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New type of solar structure cools buildings in full sunlight

28 March 2013

A new form of radiative cooling panel that works even when the sun is shining could vastly improve the daylight cooling of buildings, cars and other structures.

Professor Shanhui Fan (centre) with graduate students Aaswath Raman (left) and Eden Rephaeli (right).Photo: Norbert von der Groeben
Professor Shanhui Fan (centre) with graduate students Aaswath Raman (left) and Eden Rephaeli (right).Photo: Norbert von der Groeben

A team of researchers at Stanford University has designed an entirely new form of cooling structure that cools even when the sun is shining. Such a structure could vastly improve the daylight cooling of buildings, cars and other structures by reflecting sunlight back into space.
“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,” explained Shanhui Fan, professor of electrical engineering at Stanford. “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.”

The trick, from an engineering standpoint, is two-fold. First, the reflector has to reflect as much of the sunlight as possible. Poor reflectors absorb too much sunlight, heating up in the process and defeating the purpose of cooling.
The second challenge is that the structure must efficiently radiate heat back into space. Thus, the structure must emit thermal radiation very efficiently within a specific wavelength range in which the atmosphere is nearly transparent.

Outside this range, Earth’s atmosphere simply reflects the light back down. Most people are familiar with this phenomenon. It’s better known as the greenhouse effect.
The new structure accomplishes both goals. It is an effective a broadband mirror for solar light, reflecting most of the sunlight. It also emits thermal radiation very efficiently within the crucial wavelength range needed to escape Earth’s atmosphere.
Radiative cooling at night has been studied extensively as a mitigation strategy for climate change, yet peak demand for cooling occurs in the daytime.
“No one had yet been able to surmount the challenges of daytime radiative cooling—of cooling when the sun is shining,” said Eden Rephaeli, a doctoral candidate in Fan’s lab. “It’s a big hurdle.”
The Stanford team has succeeded by turning to nanostructured photonic materials, which can be engineered to enhance or suppress light reflection in certain wavelengths.
"We've taken a very different approach compared to previous efforts in this field," said Aaswath Raman, a doctoral candidate in Fan’s lab. "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."
Using engineered nanophotonic materials the team was able to strongly suppress how much heat-inducing sunlight the panel absorbs, while it radiates heat very efficiently in the key frequency range necessary to escape Earth’s atmosphere. The material is made of quartz and silicon carbide, both very weak absorbers of sunlight.
The new device is capable of achieving a net cooling power in excess of 100 watts per square metre. By comparison, today’s standard 10-percent-efficient solar panels generate the about the same amount of power. That means Fan’s radiative cooling panels could theoretically be substituted on rooftops where existing solar panels feed electricity to air conditioning systems needed to cool the building.
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 all other cooling strategy such as the air conditioner. It is a passive technology. It requires no energy. It has no moving parts and it is easy to maintain.

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