Study On Heat Transfer Characteristics And Optimization Of Low Temperature Cascaded Phase Change Thermal Storage Device

Renewable energy plays an important role in providing energy services in a sustainable way.However,the instability and intermittence of renewable energy are one of the main obstacles in its application process.When energy production and demand cycles do not match,renewable energy cannot provide stable and reliable energy.Latent heat thermal energy storage system(LHTESS)is characterized by high heat storage density and stable temperature during phase change.It is a technology that can help eliminate this mismatch by formulating peak load transfer strategy.However,the low thermal conductivity of phase change materials(PCM)leads to the generally low heat storage efficiency of phase change heat storage systems,which is the main factor restricting the development of latent heat thermal energy storage system.In order to solve the above problems,the thermal conductivity of the phase change material is increased by using the thermal conductivity enhancing filler,and the overall heat storage and release efficiency of the latent heat thermal energy storage system is improved by using the cascaded latent heat thermal energy storage system.In this paper,magnesium sulfate heptahydrate(MgSO4·7H2O),magnesium sulfate heptahydrate-aluminum potassium sulfate dodecahydrate(KAl(SO4)2·12H2O)binary phase change materials and magnesium sulfate heptahydrate-aluminum ammonium sulfate dodecahydrate(NH4Al(SO4)2·12H2O)binary phase change materials were prepared at first.The phase characteristics,phase change temperature,latent heat of phase change,thermal conductivity,supercooling and other thermophysical properties were compared and analyzed.Magnesium sulfate heptahydrate-potassium aluminum sulfate dodecahydrate binary phase change material has a phase change temperature of 79.4 °C,a latent heat of 189.5 kJ/kg,and a supercooling degree of 2.2 C.Compared with the other two PCM,the thermal and physical properties are more complex,and can be used as the first stage in designing the cascaded latent heat thermal energy storage system.Secondly,expanded graphite is added to the magnesium sulfate heptahydrate-potassium aluminum sulfate dodecahydrate binary phase change material to improve the thermal conductivity of the phase change material.Four methods of direct mixing,melt impregnation,ultrasonic oscillation and vacuum adsorption are used to prepare different expanded graphite-magnesium sulfate heptahydrate-aluminum potassium sulfate dodecahydrate composite phase change materials.Tests show that the melt impregnation method has less influence on the phase change temperature and latent heat of PCM and can be used as an ideal configuration method.By comparing the thermophysical properties of the composite phase change materials containing different mass fractions of expanded graphite,it is found that the addition of expanded graphite has little influence on the phase change temperature of the PCM.With the increase of mass fraction of expanded graphite,the thermal conductivity of the composite material increases,but the increase gradually decreases.The 7.5wt.%EG composite phase change material has a phase change temperature of 79.4 °C,latent heat of 175.8 kJ/kg,supercooling of 1.3 °C,and thermal conductivity of 2.53 W/(m k),which is 314% higher than that of pure magnesium sulfate heptahydrate-aluminum potassium sulfate dodecahydrate binary phase change material.After that,a cascaded latent heat thermal energy storage system was designed which used expanded graphite-magnesium sulfate heptahydrate-aluminum potassium sulfate dodecahydrate composite phase change material,stearic acid and 60# paraffin wax as three-stage with phase change temperatures of 79.4 °C,71 °C and 61.3 °C respectively.By comparing the heat storage capacity,heat storage time,heat storage efficiency and heat recovery efficiency of the single-stage latent heat thermal energy storage system and the cascaded latent heat thermal energy storage system.It is found that compared with the single-stage latent heat thermal energy storage system using only stearic acid or only 60# paraffin wax,the theoretical maximum heat storage capacity of the designed cascaded latent heat thermal energy storage system is increased by 16.85% and 11.80%,the melting time of PCM is shortened by 50.42% and 12.30%,and the solidification time is shortened by 54.64% and 49.39%,respectively.Finally,regarding the dynamic characteristics of the cascaded latent heat thermal energy storage system under variable working conditions,the simulation analysis is carried out from the angle of variable inlet flow rate.It is found that the inlet speed of heat transfer fluid has little influence on the heat recovery efficiency of the cascaded latent heat thermal energy storage system.When the flow rate of heat transfer fluid is 1 m/s,the melting process of all phases of phase change material in the cascade phase change heat storage device is almost synchronized.The time required for the complete melting of phase change material is the shortest,which is 15.15% lower than the flow rate of 0.5 m/s and 11.55% lower than the flow rate of 2.5 m/s.