In this tropical city, where humidity levels oscillate between muggy and oppressive for most of the year, Santosh Naykar’s only defense against the stickiness is a 14-year-old window air-conditioning unit.

“We don’t even want to think about going back to life without an air conditioner,” Naykar, 60, said as he directed the flimsy vents toward his face.

Window air conditioners are cheap and convenient, but are less efficient than other models.

That cold air comes at a cost. Each time Naykar and his family turn on their basic window unit — which can be among the least energy-efficient air conditioners available — it guzzles electricity and spews planet-warming emissions.

But less than 20 miles away in an apartment set to re-create the balmy conditions inside households like the Naykars’, scientists are working on new AC prototypes to cool the millions of people bearing the brunt of higher temperatures without further fueling global warming.

The key, some experts say, may lie in transforming air conditioners from cooling machines into more efficient humidity gulpers. As the planet gets hotter, warmer air in the atmosphere holds more water vapor, leading to greater humidity in some places. Combined with intensifying heat, these higher humidity levels become dangerous: The more humid it gets, the harder it is for the human body to sweat and cool itself.

“It’s a necessity product in the context of today’s world, so we have to get it right,” said Ankit Kalanki, who is working on an effort through RMI, a U.S.-based clean energy think tank, to bring more planet-friendly air conditioners to homes.

So far, the world’s roughly 1.2 billion residential ACs are largely concentrated in richer countries. But that is quickly changing as temperatures rise and more families like the Naykars can afford to buy them. In the regions most in need of cooling — such as India, Southeast Asia and Latin America — the potential for AC growth is huge, with billions of would-be users eager to install their first unit.

Cooling already accounts for nearly 4 percent of annual greenhouse gas emissions, or twice as much as planes. Much of that comes from electricity to run air conditioners and refrigerant leaks. The cooling chemicals can be hundreds of times or even more than a thousand times as potent as carbon dioxide.

By 2050, the number of air conditioners in buildings around the world is expected to triple, according to a 2018 report from the International Energy Agency, or IEA. If nothing changes, the carbon dioxide emissions they generate will almost double from 1.1 billion metric tons in 2016 to just over 2 billion metric tons in 2050, or roughly what 476 million cars spew out a year, according to the agency.

To avoid that scenario, researchers and manufacturers are revamping the century-old appliance to cool the next generation of AC users in a hotter planet.

It’s a stark challenge. The standard technology, first deployed in a Brooklyn printing plant in 1902, is not designed to tackle the higher temperatures and humidity levels common in the places expected to add the most ACs in coming decades.

The machines not only have to be more powerful to mitigate the Southern Hemisphere’s hotter and wetter climate, but to combat further global warming, they also have to do so while using less electricity, which in many of these regions is still generated by burning fossil fuels.

These improvements also have to come at an affordable price tag: If the new AC units are too expensive, buyers will opt for less efficient and more polluting models on sale today.

India, which is expected to add hundreds of millions of new AC units over the next couple of decades, offered an ideal location to test whether a single machine could check all those boxes.

The first step is reimagining ACs as more than air coolers.

Whether it’s an old window unit like the Naykars’ or the most efficient mini-split on the market, many air conditioners generally work the same way:

Full diagram of an AC unit, showing the flow of refrigerant through the coils and the fans that pull air in and push air out of the unit.

indoors

outdoors

fans

coils

air

Diagram showing how air flows into the indoor portion of an AC unit and over the coils carrying refrigerant. A closer view of the coils shows how the air comes in hot but as it passes over the coil with the refrigerant, it becomes cooler as the refrigerant absorbs the heat from the air. A third portion of the diagram shows the outdoor section of the AC unit and how the warm air from indoors passes out through the vent into the outdoor air. It also shows a drop of condensation coming out from the bottom of the unit.

coils

indoors

A fan inside the machine sucks in air from the room and pushes it through a cooling coil filled with a chemical refrigerant.

air

coils

As the balmy room air flows through the coil, the refrigerant pulls heat and moisture out of the air …

air

air

… then carries that heat outdoors, to the outside part of the AC, and rejects it.

 

As the air cools, water condenses out and is drained.

outdoors

condensation

The cycle continues until the room reaches the set point on the thermostat. But human comfort does not depend on temperature alone. In places such as Mumbai, high humidity levels can make people in air-conditioned spaces feel sticky even at temperatures that would be comfortable in a drier climate.

Many AC users deal with this by cranking down the temperature, driving their units to run longer, gobbling more energy and overcooling their rooms. More than half of the emissions from air conditioning come from units removing humidity from the air, according to a peer-reviewed study published in 2022.

“Today’s air conditioners have a giant blind spot: That’s humidity,” said Kalanki, of RMI, which was created in response to the 1970s energy crisis and has since shifted its focus to addressing global warming.

