They look like mirrors: 32 rectangles neatly arranged in eight rows on the rooftop of a supermarket called Grocery Outlet in Stockton, California. Shimmering beneath a bright sky, at first glance they could be solar panels, but the job of this rig is quite different. It keeps the store from overheating.
Tilted toward the sun, the panels absorb almost none of the warmth beating down on them; they even launch some into space, improving the performance of the systems that keep things inside cold. The feat relies on a phenomenon called radiative cooling: Everything on Earth emits heat in the form of invisible infrared rays that rise skyward. At night, in the absence of mercury-raising daylight, this can chill something enough to produce ice. When your car’s windshield frosts over, even if the thermometer hasn’t dipped below freezing? That’s radiative cooling in action.
To Aaswath Raman, who was a key mind behind Grocery Outlet’s shiny tiles, that effect seemed like an opportunity. “Your skin, your roof, the ground, all of them are cooling by sending their heat up to the sky,” he says.
Raman, a materials science and engineering professor at the University of California at Los Angeles, is the co-founder of SkyCool Systems, a startup trying to flip the script on the technology we depend on to create chill. As the world warms, demand for air conditioning and refrigeration is going up. But these systems themselves expel a tremendous amount of heat, and the chemical compounds they use can escape skyward, where they act as a planet-warming greenhouse gas. According to the Birmingham Energy Institute in the UK, these substances and the power involved accounted for at least 11 percent of global greenhouse gas emissions in 2018. By 2050, more than 4.5 billion air conditioners and 1.6 billion refrigerators are projected to consume nearly 40 percent of all electricity. If it goes mainstream, SkyCool’s tech—and similar approaches in the works from competitors and other researchers—could slow the cycle by naturally lowering building temperatures and easing the energy burden on conventional methods.
After Grocery Outlet put the panels on the roof of the 25,000-square-foot building in late 2019, energy use by the store’s refrigeration system dropped by 15 percent. That amounts to almost $6,000 in savings per year.
It’s hard to say if the installation has grabbed the infrastructural upgrade brass ring and paid for itself. Lime Energy, a national retrofitter specializing in upgrades to boost efficiency, financed the supermarket’s setup costs, which made the panels affordable. To work on a massive scale, though, radiative cooling needs to be cheap to manufacture and install. Make that happen, and it could be one way to conserve power and reduce emissions. “I was somewhat skeptical that you could gain this significant amount of cooling even under direct sun,” says Chris Atkinson, a former program director of the Advanced Research Projects Agency–Energy (ARPA-E), a division of the US Department of Energy (DOE) that funded Raman’s early research. “But once it was explained to me, it sounded plausible—and the results are remarkably compelling.”
CENTURIES AGO, desert-dwelling peoples exploited radiative cooling to make ice. In the evenings, they insulated the walls of large bowls or pits, then poured in water. During the pitch-black night, heat escaped the liquid, and by morning, it was frozen solid.
Architects and physicists were long skeptical that the effect could ever work in daylight. In the 1970s and ’80s, they made various attempts to apply it to buildings using pools of water on rooftops. But the structures were difficult to maintain and still absorbed too much of the sun’s warmth.
Raman’s own interest in the technique took hold in 2012 while he was finishing his doctorate in applied physics at Stanford University. He was fascinated with how materials interact with light and thought about pursuing a career in solar energy. Then he happened upon research discussing radiative cooling and became fixated on whether the effect could ever happen under direct sunlight.
“What’s happening at night is you are losing heat to the sky, and the sky is letting some of that go to space,” he says. “During the day, you want to continue doing that, but at the same time you want to avoid absorbing the sun’s energy.”
Luckily, Raman had nanotechnology at his disposal—a discipline that designs and produces materials through arranging molecules and atoms to behave exactly as needed.
Under the guidance of Shanhui Fan, an applied physics and electrical engineering professor at Stanford, and with a small team from the engineering department, Raman developed the material that now forms the basis of SkyCool Systems. (ARPAE helped with a $300,000 grant; later, the agency awarded the team some $2.5 million in additional funding.) In the labs, he had access to a variety of tools: physical vapor deposition machines, used for producing ultrathin multilayered coatings; scanning electron microscopes to determine the thickness of the layers; and a variety of spectrophotometers, which measured the ultraviolet and infrared properties of the substances.
In less than a year, they created a thin film composed of seven microscopic layers atop a sliver of silver. The slices alternated between hafnium oxide, an inorganic compound that acts as an electrical insulator, and silicon dioxide, or silica, a natural material that makes up quartz, sand, and nearly two-thirds of Earth’s crust.
