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To solve the problems of high energy consumed by the cooling system in the data center, this article puts forward a composite air conditioning system. The simulation platform for system operation performance is established, and the accuracy of the simulation platform is verified by experiments. The influence of operation parameters on system performance is analyzed by simulation. Finally, the optimal operation control parameters are determined. The results show that the system can adjust the condenser wind speed and the refrigerant pump operating frequency to meet the cooling requirements and solve the problem of low evaporation temperature in the heat pipe. The evaporative cooling effectively expands the working range of the heat pipe mode and raises the upper limit of outdoor temperature from 8 °C to 15 °C. In the vapor compression mode, when the ambient temperature is lower than 26 °C, the performance of the air-cooled mode is better than that of the evaporative-cooled mode. The condensation temperature can be ensured within a reasonable range by adjusting the condenser cooling wind speed and the compressor frequency. When the outdoor temperature is not less than 26 °C, the evaporative cooling can effectively improve the operation performance

To improve energy efficiency of data centers, an air conditioning system for the concept of demand-based on cooling was proposed. This system combines a heat pipe cooling cycle and a vapor compression cooling cycle. Besides, the evaporative condenser is used to further utilize the natural cooling source, and the evaporator is put into the rack to realize on-demand cooling. A mathematical model was established to perform simulations of the system. The results showed that the energy consumption of compressor and pump reduce with the rising of outdoor unit air flow and evaporator inlet temperature. The coefficient of performance (COP) was selected as a reference index. When the cooling capacity is 8000 W, 6400 W and 4800 W, the optimal outdoor unit air flow of the heat pipe mode is 2000 m3/h, 1400 m3/h and 800 m3/h, respectively, and that of the vapor compression mode is 4000 m3/h, 3600 m3/h and 2400 m3/h, respectively. With the rising of evaporator inlet temperature from 29 °C to 41 °C under rated conditions, the heat pipe mode COP increased by 30.11%, 13.52% and 6.30%, respectively, and the vapor compression mode COP increased by 74.65%, 82.12% and 90.97%, respectively.

To solve the problems of single heat source heat pump systems in severe cold regions, a dual-source hybrid heat pump unit (DSHHPU) is proposed. The mathematical models of the DSHHPU when charging R134a or its alternative refrigerants R32, R290 and R600a were established respectively, and the performance was simulated and analysed. The results showed that the four refrigerants have different performance characteristics in different aspects. In heat pipe mode, the heating capacity and evaporating pressure of R32 are 36.94% and 59.94% higher than those of R134a. The heating capacity and evaporating pressure of R290 are 5.73% and 22.99% lower than those of R134a. The heating capacity and evaporating pressure of R600a are 43.29% and 68.08% lower than those of R134a. In vapour compression heating mode, the discharge temperature of R32, R290 and R600a are 184.88, 72.98 and 66.44% of that of R134a. The coefficient of performance (COP) of R32, R290 and R600a are 72.65, 111.59 and 117.94% of that of R134a. Finally, the effects of radiation intensity and ambient temperature on key performance parameters of the different refrigerants were analysed. The research results provide a reference for research on refrigerant replacements for multi-heat source composite heat pump systems.

Aiming at the large energy consumption of the air-conditioning system in the data center, an evaporative cooling composite air-conditioning system was proposed. This paper analyzes the impact of outdoor airflow on system performance parameters under different cooling requirements through system modeling and simulation. The results show that the increasing airflow of the outdoor unit increases the energy consumption of the fan, but helps to reduce the power consumption of the compressor and the refrigerant pump. Taking COP value as objective function, the optimum outdoor airflow is obtained. Under the 100, 80, and 60% rated cooling capacity of this system, the optimal outdoor airflow in the heat pipe mode are 2000, 1400, and 800 m3/h and in the vapor compression mode are 4000, 3600, and 2400 m3/h, respectively.

Maximizing utilization of natural cold energy has become the key to reduce the energy consumption of data center air-conditioning system. A new type of evaporative condenser with water-cooled condensation is proposed and used in the data center composite air conditioning system. The model of air conditioning system is established and verified by the experimental platform. The simulation results show that although the coefficient of performance (COP) of the evaporative cooling heat pipe mode is reduced by 0.88% on average by the proposed evaporative condenser, the upper limit of temperature operation of this mode is raised from the original 15 °C to 17 °C. The heat pipe mode has been extended throughout the year, and the advantages of energy saving have been fully utilized. For the evaporative cooling vapor compression mode. The proposed evaporative condenser can reduce the energy of the composite system, and the COP is increased by 9.82% on average, showing high operating performance.