Influence of the Refrigerant on the Performance of a Heat Pump with Gas Bearings

Heat pumps have become a potential technology for future thermal management systems in space. However, conventional heat pumps lubricated with oil are currently unfeasible due to the difficulty separating oil in microgravity environments. The oil-refrigerant mixture reduces the heat transfer capacity of heat exchangers, and insufficient separation can damage the compressor. This paper proposes a new heat pump with gas bearings (HPGB) that combines a centrifugal compressor with externally pressurized gas bearings to solve lubrication problems. A numerical model is established to compare performance of the HPGB system with different refrigerants. The results show that the coefficient of performance (COP) of the HPGB system is slightly smaller than those of conventional heat pumps, but higher than those of heat pumps with the impact of an oil concentration of 5%, when evaporator power is 30 kW. The HPGB system is more suitable for high-power conditions because of the higher COP. Ammonia is the best choice for the HPGB system under high-power conditions (above 80 kW). R134a is the best choice under middle-power conditions (from 25 kW up to 80 kW). R1234yf is more suitable with the maximum COP of 2.88 under low-power conditions (below 25 kW).