“With climate change, you are seeing not just increased events of higher temperatures around the world, but also starting to see higher humidity … creating a new need for solutions that can actually manage humidity and not just temperature.”

RMI, based in Colorado, is working to jump-start a market for ACs that can fill that gap. Ongoing efforts to get this technology on store shelves are backed by the Clean Cooling Collaborative, an environmental coalition funded by the William and Flora Hewlett Foundation and the Ikea Foundation, among others.

Before they could afford their first air conditioner, the Naykars stayed cool during sweltering summer months by filling a bucket with ice water and pouring it on their apartment floor before lying down on the bare surface to sleep. Sometimes they even stuck their legs in the icy water.

“There were times when, even at midnight, I used to take long cold showers to escape the heat and sweating,” said Naykar, a barber, who shares the cramped one-bedroom apartment with his wife and son.

After years of saving, they shelled out 8,000 rupees, or about $95 in today’s dollars, for a Voltas Vertis Premium window air conditioner. Now, the aging unit is one of two they own. About three years ago, Naykar installed a mini-split in his barbershop, though he usually only turns it on if he has customers.

In 2018, only about 5 percent of India’s 300 million households, many of which are in densely populated urban slums, had air conditioning, according to a report from the International Energy Agency. But that figure is rapidly increasing.

Last year, Indians bought between 8 million and 10 million room air conditioners. For about 90 percent of buyers, it was their first AC purchase, according to RMI. As incomes continue to rise, those numbers are projected to explode: The number of air conditioners installed and sold in India will increase to more than 1 billion by 2050 — a roughly 30-fold increase from 2018, another IEA report estimates.

Those units are expected to generate 25 gigatons of cumulative emissions by 2050, equivalent to nearly 6 billion gas-powered cars being driven for a year, according to RMI. Operating them would require about 1.5 times as much power as India’s total capacity today, much of which still relies on fossil fuels.

“We must switch and try to accelerate the market toward these super-efficient ACs now,” Kalanki said. “Otherwise, we will be adding a large number of inefficient units that will only contribute to this vicious loop of warming.”

The new prototypes look like regular mini-splits, but they use a fraction of the energy.

These units are the winners of a global contest launched in 2018 by RMI in partnership with a global coalition that included the Indian government. Known as the Global Cooling Prize, it challenged participants to come up with air conditioners that don’t contribute as much to global warming.

Organizers also pushed teams to come up with models that people could buy and operate at no more than twice the cost of a standard AC in India.

Teams led by air-conditioning manufacturers Gree, from China, and Daikin, from Japan, won. To gauge how their prototypes perform, researchers have been pitting them against standard mini-splits already in many Indian households.

In one test, researchers measured how much energy it took the air conditioners to keep a space comfortable — at or below about 80 degrees Fahrenheit and 60 percent relative humidity, according to the testing parameters.

When it’s hot and balmy, it’s impossible to hit that humidity target by setting the thermostat to 80 degrees. That’s not cold enough for the water vapor in the air to condense so it can be extracted, a temperature known as dew point. Under the testing conditions, that point would be closer to 66 degrees Fahrenheit.

The only path to reach that with a traditional model is by cranking down the thermostat, which is what someone sitting in that uncomfortable room would do. The researchers mimicked that response in their tests and measured what happened.

To achieve the colder temperature, the machine furiously moved refrigerant through its coil. External monitors showed this approach removed humidity, but it also chilled the room well past comfortable levels, consuming large amounts of energy.

The prototypes have more levers they can pull to get both temperature and humidity close to the testing parameters — while limiting the time they spend operating at full capacity. Because they can sense humidity in real time, they, not users, make constant adjustments to avoid overcooling a space.

One of the winning models also has two coils instead of one, allowing it to work toward the dew point gradually. Instead of immediately kicking into high gear, as with traditional models, the prototype uses the first coil to take a first pass at cooling the air, but that doesn’t always get it all the way to the dew point. If additional cooling is needed, that’s the job of the second coil. Because the AC is doing this in steps, it can dehumidify the room without overtaxing itself.

On a typical warm and humid day in India, here’s how the old and the new models stack up, according to the researchers’ tests:

The off-the-shelf unit went through more than 16 kilowatt-hours of energy. That’s equivalent to the energy needed to run a typical clothes dryer for almost six hours. The more efficient new units drew about 75 percent less energy.

What does this mean for real Indian households? For the Bhanushalis, a family of five who cool their one-bedroom apartment in Thane with a couple of mini-splits, it would free up some of the money that now goes toward their electricity bill. In May, for example, one of the hottest months in India, they paid 8,000 to 9,000 rupees, or about 10 percent of their total monthly income.

