A Study Of Properties And Improvement Of NaNO₃-KNO₃ Based Molten Salts For Heat Transfer And Storage

In solar thermal power generation,NaNO₃-KNO₃（60-40wt%,Solar salt）and NaNO₃-KNO₃-NaNO₂（7-53-40wt%,Hitec）molten salt are mainly used as heat transfer mediums.The heat transfer property is an important indicator of the long-term stable operation of the power station.At present,the actual operating time of solar thermal power stations is relatively short,and there is little research on the heat transfer characteristics of molten salt after a certain period of high temperature operation.There is no report on the prediction of the life of solar thermal power plants.On the other hand,the specific heat capacity and thermal conductivity of nitrate are relatively small,which to a certain extent also limits its wide application in solar thermal power generation.Based on the above facts,this paper takes Solar salt and Hitec molten salt as the research object.Firstly,the change law of two kinds of molten salt heat transfer and storage properties after different temperature operation is studied,and the possible influence mechanism is analyzed.On the basis of the above analysis,nano-particles are used to enhance the heat transfer characteristics of the salts,such as the small specific heat capacity,besides the influence of nano-particles on the heat transfer and storage properties of molten salt is analyzed.Based on this,a new type of molten salt is designed and its performance analysis is carried out in order to provide a candidate molten salt system for CSP and further promote the rapid development of solar thermal power stations.Research indicates:1)In an air atmosphere,the Solar salt molten salt was heated at 25℃,300℃,565℃,and 600℃ for 240 hours,and the thermal salt properties of the Solar salt after different temperatures were measured at 250℃-480℃.With the increase of heat treatment temperature,its specific heat,density,thermal conductivity and thermal weight loss showed a trend of decrease,viscosity showed an increasing trend,nitrate ion concentration showed a decreasing trend,and nitrite ion concentration showed an increasing trend.In an air atmosphere,Hitec molten salt was heat-treated at 25℃,400℃,450℃,and 500℃ for 240 hours,and Hitec molten salt treated at different temperatures was tested for thermal properties at 200℃-400℃.It was found that:With the increase of heat treatment temperature,its specific heat,density,thermal conductivity and thermal weight loss showed an increasing trend,and the viscosity tended to decrease.Nitrate ion showed an increasing trend,and nitrite ion concentration showed a decreasing trend.2)Nitrate heat transfer and storage properties reinforcement part:Addition of different amounts（0.0625wt%,0.5wt%,and 1wt%）and different particle sizes（25 nm,40 nm,and 100 nm）of TiO₂ to Solar salt and Hitec molten salt,respectively.The experimental results showed that:When the nano-TiO₂ particle size was 25 nm,it enhanceed the specific heat and thermal conductivity of Solar salt and Hitec molten salt best.Among them,the specific heat of the molten salt of Solar salt increased by8.6%,the decomposition temperature increased by 16℃,the specific heat of Hitec molten salt increased by 9%,and the decomposition temperature increased by18℃.When the addition amount of nano-TiO₂ was 0.0625wt%,the specific heat and thermal conductivity of Hitec molten salt enhanced the best.Among them,the specific heat of Solar salt molten salt increased by 11.7%,the decomposition temperature increased by 10℃,the specific heat of Hitec molten salt increased by 12%,and the decomposition temperature increased by 23℃.At the same time,through the analysis of the phase and lattice structure,it was found that among the composites made of nano-particles in the molten salt,no other substances were generated,and there existed nano-TiO₂ particles at the same time.3)New molten salt preparation:Based on Solar salt and Hitec,LiNO₃ was added to the two molten salts,NaNO₃-KNO₃-LiNO₃ and NaNO₃-NaNO₂-KNO₃-LiNO₃,were formulated.At 250℃,the thermal conductivity of NaNO₃-KNO₃-LiNO₃ was increased by 21.5%compared with that of Solar salt,and the thermal conductivity of NaNO₃-Na NO₂-KNO₃-LiNO₃ was increased by 32.0%than that of Hitec molten salt.