Investigations On The Heat Transfer Performance Of Two-stage Phase Change Energy Storage System

The phase change energy storage(PCES)utilizing the latent heat of phase change material(PCM)to absorb and release heat,has been widely applied to refrigeration,solar energy storage and waste heat recovery.Compared with the single-stage PCES,the multi-stage PCES can keep the temperautre difference between PCM and thermal fluid,leading to more effective energy storage rate.In this dissertation,the two-stage PCES has been established and the effects of Reynolds number,Rayleigh number and inlet temperature on energy storage rate are investigaed by numerical and experimental methods.The major conclusions are as follows:(1)In this dissertation,the numerical model of two-stage PCES has been established.In order to improve the energy storage rate,the axial fin is set.The comparisions of energy storage rate are conducted.The results show that,the heat accumulation near the fin can be weakened by axial fin and the heat transfer between two PCMs can be enhanced.Besides,with the axial fin,the temperature distributes more uniformly,and the total melting times fo the two PCMs has been reduced by 21.79%and 13.87%.(2)In order to find out the heat transfer performance of two-stage PCES with axial fin,the dimensional analysis has been conducted and the effects of Reynolds number,Rayleigh number and thermal properties of PCM on energy storage rate have been investigated.The results show that,the impacts of Reynolds number or Rayleigh number are reduced with the increase of Reynolds number or Rayleigh number.When the Rayleigh number is increased from 0.8×10~5 to 1.2×10~5,the total stored energy is increased by 0.091.When the Rayleigh number is raised from 2.0×10~5 to 2.4×10~5,the total stored energy is only increased by 0.029.(3)In order to enhance the heat transfer performance of PCM,the coupled PCM is formed from expanded graphite and paraffin.The coupled PCM is utilized in two-stage PCES,and the corresponding experimental platform is establihsed.The influnces of Reynolds number and thermal fluid's inlet temperature are studied.The results show that,the stored energy is increased with the Reynolds number.When the Reynolds number is increased form 66.8 to 140.1,the stored energy is increased by 0.28×10~4 J.When the Reynolds number is increased from 250.4 to 375.6,the stored enery is raised by only 0.02×10~4 J.Besides,increasing the inlet temperature can enhance the efficiency of energy storage,and increase the stored energy.When the time is 18000 s,the stored energy of the system with inlet temperature of 353.15 K is 1.8 times more than that of323.15 K.The resutls of this dissertation are able to help to improve efficiency of the two-stage phase PCES,and extend the application of PCES in refrigeration,energy storage of solar energy.