Study On Coupled Heat Transfer Charateristics Of Thermal Energy Storage System Utilizating Typical Molten Salt

Concentrated Solar Power(CSP)is widely employed and studied as the key technology to solve the utilization of solar energy power.Thermal Energy Storage(TES)system,which can overcoming the disadvantage of intermittent by nature of solar energy,is an effective strategy to adjust the balance between supply and demand.As the most significant media in TES,the heat transfer characteristic of molten salt at high temperature has attracted more and more attention.However,because of the lack of radiative properties of molten salt and the complexity involved in solving radiative transfer equation,the influence of radiation has been omitted in the simulations about heat transfer characteristic of molten salt.In the present study,the uncertainty of measurement for spectral emissivity of typical molten salt was estimated.The results show the uncertainty is function of both temperature and wavelength.It increases with the increase of wavelength.The uncertainty of temperature of samples plays a significant role in total uncertainty in short wavelength,while the uncertainty of signal and temperature of wall is in dominant position in long wavelength.Besides,according to spectral absorption coefficient calculated in previous study,the average absorption coefficients at variable temperatures were obtained for numerical simulations subsequently.Then,a 3-D steady model of conductive,convective and radiative coupled heat transfer for a Sensible Thermal Energy Storage(STES)system was implemented.The differences of heat transfer characteristic with and without considering radiation were compared.The results revealed that the maximum differences of temperature along the axis on both sides of tube are 14% and 16% separately,and the radiation inside molten salt can enhance heat transfer.A empirical correlation considering the effect of radiative heat transfer was obtained.In addition,the effects on heat transfer characteristic of scattering albedo and refractive index were discussed.It demonstrates the temperature on side of molten salt increases with the increase of scattering albedo,contrary to the case of refractive index.Finally,a 2-D axial symmetry transient model of phase change,convective and radiative coupled heat transfer of molten salt in encapsulated capsules,which are widely used in Latent Thermal Energy Storage(LTES)system,was implemented.The model was verified with other literature.The melting rates with and without considering radiation were compared.The results show that radiation should not be omitted in numerical simulation,despite not as obvious as convection.The enhancement of radiative heat transfer is affected not only by scattering albedo and absorption coefficient of molten salt,but also by the shape of the encapsulate particles.The enhancement effects of radiation on the shape of Red Blood Cell(RBC)and annular encapsulated capsules are more obvious than that of the other shapes within the scope of the study.