Energy Asymmetry Analysis And Numerical Simulation Of Heat Transfer Process Of Phase Change Material

With the increasing energy consumption,how to use energy efficiently and reasonably has become the focus of people's attention.The energy storage with phase change material(PCM)is a kind of advanced technology which can improve the efficiency of energy use.It can be used to solve the mismatch problem between energy supply and demand.Correspondingly,the detailed heat transfer process of PCM can be analyzed deeply,which can provide theoretical guidance for the practical application of PCM.In this paper,the method of simulation and experiment was used to study the energy asymmetry and the heat transfer process of PCM during the melting and solidification processes.The main conclusions of the researches are shown as follows:Several kinds of PCMs were tested by a differential scanning calorimeter(DSC).The experimental results showed that the melting process and the solidification process of PCM were asymmetrical.Because of the energy asymmetry in the melting and solidification processes of PCM,it was found that the enthalpy and the state of PCM were not only dependent on its temperature,but also affected by its "previous state".The cooling experiments of two kinds of PCMs were conducted.The results showed that the factors that affect the asymmetry of the energy of PCM are due to the asymmetry degrees and supercooling degrees of PCM during the melting process and solidification process.Based on the lumped parameter method and the effective heat capacity method,the numerical model of step cooling process of PCM was built,and the model was validated by the experimental results.The calculation results showed that the starting solidifying time of the PCM and the total time of solidification process both had the exponential relationship with the supercooling degree.The amount of latent heat had no influence on the starting solidifying time,but the total time of solidification process was increased with the increase of the effective latent heat.Two numerical heat transfer models of PCM were built with the effective heat capacity method and the enthalpy method,respectively.The results showed that when the model was built with the effective heat capacity method,the calculation error existed when the state of PCM(solid,molten or liquid)was changed during the calculation of one time step.The calculation error was increased with the increase of the amount of heat absorbed by the PCM during the calculation of one time step,and the calculation error was significant when the phase change temperature range of the PCM was very small.However,compared with the model with the effective heat capacity method,the computing time of the model with the enthalpy method was longer.The asymmetric heat transfer model of PCM was established and the model was verified by the experiment.The effect of asymmetry degrees and supercooling degrees on the melting and solidification process of PCM were analyzed by the model.It was found that the greater the asymmetry degrees or supercooling degrees,the greater the difference between the asymmetric model and the symmetric model.Therefore,it is necessary to consider the supercooling degrees and asymmetry degrees of PCM in modeling.