March 27, 2013

Solar Structure Cools Even When The Sun Is Shining

Peter Suciu for redOrbit.com — Your Universe Online

It has long been known that light colors can reflect heat, while dark colors can absorb heat, but it is all a matter of light. This is because light is energy and darker colors have greater absorption, thus more heat. Even light-colored materials, such as cement or concrete, can absorb heat during the day and release it at night.

Because buildings are used primarily during the day, it is typically required to cool them during the summer months, especially during the daylight hours. However, a team of researchers from Stanford University has devised a new form of cooling structure that could remain cool, even when the sun is shining on it. The researchers are looking to take a cue from outerspace.

The structure could change the way buildings, cars and other structures are cooled in the day, by reflecting the sunlight back into the space.

The team published their findings in the March 5 edition of Nano Letters, published by the American Chemical Society.

“If properly designed, terrestrial structures can passively cool themselves through radiative emission of heat to outer space. For the first time, we present a metal-dielectric photonic structure capable of radiative cooling in daytime outdoor conditions. The structure behaves as a broadband mirror for solar light, while simultaneously emitting strongly in the mid-IR within the atmospheric transparency window, achieving a net cooling power in excess of 100 W/m2 at ambient temperature. This cooling persists in the presence of significant convective/conductive heat exchange and nonideal atmospheric conditions,” the researchers wrote in the paper´s Abstract.

As the sun heats the planet and makes life possible, space beyond the Earth´s atmosphere is far less inviting.

“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,” said Shanhui Fan, professor of electrical engineering 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 day time.”

To actually reflect the heat, a reflector is used that can reflect as much of the sunlight as possible, while the second challenge is for the structure to efficiently radiate the heat back into space. To accomplish this, the structure in question must emit thermal radiation as efficiently as possible within a specific wavelength range into the atmosphere.

However, outside of the specific range, the Earth´s atmosphere actually reflects the light back down, creating a so-called “greenhouse effect,” which is believed by some researchers to be the cause of climate change.

The Stanford team has created a structure that could accomplish both goals in that it could reflect the light, and then radiate it back into space. It can effectively reflect most of the sunlight, but then emit the thermal radiation within the crucial wavelength range required for it to escape Earth´s atmosphere.

While radiative cooling at night has also been considered, peak demand for cooling still occurs during the daylight hours when the sun is shining. The team has been successful in utilizing nanostructured photonic materials, which can be engineered to enhance or suppress light reflection within certain wavelengths.

“No one had yet been able to surmount the challenges of daytime radiative cooling–of cooling when the sun is shining,” said the paper´s co-first-author, Eden Rephaeli, a doctoral candidate in Fan´s lab. “It´s a big hurdle.”

The team´s unique approach has differed from other efforts in this area and has combined the thermal emitter and solar reflector into a single device, one that offers higher performance, and is thus much more robust. The team created these engineered nanophotonic materials, which were able to suppress how much heat-inducing sunlight the panels absorbed, while further radiating the heat more effectively.

The material was made of quartz and silicon carbide, which are both very weak absorbers of sunlight. This allowed for a potential of achieving a net cooling power in excess of 100 watts per square meter.

The result would mean a typical one-story, single-family home with just ten percent of its roof covered by this material could offset 35 percent of the entire air conditioning needs during even the hottest hours of the summer.

Moreover, as this is a passive technology, it requires no energy of its own, has no moving parts and should be easy to maintain.