Reduced Pressure Carbon Dioxide Cycle for Vehicle Climate Control

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 under these conditions 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 reported (Ref 1). Two companion papers will address properties of working fluids (Ref 2) and thermodynamic and cycle models (Ref 3) for the low pressure carbon dioxide cycle. 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. With this facility, the cooling capacity and coefficient of performance of the low pressure carbon dioxide system are compared to HFC-134a. Results for several different co-fluids are also presented.