet temperature from 425 °C to 465 °C. The efficiency will up to 83.6% from the level of 80.6% by the rise of mass flow rate. (3) To compare the shell and tube thermal storage system, the charging and discharging rates of PBTES are 1.8-3.2 times that of the former one. The overall efficiency of PBTES is 1.9-2.4 times that of shell and tube thermal storage system. (4) According to the numerical simulation results, the good agreements of temperature evolution are achieved between the numerical results and experimental data, and the shorter diameter is optimal to the PBTES because a larger diameter will increase the charging time. In summary, the PBTES is an efficient method of heat storage. The study proposes a design of such the PBTES for a first step implementation of the technology and the improvement of thermal performance optimization.