Research On Thermal Property Of Medium-high Temperature Phase Change Material And Heat Transfer Performance Of Thermocline Tanks

Phase change energy storage based on phase change materials is one of the effective methods to deal with the discrepancy between energy supply and demand during the renewable energy generation and industrial waste heat utilization.To meet the demand of medium-high temperature phase change energy storage system,the relatively high phase change enthalpy and low supercooling LiNO3-NaNO3 eutectic binary salt mixture is selected as phase change material and the expanded graphite(EG)with excellent chemical stability,thermal conductive property and low cost as the thermal conductive enhancer addressing the material level.The thermal properties and cycling stability of LiNO3/NaNO3-EG phase change composites are fully studied in this research.Addressing the system level,on the demand of enhancing the peak load regulation capability of combined heat and power plants,a multi-layer encapsulated phase change material packed-bed thermocline tank system is built.The dynamic heat transfer performance during single charging/discharging processes and cyclic operation of the system are investigated.The main work and results of this research are as followed.A novel LiNO3/NaNO3-EG phase change composite was prepared and several experimental methods were used to investigate and analyze the morphology,phase change properties and thermal conductive properties of the composite.Results showed that by using the melting-infiltration method,the LiNO3-NaNO3 eutectic binary salt mixture was successfully dispersed into the EG porous honeycomb structure,the onset melting temperature of the composite is barely shifted from the original LiNO3-NaNO3 eutectic binary salt,meanwhile the onset solidification temperature of the composite is raised due to the introduction of EG,indicating that EG modified the supercooling phenomenon of the eutectic binary nitrate salts.The phase change composites possess relatively high melting phase change enthalpy of 216.1-248.2 J/g and solidification enthalpy of 207.4-239.9 J/g.The thermal conductive performance of the composite was greatly enhanced by adding EG compared with the original eutectic nitrate salt and with the increment of EG content,the maximum thermal diffusivity of the composite reached 3.287 mm2/s.The adding of EG formed thermal conductive chains or networks inside the composite,therefore the thermal conductive performance of the composite was strengthened.The measured thermal diffusivity and calculated thermal conductivity of the composites were both decreased with the increased measuring temperature.The cycling thermal stability of LiNO3/NaNO3-EG phase change composite was investigated regarding the thermal properties of phase change temperature,phase change enthalpy,thermal diffusivity and the morphology and compatibility after cycling.Results showed that the composite experienced minor weight loss during the short-term thermal cycling in the simultaneous thermal analyzer,indicating that the composite possessed with good short-term thermal stability.After long-term thermal cycling test,both the onset melting and solidification temperature of the LiNO3/NaNO3-EG phase change composite was altered,meaning that the phase change behavior of the composites was postponed in the melting/solidification processes.The phase change enthalpies of composite on both melting and solidification processes decreased after thermal cycling and the maximum range of enthalpies decrement was 5.65%among the groups of composites.The thermal conductive performance of the composites also experienced varying degrees of decrement after thermal cycling test.The composites thermal diffusivity under the according measuring temperature decreased 11.15%-48.39%compared with the original phase change composites.By analyzing the SEM images of the thermal cycled LiNO3/NaNO3-EG phase change composite,it can be concluded that the cavity and void structure inside the thermal cycled composites was responsible for the mass decrement of the thermal conductive performance.The XRD patterns of the thermal cycled composites showed that EG and the eutectic salts were in good compatibility.A one-dimensional concentric-dispersion model of encapsulated phase change material packed-bed thermocline energy storage system was built.The dynamic heat transfer performance of single charging/discharging processes and the cyclic operation were investigated.Comparison between single and multi-layer encapsulated phase change material systems were analyzed.The effect of various operating parameters in the multi-layer system.such as the phase change material volume fraction and the inverse Stefan number on the system temperature distribution,heat storage and release quantity and the system volume utilization have also been analyzed.During a single charging/discharging process,the thermal performance of multi-layer system using PCM 70,PCM 50 and PCM 40 are between the single system optimal and worst performance:the single PCM 70 system is the highest performance,followed by the multi-layer system.For the multi-layer system,under the same circumstance of total phase change material quantity,the volume fraction of each layer in the multi-layer system greatly influenced the storage and release energy quantity during the single charging/discharging process.During cyclic operation.the higher the volume fraction of PCM 70 layer in the multi-layer system,the higher the heat storage and release periods,therefore the higher the energy storage and release quantity.The effect of the inverse Stefan number had a strong influence on the dynamic performance of the multi-layer system.The inverse Stefan number of the bottom layer phase change materials greatly influenced the system energy efficiency of the charging/discharging process.