Cooling structure could reduce global air-conditioning dependence

Keeping cool in the sun isn’t easy, but a group of Stanford University researchers have developed a new form of cooling structure which does exactly that. The panel, which reflects sunlight back into the chilly vacuum of space, could reduce our reliance on air conditioners in homes and cars, the researchers say.

A paper describing the device was published in Nano Letters on 5 March.

“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, a 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 daytime.”

The trick, the engineers say, is twofold. Firstly, the structure reflects as much of the sunlight as possible. Poor reflectors absorb too much sunlight, heating up in the process and defeating the goal of cooling.

The second challenge is that the structure must efficiently radiate heat (from a building, for example) 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, the thermal radiation interacts with Earth’s atmosphere. Most people are familiar with this phenomenon. It’s better known as the greenhouse effect - the cause of global climate change.

The new structure accomplishes both goals: it is an effective broadband mirror for solar light and also emits thermal radiation efficiently within the crucial wavelength range needed to escape Earth’s atmosphere.

Radiative cooling at night has been studied as a climate change mitigation strategy, but peak demand for cooling occurs in the daytime. No-one has yet been able to surmount the challenges of daytime radiative cooling, said Eden Rephaeli, first co-author of the paper.

The Stanford team has succeeded where others have come up short by turning to nanostructured photonic materials. These materials 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 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.”

Using nanophotonic material made of quartz and silicon carbide - very weak absorbers of sunlight - the team was able to 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 device can achieve a net cooling power of more than 100 watts per square metre. By comparison, today’s standard 10%-efficient solar panels generate about the same amount of power. This means the radiative cooling panels could theoretically be substituted on rooftops where existing solar panels feed electricity to air-conditioning systems needed to cool the building.

A typical one-storey, single-family house with just 10% of its roof covered by radiative cooling panels could offset 35% of its entire air conditioning needs during the hottest hours of the summer.

Radiative cooling has a profound advantage over other cooling equipment like air conditioners: it is a passive technology. It requires no energy, has no moving parts, is easy to maintain and starts working immediately.

Beyond the commercial implications, Fan’s team foresees a broad potential social impact. Much of the human population on Earth lives in sun-drenched regions huddled around the equator. Electrical demand to drive air conditioners is skyrocketing in these places, presenting an economic and environmental challenge. These areas tend to be poor and the power necessary to drive cooling usually means fossil-fuel power plants that compound the greenhouse gas problem.

“In addition to these regions, 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,” Fan said.