Investigation On The Thermal Performance Of Molten Salt Thermal Energy Storage System And The Characteristics Of Molten Salt Leakage And Migration

Solar thermal power generation combinating with thermal energy storage system improves the stability,continuity and dispatchability of power supply,and has become one of the most potential renewable energy utilization technologies.Molten salt is currently the most mature and widely used thermal storage medium,and the two-tank molten salt thermal storage system is widely used in large-scale solar thermal power plant.The deeply understanding of the thermal performance of the molten salt thermal storage system and the optimized design of the molten salt storage tank are essential to the safe and efficient operation of the thermal storage system.The leakage and migration of molten salt is an important problem in molten salt application.There is an extremely lack of experiments and mechanism research on the flow and phase change characteristics of leaking high temperature molten salt in the case of the damage of molten salt storage tank.Research on the migration characteristics of molten salt leaking from storage tanks into foundation materials is of great significance for the development of molten salt leaking detection methods and the treatment of leakage accidents.Under the above prospect,this present work was proposed and the attention was paid to the thermal performance of molten salt thermal storage system and the characteristics of molten salt leakage and migration.Firstly,the foundation structure of the storage tank was constructed with the siliceous thermal insulating material road base and the light expanded clay aggregate ceramsite.The temperature distribution and heat loss of different scales storage tank foundations were investigated.The experimental and numerical results indicated that the temperature distribution in the pilot foundation was reasonable,and the ventilation system could effectively avoid the excessive temperature at the bottom of the foundation.Applying the structure and materials to an industrial molten salt storage tank foundation,the temperature of the external surface and the concrete slab of the foundation and the heat loss through the tank bottom were greatly reduced.The structure and material could improve the temperature distribution of the inner and external surface of the molten salt storage tank foundation,and reduce the heat loss through the bottom of the molten salt storage tank.Secondly,a 1 MW_th pilot molten salt thermal energy storage system was built and the temperature distribution and required preheating time of the preheating process of molten salt tank by hot air before molten salt was injected to the storage tank were investigated.And the preheating process of the molten salt storage tank was simulated.The results showed that the temperature of both the air in the tank and the tank wall surfaces increased rapidly and then slowly reach the thermal steady temperature during the preheating process.The temperature of the air near the bottom of the tank and the wall of the tank bottom rose slowly,and the temperature stratification was observed along the radial direction of the tank bottom.The temperature of the tank foundation changed very slowly and was far from reaching the thermal steady state when the tank wall temperature had reached the steady state.The hot air at 350℃could effectively preheat the molten salt storage tank to about240℃in 1470 min reducing the thermal shock on the tank walls and avoiding the occurrence of local molten salt solidification during the injection of molten salt.Thirdly,the temperature changes and heat loss of the molten salt tank during the thermal storage process were investigated in the 1 MW_th pilot molten salt thermal energy storage system.The influences of liquid level and operating temperature of molten salt on the heat loss of tank were explored.The molten salt at different heights in the storage tank had not obvious temperature stratification,but there was a significant temperature difference in the upper gas,that the gas temperature decreased with the increase in altitude.The level of molten salt had an obvious effect on the temperature drop rate of the molten salt,but had little effect on the total heat loss of the tank.However,the molten salt operating temperature had a significant effect both on the temperature change and heat loss.As the operating temperature of molten salt dropped from 557.7℃to 376.9℃,the heat loss in the molten salt storage tank decreased by 48.2%Fourthly,the molten salt leaking and migrating experimental system was designed to study molten salt leaking into the thermal steady-state foundation material.The effects of foundation material parameters,leakage hole size,molten salt operating temperature and leaking molten salt mass on the migration and phase change process of Solar Salt molten salt were investigated.The test was divided into three processes,the heating process of the foundation material,the molten salt leakage and migration process,and the cooling process after the leakage.During the leakage process,the molten salt migrated rapidly in the porous foundation material,and the temperature of the foundation material increased rapidly.The molten salt firstly began to solidify on the underneath of the foundation and prevented subsequent molten salt from flowing further down.Compared with the molten salt transfering in the cold foundation,the migrating range of molten salt in the thermal steady-state foundation material significantly increased.The operating temperature of molten salt and foundation material had a great influence on migration characteristics.As the porosity of the foundation material decreased,the migration depth of molten salt decreased and the migration width increased.When the leaking molten salt mass changed within a certain range,the migration depth of molten salt did not change significantly,but the migration width increased with the increase of molten salt mass.When the size of leakage hole shrank,the migration depth increased significantly but the migration width decreased.The width and length of the leaking rectangular crack had little effect on the molten salt migration depth in the foundation material,but affected the migration width at different depths.The thermal stable temperatures of the foundation material after the leakage of molten salt were higher than that before the leakage,and increased as the leaking molten salt mass increased.Fifthly,to investigate the difference of leakage and migration with different molten salts,the heat transfer and flow characteristics of three commercially common molten nitrates leaking in thermal steady-state porous foundation materials were studied.During the leakage process of Solar Salt,Hitec XL and Hitec molten salt,the temperature rising rate of the foundation materials all had a trend of increasing first,then decreasing,and then increasing along the depth direction.The temperature rising rate and migration depth both increased obviously as the increase of molten salt operating temperature,while the average migration width changed little.Affected by the freezing point of molten salt,the temperature rising rate and migration depth with Hitec XL and Hitec leaking in the foundation material were significantly greater than the values with Solar Salt under the same operating temperature.And molten salt Hitec had a higher temperature rising rate and a deeper and wider migration range than Hitec XL which had a higher viscosity.Finally,a method for detecting molten salt leakage based on the conductivity of molten salt was proposed,and the method of electrode sheets to detect molten salt leakage was verified.The electrode sheets connected with the positive and negative poles were connected by the leaking molten salt to form an open circuit,so that the leakage of molten salt was found in time and the leakage position was located through the change of the branch electrical signals.It was found that the molten salt at higher temperature with stronger conductivity made the circuit with greater current.The electrode sheets could sensitively detect the leakage of molten salt.