Reduced Pressure Carbon Dioxide Cycle for Vehicle Climate Control 1999-01-0868
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.