Breakthroughs

An air conditioner powered by outer space and help from the sun

Linda Cicero, Stanford News
Engineers Shanhui Fan (left) and Zhen Chen with their dual-action device, which simultaneously harvests solar energy and cools surfaces without the need for electricity.

A team of Stanford University electrical engineers has developed a new device that could cool buildings while simultaneously generating electricity. If the prototype proves scalable, the technology could represent another possible pathway for reducing the footprint of the built environment, which accounts for almost 40 percent (PDF) of the carbon emissions of the United States alone.

Air conditioning and climate change interact in a vicious cycle: Cooling buildings thus far has relied on processes and equipment that produce significant greenhouse gas emissions — and that are projected to increase dramatically in the coming years. As the planet warms, demand for air conditioning increases, thereby driving emissions ever higher.

While the 1987 Montreal Protocol was effective in reducing emissions of chlorofluorocarbons — the inexpensive, ozone-destroying chemical refrigerants that originally underpinned air-conditioning technology — the hydrofluorocarbons (HFCs) that replaced them also contribute to global warming in their own right. Indeed, HFCs’ warming effect is 2,100 times that of carbon dioxide: They are one of the so-called super pollutants.

Meanwhile, our appetite for clean electricity continues to grow as the global population swells and more segments of the economy are electrified.

One possible solution

What to do about this conundrum? Why not address them together? In the simplest sense, that's what the device created by the Stanford scientists promises to do: the idea is to collect heat from the sun and cold from outer space at the same time — employing solar heating in tandem with a process known as radiative cooling in one integrated piece of equipment.

The solar heating element is not a new technology; it employs the same semiconductor materials in use in other rooftop solar applications. However, the researchers broke new ground in their integrated use of radiative cooling, a process by which objects shed heat by radiating infrared light.

"It is widely recognized that the sun is a perfect heat source nature offers human beings on Earth," said Zhen Chen, one of the engineers. "It is less widely recognized that nature also offers human beings outer space as a perfect heat sink."

How does radiative cooling work? Consider that human bodies are some of the objects that give off heat in the form of infrared radiation. We bundle up in the winter so as to retain that heat. The Earth’s atmosphere functions similarly to coats and hats, trapping heat like a thick blanket near the planet. A small fraction of emissions in the mid-infrared range slips through what are essentially holes in the blanket — and is jettisoned into the cold expanses of outer space. The surface temperature of the heat-emitting objects then drops below that of the surrounding air.

This phenomenon explains why frost forms on windshields overnight, for example, even when ambient temperatures haven’t reached freezing, according to the MIT Technology Review. Daytime heat from the sun typically tends to offset the cooling effect; that reality so far has hampered efforts to harness radiative cooling for beneficial applications. In an illuminating April TED talk, physicist Aaswath Raman explains the science in accessible detail.

That's where the new technology being tested at Stanford comes in. To mitigate those canceling effects of the daytime sun on radiative cooling, the top layer of the Stanford engineers’ device is designed to absorb solar energy. The team confirmed during testing that the device’s top layer was consistently hotter than the ambient air. Meanwhile, because the radiative cooling element of the device was engineered with silicon compounds and other materials that emit in the mid-infrared range, that bottom layer remained around 9 degrees Fahrenheit below ambient air temperatures on a sunny California roof.

"We’ve built the first device that one day could make energy and save energy, in the same place and at the same time, by controlling two very different properties of light," said Shanhui Fan, senior author of an article in Joule detailing the team’s research.

The team still has considerable work to do before their invention can be scaled up for commercial use. For instance, they were not able to test whether the device actually produced electricity, as they lacked some components that would have allowed such functionality. What's more, some materials the team used are also prohibitively costly, so they are investigating alternatives. For example, the team thinks the expensive zinc selenium window on the prototype's vacuum chamber could be replaced with a much cheaper silicon or germanium window without sacrificing performance.

The team feels confident their research demonstrates that renewable energy has even more rooftop potential than previously understood, not just for generating electricity but for thermal applications. "We’re trying to change how people view rooftops to harvest renewable energy," Chen explained. "On hot summer days, you could continuously harvest electricity, and at the same time, you could use the device to cool down your house."

Some experts expect that "any practical implications are very, very far away," but say the Stanford team has accomplished something new and important by showing that cooling technologies and solar panels don’t necessarily have to compete for finite rooftop space and actually can be complementary.

If they’re right, their research could reframe how air conditioners and cooling systems are designed, and could prompt further exploration of how solar energy can power other applications.

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