Efficient Air-Conditioning Beams Heat Into Space

Radiative system could send heat from AC condensers out into space, reducing energy needed to cool buildings
Photo: Aaswath Raman

Air-conditioners work hard in hot weather, hogging energy. With a warming climate and more people across the world cranking up ACs, more efficient cooling systems are going to become critical to reduce energy use and greenhouse gas emissions. 

 Stanford researchers have developed a cooling system that could cut the energy used by conventional building air-conditioning systems by over 20 percent in the middle of summer.

 The Stanford team’s passive cooling system chills water by a few degrees with the help of radiative panels that absorb heat and beam it directly into outerspace. This requires minimal electricity and no water evaporation, saving both energy and water. The researchers want to use these fluid-cooling panels to cool off AC condensers.

 They first reported their passive radiative cooling idea in 2014. In the new work reported in Nature Energy, they’ve taken the next step with a practical system that chills water. They’ve also established a startup, SkyCool Systems, to commercialize the technology.

Radiative cooling relies on the fact that most objects release heat. “The sun heats up objects during the day, and at night the Earth’s surface or building roofs all radiate that back to the sky,” says Aaswath Raman, an applied physicist at Stanford. Problem is, radiative cooling doesn’t work during the day while the sun’s beating down on the Earth, or when the ambient air temperature is very high.

So Raman and electrical engineering professor Shanhui Fan made panels containing layers of silicon dioxide and hafnium oxide on top of a thin layer of silver. These radiate in a unique way: They send heat directly into space, bypassing the Earth’s atmosphere. The panels do this by emitting heat at infrared wavelengths between 8 and 13 micrometers. To these waves, the Earth’s atmosphere is transparent. What’s more, the panels reflect nearly all the sunlight falling on them.

For the new fluid-cooling system, the researchers made radiative panels that were each one-third of a square meter in area; they attached the panels to an aluminum heat exchanger plate with copper pipes embedded in it. The setup was enclosed in an acrylic box covered with a plastic sheet.

The team tested it on a rootop on the Stanford campus. Over three days of testing, they found that water temperatures went down by between 3- and 5 °C. The only electricity it requires is what’s needed to pump water through the copper pipes. Water that flowed more slowly was cooled more.

As a practical application for the system, the researchers built a model in which the radiative water-cooling panels cool the condenser coils of a building’s air-conditioning system, providing an assist to the system’s cooling fans. The circulating fluid helps siphon more heat from the condenser, increasing efficiency. Water that’s cooled by only a few degrees can make a big difference: In general, the electricity needed for a cooling system is reduced by 3 to 5 percent for every degree Celcius the condenser temperature drops. 

The model showed that cooling a two-story commercial office building in Las Vegas with fluid-cooling panels—which covered 60 percent of the roof—cut the building’s electricity use by 21 percent compared with using only a traditional fan-based condenser during the hot summer months of May through August.

New radiant cooling systems, which use chilled water running through aluminum panels or pipes, are getting more common in Europe and China and in high-efficiency buildings in the U.S., says Raman. “If we could couple our system with such radiant cooling systems, we could get 70 percent efficiency savings.”

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