Design And Experimental Study Of Thermal Storage With Multiple Low-temperature Phase Change Material

In the context of national carbon peak and carbon neutralization,the application of renewable energy is particularly important.Solar energy has an abundance of heat,but is difficult to store and utilise due to its intermittent and unstable nature.Phase change heat storage technology has attracted much attention because of its advantages of high heat storage density,safe and easily controlled reaction.However,in the common phase change heat storage process,there are often some problems,such as the lag of phase change conditions near the exit of the heat accumulator and long storage times.Therefore,it is of great significance to design a heat accumulator with phase change materials,which can shorten the heat storage and release time and improve the overall heat storage and release capacity.In this paper,a low-temperature mulpitle phase change material heat storage experimental rig capable of observing phase changes within the chamber was constructed based on a shell-andtube structure to investigate the effects of different combinations of materials on the performance of heat storage.Three paraffins were measured and selected for different combinations of phase change temperatures;paraffin-expanded graphite(EG)composites were prepared for different combinations of thermal conductivity.The theoretical maximum heat storage capacity,heat storage and release time and heat storage and release per unit time are taken as the evaluation indexes,and the phase change is observed to analyze the heat storage and release characteristics.Combining the total heat storage and cost,the entropy value method is used to assign weights,and the approximate ideal solution ranking method is used to combine the scores and determine the best arrangement of materials.Using the optimum arrangement,the volume flow rate and inlet temperature of the fluid are varied to determine the optimum operating parameters.The results show that:(1)With the phase change temperature scale of paraffin wax,the thermal conductivity of 64 paraffin,56 paraffin and 48 paraffin are similar,which can be used as materials with different phase transition temperatures.Paraffin wax No.56 is compounded with 5% EG and 7% EG respectively,and the phase change temperatures are close to each other and can be used as materials with different thermal conductivity combinations.The low-temperature combined phase change heat accumulator designed in this paper fills the box with materials with different physical properties,which effectively improves the slow heat storage at the outlet of the traditional heat accumulator and improves the heat storage and release performance of the system.(2)When different phase change temperature materials are combined,reducing the volume of high phase change temperature materials and arranging them at the front of the accumulator is more conducive to heat storage;the combination of different phase change temperatures has little effect on the overall exothermic performance of the system.When different thermal conductivity materials are combined,increasing the volume of high thermal conductivity materials and arranging them at the rear can significantly improve the heat storage and release performance of the system.A comprehensive analysis shows that the three intervals of 56 paraffin,56 paraffin +5% EG and 56 paraffin + 7% EG are optimal,which can greatly improve the heat storage and release performance of the system under economic conditions.(3)Increasing the volumetric flow rate of the fluid can shorten the heat storage and release time of the system.When the flow rate increases from 1000 L/h to 1250 L/h,the heat storage and release time is greatly shortened;When it is increased to 1750 L/h,the change is small.Increasing the temperature difference between fluid and phase change material can increases the rate of temperature change of the material and shorten the heat storage and release time.Compared with the volume flow,the inlet temperature has a greater impact on the heat storage and release,with a 75% increase in flow rate and 8.3% reduction in time for heat storage and 21.1% increase in inlet temperature and 28.3% reduction in time.During heat release,the flow rate is increased by75% and the time is shortened by 15.6%;The inlet temperature decreases by 33.3% and the time is reduced by 50%.