How to Stay Cool Without Warming the Planet

The race is on to create climate-friendly air-con technology.

There is an insidious irony to climate change: as it gets hotter, more and more people are cranking up their air conditioners, which in turn contributes to global warming. Air conditioner (AC) sales are booming worldwide, especially in emerging economies such as China, India and Indonesia where rising incomes make them more affordable and a warmer, more humid climate a necessity. The International Energy Agency (IEA) predicts that around two thirds of the world’s households could have an AC by 2050, and the demand for energy to cool buildings could triple.

Keeping buildings cool contributes to global warming in two ways: ACs run on electricity, and they can release chemicals with a strong heat-trapping effect. ACs account for 16 percent of total electricity used in residential and commercial buildings around the world. This is significantly less than emissions caused by heating buildings—heaters run on natural gas, oil or electricity. But since the 2000s demand for air conditioning, ventilation and refrigeration systems has grown twice as fast as that for heating systems, at an average four percent per year.

Globally, there are around two billion AC units in use today—with the United States, Japan and China accounting for two thirds of them. China produces about 70 per cent of the world’s air conditioners and has experienced exceptionally growing demand in the last twenty years; around four in ten air conditioners are purchased and installed in the country.

The units precisely control the temperature and humidity in shops, laboratories or server rooms. They ensure that people feel as comfortable on a transatlantic flight as they do at home, using more electricity than any home appliance. But all that comes at a cost, and unless we switch to fully renewable electricity, the boom in air conditioning will generate more emissions and contribute to global warming, making hot summers even hotter.

With every new AC installed, the risk of a leak increases. The technology behind modern ACs hasn’t changed significantly since 1902, when a 25-year-old engineer named Willis Carrier invented a mechanical unit to control the temperature and humidity in a printing plant in New York. Carrier’s unit sent air through coils filled with cold water, and cooled the air while removing moisture from the room. His invention was soon fitted in other buildings, mainly in the textile, film and food industries, and a few decades later in people’s homes.

Today, the coils are filled with a refrigerant, a cold substance that easily changes from liquid to gas because of its low boiling point. As warm air from a room passes over the evaporator coils, the refrigerant absorbs the ambient heat, evaporates and carries away the heat through a condenser unit that sits outside a building.

These refrigerants are useful but problematic. They can seep out through joints, cracks or holes in the piping and contribute to global warming. The leakage of so-called fluorinated gases was particularly high by 2014—it accounted for about three percent of all greenhouse gas emissions in Europe—so the EU adopted a law to cap the amounts of gases sold. The conventional refrigerant R-410A is still widely used worldwide and has the potential to trap thousands of times more heat in the atmosphere than carbon dioxide.

While there are climate-friendlier AC units available on the market, most consumers purchase models that are cheaper and less efficient. In 2018, the Rocky Mountain Institute—a U.S. think tank focused on energy policy—offered a $1 million prize for residential cooling solutions that have a five times lower climate impact than a standard air conditioner, at no more than twice the cost. The prize, announced in April this year, went to two of the world’s largest manufacturers, Daikin and Gree. Their prototypes fundamentally work like existing ACs, but use refrigerants with lower global warming potential and are able to switch between operating modes depending on the outside weather conditions. Gree has also attached a small solar panel to its outdoor unit to generate some of the electricity needed to operate the AC.

But other companies want to ditch gaseous refrigerants altogether and replace them with materials that absorb heat just as well without the risk of leakage or fire. Barocal, a spin-off from the University of Cambridge, was one of the eight finalists in the competition. The team behind it, led by material scientist Xavier Moya, investigated whether solid materials could efficiently cool air. It settled on plastic crystals to generate an effect known as barocaloric cooling: the crystals are flexible so when they are squashed in a pressurized chamber, they heat up; when the pressure is released, they cool down and thus cool the air around them. The cooling capacity of these crystals is similar to that of the commercial refrigerants used in ACs, according to Moya.

If the concept works out, such a device could also be used for heating purposes by repeatedly squashing the crystals and thus heating them. “At the moment, we are working on developing the next generation of prototypes, to have something that delivers on all aspects of cooling and heating powers and efficiencies,” says Moya.

Other companies are focusing their efforts not on creating new refrigerants, but rather on designing altogether different methods of cooling—for instance, by sending excess heat to space. A Silicon Valley startup called SkyCool has developed rooftop panels that look like solar panels at first glance. They are, however, not meant to harness the energy of the sun—quite the opposite.

Every building, object or person on Earth radiates heat, which is absorbed by the atmosphere. However, radiation between eight and 13 micrometers in wavelength is not captured by the atmosphere: it simply passes through and escapes into space. Materials that radiate in that range can expel excess heat and cool themselves in the process in a phenomenon called passive radiative cooling.

SkyCool has collaborated with the manufacturing conglomerate 3M to develop an optical film that can do just that. SkyCool’s panels are covered with this film and stay up to eight degrees Celsius cooler than the air around them, even under direct sunlight. In fact, the panels work best in hot and dry climates. When it’s cloudy or humid, water vapour can trap the radiation in the atmosphere.

Beneath the film is a grid of pipes filled with a water-glycol mixture, which cools down when exposed to the panels’ colder temperature. The fluid is then pumped into a building's air conditioning system to chill down the conventional refrigerants. In this way, the amount of electricity needed to keep these systems running can be reduced. A supermarket in California installed an early version of the high-tech panels in late 2019 and is now using 15 percent less electricity.

SkyCool co-founder and CEO Eli Goldstein says that the startup is focusing on commercial and industrial facilities for now, because they tend to have higher cooling loads. “They run refrigeration systems, they have data centres, they have processes that need to be cooled, not just in the summer, but also the year round,” he says. The IEA estimates that 2 percent of all global electricity is used by data centres and transferring networks.

This year, SkyCool has installed panels on a handful of supermarkets, data centres and a dining hall. The biggest obstacle to rolling out the technology nationwide, or even internationally, are the relatively high installation costs. SkyCool doesn’t disclose its pricing but says that it charges slightly more than what it costs to install solar panels in the commercial sector (in the U.S. commercial rooftop panels are benchmarked at $1.56 per watt). The $3.5 million awarded by the Department of Energy in February 2021 could give the startup the push it needs to make the technology affordable and bring it to a larger market.

It is unlikely, though, that we will be seeing the panels on the roofs of residential buildings anytime soon. California-based startup Gradient is betting on heat pumps’ potential to decarbonise domestic cooling with high efficiency but low cost. The company has designed an electric pump, with two box-like components—one inside and one outside—connected by a platform that lies across the base of a window. It has heating and cooling functions.

Gradient uses a less potent refrigerant than the standard R-410A for its pump, which will be available in the U.S. before the summer of 2022. In the future the startup wants to use propane, which has even lower global warming potential than commonly used refrigerants. Propane is, however, more flammable and thus heavily regulated.

Heat pumps will not be completely carbon-free so long as the electricity comes from natural gas, oil or coal. “The goal is that these systems use as much renewable electricity as possible, as soon as possible,” says Gradient’s head of engineering, Santhi Analytis. That would be a good start to ensure that cooling does not heat the planet.

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