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.

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.

The new system will add costs “because it offers better cooling and an added fuel consumption lower than its competition,” said Behr CEO Markus Flik in a speech last month in Stuttgart. “R744 is the refrigerant of the future,” boasted Flik, using CO₂’s industry code name.

The CO₂ in air-conditioning systems is a fluid, compressed to as high as 140 bar – five times higher than the maximum pressure in conventional car air conditioners. The liquid CO₂ absorbs and dispenses heat more efficiently than hydrofluorocarbon refrigerants, so the system can be engineered to deliver an equivalent amount of cooling faster and, on average, using less energy.

Of course operating at higher pressure requires tougher, more expensive parts. And it also requires a more sophisticated control system. In fact, Glober says that a CO₂-based system under conventional controls can exert substantial torque on the engine when it starts up – enough to stall a small car.

Denis Clodic, director of the Center for Energy and Processes at École des Mines in Paris, points to another cost: establishing servicing networks for the high-pressure systems. “There is a lot of complexity to these systems. That means lots of training,” he says.

Clodic predicts that, at least in 2011, CO₂–based systems will appear only in high-end luxury cars, and only in Europe. “We will see some thousands of cars made by Audi, BMW, and Mercedes, to show the European Commission that they really tried to prepare something alternative,” he says. “But there won’t be a mass market for 2011.”

That is, assuming the hydrofluorocarbons from the chemical manufacturers pass muster in long-term testing. DuPont has said that it could take five years to commercialize its new hydrofluorocarbon refrigerant. By that time, companies like Behr may have overcome the remaining challenges with the CO₂ systems – identifying affordable CO₂ sensors and reducing the cost of high-pressure hoses and compressors. If so, both technologies could enter the market – initiating a classic competition based on performance and price.