Preparation Of Acetate Trihydrate Sodium Composite And Study On The Thermo-Physical Phonomena In Thermal Storage And Release

Energy storage technology with high heat storage density can solve the mismatch between energy supply and demand;also,it can realize the efficient utilization of low-grade heat,which is an effective guarantee for the sustainable development of energy and the construction of ecological civilization.Commonly,hydrated salt,as a medium and low temperature inorganic phase change material,has promoising advantages of large solubility heat,wide melting temperature zone,large heat storage density and low price.However,inherent problems limit its applications,such as low thermal conductivity,large supercooling degree and serious phase separation.Solar water heater and air source heat pump have been widely used as energy-saving and environmental protection equipments,but their efficiencies are seriously affected by weather and working temperature.At present,the hot water storage tank covers a large area,but cannot achieve cascade heating.In view of the above problems,this paper mainly includes the following aspects:(1)For the the purpose of cascade heating and efficient operation of compressor,the sodium acetate trihydrate(SAT)with high thermal storage density was selected as the phase change component.Expanded graphite(EG)and xanthan gum(XG)were utilited as high thermal conductive additive and crystallization nucleating agent,and to solve phase separation of pure SAT and the composite.Different proportions of composite phase change materials were prepared by melting blending method.The analysis showed that SAT was bound in EG pores,and melted and solidified in them under the action of capillary force and surface tension.The vermicular lamellar structure of EG was a good solid interface during freezing process of SAT.When SAT was added with more than 2wt.% EG,the supercooling degree was basically eliminated.The continuous porous structure of EG constituted a good thermal conductivity network,which significantly enhanced the thermal conductivity of the phase change component.As adding 4wt.%EG,the thermal conductivity of the composite could reach 1.81W/(m·K),which was 3 times of the thermal conductivity of pure SAT.97:3 of SAT: EG was the optimal proportion that all SAT exactly enterd into EG pores.With XG higher than 0.6wt,the composite exhibited weak even no phase sparation.After 60 times heating-cooling cycles,composite phase change materials adding 2-4 wt.% EG and 0.6-1.2 wt.% XG had melting temperatures of 57-59?,phase change enthalpies of 250-255 kJ/kg,and their volume expansion rates were about 7.38%.Condensation temperature of the composite stabilized at about 57.6?,and no degree of supercooling exited.This obtained material had good thermal stability and liquidity,which was suitable for thermal storage device and system.(2)This paper optimized the heat accumulator structure,and replaced fin with expanded graphite to simplify the heat exchange device.The thermal storage system driven by heat water was designed based on use across time of heat and the cascade heat output;another thermal storage system driven by waste heat of compressor was designed based on waste heat recovery and efficient work of the compressor.Subsequently,a thermal storage device driven by heat water was fabricated with a unit volume heat storage capacity of 410.99 kJ/L and its thermal efficiency over 90%;its working medium could be selected by operation temperature according to the consumer requipments.Another thermal storage device driven by waste heat of compressor was made based on compressor structure with the unit volume of heat storage up to 467.07 kJ/L,which was 4.8 times as the conventional heat exchanger with the sensible heat;it could realize the import fluid temperature higher than 5? within 5.25 min.(3)A two-dimensional simplified model was proposed,which ignores convection and volume expansion rate.The phase interface movement,temperature gradient and heat flow density of the material in the melting process were studied in combination with the common-used tubular heat exchanger.The the enthalpy method was verified by the corresponding visualization experiment that it could accurately described the phase transition process of composites with expanded graphite.Numerical simulation showed that the effects of temperature rise rate of the composite were the distance between the composite and the heat exchange pipe,the length of the nearby heat exchange pipe and the distance from the fluid inlet.The melting rate of the device rised sharply to a peak of 82% and then suddenly slows down,and the crystallization rate changed consistently,which was because the temperature difference between the composite and heat source reduced,and the thermal conductivity of the liquid composite phase change material was slightly lower than the solid state.The main influence on the melting rate of composite and its gradient of melting was the difference between the temperature of the fluid in the tube and the phase change temperature of the composite,while the velocity of the fluid in the tube had a little influence on the melting characteristics.The melting processes of composites in thermal storage devices with horizontal or vertical copper tubes have little difference.