Study On The Enhancement Mechanism Of The Heat Transfer And The Thermal Energy Storage Performance Of Molten Salts By Nanoparticle Dispersion

As novel heat transfer mediums,molten salts employed extensively as the heat transfer and thermal storage materials have already been applied to Concentrating Solar Power?CSP?plants due to it their wide operating temperature range in liquid state,relatively high specific heat capacity and superior capacity for thermal energy storage?TES?.The TES capacity can be significantly improved by strengthening the specific heat capacity,thermal conductivity,and latent heat of molten salts.In this paper,the nano-salts were synthesized by dispersing nanoparticles into the potassium nitrate,the sodium nitrate,their mixture-Solar salt?60 wt.%NaNO₃-40 wt.%KNO₃?and binary eutectic carbonate?62 mole.%Li₂CO₃-38 mole.%K₂CO₃?respectively.The key thermo-physical properties were measured and analyzed.The thermo-physical properties,including the melting temperature,the latent heat,the specific heat capacity,the thermal conductivity and the thermal stability were measured by Differential Scanning Calorimetry?DSC?,Thermogravimetric Analysis?TGA?and Laser Flash Analysis?LFA?.The micromorphology of Nano-salts were observed by Scanning Electron Microscope?SEM?.This study found that the enhancement on thermal properties of nitrates varies with the mass concentration of SiO₂ nanoparticles.The concentration of 0.7 wt.%can be considered as most effective in enhancing the latent heat of the potassium nitrate and sodium nitrate.Compared with the base salt,the heat of fusion of the potassium nitrate and sodium nitrate increased by 9.16%and 3.31%,while the melting temperature decreased by 2.1?and 2.20?respectively.The heat of fusion of the Solar salt with SiO₂ nanoparticles of 1.0 wt.%increased by a maximum of 3.84%,and melting temperature decreased by 2.40?.The average increases for Solar salt,sodium nitrate,potassium nitrate of the specific heat were calculated to be 19.2%,9.12%,27.16%in solid and 15.89%,11.86%,33.5%in liquid,respectively;the highest thermal conductivity were observed to be 0.584 W/?m.K?,0.573W/?m.K?,0.589 W/?m.K?,the average increase rate is 15.91%,8.86%,39.24%,respectively.The specific heat and thermal conductivity enhancement can be considered the most effective for Solar salt,sodium nitrate,potassium nitrate while the mass concentrations of SiO₂ are 0.5wt.%,1.0 wt.%,1.0 wt.%respectively.The effect of adding 1.0 wt.%SiO₂ and MgO nanoparticles on the thermal properties of the binary eutectic carbonate was also investigated experimentally and analytically.The results show that the effect of MgO nanoparticles on the specific heat capacity,latent heat of fusion and thermal conductivity of the binary carbonate is more significant.In Comparisons with that of the base salt,the specific heat capacity of Li₂CO3-K₂CO₃ was increased by 27.5%and 11%in the solid state and was increased by 34.1%and 20.7%in liquid after the dispersion of SiO₂ and MgO nanoparticles respectively;the latent heat of eutectic carbonate increased by 4.0 J/g and 30.2 J/g,the average increase rate is 1.1%and 8.6%;the thermal conductivity increased by 25.9%and 49.1%respectively.To discuss the effect of the synthesis methods,nano-salts were prepared respectively by the magnetic stirring method and the ultrasonic dispersion method,the effect of synthesis process on specific heat capacity was analyzed respectively.It was found that the specific heat capacity of the nano-salts synthesized by the magnetic stirring method was superior to that by the ultrasonic dispersion method.The combination of 750 rpm-75 min was thought of as the optimal preparation condition for preparing nano-salts by magnetic stirring method,while the specific heat capacity of nano-salts was increased by 29.09%in solid and 35.76%in liquid.Under the ultrasonic dispersion of 60min and frequency of 40 kHz,the specific heat of nano-salts reached the maximum value,increased by 11.03%in solid and 11.55%in liquid respectively.The specific heat capacity and sample mass of the two nano-salts did not varied significantly and the nano-salts still keep good thermal stability undergoing multiple solid-liquid cycles.The micromorphology of nano-salts by SEM showed that special nanostructures formed on the surface of nano-salts.shapes of these special structures,which exist only in nano-salts,were different,which is related to the type and concentration of nanoparticle added,and the type of the base salts.It is proposed that the increase of the specific heat capacity of the nano-salts may be related to the nanostructure formed and the semi-solid layer structure in the molten state between the nanoparticle and the salt surface.These special nanostructures can store more thermal energy.In addition,there is an interfacial thermal resistance between the molten salt molecules and the nanoparticles,so that the same temperature increases in the nano-salts required higher heat,and the specific heat capacity is abnormally increased.The reason for thermal conductivity enhancement is believed to be the result of the interaction of the nanoparticles with the random diffusion motion and the nanostructures formed by the nanoparticles and the salt surface.