Research On The Thermophysical Properties Enhancement Of Molten Salt Based Nanofluid

With the rapid economic development,traditional fossil energy consumption is growing with the passage of time,and the accompanying pollutant emission problem is also becoming more and more serious.Solar energy is considered to be one of the most likely energy resources to replace traditional fossil fuels due to its huge reserves and environmentally friendly characteristics.Concentrating solar power(CSP)is an important technology to utilize solar energy.The heat storage system equipped in the CSP system can buffer capacity,stabilize power output,and improve utilization efficiency.Molten salt is widely used as a heat transfer and heat storage working medium in CSP systems due to its advantages of high working temperature,low vapor pressure,moderate viscosity,and good thermal stability.Molten salts have many advantages,but still have some defects such as low thermal conductivity and specific heat capacity.Doping nanoparticles in the molten salt can improve the thermal conductive property and thermal storage property.However,the enhancement mechanisms of thermal properties are still unclear,which is not conducive to guide the selection and preparation of the molten salt nanofluid materials.In view of this,this study combined the experimental research and molecular dynamics simulation to study the thermophysical properties of molten salt nanofluids and the mechanism behind the thermophysical properties enhancement.In addition,exploratory research has been carried out for the major bottleneck(i.e.,stability of molten salt based nanofluid)restricting the development and application of molten salt based nanofluid,and the method to improve the stability have also been proposed.In this study,the most commonly used molten salt,Solar Salt,and the SiO2 nanoparticles with mature preparation technology were selected as the base salt and nano-additives,meanwhile,the molten salt-based nanofluid was prepared by the aqueous solution method.The nanoparticle loading,particle size and the concentration of the salt solution are set as influencing factor to explore its influence on the thermophysical properties.The experimental results show that the addition of SiO2 nanoparticles can indeed enhance the specific heat capacity and thermal conductivity of the molten salt base nanofluid significantly,and has no effect on other key thermal properties such as melting point and decomposition temperature.In addition,the optimal nanoparticle loading of SiO2 nanoparticles is 1 wt.%,which corresponds to an increment in specific heat capacity of about 22%,and in thermal conductivity of about 6%.However,the thermal conductivity and heat storage performance will both decrease when the nanoparticle loading exceeds the optimal loading.Among the SiO2 nanoparticles,the thermal properties of the sample prepared using the nanoparticle with the size of 30,50,and 80 nm particles are not much different and are better than that of the 15 nm sample.As we all know,the concentration of the salt solution is one of the most important parameters in the preparation process,and the research results show that the greater the concentration of the salt solution,the worse the thermophysical properties will be.Furthermore,the results of microscopic morphology observation show that the dispersibility of nanoparticles in the base fluid directly affects the enhancement of thermophysical properties,and its impact on the thermal storage performance and thermal conductivity is consistent and synergistic.Subsequently,the molecular dynamics simulation(MD simulation)was used to study the mechanism behind the thermophysical properties enhancement.Regarding the thermal conductivity,the inferences that have been widely accepted and applied by researchers over the years(i.e.,Brownian motion and induced micro-convection,interface effects,and agglomeration effects)have been verified one by one.The simulation results show that the Brownian motion of the nanoparticles cannot provide extra heat transport.In addition,the micro-convection is not related to the Brownian motion.Adversely,it may even weaken the diffusion strength of the base fluid particles.Besides,there is indeed a compressed layer around the nanoparticles,but its thickness is extremely small,and the particle configuration in the compressed layer is highly disordered.According to the size effect of thermal conductivity and the mean free path theory,the compressed layer has a relatively lower thermal conductivity,and the compressed layer cannot bring extra heat conduction to the base fluid.At the same time,there is an interface thermal resistance between the nanoparticle and the compressed layer,which may further have a negative effect on the system thermal conductivity.Besides,the agglomeration of nanoparticles has a positive effect on the thermal conductivity enhancement of the system.The agglomerate constructs a low thermal resistance transmission channel for heat conduction and enhance the macroscopic thermal conductivity of the system.In addition to the verification of the above inferences,the mechanism behind thermal conductivity enhancement is also proposed from the perspective of hot carriers,that is,the addition of nanoparticles increases the probability and frequency of particle collisions in the base fluid,thereby enhancing the thermal conductivity of the molten salt base fluid.Similar to the path of the exploration on the mechanism-behind thermal conductivity enhancement,the inferences of mechanism(i.e.,the specific heat capacity of nanoparticles is higher than that of the corresponding bulk material,interface thermal resistance theory,and compressed layer theory)are also verified firstly.The simulation results show that although the specific heat of the nanoparticle is higher than the specific heat capacity of the bulk material,but still lower than the specific heat capacity of the base liquid,and cannot provide more heat storage for the system.Although there is interface thermal resistance and compressed layer,they are only the appearance of the interaction between the nanoparticle and the base fluid particles.The mechanism of thermal performance is further analyzed,and the real reason for the increase in the specific heat of the system should be that the interaction between the particles and the base fluid constitutes a constraint on the base fluid particles.When the temperature rises,more energy is required to break this constraint,and this process is similar to the phase change process,so that the heat storage performance of the base fluid is enhanced.In order to simulate the actual operating conditions of molten salt in the CSP system,the long-term high temperature condition and high-low temperature circulation condition are set up in this study to study the stability of molten salt-based nanofluids prepared by the aqueous solution method.The experimental results show that the thermophysical properties have reduced significantly after 100 hours high temperature and 100 times high-low temperature cycles heat treatment.And the thermophysical properties enhancement has been nearly lost after 500 hours high temperature and 500 times high-low temperature cycles heat treatment.Further analysis of the precipitate in the sample shows that the stability of the sample under high-low temperature circulation condition is slightly better than that under long-term high temperature condition,but they both fail to meet the requirements of industrial applications.Therefore,further stability improvement studies are needed.Further researches start from two perspectives:preparation method and material selection.The samples were prepared by the high-temperature melting method and suffered corresponding heat treatment.The results showed that compared with the aqueous solution method,the attenuation range of the thermophysical properties was reduced,that is,the stability was improved to a certain extent.Meanwhile,the three kinds of hybrid nanofluid are prepared using Al2O3-SiO2,TiO2-SiO2,and CuO-SiO2 hybrid nanoparticle.After heat treatment at long-term high temperature conditions,the thermophysical properties of these three samples all have reduced,but the attenuation amplitude of the Al2O3-SiO2 hybrid nanofluid is further reduced compared with that of the SiO2 nanofluid,white for the other two hybrid nanofluids,the ability enhancing the thermophysical properties has been lost after 100 hours of high temperature condition.In other words,the Al2O3-SiO2 hybrid nanoparticles can further improve the stability of the molten salt based nanofluid.