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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