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Developing countries in hot, tropical locations—places like India, Pakistan, and Nigeria—are going to be buying air conditioners and heat pumps by the hundreds of millions over the next decade.
The climate has a refrigerant problem. These chemicals, used across industries and homes, are often super-powerful greenhouse gases—thousands or even tens of thousands of times stronger than carbon dioxide—that contribute significantly to climate change. And with temperatures rising across the globe, the demand for air conditioning is going to explode.
For nearly a century, the chemical industry has been playing whack-a-mole—coming up with new refrigerants to address environmental problems, only to create new ones. The latest generation of refrigerants has at last solved the major environmental problems, but at the cost of lower efficiency.
Dealing with refrigerants is a modest but important area of climate policy—and one where solutions are already available. It’s time for governments and regulators to consider bringing back the very first refrigerants—above all, propane.
Let’s start with some science and engineering. An air conditioner, for instance, works by manipulating the properties of a refrigerant. Inside the machine, a compressor puts this substance under enough pressure to condense it into a liquid, which heats it up, and then it’s pumped through a heat sink that is placed outside. At the other end of the heat sink, the refrigerant passes through a tiny valve that lowers the pressure on the other side, causing the liquid to evaporate and absorb heat. (A heat pump works exactly the same way, except that it can also go in reverse to provide heat as well as cooling.)
A refrigerant can’t be just any liquid, however. It needs to boil at a low temperature, for example, so that rules out water. Small hydrocarbons like propane work well, as does ammonia, but those are both flammable and the latter is toxic. Starting in the 1920s, industrial chemists invented refrigerants without these properties, initially settling on short carbon chains with their bonds filled out with halogens like chlorine and fluorine—the now-infamous chlorofluorocarbons (CFCs). One of the most widely used was dichlorodifluoromethane (or R-12, an industry label), which is a single carbon atom bonded to two fluorines and two chlorines. Bonds between carbon and halogens tend to be very strong, so the compounds were stable, nontoxic, and fire-resistant.
Unfortunately, this very stability created a problem. When CFCs get into the atmosphere, they don’t fall apart quickly. Instead, they get circulated up to the stratosphere, where they get blasted apart by high-energy light frequencies from the sun—causing them to release highly reactive chlorine radicals, which eat up the ozone layer. They are also stupendously powerful greenhouse gases; R-12 has a “global warming potential” of about 10,800—a technical shorthand meaning that in the course of a century, it traps 10,800 times the heat that carbon dioxide does.
Additionally, any refrigerant used as a propellant, say for hairspray or blown insulation, gets into the atmosphere. Cooling systems can also leak. Portable or window-mounted air conditioners, or refrigerators and freezers, are sealed tightly at the factory and so generally don’t leak much when operating, but they will eventually be thrown out or scrapped. American-style central-air systems should be sealed thanks to brazed or soldered fittings, but sometimes they are installed poorly or break.
Less efficient refrigerants could actually worsen the effects of climate change, depending on the local source of electricity.
European-style mini-split heat pumps (with an outdoor heat sink and several wall-mounted cooling/heating units) are rather leak-prone, because of the connectors on the pipes. “Those have flare fittings, and they are atrocious,” Chuck Booten, a senior engineer at the National Renewable Energy Laboratory, told the Prospect. “They are notorious for leaking all the time.” Sure enough, a U.K. study found that home-installed heat pumps leak an average of 3.8 percent of their refrigerant charge every year, the vast majority (92 percent) coming from large leaks. The same is true with the large coolers and freezers at supermarkets—the EPA’s GreenChill program, which helps supermarkets cut down their greenhouse emissions, estimates that typical grocery store systems leak an average of 25 percent of their refrigerant every year.
In response, the nations of the world came together and negotiated the Montreal Protocol in 1987 to phase out CFCs. This worked well: The atmospheric concentration of most CFCs has declined since the mid-2000s, and the ozone layer is slowly healing.
Thanks to this global regulation, the refrigerant industry was forced to invent yet more molecules to replace CFCs. One idea was obvious: Because fluorine radicals don’t interact with ozone, just get rid of the chlorine to create hydrofluorocarbons (HFCs). For instance, R-134a is a two-carbon chain bonded to four fluorines and two hydrogens, and it is stable, nontoxic, nonflammable, and ozone-safe. By the late 1990s, this kind of refrigerant was common in all kinds of applications.
Unfortunately, these HFCs weren’t perfect: They are still very powerful greenhouse gases. R-134a has a global warming potential of 1,430, and lasts an average of 14 years in the atmosphere. Others are even worse. “In a typical supermarket you can find 2,000 pounds of R-404a, with a global warming potential close to 4,000,” Omar Abdelaziz, a professor at the American University in Cairo, told the Prospect, “and it is leaking 500 pounds of refrigerant every year.”
Overall, fluorinated gases account for about 3 percent of U.S. greenhouse gas emissions, and about 2 percent of global emissions. If future air conditioners in Asia and Africa run on HFCs, that latter percentage will probably increase.
Governments once more took notice. In 2006, European Union regulators required automotive refrigerants to have a global warming potential of no more than 150, and in 2016, the Montreal Protocol was amended at a conference in Kigali, Rwanda, to add a phaseout of HFCs (though the U.S. has not yet ratified it). Rich countries were required to start phasing out HFCs as of 2019, and cut down to 15 percent of 2012 levels by 2026; middle-income countries like China must start phasing them out in 2024 and get down to 20 percent of 2021 levels by 2045; and low-income countries like India get until 2028 to start phasing out and must get down to 2024-2026 levels by 2047.
