A new ultrathin multilayered material can cool buildings without air conditioning by radiating warmth from inside the buildings into space while also reflecting sunlight to reduce incoming heat.Stanford engineers have invented a material designed to help cool buildings. The material reflects incoming sunlight, and it sends heat from inside the structure directly into space as infrared radiation (represented by reddish rays).
A team led by electrical engineering Professor Shanhui Fan and research associate Aaswath Raman reported this energy-saving breakthrough in the journal Nature.
The heart of the invention is an ultrathin, multilayered material that deals with light, both invisible and visible, in a new way.
Invisible light in the form of infrared radiation is one of the ways that all objects and living things throw off heat. When we stand in front of a closed oven without touching it, the heat we feel is infrared light. This invisible, heat-bearing light is what the Stanford invention shunts away from buildings and sends into space.
Of course, sunshine also warms buildings. The new material, in addition dealing with infrared light, is also a stunningly efficient mirror that reflects virtually all of the incoming sunlight that strikes it.
The result is what the Stanford team calls photonic radiative cooling – a one-two punch that offloads infrared heat from within a building while also reflecting the sunlight that would otherwise warm it up. The result is cooler buildings that require less air conditioning.
"This is very novel and an extraordinarily simple idea," said Eli Yablonovitch, a professor of engineering at the University of California, Berkeley, and a pioneer of photonics who directs the Center for Energy Efficient Electronics Science. "As a result of professor Fan's work, we can now [use radiative cooling], not only at night but counter-intuitively in the daytime as well."
The researchers say they designed the material to be cost-effective for large-scale deployment on building rooftops. Though still a young technology, they believe it could one day reduce demand for electricity. As much as 15 percent of the energy used in buildings in the United States is spent powering air conditioning systems.
In practice the researchers think the coating might be sprayed on a more solid material to make it suitable for withstanding the elements.
"This team has shown how to passively cool structures by simply radiating heat into the cold darkness of space," said Nobel Prize-winning physicist Burton Richter, professor emeritus at Stanford and former director of the research facility now called the SLAC National Accelerator Laboratory.
A warming world needs cooling technologies that don't require power, according to Raman, lead author of the Nature paper.
"Across the developing world, photonic radiative cooling makes off-grid cooling a possibility in rural regions, in addition to meeting skyrocketing demand for air conditioning in urban areas," he said.
Using a window into spaceThe real breakthrough is how the Stanford material radiates heat away from buildings.
Doctoral candidate Linxiao Zhu, Professor Shanhui Fan and research associate
Aaswath Raman are members of the team that invented the breakthrough energy-saving material.
As science students know, heat can be transferred in three ways: conduction, convection and radiation. Conduction transfers heat by touch. That's why you don't touch an oven pan without wearing a mitt. Convection transfers heat by movement of fluids or air. It's the warm rush of air when the oven is opened. Radiation transfers heat in the form of infrared light that emanates outward from objects, sight unseen.
"Think about it like having a window into space," said Fan.
Aiming the mirrorBut transmitting heat into space is not enough on its own.
This multilayered coating also acts as a highly efficient mirror, preventing 97 percent of sunlight from striking the building and heating it up.
"We've created something that's a radiator that also happens to be an excellent mirror," said Raman.
Together, the radiation and reflection make the photonic radiative cooler nearly 9 degrees Fahrenheit cooler than the surrounding air during the day.
From prototype to building panel
Making photonic radiative cooling practical requires solving at least two technical problems.
The first is how to conduct the heat inside the building to this exterior coating. Once it gets there, the coating can direct the heat into space, but engineers must first figure out how to efficiently deliver the building heat to the coating.
The second problem is production. Right now the Stanford team's prototype is the size of a personal pizza. Cooling buildings will require large panels. The researchers say there exist large-area fabrication facilities that can make their panels at the scales needed.
The cosmic fridgeMore broadly, the team sees this project as a first step toward using the cold of space as a resource. In the same way that sunlight provides a renewable source of solar energy, the cold universe supplies a nearly unlimited expanse to dump heat.
"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."
In addition to Fan, Raman and Zhu, this paper has two additional co-authors: Marc Abou Anoma, a master's student in mechanical engineering who has graduated; and Eden Rephaeli, a doctoral student in applied physics who has graduated.