| Energy storage is a key measure to solve the contradiction between energy supply and demand as well as power load peak regulation.Phase change heat storage is an important form of energy storage technology.Hydrated inorganic salts,as medium and low temperature phase change heat storage materials,have the advantages of high heat storage density,wide phase change temperature range,low price,etc.,but also have fatal defects such as large supercooling degree,serious phase separation,and low thermal conductivity.In this paper,based on the modified inorganic salt/expanded graphite composite phase change material,the heat transfer characteristics of the material were studied numerically and experimentally.A shape-stable composite phase change material(CPCM)was firstly prepared by melt blending method using sodium acetate trihydrate(SAT)as the main phase change material,supplemented by nucleating agent,thickening agent,and thermal conductivity enhancer.The thermophysical properties of the material were measured by the step cooling curve method and the DSC differential scanning method.The results show that EG can act as a thermal bridge.When the content of EG increases,the phase transition temperature of CPCM decreases slightly,the subcooling degree is effectively reduced,the thermal conductivity is greatly improved,and the heat storage performance does not change significantly.Comprehensive analysis shows that the optimal ratio of CPCM is 88.50%sodium acetate trihydrate+3.54%sodium hydrogen phosphate dodecahydrate+0.88%sodium polyacrylate+7.08%expanded graphite.The phase change temperature is 55.3-57.1℃,the latent heat is 219.2k J/kg,the thermal conductivity is1.6345 W/(m·K),and the mass and latent heat loss rate after multiple cycles are within 5%.The VOF model combined with the solidification/melting model is used to describe the melting process of inorganic salts,and the accuracy of the interface changes in the simulation is verified by visualization experiments.The results show that the melting process is divided into two stages dominated by thermal conduction and convection,the average melting rate shows a trend of increasing first and then decreasing.The larger the mushy coefficient C,the faster the melting rate,and C is determined as 10~6 in the subsequent simulation.For the inorganic salt/EG composite phase change material,the porous medium model was established to describe the flow and heat transfer behavior of CPCM in the porous graphite framework,and the solidification/melting model was used to describe the change of the inorganic salt solid/liquid interface.The average error rates of the simulated temperature values for the storage and release processes are 18.7%and 11.6%,respectively,which verifies that the coupled model is reasonable and reliable to study the phase change heat transfer process of inorganic salt/EG composites.The numerical model of CPCM in the phase change heat exchanger is established.Except three forms of the internal heat exchanger,the parameters of the inlet fluid and the structure of the internal heat exchanger were also be considered.The index included average temperature,the liquid phase ratio,the improvement of the storage/release efficiency,etc..Combined with four-factor and three-level orthogonal test,heat transfer characteristics of CPCM in the phase change heat exchanger were researched.The results show that the heat exchange with longitudinal fins has the best comprehensive performance.In order to improve the heat storage efficiency,increasing the inlet temperature is more effective than increasing the inlet flow rate.And the fin height has the most significant effect on the heat storage and release efficiency,so the heat storage and release performance of the phase changer can be improved by ensuring that the fin height is increasing and the number of fins and the tube spacing are within a reasonable range.A CPCM-shell-tube heat exchanger test is developed to analyze the operating characteristics of the system in detail,including temperature,heat storage and release rate.Then economics of the device is also be analyzed.The results show that in the process of heat storage/release,the change trend of the temperature at the monitoring point in the simulation and experiment is relatively consistent.Specifically,the simulation errors of heat storage and heat storage rate are 10.9%and 13.8%respectively in the process of heat storage,while the errors of heat release and heat release rate are 14.7%and 8.0%respectively in the process of heat release.The economic analysis results show that the phase change heat exchanger studied is much cheaper compared with ordinary electric heaters and phase change heat exchangers filled with other media. |
