Study On Leakage Characteristics And Detection Methods Of Molten Salt Tank And Thermal Performance Of Receiver

The increasingly prominent problems of fossil energy shortage and environmental pollution have brought serious challenges to human survival and development,and the use of renewable energy is the fundamental way to solve this problem.Solar thermal power technology equipped with molten salt heat storage can overcome the intermittent and volatile problems of wind and photovoltaic power generation,and is an important supporting technology for achieving a high proportion of renewable energy power generation systems,with broad development prospects.As the core equipment for heat storage and transfer in solar thermal power plants,indepth research,understanding,and solving the design and operation issues of molten salt heat storage systems and receivers are key to ensuring long-term safe and efficient operation of the whole plants.Leakage of molten salt storage tanks has become a relatively common operational accident in solar thermal power plants.During operation,tube wall bending and cracking caused by highly uneven temperature distribution and cyclic thermal stress are the main failures of heat absorbers.Based on the above background,this work focuses on and carries out research on the migration and heat transfer characteristics of molten salt after leakage from molten salt storage tanks,the development and verification of leakage detection methods,and the thermal performance of molten salt receivers.Firstly,the sand tank foundation,the ceramsite tank foundation,and the sand+ceramsite double layer tank foundation with different sand layer thicknesses were constructed in a lab-scale molten salt leakage test system.The migration and heat transfer characteristics of Solar Salt leaking into different thermal tank foundations were studied,and the effects of operating and structural parameters on the migration range of molten salt after leakage.The experimental and theoretical analysis results indicate that after molten salt leakage,the temperature in the area where molten slat flows first rises rapidly,then decreases rapidly,and finally remains stable.The temperature in the area where the molten salt does not flow slowly and slightly rises before reaching stability.Due to the fact that the thermal conductivity of molten salt is far superior to that of filler and air,the stable temperature of filler after molten salt leakage is higher than that before leakage.When the operating temperature,molten salt leakage mass,and filler porosity increase,the maximum migration depth and width after molten salt leakage increase.When the leakage aperture increases,the maximum migration depth and width decrease and increase,respectively.In the double layer tank foundation,with the increase of the sand layer thickness,the maximum temperature rise of the tank foundation in steady-state after leakage increases,and the maximum migration depth and width both first increase and then decrease.After molten salt leakage,solid molten salt block will eventually be formed in the tank foundation.When the sand layer thickness in the double layer tank foundation is too small to cause the molten salt to solidify and agglomerate in the lower ceramsite layer,the shape of the solid molten salt block is similar to the shape in the ceramsite tank foundation.Otherwise,the shape of solid molten salt block in the double layer tank foundation is similar to the combined form of the shape in the sand tank foundation and the shape in the ceramsite tank foundation.Secondly,based on the above molten salt leakage test system,the migration and heat transfer characteristics of three types of molten salt with great potential for future application leaking into the ceramsite tank foundation were experimentally studied and compared with Solar Salt.It was found that the operating temperature and molten salt type significantly affect the temperature rising rate of the tank foundation and the migration and heat transfer characteristics of molten salt.The molten salt with better thermal conductivity leads to a higher temperature rising rate during migration.Affected by the melting point,viscosity,and thermal conductivity of the molten salt,compared to Solar Salt,the maximum migration depths of quaternary nitrate blends,ternary chloride blends,and ternary carbonate blends increased by 56.3%,-11.6%,and-50.2%,respectively,and the maximum migration widths increased by-22.7%,-16.7%,and-2.7%,respectively.In addition,a solidification agglomeration model for molten salt leaking into the tank foundation material was proposed and validated.After molten salt leakage,the tank foundation can be divided into non solidification zone,main solidification zone,secondary solidification zone,and non molten salt zone.The final solid molten salt block will be formed in the main solidification zone and secondary solidification zone,or only in the main solidification zone.Thirdly,an innovative leakage detection circuit for molten salt tanks based on the conductivity of high-temperature molten salt is proposed.Experiments have verified the good feasibility and sensitivity of this detection method,and parameter analysis has optimized the layout of the detection circuit.When molten salt leakage is detected,the voltage of the constant resistance in the detection circuit first rises,then decreases,and finally remains stable.Vertical spacing,layout depth,and operating temperature have significant influence on the detection effect.To achieve rapid and continuous detection of molten salt tank leakage accidents,the vertical spacing should be less than 50 mm,and the layout depth should be as close to the bottom plate of the molten salt tank as possible,preferably within 150 mm.Lower operating temperatures are not conducive to the detection effect.Fourthly,by