Preparation And Physical Property Analysis Of High Temperature Mixed Molten Salt With Application Temperature Above 600℃

According to the"Third Generation Solar Power Gen3 Demonstration Roadmap",replacing the Rankine cycle with the Bratton cycle using supercritical carbon dioxide（s CO₂）can improve the thermoelectric efficiency of the system.The target of 5.9 cents/k Wh for solar thermal power generation is expected to be achieved,while the target for solar thermoelectric efficiency is above 50%at this time.In order to achieve this power generation goal,it is necessary to raise the temperature of s CO₂at the inlet of the Bretton cycle turbine to 600℃,and then the temperature of molten salt at the outlet of the collector matching it needs to increase from 565℃to over600℃.As a result,the current general heat transfer and storage medium（solar salt）cannot meet the needs of supercritical CO₂Bretton cycle power generation.We need to develop a heat transfer and storage medium with a high temperature of over 600℃.In this article,three of ternary carbonate containing different proportion of lithium ion were prepared.The content of lithium ion was not more than 30%,and increased by 10%successively,which were 10%,20%and 30%respectively.Several preferred samples are sample No.1（K₂CO₃:Li₂CO₃:Na₂CO₃=3:1:6）,sample No.2（K₂CO₃:Li₂CO₃:Na₂CO₃=3:2:5）,and sample No.3（K₂CO₃:Li₂CO₃:Na₂CO₃=2:3:5）.The melting point,crystallization point,latent heat,decomposition temperature,specific heat,thermal diffusivity,thermal conductivity and liquid viscosity of the three samples were tested.The melting point of the three samples were all below400℃,while the decomposition temperature was above 900℃,and the crystallization point was higher,which were 639.7℃,535.4℃and 454.9℃respectively.The thermal conductivity coefficient is as high as 1.024W/（m·K）,which can meet the demand of the third generation of concentrated solar power station heat storage medium.Temperature-matched ternary mixed carbonate contains lithium salt,which is more expensive.In order to greatly reduce the cost of solar thermal power generation,two kinds of binary salts,K₂CO₃-KNO₃and Na₂CO₃-Na NO₃,were prepared by using the method of the same cation and the different anions,on the premise of high upper limit temperature and good heat storage and thermal conductivity.The new mixed molten salt（the same cationic salt）was obtained by eutectic mixing.The melting point,decomposition temperature,specific heat,density,thermal diffusivity,thermal conductivity and liquid viscosity were tested.The results show that the optimal melting point of the same cationic salt is 300℃,which is nearly 100℃lower than the melting point of ternary carbonate,and the decomposition temperature is above650℃.The specific heat capacity is 1.56J/（g·K）,the average density is 1.87g/cm~3,the average thermal diffusivity is 0.28mm~2/s,and the average thermal conductivity is0.79W/（m·K）.The sensible heat storage cost is 19.64￥/k Wh,which is obviously lower than that of ternary carbonate,quaternary nitrate,Solar salt and Hitec salt.After 500 times of thermal shock experiments at 630℃,thermophysical properties have little change,composition has not changed,and stability is good.some other kinds of salt,including K₂CO₃,Na Cl,Ca（NO₃）₂and KNO₃,are added on the basis of the preferred same cationic salt.In addition,three kinds of binary salts including Na NO₃-Na NO₂,K₂CO₃-Na NO₃and Na₂CO₃-KNO₃were also prepared.the thermogravimetric test were made for the multicomponent and binary salts,The results show that the decomposition temperature of the all mixed salts was between 615 and 685℃.