Study On Thermal Storage And Heat Transfer Characteristics Of Solar Salt Doped With Nanoparticles

With the running out of fossil energy,the development of renewable energy such as,solar enregy,wind energy,nuclear energy and so on,has drawn much attention of humanity.Among them,solar energy is regarded as one of the most potential energies to replace the fossil fuel since its huge amount and hospitality to the environment.One of the most talented technology using solar energy is the concentrating solar power.However,due to the discontinuity and instability of solar energy,the solar power plants have to combine with a thermal storage system.Through controling the generated power by thermal storage technology,the output of continuous and stable electric energy becomes reality.Molten salts are widely used in thermal storage systems of conc entrating solar power plants,for the characteristics such as high operating temperature,low vapour pressure at high temperature,comparable viscosity with synthetic heat transfer oil,good stability and really low cost.However,the low thermal conductiv ity and specific heat capacity hamper the further utilization of molten salt and reduce the operating efficiency of pow plants.Doping nanoparticles into base materials is benefit to improve the physical properties of thermal storage and heat transfer materials.Nevertheless,the investigation on molten salt doped with nanoparticles is still deficient.Hence,it is really needed to conduct the study of modifing the heat transfer and thermal storage characteristics of molten salts using nanotechnology.In this paper,both numerical and experimental methods were used to analyse the thermal storage characteristics of solar salt doped with nanoparticles and its potential application in solar energy power systems.A new method,freeze-drting method,formulating the solar salt doped with nanoparticles was used.The effects of different nanoparticle fractions,nanoparticle diameters,nanoparticle materials on thermal storage characteristics were analyzed.Results show that there is an optimal nanoparticle mass fract ion for Si O2 nanoparticles,about 1.0 wt.%,and the maximum enhancement of specific heat capacity is about 27.0%.Beyond this mass fraction,the specific heat capacity gets reduced.For different Si O2 nanoparticle sizes,the specific heat capacity changes nonmonotonic,specific heat capacity of solar salt with 20 nm nanoparticles gets the greatest enhancement while with 10 nm gets the least.And for Al 2O3 nanoparticles,the specific heat capacity of samples increases with the increase of mass fractions and maximum enhancement is about 12.3% with the nanoparticle mass fraction about 2.0 wt.%.Morphology analysis was done and it shows that nano/micro structures appear in samples doped with nanoparticles which maybe one of the reasons for the enhancement of specific heat capacity.A molecular dynamics simulation model,considering interaction in moleculars and interaction between atoms,was established for thermal storage characteristics of solar salt doped with nanoparticles.The effects of nanoparticle mass fr action,nanoparticle size and nanoparticle materisals on specific heat capacity were analyzed.Results show that density and specific heat capacity calculated by molecular dynamics simulation show good agreement with theoretical and experimental results.According to the potential energy analysis in the nanoparticle-solar slat system,the addition of nanoparticles influences the Coulombic energy of per atoms in the system,resulting in the change of specific heat capacit y.Furthermore,there is a tendency to formulate a double electric layer on the surface of nanoparticles and different cations show different responds to the charge in nanoparticle surface and leads to a separation of cations resulting in the change of Coulombic energy and changing the specific heat capacity of materials.Combined with a mass modified scheme for the outlet boundary condition,a double distribution function lattice Boltzmann model coupled flow and heat transfer was proposed.The force convection heat tranfer of solar salt doped with nanoparticles in a tube was simulated.The effects of Re numbers,nanoparticle mass fractions,heat flux applied in the tube surface and nanoparticle materials on heat transfer were analyzed.Results show that increasing Reynolds numbers and adding nanoparticles will enhance the force convection heat transfer.Comparing the heat transfer coefficients,Nusselt numbers and the properties of solar salt doped with nanoparticles,it indicates that the specific heat capacity of materials contributes to the enhancement of heat transfer for the most part.A two-components lattice Boltzmann model was established to simulate the natural convection heat transfer of nanofluids in retangular cavity.The effects of nanoparticle mass fraction,nanoparticle materials,Ra and aspect ratio on heat transfer were analysed.Results show that for all cases,doping Si O 2 nanoparticles enhances the natural convection and there is an optimum mass fraction 1.0 wt.%,the maximum enhancement of Nu number is 1.4%.For Al2O3 nanoparticles,natural convection heat transfer is influenced by Ra number a lot.At high Ra numbers,the convection heat transfer is enhanced.