| In order to solve the problems of excessive heat in sunny days and insufficient heat in rainy days in the traditional solar hot water system,and to achieve the purpose of "peak shaving and valley filling",a new solar hot water system was designed by using phase-change energy storage technology based on the experimental study on the thermal properties of phase-change materials.Then numerical simulation was adopted to study the phase change process of the phase change tank in the hot water system under different conditions.According to the daily hot water temperature of residents,it is determined that the phase change temperature of phase change materials should be between 50℃to 60℃.Myristic acid with phase change temperature about 53 was chosen to be the phase change material used in ℃this system.Then,the solid liquid density,the volume expansion rate before and after phase change are measured.In addition,the thermal property parameters such as phase change temperature,enthalpy,and thermal conductivity were experimentally studied of the by DSC and Hot Disk,respectively.Secondly,taking a two-unit and nine-story residential building area in Beijing as an example,combined with phase change energy storage technology,two types of tube-fin heat exchanger filled with myristic acid with different configurations in the water tank,7 tubes 4 fins and 19 tubes 8 fins were designed,according to the relevant heat transfer theory of phase change.Phase change process was simulated using FLUENT based on physical models and mathematical models.Heat storage processes with and without natural convection or with and without fins of myristic acid were discussed,in this paper,respectively.Discusses the heat storage process of myristic acid without natural convection or fin-free,and analyzes the influence of the number of heat exchanger,the temperature of heat source and the number of fins on the heat storage process of myristic acid.At the same time,the exothermic process of myristic acid is simulated based on the 19 tubes 8 fins model,which provides a theoretical basis for the optimal design of phase change water tank.The simulation results show that the natural convection phenomenon plays a positive role in promoting the melting rate of myristic acid during the heat storage process of phase change materials,so it should not be neglected.Moreover,under the premise of ensuring the stored heat is constant,the increase in the number of fins and heat exchange tubes can effectively increase the heat exchange area of the phase change water tank and improve the heat exchange efficiency.At the same time,the different heat source temperature and the number of fins have a great influence on the heat storage process of myristic acid,according to the simulation results,the model at 90℃ completes the heat storage in only 453 min compared to the model with a heat source temperature of 70℃,which can reduce the heat storage time of myristic acid by 43%.In addition,with the increase of the number of fins,the time when myristic acid began to melt was prolonged,but the overall heat storage time was obviously reduced,and the time of 19 tubes 8 fins model was 99 min and 57 min earlier than that of 4 fins and 6 fins model respectively.In the exothermic process of the phase change material,the exothermic phenomenon was obvious due to the large temperature difference at the beginning stage.As the heat transfer progresses,the temperature difference decreased gradually,the solidification phenomenon weakened gradually,and the solidification rate approached the level gradually. |
