Study On The Latent Heat Storage Coupled With Closed Loop Two-phase Thermosyphon

Thermal energy storage can solve the mismatch of time and space between supply and demand,leading to great research and application prospects.As one of the thermal energy storage technologies,latent heat storage is outstanding with the advantages of large heat storage density and small temperature fluctuation;however,the thermal conductivity of phase change materials is generally low,which limits the large-scale application of latent heat storage.As a result,enhancement of latent heat storage is required accordingly.Closed loop two-phase thermosyphon is a simple and efficient heat transfer unit without wick or external pumping power.With the above characteristics,closed loop two-phase thermosyphon can efficiently enhance the latent heat storage.This research focuses on the closed loop two-phase thermosyphon and its coupling system with latent heat storage.Firstly,experimental research of closed loop two-phase thermosyphon is conducted.A test rig is built,with which the effects of filling mass,cold bath temperature,heat load and height difference between condenser and evaporator on the heat transfer performance of a closed loop two-phase thermosyphon are investigated,covering single-phase state,two-phase state and supercritical state.It is concluded that the thermal resistance increases with the increase of heat load and height difference,leading to worse heat transfer performance.Filling mass is one of the key parameters.Both superheating phenomenon and subcooling phenomenon will worsen the heat transfer performance of closed loop two-phase thermosyphon.Then,simulation research of closed loop two-phase thermosyphon is conducted.One-dimensional model is built based on mass,momentum and energy conservation equations.It can simulate the performance under single-phase state and two-phase state,as well as supercritical state.The model is verified with experimental results,with the errors less than ±10%.The effects of pipe diameter,filling ratio,height difference between condenser and evaporator,heat load,cold bath temperature and fluid type are investigated.The results shows that the thermal resistance is low for large pipe diameter and medium filling ratio;with the increase of height difference between condenser and evaporator,the thermal resistance increases firstly and then decreases;although mass flux increases with the increase of heat load(leading to low thermal resistance),subcooling region and superheating region also increase(leading to high thermal resistance),where the thermal resistance is determined with those factors simultaneously;the vapor quality at the outlet of evaporator increases with the increase of cold bath temperature,leading to low resistance;the working fluid in the loop should have the characteristics of low viscosity and large density difference between vapor and liquid to ensure favorable heat transfer performance.It is also concluded that,thermal resistance increases sharply when supercritical state occurs due to the disappearance of two-phase state.According to the numerical research,fluid type has great impact on the heat transfer performance of closed loop two-phase thermosyphon.As a result,selecting the right fluid can efficiently enhance the heat transfer performance for certain working condition.Based on the model built above,the research of working fluid in the loop is conducted.Firstly,three refrigerants are analyzed with different heat load and condenser temperature,namely R-125,R-134 a and R-600.Then,nine fluids are investigated with difference evaporator temperature and condenser temperature,namely R-116,CO2,R-125,R-134 a,R-1234 ze,R-600 a,R-600,R-123 and ethanol.Based on the results,the map describing the optimum operating temperature range is drawn,with which the right fluid can be selected in terms of evaporator temperature and condenser temperature.Additionally,the selection criteria of refrigerants is concluded with dimensionless analysis,providing guidance for the practical application of closed loop two-phase thermosyphon.Finally,based on the research of closed loop two-phase thermosyphon,a test rig of latent heat storage coupling closed loop two-phase thermosyphon is set up.The appropriate working medium R-600 and the appropriate filling rate of 50% are selected.Erythritol is selected as phase change material.The storage process is heated with constant heat flux while the release process is cooled with water.First,the experiment analyzes the working characteristics of the coupled system.The equivalent thermal resistance of the system can be as low as 0.16 oC/W,indicating its favorable heat transfer performance and stable operating characteristics.Subsequently,the heat storage/release characteristics under different heating loads is studied.The results show that with the increase of heat load,the storage rate increases,but the thermal stratification in the storage tank intensifies.Finally,the heat storage and release efficiency of the system is calculated,which is found to have a great relationship with the thermal insulation.It is concluded that it is necessary to strengthen the system insulation and choose a medium heat load.