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Tech + EngineeringTech & Engineering

The Forecast on Energy-Efficient Cooling Just Got a Bit Sunnier

Scientists are pioneering technology that may someday cool buildings without electricity, while harvesting solar energy at the same time.

ByKatherine J. WuNOVA NextNOVA Next
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Engineers Shanhui Fan (left) and Zhen Chen (right) with their dual-action device, which simultaneously harvests solar energy and cools surfaces without the need for electricity. Photo Credit: Linda Cicero, Stanford News

As autumn gives way to winter in the northern hemisphere, it’s sometimes easy to forget that the planet is getting hotter. Air conditioners and electric fans account for about 10 percent of global electricity consumption today—and as average global temperatures continue to rise, the world’s reliance on air conditioners will likely more than triple by 2050.

This has created a bit of a vicious cycle. Homes in the United States alone expel over 100 million tons of carbon dioxide into the atmosphere each year from AC alone, nudging temps further upwards—and prompting people around the world to seek even more solace in temperature-controlled buildings.

But researchers are now developing tools that allow us to boot unwanted heat into outer space without using any of the power air conditioning requires. Today, in the journal Joule, a team from Stanford University takes the innovation one step further with a device that could not only keep us cool in the absence of air conditioning, but also simultaneously harness solar power to meet energy demands elsewhere in the grid.

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The cooling half of this new technology, pioneered by electrical engineer Shanhui Fan of Stanford University, takes advantage of the natural ability of all objects and organisms to emanate heat through radiation. Unlike conduction and convection—the other methods through which thermal energy is transferred—radiation doesn’t require direct contact or the movement of fluids to move heat from place to place: It simply emanates from all matter that’s at a temperature above absolute zero (or -459.67 degrees Fahrenheit… so, for all intents and purposes, let’s call it “all matter”). For example, radiation is responsible for the heat you feel wafting out from a toasty oven even when its door is shut.

Through radiation, heat can be jettisoned spaceward—and if it clears the hurdle of the atmosphere, then it’s past the point of no return: Outer space is pretty much a vast, insatiable heat sink. Unfortunately, though, outbound radiation of most wavelengths doesn’t always make it that far. Some of it ricochets off water molecules in the atmosphere and boomerangs right back to Earth; another portion in the is absorbed and retained in the atmosphere by carbon dioxide and other greenhouse gases.

But not all materials emit radiation at the same wavelength. If a surface emits radiation with a wavelength between eight and 13 micrometers, it hits a sweet spot that bypasses many of these atmospheric hurdles. By slipping through this mid-infrared “window,” heat can finally enter cold, cold space—allowing the radiating object to dip below the temperature of the surrounding air. Voilà: cooling achievement unlocked.

Fan and his colleagues first exploited this window and its associated chill factor in 2014, with the design of a so-called radiative cooler. Because the device was engineered with silicon compounds and other materials that emit in the mid-infrared range, it was able to remain around 5 degrees Celsius (or 9 degrees Fahrenheit) below ambient air temperatures while atop the roof of a building in sunny California. Two years later, Fan’s lab published a follow-up that used a similar setup to cool flowing water—a breakthrough that, if scaled up appropriately, could reduce the reliance of air conditioning and refrigeration on electricity.

For Fan, outer space is the ultimate untapped well of renewable energy. One of the most exciting potential applications of this technology involves speckling the world’s rooftops with radiative coolers. Unfortunately, this move could ruffle the feathers of those attempting to do the same with solar absorbers and solar cells, which typically colonize that same space to harvest energy from the Sun.

At first glance, the two strategies seem incompatible: one hungers for heat, while the other is keen on keeping cool: Placing conventional radiative coolers and solar panels side-by-side could even reduce the efficiency of both processes. But as humankind struggles to curb carbon emissions, it’s pretty clear we need all the help we can get. Together with lead author Zhen Chen, a mechanical engineer at the Southeast University of China, Fan assembled a team of researchers to engineer a peaceful coexistence between the two technologies.

“The solar energy that’s coming into Earth must eventually come out of Earth,” Fan explains. “So these technologies are complementary: Solar energy harvesting is trying to benefit from the incoming flux of energy, while radiative cooling is trying to benefit from outgoing flux.”

