And outer space can help, Stanford researchers say. They designed an entirely new form of cooling structure that cools even when the sun is shining - by reflecting sunlight back into the chilly vacuum of space.
This required jumping over two engineering hurdles: the reflector had to reflect as much of the sunlight as possible, because poor reflectors absorb too much sunlight, heating up in the process and defeating the goal of cooling, and; the structure must efficiently radiate heat (from a building, for example) back into space, which means emitting thermal radiation very efficiently within a specific wavelength range in which the atmosphere is nearly transparent. Outside this range, the thermal radiation interacts with Earth's atmosphere. Most people recognize this phenomenon as the greenhouse effect.
The new structure accomplishes both goals. It is an effective broadband mirror for solar light and it emits thermal radiation very efficiently within the crucial wavelength range needed to escape Earth's atmosphere. 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.
Radiative cooling at nighttime has been studied extensively as a mitigation strategy for climate change, but peak demand for cooling occurs in the daytime. Radiative cooling has a big advantage over other cooling equipment, such as air conditioners. It is a passive technology. It requires no energy and has no moving parts. It is easy to maintain because you put it on the roof or the sides of buildings and it starts working immediately.
"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, a professor of electrical engineering at Stanford 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 daytime."
The team turned 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 co-first-author of the paper. "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."
Net cooling during daylight
The new device is capable of achieving a net cooling power in excess of 100 watts per square meter. By comparison, today's standard 10-percent-efficient solar panels generate about the same amount of power. That means these radiative cooling panels could theoretically be substituted on rooftops where existing solar panels feed electricity to air conditioning systems needed to cool the building.
Using that math, a typical one-story, single-family home 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.
Published in Nano Letters.