Passive heating based on harnessing solar rays is a common strategy used to warm structures in winter, but what about passive cooling? This seemingly contradictory term has been used in reference to removing heat via ventilation or slowing down heat transfer—not directly cooling, per se, which has seemed like an insurmountable challenge.
A team of researchers at Stanford University has found the secret to passive solar cooling, however, based on the use of nanostructured photonic materials. The new radiative cooling technology takes advantage of these materials' ability to limit the absorption of heat while harvesting energy from sunlight. Moreover, these materials are tuned to reflect the specific solar wavelength capable of escaping the Earth's atmosphere back into space—thus avoiding the greenhouse effect.
“No one had yet been able to surmount the challenges of daytime radiative cooling—of cooling when the sun is shining,” said PhD candidate Eden Rephaeli in a university press release. “It’s a big hurdle.”
In their prototype solar cooling panel, the scientists combined a solar reflector and thermal emitter in a single device, which they found to increase both performance and durability. The technology is already competitive with conventional solar panels rated at 10 percent efficiency, and is capable of reducing the cooling needs of a typical one-story single family residence by 35 percent, with only 10 percent roof coverage.
If scaled to commercial production, radiative cooling has far-reaching implications, based on its ability to provide effective cooling without electricity. In addition to a significant global reduction in energy consumption, the research team anticipates the ability to enhance the lives of populations in hot climates without access to electricity. According to Stanford professor and project leader Shanhui Fan, "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."
· Blaine Brownell, AIA, is a regularly featured columnist whose stories appear on this website each week. His views and conclusions are not necessarily those of ARCHITECT magazine nor of the American Institute of Architects.