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The supercritical carbon dioxide (sCO2) Brayton power cycle is more effective than the conventional power cycle and is more widely applicable to heat sources. The inlet working conditions of the compressor have a higher influence on their operating performance because the thermophysical properties of the CO2 vary dramatically close to the critical point. The flow in the sCO2 compressor is simulated and the compressor performance is analyzed. The results show that the sCO2 centrifugal compressor operates outside of its intended parameters due to the change in inlet temperature. The sCO2 compressor requires more power as the inlet temperature increases. The compressor power is 582 kW when the inlet temperature is at 304 K. But the power is doubled when the inlet temperature increases to 314 K, and the change in the isentropic efficiency is within 5%. The increase in the inlet temperature significantly reduces the risk of condensation in centrifugal compressors. When the heat load of the sCO2 power system changes, the inlet pressure to the turbine can be kept constant by regulating the rotational speed of compressors. With the increase in rotational speed, the incidence loss and condensation risk increase.

As an important device for water chillers, the centrifugal refrigeration compressor with gas bearings has new features compared with traditional oil and magnetic bearings. It is an important development direction for water chillers and needs to be studied. To research the performance of compressors for water chillers, the R134a centrifugal refrigeration compressor with gas bearings is researched by the computation fluid dynamics (CFD) method. The prototype of the two-stage centrifugal compressor was tested in the water chiller. The test bench of the centrifugal chiller has two kinds of gas supply systems for gas bearings. The axial force of impellers is simulated under different working conditions by the CFD method. Compared with the two-stage impeller in the same direction arrangement, the opposite direction arrangement can reduce the total axial force of the rotor by up to 98.3%. The simulation performance of gas bearings and test results show that the gas bearings can bear the axial force caused by the gas force of two impellers. The results from the simulation and experiment are compared with the average error of 4.3% when the rotating speed is 21000 r⋅ min−1.

This paper proposes a NH3/CO2 cascade refrigeration system that sets an auxiliary refrigeration loop in the high-temperature cycle to increase the subcooling degree in the low-temperature cycle (CRSS). Based on the basic principles of thermodynamics, a mathematical model is established and theoretical simulation is carried out to obtain the influence of key parameters on the cycle performance. Compared with the conventional cascade refrigeration system (CCRS), the conclusions find that there exists an optimal condensation temperature of low-temperature cycle (TMC.opt) to maximize COP. The maximum COP of CRSS is 4.58% higher than that of CCRS when the subcooling degree is 10 °C. The maximum exergy efficiency is 0.391, increasing by 4.40%. With the increase of the subcooling degree, both the COP and the TMC.optof CRSS increase. When the subcooling degree increases from 5 °C to 15 °C, the performance increment increases from 2.73% to 6.00%. When the evaporation temperature of the system changes, both the COP and exergy efficiency decreases slightly and it is found that the performance is better when the evaporation temperature is lower when condensation temperature is kept constant. Besides, the discharge temperature of the NH3 compressor in CRSS can be reduced by 9.9 °C when the evaporation temperature is −30 °C.

The power system using carbon dioxide (CO2) as the working fluid has been the research hotspot due to its high performance and potential for use in many energy fields. In this paper, the difference of compression processes between supercritical CO2 (S–CO2) and transcritical CO2 (T-CO2) power cycles is researched, and their effects on the thermodynamic performances of power cycles and equipment are discussed. The results show that the thermal efficiency and net output power increase with the decreasing of the compressor/pump inlet temperature, and there is a big jump near the critical temperature. The condensation margin is more sensitive to the temperature than the pressure near the critical point. To minimize the compressor input power and to guarantee the reliability of equipment, the inlet parameters of compressors should be designed and operated carefully. And the inlet velocity of compressors should be set below the usually recommend value.