Energy-Saving, Low-Cost Air Conditioning

Two new technologies could provide an eco-friendly cooling solution.

House with man inside, cooled by an air conditioner

Illustration by Eva Vasquéz

Turning on an air-conditioning unit is a catch-22. As climate change increases global temperature, world cooling demands are expected to triple by 2050. This translates to 10 new air-conditioning units deployed every second for the next three decades. But traditional AC systems are far from climate friendly. They shuttle a refrigerant between the liquid and vapor phases to remove heat and moisture from incoming air—a process that requires huge amounts of energy and utilizes chemicals with a greenhouse effect up to 10,000 times more potent than carbon dioxide. AC might soothe the symptoms of climate change, but it exacerbates the problem.

It doesn’t have to be this way, though. “We’re reimagining air conditioning to meet the global cooling demand increase and to combat climate change,” says assistant professor of architecture Jonathan Grinham, a core faculty member at the Harvard Center for Green Buildings and Cities (CGBC). A collaborative team of researchers representing the Wyss Institute, the Harvard Paulson School of Engineering and Applied Sciences (SEAS), and the Graduate School of Design (GSD), and supported in part by Harvard’s Climate Change Solutions Fund, developed two climate systems that efficiently cool buildings, contain zero refrigerants, and use far less energy than traditional AC units.

One of these systems is being field-tested in a Harvard facility. High on the third floor of CGBC’s HouseZero (see, coldSNAP (cold Superhydrophobic Nano-Architectured Process) hums in the window. Designed by Grinham, Kumagai professor of architectural technology Martin Bechtold, Wyss Institute senior staff scientist Jack Alvarenga, and Berylson professor of materials science Joanna Aizenberg, coldSNAP is a new take on evaporative cooling.

Evaporative cooling (EC) is an ancient technology that puts hot air in contact with water, which cools the air as it evaporates. Ancient Egyptians used EC, placing wet linens or jugs of water in their windows to catch the breeze, and modern-day swamp coolers (a window AC unit with a fan that blows air over moist pads) use it, too. EC units use 75 percent less energy than traditional vapor compression air conditioners, but because they add humidity to the air, they’ve only been suitable in dry desert climates. Now, coldSNAP is changing that.

The device—a unit of terracotta heat exchangers paired with fans and water pumps—is the size of a window. The ceramic part resembles a scaled-down mail cubby, with about 30 rectangular channels carved through it. Every other of these channels has been coated with a novel water-repellent material, while the rest is simply naturally porous, uncoated terracotta. Fans bring fresh air from the outside through the coated, cold terracotta channels, which cools the air. What makes the terracotta cool is an engineering trick: coldSNAP redirects some of that freshly chilled air into the uncoated channels, where water runs over the terracotta and cools the ceramic as it evaporates—like perspiration evaporating to cool the skin. The team’s water-repellent coating permits this heat exchange but is impenetrable to water—preventing moisture from entering the cooled dry air directed into the building. Right now, coldSNAP fits in windows, but eventually, the team hopes to integrate the device into building facades.

“If we think about desert beetles or cacti, they’ve evolved strategies to deal with high temperatures in an interesting way—in particular, strategies that combine hydrophobic [water-repellent] and hydrophilic [water-absorbent] areas in the same organism,” Aizenberg said in an interview released by the Wyss Institute. Like the Namib Desert beetle, which uses hydrophilic portions on its wings to attract moisture from the air and hydrophobic areas to condense it to drinking water, coldSNAP has two different relationships to water. It separates the evaporation and cooling of “evaporative cooling” into different channels—producing comfortable air without the bothersome added humidity. Its design across scale—from the meter-scale ceramic unit down to the nano-scale barrier—means it can conduct the minute process of minimizing water diffusion while also cooling an entire room.

This summer the team tested the product at HouseZero during a brutal Boston heat wave. Even with the extreme humidity and high temperatures, preliminary data showed that coldSNAP cooled the third-floor room by more than 10 degrees Celsius without increasing the moisture content in the air. The researchers haven’t released specific energy usage data yet (because they plan to improve it even more), but so far, Grinham says coldSNAP is using “much, much less energy to do the equivalent cooling of an AC unit you’d have in your house. And all that, without refrigerants.”

The technology could be a step toward righting global cooling inequities. Research from the Harvard China Project found that, by 2050, cooling needs in poorer countries like Indonesia and India could rise to become as much as 75 percent of their current total annual electricity demand, placing serious strain on their electrical infrastructure. The coldSNAP system’s minimal energy use and lower operating cost could be an affordable way to reduce that grid strain and cool spaces in some of the world’s hottest climates.

Dryscreen delivers dehumidified comfort at a fraction of the environmental cost of traditional AC units.

Next summer, the researchers plan to install their other novel device at HouseZero, a dehumidifier called Dryscreen. Currently in Harvard laboratory testing, it’s a series of plates, each with a 20-micron-thick membrane sandwiched on either side. The membrane rejects oxygen and nitrogen molecules but permits the diffusion of water molecules. “We’re allowing air to come in, and we’re pulling moisture out,” Grinham explains. A proprietary vacuum technology then sucks away the moisture on the membrane, removing humidity from the air. He says that in humid climates like Miami, the hot outside air would often feel comfortable if it weren’t for the humidity. Dryscreen delivers that dehumidified comfort at a fraction of the environmental cost of a traditional AC unit.

The team plans to work with industry partners to bring coldSNAP and Dryscreen to market as separate technologies—ideally in the next five years. Eventually, Grinham hopes the two technologies could merge. “If we were to put Dryscreen outside to pull moisture from the air as it comes in, and then cool it through the evaporative cooling of coldSNAP—now that would be the ultimate evaporative cooling system.”

Published in the print edition of the November-December 2022 issue under the headline "Low-Energy, Low-Cost Cooling for the Future."

Read more articles by: Nancy Walecki

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