combining the Analytical Approximation Method,Finite Volume Method,and Two-dimensional Thermoelastic Method,the coupled calculation model for the optical performance,thermal performance,and thermal stress of the salt circulation process of the on-site receiver was developed,and the coupling analysis of the heliostat field and the receiver was realized.Based on the numerical model,the thermal performance and stress of the HX 50 MWe on-site receiver under design conditions were studied,and the effects of different parameters,uniformity of incident flux,and molten salt flow pattern on the thermal performance and thermal stress were investigated.The results show that the maximum wall temperature and maximum thermal stress of the entire receiver occur near the center of the front outer wall of the last and first tube panels,respectively.The circumferential positions of the maximum and minimum thermal stresses locate at θ = 0° and θ = 75°,respectively.Among the component stresses,the radial thermal stress is very small and almost negligible,and the equivalent thermal stress mainly depends on the axial thermal stress.Compared to non-uniform incident flux,accurate salt outlet temperature can also be obtained under uniform incident flux,but thermal efficiency,maximum wall temperature,and maximum thermal stress are underestimated by 0.5%,5.6%,and 30.0%,respectively.Compared with the pattern of molten salt flowing into the receiver from the north,the maximum wall temperature and maximum thermal stress under the pattern of molten salt flowing into the receiver from the south simultaneously occur on the north tube panel,which is not conducive to the safe operation of the receiver.Fifthly,by combining the Finite Volume Method and Two-dimensional Thermoelastic Method,the numerical model for the preheating of a single receiver tube was established to study the distribution and evolution characteristics of wall temperature and thermal stress of the HX 50 MWe on-site receiver during preheating and salt filling in windy conditions.The results show that the temperature rise rate on the tube front side during preheating is significantly higher than that on the tube back side.Increasing heat flux and ambient temperature or decreasing wind speed can shorten preheating time.The wind condition from the 60 ° direction has the greatest impact on the preheating process.During preheating,the radial temperature gradient of the tube wall is very small,and the thermal stress is mainly determined by the circumferential temperature gradient.The evolution of the equivalent thermal stress at each circumferential position of the tube wall follows the same pattern,which increases first,then decreases,and finally remains stable.The maximum thermal stress reaches within the first few minutes after preheating begins.As the heat flux and ambient temperature increase,the maximum thermal stress increases and decreases,respectively.Wind speed and direction can change the magnitude and location of maximum thermal stress.Compared to the windless condition,the maximum thermal stress under windy condition is significantly reduced.Under windless and upwind conditions,the maximum thermal stress is located at θ = 180 °.When the wind speed exceeds 1.5 m/s or other wind directions,the maximum thermal stress is located at θ = 45 °.The axial distribution of the tube wall temperature and equivalent thermal stress is consistent with the heat flux distribution,and an uneven heat flux distribution will lead to a significant increase in the maximum wall temperature and maximum thermal stress.After preheating,when molten salt is filled,the wall temperature at each circumferential position quickly approaches the salt inlet temperature,and the thermal stress rapidly decreases.The salt inlet temperature and the molten salt mass flow rate affect the magnitude of stable thermal stress and the thermal stress reduction rate,respectively.Sixth,a lab-scale receiver preheating experimental system was established to study and obtain the variation of the temperature distribution on the receiver surface over time during the preheating process,as well as the impact of the wind speed,wind direction,xenon lamp load,ambient temperature,and midway increase in xenon lamp load on the preheating performance.In addition,the above receiver tube preheating model was applied to the HM 50 MWe on-site receiver to numerically analyze its preheating process under extreme environmental conditions.It is found that strong wind has a significant impact on the preheating,while the impact of extreme ambient temperature is small.Under strong wind conditions of 10 m/s,the maximum equivalent thermal stress decreases by 20.5% compared to that under no wind conditions.The maximum thermal stress is linearly related to ambient temperature.The maximum thermal stress decreases by 6 MPa for each 10 ℃ increase in ambient temperature.Finally,through a self built molten salt receiver tube experimental system using an electromagnetic induction heating device as the source of surface heat flux,the preheating performance and salt circulation performance of the receiver tube under different heat flux,wind speed,and molten salt inlet mass flow rate were experimentally studied.The experimental results have verified that the electromagnetic induction heating device equipped with a rectangular coil well reproduces the heating condition of the half circumference surface of the receiver tube.Under smaller heat flux and larger wind speed,the circumferential temperature distribution of the tube wall is more uniform,and it is easier to successfully preheat the front and rear sides of the tube at the same time.The greater the heat flux,the smaller the wind speed,or the smaller the inlet mass flow rate,the greater the temperature rise of the molten salt at the inlet and outlet of the tube.