Tech
Nanostructured Photonic Materials Revolutionize Cooling by Reflecting Heat Back to Space
Mark Hoffman
First Posted: Apr 15, 2013 03:26 PM EDT
Scientists developed nanostructured photonic materials that could vastly improve the daylight cooling of buildings, cars, and other structures by reflecting sunlight directly back into the chilly vacuum of space. A paper describing the device was now published in Nano Letters.
“People usually see space as a source of heat from the sun, but away from the sun outer space is really a cold, cold place,” explains Shanhui Fan, a professor of electrical engineering at Stanford University and the paper’s senior author. “We’ve developed a new type of structure that reflects the vast majority of sunlight, while at the same time it sends heat into that coldness, which cools manmade structures even in the daytime.”
The trick, from an engineering standpoint, is twofold, he explains. Apart from reflect as much incoming sunlight as possible, the structure must efficiently radiate heat (from a building, for example) back into space. Thus, the structure must emit thermal radiation very efficiently within a specific wavelength range in which the atmosphere is nearly transparent.
The new structure accomplishes both goals. It is an effective broadband mirror for solar light—it reflects most of the sunlight. It also emits thermal radiation very efficiently within the crucial wavelength range needed to escape Earth’s atmosphere.
The researchers succeeded by turning to nanostructured photonic materials, which can be engineered to enhance or suppress light reflection in certain wavelengths. The material is made of quartz and silicon carbide, both very weak absorbers of sunlight.
“We’ve taken a very different approach compared to previous efforts in this field,” says Aaswath Raman, a doctoral candidate in Fan’s lab and a co-first-author of the paper. “We combine the thermal emitter and solar reflector into one device, making it both higher performance and much more robust and practically relevant. In particular, we’re very excited because this design makes viable both industrial-scale and off-grid applications.”
The new device is capable of achieving a net cooling power in excess of 100 watts per square meter. To put it a different way, a typical one-story, single-family house with just 10 percent of its roof covered by radiative cooling panels could offset 35 percent its entire air conditioning needs during the hottest hours of the summer.
Radiative cooling has another profound advantage over other cooling equipment, such as air conditioners. It is a passive technology. It requires no energy. It has no moving parts. It is easy to maintain.
Beyond the commercial implications, the researchers foresee a broad potential social impact. Much of the human population on Earth lives in sun-drenched regions huddled around the equator.
Electrical demand to drive air conditioners is skyrocketing in these places, presenting an economic and environmental challenge. These areas tend to be poor and the power necessary to drive cooling usually means fossil-fuel power plants that compound the greenhouse gas problem.
“In addition to these regions, we can foresee applications for radiative cooling in off-the-grid areas of the developing world where air conditioning is not even possible at this time. There are large numbers of people who could benefit from such systems,” Fan says.
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First Posted: Apr 15, 2013 03:26 PM EDT
Scientists developed nanostructured photonic materials that could vastly improve the daylight cooling of buildings, cars, and other structures by reflecting sunlight directly back into the chilly vacuum of space. A paper describing the device was now published in Nano Letters.
“People usually see space as a source of heat from the sun, but away from the sun outer space is really a cold, cold place,” explains Shanhui Fan, a professor of electrical engineering at Stanford University and the paper’s senior author. “We’ve developed a new type of structure that reflects the vast majority of sunlight, while at the same time it sends heat into that coldness, which cools manmade structures even in the daytime.”
The trick, from an engineering standpoint, is twofold, he explains. Apart from reflect as much incoming sunlight as possible, the structure must efficiently radiate heat (from a building, for example) back into space. Thus, the structure must emit thermal radiation very efficiently within a specific wavelength range in which the atmosphere is nearly transparent.
The new structure accomplishes both goals. It is an effective broadband mirror for solar light—it reflects most of the sunlight. It also emits thermal radiation very efficiently within the crucial wavelength range needed to escape Earth’s atmosphere.
The researchers succeeded by turning to nanostructured photonic materials, which can be engineered to enhance or suppress light reflection in certain wavelengths. The material is made of quartz and silicon carbide, both very weak absorbers of sunlight.
“We’ve taken a very different approach compared to previous efforts in this field,” says Aaswath Raman, a doctoral candidate in Fan’s lab and a co-first-author of the paper. “We combine the thermal emitter and solar reflector into one device, making it both higher performance and much more robust and practically relevant. In particular, we’re very excited because this design makes viable both industrial-scale and off-grid applications.”
The new device is capable of achieving a net cooling power in excess of 100 watts per square meter. To put it a different way, a typical one-story, single-family house with just 10 percent of its roof covered by radiative cooling panels could offset 35 percent its entire air conditioning needs during the hottest hours of the summer.
Radiative cooling has another profound advantage over other cooling equipment, such as air conditioners. It is a passive technology. It requires no energy. It has no moving parts. It is easy to maintain.
Beyond the commercial implications, the researchers foresee a broad potential social impact. Much of the human population on Earth lives in sun-drenched regions huddled around the equator.
Electrical demand to drive air conditioners is skyrocketing in these places, presenting an economic and environmental challenge. These areas tend to be poor and the power necessary to drive cooling usually means fossil-fuel power plants that compound the greenhouse gas problem.
“In addition to these regions, we can foresee applications for radiative cooling in off-the-grid areas of the developing world where air conditioning is not even possible at this time. There are large numbers of people who could benefit from such systems,” Fan says.
See Now: NASA's Juno Spacecraft's Rendezvous With Jupiter's Mammoth Cyclone