The automotive industry took an important step when the air conditioning systems of vehicles sold in many countries were changed from a refrigerant containing cluorofluorocarbons (CFC-12) to hydrofluorocarbons (HFC-134a) less harmful to the ozone layer. .
However, in view of the objectives set by the Kyoto Protocol and the marginalization of on-board air-conditioning systems, the replacement of HFC-134a may represent an important issue for reducing greenhouse gas emissions: Indeed, HFC-134a has an impact 1300 times more important on the heating of the planet than the CO2 has equal quantity in weight.
The operation of an air conditioner depends on the compression of a gas and its expansion. A compressor compresses the hot gas at very high pressure which passes through a condenser and an internal heat exchanger (which allows heat exchanges with the low pressure zone) to be cooled and then passes into the expansion valve. A liquid comes out which cools the passenger compartment by passing through the evaporator. The low pressure gas is then accumulated in a condenser before circulating in the heat exchanger and returning to the compressor for a new cycle.
CO2 is a possible gas as a refrigerant in air conditioning systems to replace HFC-134a in the near future. The use of CO2 raises several difficulties related to the pressure at which it must be employed to be used as a refrigerant. In fact, the critical temperature of CO2 is lower than that of HFC-134 and its critical pressure is higher, which forces the air conditioning system to operate in conditions more difficult to achieve. This implies more resistant materials, therefore heavier and more expensive, which hinders the commercialization of this type of system at the present time.
However, Denso, a Japanese supplier, in 2002 fitted Toyota's fuel cell experimental vehicle with a CO2 air conditioning system.
The air conditioner can operate to heat the passenger compartment, which is a significant factor if one considers the development in the future of fuel cell vehicles which do not have a hot source (heat engine) to serve as heating.
Editor: Etienne Joly, Service for Science and Technology
Embassy of France in Japan