Acting together, the substances enable a special set of optical properties. For starters, they’re especially good at emitting infrared light. Greenhouse gases and water molecules in the atmosphere usually absorb most of these rays and send them back to Earth. Infrared between 8 and 13 micrometers in wavelength, however, isn’t absorbed by the atmosphere and instead slips into space, so Raman tuned the film to radiate only within that narrow range. What’s more, the material reflects 97 percent of the sun’s beams, enough to generate a cooling effect during the day. “It’s actually sending more heat out to the sky than the sky as a whole sends back,” says Raman.
[Related: How to stay cool without blasting the AC.]
Around 2013, Raman began testing his specialized reflector in the real world. That summer, he set up a small array of panels atop the university’s electrical engineering building. One morning, while checking instruments measuring radiation and reflectivity, Raman placed his hand on one of the sheets baking in the sunshine. It felt cold.
“That was immediately pretty exciting,” Raman allows. In fact, the material was about 9°F cooler than the ambient air temperature, which was well above 80°F, a result he subsequently published in Nature.
Around that same time, mutual friends connected him with Eli Goldstein, who was at Stanford finishing a doctorate in mechanical engineering. The men spent the next two years giving themselves a practical education in air conditioning and refrigeration, talking to manufacturers as well as their customers. In 2016, the pair founded SkyCool Systems—its name a nod to night sky cooling, another term for the physics phenomenon—and set out to commercialize their tech.
ON A TRIP TO MUMBAI to visit his grandmother, Raman had glimpsed just how influential the new film might be. More homes than he remembered from childhood had AC units installed in their windows.
While only an estimated 15 percent of the world’s population—mostly in the US, Japan, Korea, and China—has air conditioning, its use is precipitously expanding. According to 2015 projections, sales in countries like Brazil and Indonesia are increasing by upwards of 15 percent every year, although India is estimated to be the fastest-growing sector: When Raman was testing his film, residents owned more than 20 million air conditioners; by 2020, it was 48 million. The International Energy Agency (IEA), a Paris-based group that makes policy recommendations to national governments on sustainable energy solutions, projects that Indians will own more than 1 billion units by 2050. It’s a victory for public health—in 2015, more than 2,300 people in the country died in a crippling heat wave—but a harbinger of climate consequences.
Meanwhile, the basics of cooling technology are about the same as when the first electric air-conditioning unit was designed in 1902. AC systems pump a refrigerant—the chemical compound that moves heat and, in turn, causes cooling—through a mechanical system that forces it through several phase changes. The refrigerant funnels indoors through a coil as a liquid, where it turns into a vapor as it absorbs heat. It exits the building and enters a condenser, where the refrigerant is compressed and expels heat to the outside as it turns from vapor back into liquid. That process repeats until the inside is at the temperature set by the thermostat.
The fossil fuels that power this choreography and the chemicals required, however, are major contributors to global warming. In 2016, the DOE reported that stationary AC systems accounted for nearly 700 million metric tons of greenhouse gas emissions globally every year. The pollution comes from the combustion to produce the power that runs the units and the hydrofluorocarbons used as refrigerants, which are prone to escaping during repairs or retirement. Their effect is “several thousand times higher than that of carbon dioxide,” the report concluded.
In broad strokes, the world is simply starting to use too much air conditioning too quickly—and yet it must, as temperatures rise. This is leading, as the IEA put it in 2018, to an impending “cold crunch.” By midcentury, the juice required to run ACs will become one of the top drivers of worldwide electricity demand, helping to push the planet beyond the point of irrevocable ecological damage. Indeed, our increasing need to lower indoor temperatures is speeding us toward the 1.5°C warming threshold established in 2018 by the United Nations Intergovernmental Panel on Climate Change.
“Temperatures are getting hotter, and that’s inherently going to mean that air conditioners and refrigeration systems become less efficient,” says SkyCool’s Goldstein.
Eventually, he and Raman realized their panels could have a greater impact on energy usage if they augmented existing climate-control systems. Around 2016, the team ran another trial at Stanford; this time, it set up a rig with thin water pipes running directly underneath. Over three days, radiative cooling lowered the temperature of the water by upwards of 9°F. It wouldn’t be hard to connect the pipes to the condenser of a conventional AC or refrigeration setup, where the superchilled water would help to chill the refrigerant, reducing the overall energy load. One of their models also showed that integrating the technique into a two-story office building in Las Vegas would lower electricity demand by 21 percent during the summer.
