Physical Insights On Nanoparticle-induced Enhancement Of Thermal Conduction And Heat Storage Properties Of Molten Salts

The development and utilization of solar energy has become an important part of the global energy strategy along with the increasing severity of the energy crisis.The current concentrated solar power with integrated thermal storage systems can effectively address the volatility and intermittency of solar energy.The thermal storage systems help to improve the efficiency of solar energy utilization,regulate the balance of power supply and demand,and ensure the safe and stable operation of the power system.Molten salt is the most commonly used heat transfer and storage medium for solar thermal power generation due to its excellent chemical stability,low vapor pressure,low toxicity,and low cost.However,the relatively low thermal conductivity and specific heat capacity of molten salts hinder the cost reduction and efficiency of solar thermal power systems.The addition of nanoparticles to molten salts is an effective way to enhance their thermal properties.However,researchers still lack an in-depth understanding of the thermal conductivity,thermal storage mechanisms and regulatory mechanisms of molten salt-based nanofluids.This paper presents a systematic study of the thermal properties of molten salt-based nanofluids with the aim of providing principles and parameters for the design and performance tuning of photothermal molten salts.In this paper,the thermal conductivity and thermal storage properties of molten salt loaded with nanoparticle are investigated by molecular dynamics simulations and experiments.The radial distribution function,diffusion coefficient,local heat flux,number density distribution,contributions from particles(kinetic energy,potential energy and collision term)and base fluid,and contributions from van der Waals energy,coulomb energy,bond energy and angle energy are analyzed to validate the conventional thermal transport mechanisms of nanofluids.These mechanisms including Brownian motion of nanoparticles,microconvection of base liquid and interfacial layer.New insights into heat conduction performance of molten salt based nanofluid are given from the perspectives of material components,heat flux fluctuation modes and potential energy contributions.The microscopic heat storage mechanisms of molten salt-based nanofluid are revealed from three aspects:nanoparticles,base fluid and their interactions,based on the analysis of atomic radial distribution function,atomic number density and phonon density of states at different temperatures.In addition,The effects of temperature change,nanoparticle type and mass fraction on the thermal properties of molten salt nanofluid was explored.The results show that the addition of Al2O3 nanoparticles improves the thermal conductivity of solar salt,and the enhancement of thermal conductivity reaches 40.3%at a particle mass fraction of 10 wt.%.The addition of Al2O3,SiO2 nanoparticles or multi-walled carbon nanotubes(MWCNT)significantly enhances the specific heat of ternary carbonates.The enhancement of specific heat reaches up to 41.2%at a temperature of 808～833 K.Regarding the thermal conductivity mechanism,the increased thermal conductivity cannot be explained by the previously proposed hypothesis,such as the Brownian motion of nanoparticles,microconvection of base liquid,ordered layer of base liquid and coulombic potential energy.It is found that the nanofluid thermal conductivity is mainly contributed by the atom motions and interactions between atoms in Al2O3 nanoparticle,rather than in base fluid.The heat flux is concentrated in nanoparticle,which promotes the heat transfer.Furthermore,the collision involving the work done by the interatomic forces or the virial interaction dominates the heat conduction in nanofluid with lower mass fraction,while the potential energy and collision both contribute to the nanofluid with higher mass fraction.Regarding the thermal storage mechanism,it is found that the freedom degrees and surface energy of surface atoms of nanoadditive,the ion spacing of base salt,and the thermal resistance between base salt and nanoparticle cannot result in the enhanced specific heat capacity of molten salt-based nanofluid.The increase of specific heat capacity of molten salt-based nanofluid is mainly attributed to the interfacial layer.Moreover,the thickness of interfacial layer changes with temperature.The thicker interfacial layer leads to the higher specific heat capacity of molten salt-based nanofluid.In addition,the thermal conductivity of Al2O3 particle-solar salt nanofluid increased with the increase of nanoparticle mass fraction.Compared with the addition of Al2O3 or SiO2 particles,the addition of SWCNT to ternary carbonates is more effective in specific heat enhancement.In contrast to pure carbonates,the specific heat of SWCNT-carbonate nanofluid gradually decreases with increasing temperature.The specific heat of Al2O3 particle-carbonate nanofluid and SiO2 particle-carbonate nanofluid shows a increasing trend with decreasing temperature.The results of this paper provide a scientific basis for the development of new heat transfer and storage media,and help to promote the reduction of cost and efficiency of solar thermal power generation.