Study On Performance Of Low Temperature Frozen Storage Refrigeration System Using LNG Wasted Cold

With the development of China’s food,medical and hotel industries,low temperature refrigeration system plays an important role in people’s life.At present,ammonia compression refrigeration system is the most widely used in low temperature storage refrigeration.However,the system consumes a lot of power and has a great indirect impact on the environment.Therefore,it is necessary to develop a more environmentally friendly,energy-saving,safe and reliable system for low temperature refrigeration.Natural gas has become one of the most favored clean energy sources because of its high calorific value,safety and reliability,cleanness and reliability.In order to reduce the cost of natural gas,it is usually stored and transported in the form of liquefied natural gas（LNG）,and the cold energy released in the process of LNG gasification is very considerable.Therefore,it is of practical significance to apply LNG cold energy to the refrigeration system of low temperature frozen storage and analyze the heat transfer performance of the key equipment waster cold recovery heat exchanger.In this paper,a low temperature frozen storage refrigeration system using LNG wasted cold is proposed,and the whole system and key components are theoretically studied and simulated.The specific work contents are as follows:Firstly,in this paper the physical models of low temperature frozen storage refrigeration system using LNG wasted cold and traditional ammonia low temperature frozen storage refrigeration system are constructed,and the mathematical models of the two systems from the perspective of thermodynamics and economics are established.40%ethylene glycol aqueous solution is selected as the refrigerant of low temperature storage refrigeration system using LNG wasted cold.Under the condition of changing the cold storage temperature and refrigerating capacity,the two systems are compared and analyzed.The results show that when the cold storage temperature increases,the mass flow of working medium,power consumption and investment cost of the two systems decrease,while the coefficient of refrigeration performance（COP）of the system increase.When the refrigerating capacity increases,the working medium mass flow,power consumption and investment cost of the two systems increase,while the COP of ammonia low temperature frozen storage refrigeration system remains unchanged and the COP of LNG low temperature frozen storage refrigeration system increases.At 5～12 k W,the COP of LNG low temperature frozen storage refrigeration system is lower than that of ammonia low temperature frozen storage refrigeration system,while it is the opposite at 12～30 k W.The comparison of economic performance shows that the investment cost of LNG low temperature frozen storage refrigeration system is much lower than that of ammonia low temperature frozen storage refrigeration system.The comprehensive conclusion is that under the same working conditions,the thermodynamic and economic performance of the low-temperature refrigeration system using LNG wasted cold is better than that of the traditional ammonia low temperature frozen storage refrigeration system.Secondly,in order to further use the recovered LNG wasted cold efficiently for low temperature storage in cold storage,combined with the feasibility of existing simulation conditions,Fluent is used to simulate and analyze the influence of structural characteristics such as different placement methods and different inlet and outlet sizes on the heat transfer performance of waster cold recovery heat exchanger.The results show that when the waster cold recovery heat exchanger is placed laterally,the outlet temperature of ethylene glycol is lower and the cooling effect is better.At the same time,the outlet velocity of the waster cold recovery heat exchanger placed laterally is lower than that of the longitudinal waster cold recovery heat exchanger.When the inlet and outlet diameter of ethylene glycol is large,ethylene glycol can exchange heat with LNG more fully and evenly,with greater cooling range and more obvious heat exchange effect.At the same time,the inlet and outlet diameter of ethylene glycol increases,which makes the velocity jet of ethylene glycol disperse and the velocity changes sharply.Based on the structural characteristics,the heat transfer simulation calculation of the waster cold recovery heat exchanger is carried out by changing the key parameters such as LNG speed,ethylene glycol inlet mass fraction,inlet speed and temperature.It is concluded that when the speed of LNG and ethylene glycol increases,the heat transfer coefficient,thermal efficiency and pressure drop increase.In contrast,the increase of ethylene glycol speed has a greater impact on the heat transfer performance.When the mass fraction of ethylene glycol increases,the heat transfer coefficient and heat transfer efficiency decrease,but the pressure increases.When the mass fraction of ethylene glycol and inlet velocity remain unchanged,the heat transfer coefficient,thermal efficiency and pressure drop of the waster cold recovery heat exchanger decrease with the increase of inlet temperature.