[CO.sub.2] as global coolant

Usually when you hear [CO.sub.2] mentioned in the context of global warming it is the villain, but engineers at the Denso Corp.'s Thermal System Group have recently cast it against type. Along with Toyota, Denso developed the world's first car air conditioner that replaces fluorocarbons with [CO.sub.2] as a natural refrigerant. (It debuted in Toyota's fuel cell hybrid vehicle FCHV.) "But wait," you say, "I thought we took care of the environmental problems of air conditioning refrigerants a long time ago when we replaced ozone-depleting chlorofluorocarbons (CHC-12) with hydrofluorocarbons (HFC-134a)." Yes and no. HFC-134a doesn't chew up the ozone layer, but it does contribute to global warming. [CO.sub.2] does too, of course, but at only 1/1,300th the rate. So using more [CO.sub.2] refrigerant can have a big positive environmental impact.

Also, while a traditional hydrofluorocarbon air conditioner requires a heater core to transform engine heat into warm cabin air, Denso's A/C unit can quickly raise cabin temperature through the heat exchange between the [CO.sub.2] refrigerant and the air, without using engine heat at all. This not only reduces parts count, weight and complexity, it separates heater performance from engine heat output. Vehicles with marginal conventional heaters, or none at all (like fuel cell vehicles), stand to gain. And it doesn't hurt that producing the [CO.sub.2] that charges the unit is a much simpler process than the manufacture of HFC-134a.

The trade-off is that in the [CO.sub.2] unit the refrigerating cycle requires higher pressure than with hydrofluorocarbon, meaning the tubes and the tanks of the heat exchanger must be resistant to high-pressure, and the compressor structure has to be able to take higher loads without leaking--reducing the weight and cost advantages.

So, how likely are we to see [CO.sub.2] air conditioners in a marketplace a little broader than that of prototype fuel cell vehicles? Well, Denso says it is looking to "widely apply" the technology. Which means little until you realize that it just happens to be the number one maker of automotive A/C units in the world.

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