In many parts of the world, the cool blast of an air-conditioner on a hot day is nowadays seen as a luxury rather than a necessity. Climate change is tipping the balance. Average global temperatures are now roughly 1.2°C higher than they were before the Industrial Revolution: by mid-century they are projected to be about 2°C higher. Air-conditioning (AC) use, correspondingly, is set to soar. By one estimate, the number of room-cooling ACs could nearly triple between now and 2050.

These additional units will save lives, make cities liveable and stave off losses in economic productivity. The Lancet, a medical journal, estimates that access to air-conditioning averted nearly 200,000 deaths worldwide among people aged 65 or older in 2019, for instance, cutting the group’s heat-related mortality by 37%. But expanding those benefits more widely will come at a cost. The electricity needed for air-conditioning is already responsible for more carbon-dioxide (CO₂) emissions than the entire aviation industry. And as the laws of physics unhelpfully dictate that a single degree of cooling becomes more energy-intensive as the outside temperature rises, additional cooling will require more power per unit, risking a great deal more planetary heating.

To provide this cooling while doing minimal harm, decarbonising electricity is crucial. But making air conditioning more efficient is important too. Because commercial ACs tend to be used for up to two decades, those installed in the next few years will influence how many emissions are produced by 2050. Fortunately, a flurry of new technologies are emerging to make them less polluting and wasteful.

Making these improvements means struggling against one of nature’s most rigid constraints: the second law of thermodynamics. “You just can’t get round it,” says Russ Wilcox, the chief executive of Trellis Air, an air-conditioning startup. In simple terms, the law decrees that heat cannot move spontaneously from a colder object to a hotter one. This has two sobering implications for air-conditioning engineers: first, all machines generate inefficient waste heat; and second, deliberately cooling a room requires power. (Though deliberately warming a room also takes power, heating engineers are not as bothered by the incidental production of waste heat.)

Cool running

Air-conditioners currently use this power to draw in warm air and pass it over a cold refrigerant, which evaporates as it absorbs the air’s heat . The cooled air is returned to the room, and the refrigerant is condensed back into a liquid, with its trapped heat released outside.

Conventional machines also dehumidify air as they chill it. This is partly out of necessity: the water that naturally condenses on the metal around the refrigerant must be removed before it can do any damage. But this incidental dehumidification also helps make a space feel cooler, as it is easier for human sweat to evaporate—and thereby keep skin cool—in low humidity.

Doing both jobs at the same time, though, is wildly inefficient. Research published in Joule, an energy journal, by American scientists in 2022 concluded that almost a third of the electricity used for air-conditioning goes toward removing moisture. That fraction could become larger over time. Air-conditioning is growing fastest in developing countries like India and Indonesia, which tend to be more humid than richer places. And as hot air can also hold more moisture, average humidity is expected to rise worldwide.

One way of minimising the dehumidification work an AC needs to do is to expose the incoming air to a water-absorbing substance known as a desiccant. Though desiccants have been tried in the past, most are either limited in the amount of water they can absorb or require substantial amounts of energy to be made reusable. Transaera, a startup based in Massachusetts, has, instead, looked to crystalline structures called metal-organic frameworks (MOFs). These are molecular cages that can be finely tuned to catch specific “guest” particles and subsequently release them when exposed to heat.

The company spreads a MOF-based coating on a wheel inside the unit through which incoming air passes. As the wheel rotates, the water captured by the MOF is carried away and emptied by the low-level waste heat generated by the machine. Field tests conducted over the summer suggest that an air-conditioner built with this system uses 40% less energy than one without. Transaera is working with existing air-conditioning manufacturers to incorporate its technology into their designs.

Trellis is exploring alternative means of dehumidification, too. It is working on a system that filters air through a selectively permeable membrane. This, the company hopes, should help a room feel cool with little need for active cooling.

Cold-blooded calculation

Another priority is cutting down on the refrigerants that conventional ACs need to function. The most popular options—also used by fridges and heat pumps—are hydrofluorocarbons (HFCs), potent greenhouse gases which are between hundreds and thousands of times more effective at warming the atmosphere than CO₂. These gases sometimes leak out during a unit’s life, but much more can be released when it is thrown away. By one estimate, the yearly warming AC gases generate is equivalent to that produced by 720m extra tonnes of CO₂, a figure higher than that of the emissions produced by Canada’s whole economy in 2022.

International agreements aim to reduce HFC use by 85% by 2050, but their implementation is still piecemeal and patchy. That is why some companies are trying to build units that do away with refrigerants entirely. That means coming up with a totally different method of absorbing heat.

Blue Frontier, a company based in Florida, is attempting to harness the cooling effects of evaporating water. Its approach uses a liquid desiccant, similar to ultra-salty brine, to remove moisture. The dried air is then split into two streams, one of which is passed over a thin layer of water to induce evaporation. This lowers the temperature of the surrounding metal, which in turn cools the other airstream before it is directed back into the room.

Blue Frontier currently has three units installed in commercial buildings; Daniel Betts, the company’s chief executive, thinks three more will be operational by the end of 2024. Users, the company says, get to control humidity and temperature independently of one another, while reducing energy use by between 50% and 90% and the environmental impact of refrigerants by 85%. The remaining energy goes toward powering the heat pump that regenerates the desiccant.

Dr Betts points to benefits beyond efficiency. Because evaporative cooling takes advantage of heat’s propensity to flow from hot to cold, it becomes more efficient as outside temperatures rise. The system can therefore recharge the desiccant by connecting to the grid at night, when demand is lowest, and use minimal electricity to cool during the heat of the day.

Such flexibility in energy demand can help air-conditioners minimise the strain they put on power grids—another crucial problem. At present, widespread synchronised demand for cooling in hot weather leads to sharp spikes in electricity consumption. These can lead to deadly outages, especially in hotter, poorer countries with weak grids. Even rich countries frequently resort to whatever energy is available to cope with the demand in the summer: in June 2023 Britain’s National Grid turned a coal plant back on to cope with a hot spell. A few months later a severe heatwave meant California only narrowly avoided rolling blackouts.

As a result, some Californian businesses are actively experimenting with more grid-friendly approaches. The Beverly Hilton and the Waldorf Astoria Beverly Hills, two swanky hotels in suburban Los Angeles, have begun using “IceBricks” designed by Nostromo, an Israeli company. These contain hundreds of capsules of water that can be frozen when electricity demand is low, then used when it is high. Doing so, Nostromo claims, will reduce the hotels’ cooling-electricity costs by 50% and lessen their carbon emissions.

Much more is needed to make air-conditioning cheaper, cleaner and more reliable. But clever engineering of this kind is a good start. Done right, keeping humankind cool may not have to come at the planet’s expense.