Study On The Train Air-conditioning System Using Carbon Dioxide Refrigerant

High Global Warming Potential（GWP）refrigerants,such as R407C and R410A,used in the train AC system are about to be banned.As a natural and environmentally friendly refrigerant with GWP of 1,CO₂ has become the most feasible alternative for train AC system.However,the application of CO₂ as a refrigerant for air-conditioning has two major challenges:high system pressure and cooling performance attenuation under high temperature conditions.According to the survey,there is little research in the field of CO₂ train AC system in China,and there are few successful experimental test cases abroad.Therefore,this paper conducts an in-depth study on the CO₂ train air conditioning system from two aspects of simulation and experiment.The research contents and results are as follows:1)A set of double-loop CO₂ train AC system is set up and testedUnder the railway standard working conditions,the cooling capacity of the double-loop air conditioning system is 42k W and COP is 2.4,which meet the design requirements.At the same time,the refrigeration cycle mechanism of single loop air conditioning system under different charge amount,outdoor side temperature,compressor working frequency and indoor side air volume was studied.In the single-loop system test,the maximum cooling capacity was 24.3k W,and the maximum COP was 2.7.According to the linear correlation between the outlet temperature of the gas cooler and the optimal high pressure of the system,the optimal high pressure is controlled.2)The numerical components model of the CO₂ train AC system is establishedThe results of the simulation model guide the selection of compressor and the design of heat exchanger.In the design of tube-fin heat exchanger,the maximum number of tube-fin heat exchanger is 9 rows of 20 tubes,and the liquid collecting tube is 1 minute and 4 minutes.Compared with the traditional design of tube-fin heat exchanger,the cooling capacity and COP of tube-fin heat exchanger are respectively 4.1%and 4.2%higher.3)The steady-state simulation model of CO₂ train AC system is establishedUnder the same working condition,the steady-state simulation results are basically consistent with the experimental results,and the refrigerating capacity and COP errors are within 5%.The steady-state simulation model can be used to guide the prediction and performance optimization of the system.4)The high pressure protection mechanism of CO₂ train AC system is established.On one hand,the system is protected by the protection control strategy of the accumulator and electronic expansion valve in an active way.On the other hand,the system is protected by the high and low pressure switch and safety valve in a passive way.