In the 1990s, air conditioning suppliers switched from the chlorofluorocarbon Freon to an equally troublesome hydrofluorocarbon called R-134a; while easy on the ozone, R-134a is a greenhouse gas that's 1,300 times more potent than CO₂.

Automotive equipment maker Delphi will opt for this trial-tested carbon dioxide air-conditioning system, if more experimental hydrofluorocarbon refrigerants don't pan out. (Photo courtesy of Delphi.)

The impact has been most acute in automotive applications, where refrigerants often leak out. Indeed, by 2010, such leakage will contribute more than 4 percent of the total climate change impact from motor vehicles. Add in the extra fuel consumption to run the AC, and AC's share rises to 7 percent.

Little surprise, then, that the European Union decided this January 31 to begin phasing out the use of R-134a in new model cars beginning in 2011, and that regulators in California are preparing to follow suit. Until this spring, the most likely replacement looked to be novel high-pressure systems employing, ironically, CO₂ as the refrigerant. Behr GmbH -- Europe's leading AC supplier for cars -- announced last month that they would begin selling CO₂-based systems ahead of the EU's 2011 deadline.

But Behr's competitors, such as Troy, MI-based Delphi and Germany's Robert Bosch GmbH, have been backing away from CO₂ since February, when DuPont and Honeywell unveiled new hydrofluorocarbon refrigerants that may be clean enough to squeak by the regulators. According to the chemical companies, the new kinds of hydrofluorocarbons are no more than 150 more potent as greenhouse gases than CO_{2 --} the limit set by the EU for auto refrigerants after 2011. What's more, these refrigerants can be dropped into existing AC equipment. "The prospect of having a new drop-in refrigerant that would satisfy the 2011 legislation is incredible -- it's enormous," says Stefan Glober, director of engineering for Delphi's thermal and interior division.

Many questions remain for both options, however. The new hydrofluorocarbon-based refrigerants offered by DuPont and Honeywell must complete a host of long-term tests, including for the stability of the compounds under heavy use and for toxicity. That could take at least three years. And it's unknown how much the new refrigerants will cost to manufacture. This means that AC manufacturers must also continue to develop their new CO₂ systems. "These alternatives have appeared relatively late. That's the dilemma we're in right now," says Glober.

The CO₂ systems have their own hurdles. One is detecting leaks: cheap, effective CO₂ sensors don't exist yet. "That's a huge headache," says Glober. Another is cost. And it's here that Behr and its competitors part ways. Glober says the industry consensus is that the first CO₂ systems will sell for €150-200 more than conventional AC systems, doubling their costs. Behr, in contrast, says it will be able to keep down the added cost to less than €100 in the first-generation system and half that by 2015 -- sums that the firm predicts will be justified by higher performance.