Mini-splits are common in Indian households with air conditioning.

If the Bhanushalis had a much more efficient air conditioner, such as a model similar to the prototypes, they might also be willing to use their unit more liberally. But experts say the family would still see significant energy savings over using traditional ACs.

Despite enduring stretches of 100-degree-plus outdoor temperatures and humidity levels surpassing 70 percent, Hansa Bhanushali usually only allows the units to be on from 11 p.m. until dawn, when the family is asleep.

“Using air conditioning is costly,” said Bhanushali, sitting in her living room, a light sheen of sweat covering her face. Above her, the air conditioner was quiet, vents closed.

The prototypes have additional features that make them more climate-friendly. For example, the units use new refrigerants that won’t warm the atmosphere as much if they leak.

In 2018, RMI calculated that if by 2040 nearly all the units sold globally met the contest parameters, the technology could prevent up to half a degree Celsius of global warming by the end of the century.

Achieving this goal, though, hinges on these units being installed in enough homes, which won’t happen unless their cost goes down and buyers are aware of their energy savings, said Nihar Shah, presidential director of the Global Cooling Efficiency Program at Lawrence Berkeley National Laboratory.

“It’s not just going to magically happen,” said Shah, who served on the prize’s technical review committee.

Still, Shah said, air conditioners designed to handle humidity are critical for the future, calling them “absolutely one of the most key things that we can do.”

But experts at the National Renewable Energy Laboratory, part of the Energy Department, say there is still much work to be done to create ACs for the climate change era.

“They probably took a little step toward the goal of climbing a much bigger mountain,” said Eric Kozubal, a senior mechanical engineer and researcher at NREL, which was not involved in RMI’s air conditioner contest and testing.

Price, not technology, probably will remain one of the main hurdles in building a carbon-neutral AC, his colleague Chuck Booten said.

“If you said, ‘Look, I’ll pay you $100 million for the single best air conditioner that can do absolutely anything in the world …’ Yes, no problem. It’ll happen,” said Booten, a senior engineer at NREL. “It becomes kind of a practical limit on how do you make it better without making it so expensive or so complicated or unreliable that it’s just not a mass-market solution?”

Before they make it to homes, these units first have to pass a battery of tests.

In a lofty residential tower less than 20 miles southeast of the Bhanushalis’ apartment, the winning prototypes hummed in single-bedroom units day and night for about eight months. There were no living residents in these apartments.

Instead, cold air blew over four heat-generating silver cylinders, each about 5 feet 10 inches tall, which simulated an average household of four. Wisps of smokelike water vapor billowed from holes in a metal tube, mimicking real-life humidity loads.

Over a 24-hour period, these machines replicated the changing levels of heat and humidity in a typical household, testing the air conditioner’s ability to adjust. For example, when several steaming pots are bubbling on the stovetop before dinner and everyone is home ready to eat, heat and humidity levels can shoot up. But those levels go down during the middle of the day when people aren’t home.

“There are so many conditions in which the unit has to operate,” said Yashkumar Shukla, principal researcher at the Center for Advanced Research in Building Science and Energy at CEPT University, an academic institution in Ahmedabad that partnered with RMI to test the prototypes. “You can only know whether it performs in all conditions once you test it.”

These tests are not only meant to ensure the ACs will work when they’re installed in homes, but to inform the development of new testing standards that can push manufacturers to produce these kinds of units. Current testing protocols tend to reflect the more temperate and less humid climate of affluent countries in the Northern Hemisphere.

But creating new standards is just a first step in getting these ACs into more households. Even if the prototypes hit store shelves, early models would still be out of reach for many Indians.

“It’s not enough, because once consumers see the energy savings, they still need to be able to afford to buy those,” Shah said. “Bringing that price point down, of course, is another part of it.”

RMI is working with standard-setting groups that can use the findings from the tests to update protocols that are used to assess and rate ACs, Kalanki said. The clean energy think tank is also in contact with potential big buyers, such as governments and developers, which could purchase the new ACs in bulk and help make these units viable. He said he is hopeful that some of the units could be cooling spaces in less than five years.

But as Kalanki and others push to get the units out into the world, the Naykars are relying on their trusty window AC. After coming back to his apartment for lunch and a daily nap on an afternoon in June, Santosh Naykar made a beeline for the bedroom to turn on the air conditioner.

The unit rattled to life. As the bedroom cooled, Naykar’s wife laid blankets on the floor while their son stretched out on a small Murphy bed, idly scrolling on his phone. Naykar climbed in next to him, taking the spot closest to the air conditioner. He covered his legs with a small throw blanket and settled in, throwing his arms over his head.

“Feels like heaven,” Naykar said, closing his eyes.