Modelers estimate that if the agreement holds, it will cut global warming over this century by as much as half a degree Celsius.
So the beleaguered chemists of the refrigerant industry went back to the drawing board once more. This time, they added a double bond to their fluorine-bearing carbon chain to create hydrofluoroolefins (or HFOs), which makes the molecules decompose rapidly in the atmosphere. If you add a double-bonded carbon to R-134a, for instance, you get R-1234yf—which is quickly becoming the international standard for automobiles—and that has no effect on the ozone or on the climate.
Alas, this solution is not perfect either. HFOs are generally less efficient than HFCs at refrigeration, and for some purposes they are simply no good. As a result, many of the newer refrigerants are actually just mixtures of HFOs and HFCs (like R-454b), which is only a modest improvement in terms of leak risk and still comes with a hit to efficiency.
Indeed, less efficient refrigerants could actually worsen the effects of climate change, depending on the local source of electricity. Studies of air conditioners have found that even when charged with older refrigerants, leaks currently only account for about 10 percent of their overall global warming effect. If the electric grid is mainly powered by fossil fuels, replacing an HFC charge with an HFO will accomplish little.
Moreover, in the environment HFOs degrade into trifluoroacetic acid, which accumulates in water supplies, is very difficult to remove, and can be toxic in sufficient concentrations. Initial studies have predicted that it shouldn’t build up to harmful levels given reasonable assumptions, but you’d hate to be wrong about that.
In any case, the Kigali agreement gives many big, developing countries in hot, tropical locations—places like India, Pakistan, and Nigeria that are going to be buying air conditioners and heat pumps by the hundreds of millions over the next decade—six years before they even have to start cutting back on HFCs. Now, it’s only fair that these countries get more time to adjust than richer ones, given their lack of resources, but it would be very unfortunate if they locked in millions of tons of HFCs in equipment that is bound to leak out sooner or later.
Fortunately, there are options. If the world stops trying to engineer around the problem with chemistry and instead takes advantage of the oldest refrigerants out there, namely ammonia, propane, and carbon dioxide, rich countries could cut down on their emissions and poor ones could jump directly to climate-friendly, efficient air-conditioning.
Both ammonia and propane are relatively environmentally safe, with no long-term by-products, and have global warming potentials of zero and 3.3, respectively. Carbon dioxide is virtually harmless, and has a global warming potential of one (by definition, since it is the reference point), though it does require high pressure and therefore more expensive equipment. All three are also much cheaper to obtain than synthetic refrigerants.
Indeed, in heavy industrial applications like meatpacking plants or frozen food factories, ammonia is still used because its excellent efficiency (something like 10 to 40 percent better than synthetic refrigerants, depending on the application) and cheaper price make up for the heavier equipment it requires. The danger of leaks is also mitigated by ammonia’s sharp smell, which people notice very quickly.
American and European regulators don’t have direct influence over the rest of the world, but they do have a lot of indirect influence.
Ammonia is probably impractical for most home uses, as is carbon dioxide because of pressure requirements, but propane could be used in most home refrigerators (as it’s already used in most small bar fridges), air conditioners, and heat pumps with only slight modifications. The fire risk would be negligible in the case of refrigerators and window units, because they are factory-sealed and only need a few hundred grams of refrigerant charge in any case.
The problem is that the EPA limits propane charges to just 150 grams per circuit (which was actually increased in 2018 from 57 grams). “Roughly speaking, you can only get about a 6,000 to maybe a 9,000 BTU system on 150 grams, if you’re really careful, which is on the low end of room air conditioners,” Booten said—not enough for a home-scale central-air unit or heat pump, much less a big commercial setting. Manufacturers are also leery of liability risk. Some companies have started producing propane-charged refrigerants for supermarkets, with reportedly greater efficiency and less leakage, but each unit has to be separated to stay under the 150-gram cap.
The EU recently increased its charge limit to 500 grams, which is a good start, but still probably not enough for a large, centralized (and hence maximally efficient) commercial system.
Incidentally, it’s rather odd to be hyper-cautious about small quantities of refrigerant when a large fraction of American homes are already connected to natural gas lines (powering appliances that run inside the house) that can deliver limitless quantities of highly flammable gas if a pipe or connection fails. Leak-caused natural gas explosions that destroy homes and businesses (if not half a city block) are all too common.
Still, the risk is real, and regulators could require that new propane-charged units be fitted with a sensor that would trigger an alarm in case of a leak, just in case—especially for heat pumps and commercial coolers, given their history. They also could require installers to use leak-proof connectors, and mandate propane detectors in homes and businesses that use propane refrigerants. Even simple outreach programs can help—the GreenChill program mentioned above reports that supermarkets that enroll in their program cut their leak rate by more than half, which saves money.
American and European regulators don’t have direct influence over the rest of the world—particularly China, where the vast majority of heat pumps are manufactured—but they do have a lot of indirect influence. Chinese industry statistics show a marked movement away from the hydrofluorocarbon R-22 starting in 2020, thanks to it being banned outright in America and the EU. If propane becomes the de facto standard for its best use cases, the developing world will buy whatever is on the shelf.
At any rate, propane isn’t a magic bullet. The best refrigerant for any particular setting will depend on the amount of heating or cooling needed, the difference between outdoor and indoor temperature, the available space, the global warming potential of each option, and numerous other factors. But propane certainly deserves a much larger share of the refrigerant market than it currently has, particularly for domestic uses. With just a little attention from regulators, the global heating and cooling industry could be tipped into a more efficient, more climate-friendly status.
