A team of Stanford electrical engineers has designed a cooling panel that radiates sunlight away from buildings and back into space. The panels could pave the way for buildings or cars that don't heat up in the sun and don't need air conditioners. "We've developed a new type of structure -- called Fan -- that reflects the vast majority of sunlight, while at the same time it sends heat into that coldness [of space], which cools manmade structures even in the daytime," said Shanhui Fan, a professor of electrical engineering and author of the paper.
It was critical to make the panel reflect as much sunlight as possible -- poor reflectors absorb too much sunlight and heat up.
The structure also had to efficiently emit thermal radiation within a specific wavelength range that let it escape the Earth's atmosphere and dissipate in the cold of space. Any radiation outside of this range interacts with the atmosphere and contributes to the greenhouse effect.
The idea of radiative cooling of the Earth has been studied extensively as a means to mitigate climate change, but the focus has been on night time activity. It's of particular interest as it's a passive technology, meaning it requires no energy.
However, until now, no one has cracked daytime radiative cooling when the sun is shining. The Stanford team got around this by using nanostructured photonic materials -- made from quartz and silicon carbide -- which can be engineered to enhance light reflection in certain wavelengths.
They combined a thermal emitter and a solar reflector into a single device to maximise performance. The nanophotonic materials could supress how much heat-generating sunlight the panel absorbs, while radiating heat very efficiently in the key frequency range necessary to escape the Earth's atmosphere.
The panels can achieve a net cooling power of more than 100 watts per square metre. This means that if a typical single family house had 10 percent of its roof covered, it could reduce the house's air conditioning energy requirements by 35 percent during summer.
The paper was published in Nano Letters