Safety Issues Study And Universal Exergy Analysis Of Molten Salt Thermal Storage System

Development and utilization of renewable energy has become an efficient way to solve a series of energy and environmental problems that the world is facing right now.It is also considered as the important measure to promote energy restructuring of all countries.As an inexhaustible energy source,solar energy has been attracted popul ar attention intensively.Because of the ability of combining with the large-scale thermal storage system,solar thermal plant becomes of a grid friendly renewable energy generation technology,that has rapidly developed this years.Molten salt thermal storage system has been the most recognizable among the many heat storage technologies.As one of the most important part of the CSP system,although molten salt thermal storage technology has been successfully commercial operation,there are still some problems needed further study.To further improve the safety of molten salt thermal storage and complete the evaluation system of CSP,different methods are used to research the problems of structural strength of large-scale molten salt tank,flow corrosion of high temperature molten salt and evaluation method of CSP.Main conclusions are obtained that:(1)In this article,the molten salt tank with the volume of 2×10⁴m³had been designed based on the API650 and GB50341 standard,which provide the analysis model for the thermal and structural strength analysis.(2)For the deficiency of the temperature distribution data and indeterminacy of the thermal stratification,a small-scale of molten salt experimental facility is designed and build to study the temperature distribution and thermal losses at the cooling down process of 550??180?.The experimental results shows that:1)Thermal stratification appears at 470?during cooling down process with height of the thermal stratification zone staying about 1m.2)The lowest temperature appears at the lower edge with the maximum temperature difference of 16.1?.3)The result is found that the temperature distribution of the insulation foundation bottom is not uniformity,which is different with the boundary conditions of previous literature.4)Thermal resistance of the tank bottom increases by the existence of thermal stratification,which reduces the heat losses of the tank.(3)The CFD model of large-scale molten salt tank is build by the designed result.Temperature distribution of large-scale molten salt tank is calculated by setting the boundary conditions based on the experiment.The results shows that:1)Temperature of the molten salt in the hot and cold tank declines about 1.5?and 0.8?during 15h cooling down process.Meanwhile,thermal stratification is obvious observed in cold tank.2)The conclusion that the boundary of wind speed,heat transfer coefficient and environment temperature only reduces the temperature,but can no effect the temperature distribution of the molten salt in the tank during the cooling down process were studied.3)Under the designed thickness of insulation,the heat losses of tank wall,tank bottom and tank top surface of hot and cold tank are drawn by the analysis of CFD.4)Two high velocity zones are observed near the tank wall and the center of the molten salt with the maximum velocities of 23?32mm during the process.(4)The strength analysis of tank is studied by the ANSYS Workbench software,drawn the result that the maximum stress intensity is found at the filled weld.At the same time,the first two layer tank wall are also needed focus on because of the appear of stress concentration.Moreover,the stress level of the designed hot and cold molten salt storage tank is evaluated by the evaluation principle of the third strength theory.The result showed that the stress level meet the evaluation criteria,so the design tank can satisfy the strength requirements.The thermal stress of hot and low molten salt tanks are researched under the steady static load condition by loading the temperature capacity,which shows the significant influence on stress intensity of the tank,while were still meet the stress intensity evaluations.(5)Although there are a lot of reports about the molten salt corrosion,the flow corrosion of the molten salt is more valuable,because molten slat is flowing in practical engineering.In this work,the dynamic corrosion behavior of 316 and 321stainless steels in binary molten salt,also known as Solar Salt,has been tested at565?C in the self-design and built experimental equipment.The results show that:1)The corrosion rate increases about 35?40%at the molten salt velocity of 3m/s.2)The transfer process of corrosion products of Fe₂O₃?Fe₃O₄?Mg Fe₂O₄and Na Fe O₂to(Fe,Cr,Ni)₃O₄with the thickness of the oxidation layers.3)The eroding effect of flowing molten salt also influences the electrochemical process.(6)Combining with the SAM software,the parabolic trough solar power system model with 6h thermal storage system is studied by the methods of energy,exergy and universal exergy analysis.The results showe that:1)The energy coefficient,exergy coefficient and universal exergy utilization factor of the model are respectively 13.1%,14.0%and 8.6%at the optimum economic point.So the CSP plant 2)The vulnerable spots of energy utilization,exergy utilization and resource utilization process of the system are respectively heat exchanger-power block subsystem,concentrated heat collection subsystem and thermal energy storage subsystem.3)The factors of area of solar field,geometric concentration ratio,full load hours of thermal energy storage and loan rate had different influence on he energy coefficient,exergy coefficient and universal exergy utilization factor.