Coating Boomerangs Heat into Space
WEDNESDAY, DECEMBER 3, 2014
It sounds like a comic book for coating junkies, in which Our Hero zaps accumulated building heat into space without warming the atmosphere, then casts a shiny shield to protect the building from sunlight.
But this is no Marvel fiction: A high-tech coated mirror that can beam heat away from buildings and into space is, in fact, in the works at Stanford University.
Stanford's engineers say their new coating material "can help cool buildings, even on sunny days, by radiating heat away from the buildings and sending it directly into space."
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| Stanford University / Norbert von der Groeben |
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The team behind Stanford's energy-saving coating includes (from left) doctoral candidate Linxiao Zhu, Professor Shanhui Fan and research associate Aaswath Raman. |
The system involves a rooftop apparatus and a technology called photonic radiative cooling—a double-whammy energy saver that not only "offloads infrared heat from within a building" but also reflects sunlight that would heat up the building further.
"The result is cooler buildings that require less air conditioning," the university said in a research announcement.
Cutting Cooling Costs
And reducing the world's air-conditioning bill is no small thing, the team says.
Air conditioning accounts for nearly 15 percent of the primary energy used by buildings in the United States alone, the group reports in "Passive radiative cooling below ambient air temperature under direct sunlight," published Nov. 27 in the journal Nature.
"Cooling is a significant end-use of energy globally and a major driver of peak electricity demand. ... A passive cooling strategy that cools without any electricity input could therefore have a significant impact on global energy consumption."
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| Stanford University / Fan Lab |
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The rooftop cooler, with an ultra-thin eight-layer coating, reflects incoming sunlight and sends heat from inside the structure directly into space as infrared radiation (represented by the reddish rays). |
Passive cooling has worked at night, but the team led by Stanford's Shanhui Fan is the first to demonstrate it during the day under direct sunlight.
How Cool is It?
The group's experiments showed cooling of nearly five degrees Celsius (about nine degrees Fahrenheit) below the ambient air temperature under direct sunlight.
The team says the liquid coating could be spray-applied to a more solid material to make it better able to withstand the elements. The material was designed to be cost-effective for widespread rooftop use.
"These results demonstrate that a tailored, photonic approach can fundamentally enable new technological possibilities for energy efficiency," the team writes.
"Further, the cold darkness of the Universe can be used as a renewable thermodynamic resource, even during the hottest hours of the day."
'A Window into Space''
As Stanford explains it, the coating was constructed to radiate infrared light away from buildings "at the precise frequency that allows it to pass through the atmosphere without warming the air"—a concept that Fan likens to "having a window into space."
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| ©iStock.com / 4FR |
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The passive cooling coating technology could help relieve the drain on America's air conditioning, researchers say. Air conditioning accounts for nearly 15 percent of the primary energy used by U.S. buildings.   |
In addition, the eight-layer coating acts as a massive and highly efficient mirror, reflecting 97 percent of sunlight away from the building. For all of that muscle, however, the coating is only 1.8 microns thick—"thinner than the thinnest aluminum foil," Stanford reports.
Problems and Pizzas
The technology still faces challenges on the road to commercialization. First, how to conduct the heat inside the building to the exterior coating?
And second, how to scale up production? The team's prototype is "the size of a personal pizza." Widespread deployment will demand huge panels.
Still, the team is optimistic.
"Every object that produces heat has to dump that heat into a heat sink," Fan said.
"What we've done is to create a way that should allow us to use the coldness of the universe as a heat sink during the day."
Tagged categories: Architectural coatings; Coating Materials; Commercial Buildings; Energy efficiency; Reflective coatings; Research; Solar reflectance
