Study On Preparation,heat Storage And Enhanced Heat Transfer Of Ceramic Coupling Medium-and High-Temperature Phase Change Materials

Industrial waste heat is the most important energy generated in the industrial production process.Due to the temporal and spatial fluctuations of the amount and temperature of waste heat in many occasions,efficiently recovering and utilizing waste heat is difficult.The utilization rate of many industrial waste heat resources is still at a low level.Taking the iron and steel industry as an example,its energy-consumption accounts for 13%of China's total industrial energy consumption,and the total waste heat per ton of steel accounts for 33%of the total energy consumption per ton of steel.However,the utilization rate of waste heat of steel is only 30-50%.The inefficient use of industrial waste heat not only causes a lot of energy waste,but also puts a lot of pressure on the environment.Therefore,there is an urgent need to develop new technologies for the efficient use of the unsteady waste heat,which is an important issue in energy conservation and emission reduction.To efficiently use unsteady-state waste heat,the unsteady-state heat must first be transformed into steady-state heat.Heat storage technology can to realize the above process,that is,unsteady heat is stored by heat storage materials and is reused in a stable manner when needed.This technology overcomes the intermittent of the waste heat and effectively solves the contradiction between mismatch of heat supply and demand.Heat storage materials are one of the most critical elements in heat storage technology.Inorganic salt-based phase change materials are considered to be an ideal medium-and high-temperature(>200?)heat storage material due to their large heat storage temperature region,high heat storage density,stable physical/chemical properties,and low price,which has potential value in the utilization of unsteady-state waste heat.However,low thermal conductivity and susceptibility to corrosion of containers have greatly limited applications of inorganic salt-based phase change materials.Developing salt-based composite phase change materials with high performances is thus imperative.In this paper,NaNO3 and porous ceramic is used as a phase change material and a carrier,respectively,to prepare a ceramic coupling medium-and high-temperature phase change material,which has strong structural stability,high heat storage density and good heat transfer performance.Preparation of skeleton structure of carrier,preparation of composite material,and heat storage and enhanced heat transfer properties of composite material are carried out,and the results are obtained as follows:(1)Sintering temperature of porous ceramic should be lower than its densification temperature.The apparent porosity of fly ash cenosphere-based porous ceramic at 1250? is 41.51%,and the highest apparent porosity after modification with 30wt.%CaCO3 can reach 59.25%;the effect of CaCO3 is that the decomposition product CO2 is beneficial to increase the number of pores,and the other decomposition product of CaO helps fly ash cenosphere form a porous shell structure.The apparent porosity of diatomite-based porous ceramic at the at 1150?is 61.61%,and it has an adjustable pore structure after modification with 0-50wt.%CaCO3:apparent porosity of 61.61-67.53%,average pore size of 0.73-26.6?m;The effect of CaCO3 is that the space vacated by its pyrolysis contributes to the formation of pores,and transforms the newly generated pores from an isolated state to a connected state.It is more suitable to prepare low-cost and high heat storage density ceramic coupling medium-and high-temperature phase change material using diatomite-based porous ceramic.(2)The spontaneous infiltration method requires that average pore size of the carrier is below a critical value;on the experimental condition,the maximum pore size of the diatomite-based porous ceramic carrier used to load NaNO3 is between 10.92-26.61 ?m.On the premise of meeting pore size requirements of the carrier,the process of impregnating the carrier with molten phase change material is also affected by kinetic factors(eg.infiltration temperature,infiltration time);on the experimental condition,the optimum conditions for molten NaNO3 to infiltrate diatomite-based porous ceramic are 340? for infiltration temperature and 1h for infiltration time.On that condition,the skeleton of diatomite-based porous ceramic modified by CaCO3 has been relatively filled with NaNO3-based phase change material,achieving a near-ideal infiltration result.(3)Unmodified and modified diatomite-based porous ceramic by CaCO3 both have good chemical compatibility with NaNO3-based phase change material.Porous ceramic carrier does not the change phase change temperature of phase change material,but will reduce the enthalpy of composite material and improve the thermal stability of phase change material.CaCO3 modification of diatomite-based porous ceramic can increase the apparent porosity of the carrier(67.53%),thereby enhancing its capacity to load NaNO3(58.67wt.%),its loading rate is higher than that of the same type skeleton for 10 wt.%,and increasing the heat storage density(297.13J/g,200-330?)and heat storage efficiency(58.71%)of composite phase change material.CaCO3 modification of diatomite-based porous ceramic can also increase the pore size of the carrier(10.92 ?m),thereby promoting the micro-flow of NaNO3 melt in the framework of carrier,effectively avoiding the destruction of repeated volume expansion of the solid-liquid(liquid-solid)phase change process to the rigid structure of the carrier,therefore significantly improving the thermal cycling stability of composite phase change material,from less than 100 times to more than 500 times.(4)High-temperature modification of diatomite-based porous ceramic can obtain a denser,continuous and low thermal resistance heat transfer path,which can significantly improve the thermal conductivity of its loaded salt,up to 1.33 W/(m·K)(25?),close to twice the thermal conductivity of the same type of composite material.CaCO3 modification of diatomite-based porous ceramic can effectively avoid the damage of the carrier structure by the volume change of salt,thus providing a stable heat transfer path for the composite;Meanwhile,wollastonite and larnite with low thermal conductivity are generated in the skeleton.On the basis of CaCO3 modification,the use of SiC to further modify the skeleton of diatomite-based porous ceramic can significantly increase the thermal conductivity of loaded salt,up to 2.06W/(m-K)(25?),the increase rate is about 78%.The mechanism of SiC modification to improve the thermal conductivity is as follows?introduced heat conduction enhancement phase(moissanite phase);ensured low contact thermal resistance between heat conduction enhancement phase and phase change material;increasesd heat transfer area between heat conduction enhancement phase and phase change material.This paper provides an idea for the application and promotion of inorganic salt-based phase change materials,and lays a theoretical foundation for the recovery and utilization of industrial unsteady-state waste heat with medium-and high-temperature.