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This past July was the hottest recorded month in human history. Heat waves smashed temperature records worldwide and even brought summer temperatures to Chile and Argentina during the Southern Hemisphere’s winter. It’s more than just a matter of sweaty discomfort. Severe heat is the deadliest of all weather events; in the U.S. alone, it kills more people each year than floods, tornadoes and hurricanes combined. As climate change worsens, access to artificially cooled spaces is rapidly becoming a health necessity—and an issue of basic human rights.
Yet standard air-conditioning systems have ensnared us in a negative feedback loop: the hotter it is, the more people crank the AC—and the more energy is used (and greenhouse gases are emitted) as a result. “We’re in a vicious cycle,” says Nicole Miranda, an engineer researching sustainable cooling at the University of Oxford. And “it’s not only a vicious cycle, but it’s an accelerating one.” Cooling is the fastest-growing single source of energy use in buildings, according to 2018 data from the International Energy Agency (IEA). Following a business-as-usual scenario, the IEA projects that worldwide annual energy demand from cooling will more than triple by 2050. That’s an increase of more than 4,000 terawatt-hours, which is about how much energy the entire U.S. uses in a year.
It’s becoming increasingly clear that humans cannot outrun climate change with the same air-conditioning technology we’ve been using for nearly a century. Breaking the cycle requires new innovations that will help bring cooler air to more people with less environmental impact.
One well-known problem with current AC systems is their reliance on refrigerant chemicals, many of which are potent greenhouse gases. Some projects aim to replace these substances with less-harmful coolants—but even if they do, the refrigerants make up only a fraction of air-conditioning’s climate toll. About 80 percent of a standard AC unit’s climate-warming emissions currently come from the energy used to power it, says Nihar Shah, director of the Global Cooling Efficiency Program at Lawrence Berkeley National Laboratory. A lot of recent work has gone into boosting the energy efficiency of compressors and heat exchangers, which are parts of standard AC designs, Shah explains. Yet more ambitious projects aim to reduce the amount of work those components must do in the first place.
Standard air-conditioning systems simultaneously cool and dehumidify through a relatively inefficient mechanism: in order to condense water out of the air, Shah says, they overcool that air past the point of comfort. Many new designs therefore separate the dehumidification and cooling processes, which avoids the need to overcool.
For example, some newer air conditioner designs pull moisture from the air with desiccant materials (similar to the silica gel in the packets you might find in a bag of jerky or a bottle of pills). The dried air can then be cooled to a more reasonable temperature. This process can require some additional energy because the desiccant needs to be “recharged” using heat. But some companies, including the Somerville, Mass.–based start-up Transaera, recycle the heat generated by the cooling process to recharge the desiccant. Transaera claims that the system it is developing could use 35 percent less energy than the average standard AC unit.
Even bigger efficiency gains are possible when dehumidification is paired with evaporative cooling, which takes the energy-intensive process called vapor compression out of the equation altogether. Vapor compression—the system by which standard AC works—moves a refrigerant through a cycle in which it is variably condensed and expanded, enabling it to absorb heat from inside and release that heat outside. Conversely, evaporative cooling is a simpler process. It’s the same one through which sweating cools our skin: as water goes from liquid to gas, it absorbs heat. Swamp coolers, DIY devices in which a fan blows air over ice, work the same way. And in dry climates, people have used evaporative cooling for thousands of years. In ancient Iran, for instance, people engineered yakhchāls—large, cone-shaped clay structures with solar chimneys—which harnessed air circulation and the evaporation of adjacent water to lower temperatures so much that they could make ice in winter and store it through summer.
But this strategy also increases air’s humidity, so as a cooling system, it tends to work only when the weather is hot and dry; if humidity rises beyond a certain point, it cancels out the comfort gains of reduced temperature. To solve this, research groups, including Harvard University’s cSNAP team, have designed AC devices that use a hydrophobic barrier to perform evaporative cooling while holding back humidity. As a bonus, refrigerants—which are often greenhouse gases that are many times more potent than carbon dioxide—aren’t involved at all. “We expect to provide a 75 percent more energy-efficient air conditioner,” says Jonathan Grinham, an assistant professor of architecture at Harvard and one of cSNAP’s lead designers.
Meanwhile Florida-based company Blue Frontier is trialing a commercial air-conditioning system based on both a desiccant (in this case, a liquid salt solution) and evaporative cooling. This design dries the air and then splits it into two adjacent streams, explains the company’s CEO, Daniel Betts. The air in one stream is directly cooled through the reintroduction of moisture and evaporation. The other airstream is kept dry, and it is cooled by being run across a thin aluminum wall that pulls in the cold—but not the humidity—from the first stream. The liquid salt desiccant then runs through a heat pump system to be recharged. To maximize efficiency, the heat pump can be run at night, when the power grid is least stressed, and the desiccant can then be stored for use in the hottest part of the day. Based on the company’s field trials, “we’re looking at 50 to 90 percent reductions in energy consumption,” Betts claims.
But Blue Frontier, cSNAP and Transaera have yet to go from testing to market. All three groups predict they’re at least a couple of years away from commercial launch. And even then, there will be obstacles that could prevent the new systems from replacing traditional ACs. These include relatively higher manufacturing and installation costs, industry inertia and policies that incentivize cheap systems over efficient ones.
Even with some of the best technologies available, the gains in efficiency alone might not be enough to offset the widely expected uptick in air-conditioning use. Under the best-case model, the IEA projects that cooling worldwide will require 50 percent more energy in the next 25 years than it does now because of rising demand, Shah says. It will not work to simply replace every existing air conditioner with a better model and call it a day. Instead a truly cooler future will have to employ other, passive strategies that rely on urban planning and building design to minimize the need for cooling in the first place. Bringing greenery and water bodies into cityscapes, shading windows, positioning new buildings to take advantage of natural airflow and retrofitting buildings with better insulation and reflective panels that can send heat into space are all critical, both Shah and Miranda say.
“Cooling is a multi-faceted challenge,” says Sneha Sachar, an energy efficiency expert at the nonprofit organization ClimateWorks. “There isn’t one strategy or one answer.” We need a combination of better buildings and cities, better technology and a better understanding that the true cost of air-conditioning extends beyond electric bills. “What we do in one part of the world impacts the whole global environment,” Sachar says.
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