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Making air-conditioning less harmful to the planet

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 livable 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 cohort’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, decarbonizing 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 towards 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 metalorganic

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, which

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 makes use of 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 towards 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 minimize the

strain they put on power grids—another crucial problem. At present,

widespread synchronized 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.


Source: The Economist

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