In fact, the spectrum of the Sun’s energy most relevant for a solar panel doesn’t actually overlap with the mid-infrared window—meaning that even if the two devices were stacked atop one another, they wouldn’t have to cancel each other out. To put this idea to the test, Chen, Fan, and their colleagues slid a silicon-based radiative cooler beneath a germanium solar absorber, which soaks in sunlight for the purposes of energy-efficient heating (not to be confused with a solar cell, which adds in the extra step of converting this energy into electricity). Unlike conventional solar absorbers, however, this particular panel is transparent to radiation in that mid-infrared range of eight to 13 micrometers. This allows the cooler to pass infrared heat through the solar absorber and eject it into space unimpeded.

Importantly, the two layers of this device operate completely independently of each other. The solar absorber hoards heat—but none of this energy needs to be funneled to the radiative cooler below, which, by definition, works power-free. This means that the solar payload collected above can, theoretically, be routed elsewhere.

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The researchers' double-layered rooftop panel. On the bottom is a radiative cooler that emits heat in the mid-infrared range into outer space. On top is a solar absorber that is transparent in the aforementioned infrared range. Photo Credit: Linda Cicero, Stanford News

While the two halves of this device aren’t interdependent, they can still benefit from each other’s company. Many conventional radiative coolers function best at night, out of the heat of the Sun; for efficient use in the daytime, the devices need highly reflective surfaces capable of deflecting light. While this keeps the temperature down, it also wastes what could otherwise be a serious kick of solar power. With the solar panel on top sponging up sunlight, though, the cooler doesn’t need this reflective boost.

Shielded by the solar panel, the temperature of the team’s radiative cooler plunged as low as 29 degrees Celsius (52 degrees Fahrenheit) below ambient temperature, while the absorber roasted at a pleasant 24 degrees Celsius (43 degrees Fahrenheit) above the surrounding air—an enormous differential. One immediate application, Chen says, has already revealed itself: The temperature gradient created by these two extremes could eventually be harnessed to power a thermal engine.

Even more exciting to Chen and Fan is the idea of replacing the solar absorber half of their device with an electricity-generating solar cell. If this switch pans out, the dual-action technology could serve as a supplement to our conventional, electricity-sapping cooling methods while simultaneously supplying a source of renewable energy.

“We’re trying to change how people view rooftops to harvest renewable energy,” Chen explains. “On hot summer days, you could continuously harvest electricity, and at the same time, you could use the device to cool down your house.”

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However, Chen doesn’t think radiative cooling will ever progress to the point of completely replacing air conditioning or refrigeration. Even the highest-efficiency coolers would probably need to be scaled up to cover entire rooftops or more—probably an untenable engineering ask, especially given the costs of the materials the team used in their prototype. The mid-infrared window also bangs shut on cloudy or rainy days, making this technology the most literal of fair-weather friends. But, Chen explains, radiative cooling could still have its day in the Sun (on a small scale, at least) in arid climes or regions with inconsistent access to power. The researchers are also looking into rebuilding their apparatus with more cost-effective materials.

In the meantime, there are still many technological roadblocks to clear. While the theory behind the methods is elegant, explains Emily Warmann, a mechanical engineer studying solar cells at the California Institute of Technology who did not participate in the study, “any practical implications are very, very far away.” Simple though they sound, the necessary tweaks to boost the device’s efficiency, or make the swap from solar absorber to solar cell, could very well compromise the panel’s critical infrared transparency.

Furthermore, the prototype’s radiative cooler is sealed up in a vacuum chamber to minimize unwanted heat loss through conduction and convection. To function as an air conditioning or refrigeration alternative, as in their previous work, it would need access to water—but the exposure to the surrounding air would slash its cooling capabilities back down to about 5 degrees Celsius (9 degrees Fahrenheit) below ambient temperature, as opposed to the 29 degrees Celsius (52 degrees Fahrenheit) achieved within the vacuum.

Despite these obstacles, Chen, Fan, and their colleagues have demonstrated something new and important, says Mark Nurge, a physicist at NASA’s Applied Physics Lab who did not contribute to the new finding. For the first time, it’s clear from the one-two punch of this device that these two sources of renewable energy don’t have to jostle for space—and with radiative cooling on our side, even the sky is no longer the limit.

“This is a creative idea,” says Gabriela Schlau-Cohen, a chemist studying light harvesting at the Massachusetts Institute of Technology who was not involved in the study. “If we want to actually address our growing energy needs, we need to do it in a way that’s paradigm shifting. And that means ideas that are different than incremental improvements on our current structure.”