Using outer space to help cool buildings on Earth

Using a phenomenon known as radiative sky cooling, a team of Stanford researchers has developed rooftop panels that could be used to passively cool buildings.

Norbert von der Groeben
This photo from 2014 shows the reflectivity of the mirror-like optical surface that Stanford University researchers have been studying, which allows for daytime radiative sky cooling by sending thermal energy into the sky while also blocking sunlight. The people in this photo (left to right) are Linxiano Zhu, Shanhui Fan, and Aaswath Raman.

Researchers may have found a way to make refrigerators and air conditioners more efficient: Just shoot the heat into space.

Using a natural optical phenomenon called radiative sky cooling, a group of scientists-turned-entrepreneurs has developed roof panels that they say could reduce the energy needed to cool homes, offices, supermarkets, and data centers. Elegant in its passivity and its simplicity, their new application may represent a considerable, if incremental, step toward rethinking how we build our homes and places of business. And it could even provide clues about how to make existing structures more efficient.

“I think the elegance of it appealed to me,” says Aaswath Raman, a research associate at Stanford University in California and co-author of a paper published in the journal Nature Energy last month that describes the radiative cooling panels. “It’s to make something that’s a very good non-absorber of sunlight and, at the same time, a very good emitter of heat away.”

Most of the heat radiated by objects on Earth is absorbed by greenhouse gases in the atmosphere and re-emitted back to the surface. But Earth’s greenhouse effect isn’t a perfect seal. Some infrared wavelengths of radiation can slip through the atmosphere and pass into space. That heat loss produces an effect known as radiative sky cooling.

For the cooling effect to work, an object must face the sky directly – a physical obstruction, such as a tree or building, will absorb and re-emit any thermal energy that is released. Under certain conditions, the cooling phenomenon can bring an object’s temperature below that of the surrounding air. That’s why frost can form on cloudless nights, even when ambient temperatures stay above freezing. To harness this natural quirk is to use deep space as a kind of heat sink.

The panels developed by Dr. Raman and his colleagues exploit this phenomenon, cooling water to temperatures below that of the ambient air, no electricity needed. The water could then be circulated through the building to cool its interior.

Sophisticated passive cooling technologies have existed for millennia. By 400 BC, engineers in present-day Iran had mastered the art of building yakhchāls, or ice pits. By running water along the inside of clay domes – an early evaporative cooling system – one could keep the pit cool enough to store ice in the sweltering summer months.

Passive cooling has its limitations, however. On a clear day, the sun’s heat will usually offset any cooling effect.

“Some architects started looking at it in the ’60s and ’70s, with the main caveat being that it only worked at night,” says Raman. “It never really took off as a technology, because it became rather complex to use something that only worked at night to deliver cooling during the day.”

In 2014, Mr. Raman and his Stanford colleagues found a workaround. With $3 million in funding from the US Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E) program, they developed highly reflective panels that could harness the sky cooling effect while blocking up to 97 percent of solar light. Placed on a roof under direct sunlight, their panels stayed 8.8 degrees F. cooler than the surrounding air – a cooling power of 40.1 watts per square meter. Last year, three of those researchers founded SkyCool Systems, of which Raman is chief executive officer.

Researchers generally agree that radiative cooling technologies such as the one developed by SkyCool could translate to significant long-term energy savings in new buildings. But retrofitting still presents a considerable challenge: In a 2015 analysis by the Pacific Northwest National Laboratory in Richland, Wash., researchers found that material and installation costs made SkyCool’s technology unsuitable for existing structures.

“The use of radiative cooling technology requires completely different types of building components,” says Srinivas Katipamula, who co-authored the analysis. “Only when existing buildings are completely torn down for a retrofit is it possible to use radiative cooling technologies. We have to look at existing buildings if we are to make a big dent in reducing energy consumption, because much of the consumption is associated with those buildings.”

To that end, Raman’s company has shifted focus toward a new mode of integration: solar water coolers. Such a system would use the same radiative panels to cool fluid in a closed loop. In recent field trials, his team successfully retrofitted the condenser component of off-the-shelf commercial refrigeration and air conditioning systems.

“In this particular trial, we’re targeting something like a 10 to 20 percent improvement in efficiency on an annual basis,” says Raman. “For something like refrigeration, that’s actually a big deal. If you think about supermarkets and other cold storage facilities, keeping things quite cold or even frozen is an expensive proposition.”

With potential savings like that, radiative cooling could represent a step forward in a growing “zero-energy design” movement, which advocates building structures that produce at least as much energy as they consume.

The movement has grown steadily in the last decade. Earlier this year, Austin, Texas, unveiled a new housing development of 7,500 net-zero-energy homes – the largest in the country. That growth can be partially attributed to trends in solar energy, says Nathan Johnson, an assistant professor at Arizona State University who studies energy markets and renewables.

“Solar prices have dropped significantly, and subsidies are set to remain until at least 2020, so the incentives to install distributed solar are strong,” says Professor Johnson. “The market has grown sufficiently that standard business practices in an open market will take things forward.”

Residential and commercial buildings accounted for about 40 percent of the country’s total energy consumption in 2016, according to the US Energy Information Administration. Of that, 6 to 8 percent was used specifically for cooling, while at the same time creating emissions that contribute to global warming.

But cutting-edge technology can’t always replace good engineering, says L.D. Danny Harvey, a University of Toronto climatology professor and author of “A Handbook on Low-Energy Buildings and District-Energy Systems.” By reducing the need for cooling in the first place, developers can make simple choices to improve efficiency in the long-run.

“You minimize solar heat gain by minimizing the window areas facing west, and you use windows that permit very little solar heat to get in,” says Professor Harvey. “If it’s west-facing, you have adjustable external blinds. If it’s south-facing, you have an overhead which will shade it in the summer and let in sunlight in the winter. You do sensible things like that to reduce the need for both heating and cooling.”

Existing buildings can be improved through window glazing and double-skin facades, which reduce solar penetration and improve ventilation. It’s a scaled-back approach, Harvey admits, and it won’t make an old building energy neutral. But since more than half of the US housing stock was built before 1980, according to a report by the National Association of Home Builders, even small gains in efficiency could add up.

“We’re going to have to retrofit the entire building stock of all the countries in the world,” says Harvey. “Maybe you can renovate 2 percent a year – you’re talking a 40-50 year period, so that takes us to 2060 or so. But at the same time we’re converting the electricity grid entirely to renewable energy. And we’re implementing ever-more stringent standards for new buildings, and we’re making more and more of the new buildings net-zero or close to it.”

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