| Natural gas is widely used in recent years to solve the problems of shortage of petroleum resources and the severe environment pollution in many areas. Air pollution caused by auto exhaust gas will be decreased when natural gas is used as the auto fuel. At present, CNGV (Compressed Natural Gas Vehicle) has rapidly developed at home and abroad. LNG (Liquefied Natural Gas) has many advantages such as higher energy density, longer driving running distance and better safety performance in comparison with CNG. LNG can provide plenty of cold energy when it is used as the fuel. The cold energy can be recovered and utilized in vehicle refrigeration. The idea for recovering and utilizing the cold energy of LNG in automobile air-conditioning has been proposed in this paper for the first time. It can effectively save energy and avoid the ozone layer depletion and greenhouse effect for the leak of CFCs such as R-12.Analysis of the cold exergy, pressure exergy and the characteristics of Liquefied Natural Gas has been conducted from the viewpoint of thermodynamics. It is shown that cold exergy includes two parts, sensible cold exergy and latent cold exergy. The environment temperature and the system pressure have great effects on the cold exergy of fixed component LNG. The cold exergy of LNG increases constantly with the increase of ambient temperature T0 and decreases with the increase of the system pressure, meanwhile the pressure exergy of LNG increases proportionally with its system pressure. When the system pressure exceeds over 2 MPa, the cold exergy becomes very little. The lower system pressure is benefit to recovery the cold energy of LNG. The exergy analysis on mechanical refrigeration system has been conducted to demonstrate the practical energy saving effect for replacing the cold exergy of refrigerant with the cold exergy of LNG.Based on the demonstration of the feasibility for recovering and applying LNG cold energy in auto air-conditioning, a reasonable auto air-conditioning system has been developed. The system comprises a low temperature multi-stage heat exchange system with several heat exchangers and a cold storage system with a glycol solution circulation loop. The heat exchanger devices have been designed according to the system parameters. Numerical analysis of the cold energy recovery system has been conducted. The effects of the system pressure, the flow rate of natural and the flow rate of glycol solution on the system performance have been analyzed.An experimental device for recovery the cold energy of LNG has been established to verify the reliability of the cycle. For the experimental study, an intellective data collecting system with high precision has been developed based on the computer. It can measure the parameters of the pressure, the flow rate and the temperature accurately.Natural gas is a dangerous material. There must have better experimental conditions and environment to recovery the cold energy of LNG. As the first step of the experimental study, nitrogen was used to replace LNG. The property of nitrogen and natural gas are compared and analyzed. Based on the theory of similar modeling, the modeling analysis of the nitrogen simulative cold energy recovery system of LNG is completed.The parameters affecting the performance of the system of simulative experiment of Liquefied Nitrogen (LN2) are investigated. The results show that the system has successfully recovered the cold energy of LNG with the method of multi-stage recovery. Some key technologies have been solved such as heat exchange with a significant temperature difference under low temperature conditions and auto-adjustment of the supplying cold capacity according to the automobile cold load. The system has good performance of both transient state and steady state. The recovering efficiency of the cold energy is about 80%, which can meet the requirement cold load of automobile air-conditioning. The numerical analysis results are compared with the experimental results. They are basically consistent which indicate that the mathematical model and the calculating method are reliable. |
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