Raman and Goldstein decided to launch the business with locations that use refrigeration systems, since—unlike AC—those need to run every hour of every day. Their estimates indicated that electricity savings for cooling infrastructure that runs constantly are greater than 500 kilowatt-hours per year per square meter of SkyCool film. From 2017 through 2019, the company signed up several California customers interested in trying out anything that might lower their bills: a convenience store, a data center, and Grocery Outlet. The 32 panes atop that market cover 62 square meters; data collected so far shows the store is using 100 fewer kilowatt-hours per day, or 36,500 fewer kilowatt-hours every year.
There is a limit, however, to the overall effectiveness of radiative cooling. The best climates in which to deploy the technology are relatively dry with clear skies: California, Arizona, Nevada, and the like. Cloud cover and high humidity reduce the effect during the day, as water molecules in the air trap some of the emitted infrared.
SkyCool is in discussions with the California State University system to use its tech to chill water that will be piped through the ceilings of three classrooms at Cal Maritime. (Goldstein hopes the project will launch in 2022.) But in dry climates, it might be enough to use the panels by themselves. “Imagine instead of having to buy an air conditioner in a small house in India or Africa, you could just put this on the roof,” says Goldstein.
BECAUSE OF THE PROMISE of radiative cooling, other startups have rushed into the field. Engineers from the University of Colorado, Boulder and the University of Wyoming teamed up to create their own film-like material in 2017. Engineers at the University of Buffalo published research in February 2021 on their own version: two mirrors composed of 10 thin layers of silver and silicon dioxide. They’re now trying to bring it to market through their company, Sunny Clean Water.
The big question is how likely people are to implement a brand-new product. “The technology makes sense,” says Jeremy Munday, a University of California, Davis professor who studies clean-energy innovations. “It really comes down to things like the market, the cost, and then just having the motivation to adopt it.”
Raman and Goldstein aren’t disclosing their pricing, but they admit that SkyCool’s future challenges will be on the manufacturing—not the scientific—side of things. A 2015 study by the Pacific Northwest National Laboratory, part of the DOE, estimated that if rooftop materials like SkyCool’s could be built and installed for less than $6.25 a square meter, the costs would be covered by energy savings over five years.
The pair think they can hit a worthwhile price inside three years, in part because they’ve further refined the film they originally tested at Stanford. These days, the precise makeup is proprietary, although it still contains a mix of polymers and inorganic materials. “We’ve figured out ways to do it that are lower cost and better suited to manufacturing,” Raman says.
With the help of a $3.5 million federal energy grant, SkyCool soon hopes to have the sort of connections that could make its film cost-effective. The startup is collaborating with the 3M Company to devise an affordable means of making hundreds of thousands of its films. The goal is to drive down the price enough by 2023 that customers with persistent cooling needs can recoup installation costs in three to five years.
On top of those challenges, other researchers say they can get the same result with paint. The white version tried decades ago didn’t reflect enough rays to create a cooling effect. In 2020, though, Purdue University engineers created an ultrawhite variety that works like SkyCool’s mirrorlike material. According to Xiulin Ruan, a professor of mechanical engineering involved in its development, the product reflects 98.1 percent of sunlight and radiates infrared at the right wavelength to escape into space—cooling buildings midday to 8°F below the ambient temperature.
Ruan admits that the paint is more a supplemental measure. “You still need to turn on air conditioners, but it can offset a lot of the heat from the sun and reduce demand,” he says. In that sense, it’s missing one component of SkyCool: the ability to connect to existing systems and boost their performance. Still, paints have caught Raman’s eye. Last year he co-published an article in the journal Joule discussing the possibility of modifying off-the-shelf paints so they too carry out radiative cooling.
If any of these methods do catch on, no one seems too concerned about sending heat into the final frontier. “If all that energy was emitted back into space, it would not have any noticeable effects anywhere at all,” says Atkinson, the former DOE official who backed Raman’s early research.
For now, SkyCool is trying to win over more businesses. Soon it plans to deploy its panels in office buildings to augment commercial AC. In March, a big-box retailer in Southern California became the latest customer. On the roof, five full rows of the creaseless, mirrorlike films sit between two columns of solar panels—a fitting juxtaposition, considering Raman’s prior interests. Now he wants to cut energy, not produce it.
“All you have to do,” he says, “is put the material outside, and it stays cool.”
Correction: July 21, 2021. An earlier version of this story incorrectly stated the temperature change in early tests of the radiative cooling film as 40 degrees F, and put an end date on SkyCool’s relationship with 3M when their collaboration is ongoing.