Reduced Pressure Carbon Dioxide Cycle for Vehicle Climate Control: Progress Since 1999

Environmental concerns have spawned renewed interest in naturally occurring refrigerants such as carbon dioxide. CO₂ has attractive features such as high enthalpy of evaporation and low cost compared to halocarbons. However, the vapor pressure of CO₂ is high at temperatures normally encountered in refrigeration and air conditioning systems when compared to traditional and alternative refrigerants such as CFC-12 and HFC-134a. Major research efforts are underway to investigate the transcritical CO₂ cycle, in which a gas cooler instead of a condenser accomplishes heat rejection to ambient, since carbon dioxide in this cycle is above the critical point. The vapor pressure in the gas cooler may exceed 120 bar (1,740 lb/in²).

In this paper a reduced pressure carbon dioxide system is revisited^{1, 2}. The working fluid is a mixture of CO₂ and a non-volatile liquid, referred to as a co-fluid, in which CO₂ is highly soluble and readily absorbed and desorbed. This mixture is circulated throughout the system and the vapor pressure is greatly reduced to less than 35 bar (507.5 lb/in²) in the condenser (referred to as the resorber). A full-scale test stand has been constructed, which simulates operating conditions encountered by automotive air-conditioning systems. In this test stand, the cooling capacity and coefficient of performance of the low-pressure carbon dioxide system are measured and they compare favorably with those of HFC-134a.