Study And Optimization On Charging/Discharging Characteristics Of Phase Change Cold Storage Unit For Air Conditioning

The use of conventional central air-conditioning systems has continued to increase in recent years,which leads to high electrical costs.Air-conditioning systems using thermal energy storage seem to be the right solution to this problem.The thermal energy storage utilizes the phase change materials(PCMs).PCMs store energy during the off-peak electrical load periods,and release it during the on-peak electrical load periods,which shifts the on-peak demand to off-peak periods,thus,resulting in the use of cheaper electric power.At present,ice-based latent heat thermal energy storage is considered more efficient.However,the latent heat and the thermal conductivity of PCM used in conventional air-conditioning systems are small.A decrease in latent heat and thermal conductivity leads to a decrease in the charging/discharging capacity and the rate of charging/discharging,respectively.To overcome this problem,air conditioning using multi-PCM TES has been proposed.In recent years,several studies have investigated the advantages of using multi-PCM instead of single-PCM.Meanwhile,the tubes are also optimized to improve the cold storage performance and the cold release performance.The main work and research results are as follows:(1)The physical model and mathematical model of the multi-PCMs thermal storage tank were established.To validate the theoretical analyses,an experimental multi-PCM TES system was set up.(2)The influence of the structural parameters of the PCM heat transfer tubes in the tank on the heat transfer performance of the charging/discharging process was studied and optimized.Numerical calculations were conducted to investigate the heat transfer effect of the PCM regenerator,the inner tube radius,and the fin group number in the PCM heat exchange tube.The analysis results show that the reasonable selection of the segmentation ratio,the inner tube radius and the fin group number will shorten the time for the charging process and increase the charging rate.The HTF outlet temperature,which is maintained at the temperature of the air conditioning cold water supply temperature,is prolong.(3)The research on the charging characteristics after optimizing the structural parameters.The effects of HTF inlet flow rate and inlet temperature on the volume fraction of liquid phase,the charging capacity,the temperature distribution and the phase interface distribution in the tank were analyzed.It is concluded that the increase of the inlet flow and the decrease of the inlet temperature will lead to the conclusion that the heat thransfer performance is improved.(4)The effects of inlet flow rate and inlet temperature on the discharging characteristics after optimizing were studied.The results indicate that the discharging performance increases as the inlet flow rate decreases and the inlet temperature decreases,but the inlet temperature that is too low also increases the power consumption of the refrigeration unit.