Cool car climate with CO2

By Jostein Pettersen
Illustration: O. R. Valmot

Design of the CO2 system compared to a conventional circuit.

Researchers at SINTEF have developed a new, efficient and environmentally benign automobile air-conditioning system that uses carbon dioxide as the refrigerant.

The new system, which is based on an invention by NTH Professor Gustav Lorentzen, has been developed in the course of the past four years in a prototype project in cooperation with Hydro Aluminium. Extensive laboratory testing has shown the new system to be highly competitive with current technology based on CFC-12 or HFC134a refrigerants.

The use of a natural substance like CO2 as a refrigerant offers a number of environmental and practical advantages. Fluids that already play a natural role in our ecosystem are less likely to have unexpected long-term effects.

Annual consumption of CFC-12 in vehicle air-conditioning systems is about 130,000 tons. This is about 20% of the worldΔs total CFC consumption and more than 50% of the CFCs used in cooling systems. There is no doubt that the worst losses of cooling medium to the atmosphere come from vehicle air-conditioning systems.

Most alternative fluorocarbon refrigerants are totally new chemical compounds, foreign to nature and with both known and possibly unforeseen environmental effects. For example, 1,1,1,2-tetrafluoroethane or HFC-134a, which are now being employed as replacements for CFC-12 in automobile air-conditioning systems, have a significant global warming potential. Losses of HFC-134a from the air-conditioning system could increase the global warming impact of a car by as much as 20%.

Although carbon dioxide is a greenhouse gas as well, its specific (per kg) effect is three orders of magnitude lower than of the fluorocarbon chemicals. In practice, however, it is not necessary to produce new CO2 for refrigeration purposes, as abundant amounts are recovered from waste gas, for instance in oil refining or ammonia production. The net global warming contribution made by the recovered CO2 is therefore zero.

Large amounts of carbon dioxide are already used in soft drink production, fire extinguishers, welding equipment, the metallurgical industry and water treatment. The properties and characteristics of CO2 are all well known and documented, and further toxicity testing is not required. Use of this low-cost fluid will eliminate the refrigerant producer monopolies and terminate the present high-cost and exclusivity trend in fluorocarbon replacements and their lubricants. Furthermore, recycling or recovery will not be necessary when using CO2, whether for environmental or economic reasons. This will reduce costs and simplify work immensely during the lifecycle of the system.

Measured energy efficiency (Coefficient of Performance) of the CO2 laboratory prototype and a standard CFC-12 system.

CO2 was once widely used as a refrigerant, particularly in marine systems, due to its inherent safety and compact refrigeration machinery. After 1950, however, it was abandoned in favour of the CFCs as a result of its high power consumption and loss of capacity at high ambient temperatures. The new concept avoids these disadvantages by using a modified vapour compression system, and by improved operation and control. High compressor efficiency due to operation at low pressure ratios, improved heat transfer with high-pressure CO2 and efficient regulation of the high-side pressure are among the advantages of the new system. Laboratory prototypes tested under realistic conditions have demonstrated slightly lower energy consumption than ordinary commercial CFC-12 systems.

Some 45 million cars and commercial vehicles are manufactured every year around the world, and about half of them have factory-installed air-conditioning systems. The proportion of vehicles sold with such systems as part of their standard equipment is expected to rise, particularly in Europe. There is also a considerable retrofitting market.

Development of the new concept has been financed by Hydro Aluminium in Norway, which has licensed the commercial rights to the technology.