The power of radiative cooling: will it be the next big energy saver?

Andrew Tunnicliffe 13 March 2019 (Last Updated February 6th, 2020 13:13)

For a few years a team of academics in the US has been on a quest to revolutionise the way buildings are cooled. Now, ongoing research by one of them could be about to deliver one of the biggest gamechangers to building management, as Andrew Tunnicliffe finds out.

The power of radiative cooling: will it be the next big energy saver?
Professor Shanhui Fan and postdoctoral scholar Wei Li atop the Packard Electrical Engineering building with the apparatus that is proving the efficacy of a double-layered solar panel. (Image credit: L.A. Cicero)

Responsible for as much as 8% of global greenhouse gas emissions, air conditioning and refrigeration systems consume 17% of all electricity generated worldwide, according to SkyCool Systems. The company says the “status quo is no longer an option”, warning the use of electricity for cooling is set to grow six-fold by 2050.

Stanford University Professor Shanhui Fan and his team of research students are working on a device that could not only help address the problem, but also harness the sun’s power to generate electricity.

In late 2018, Fan’s research lab announced it had achieved a world first, something the university says could “revolutionise energy-producing rooftop arrays”. “We’ve built the first device that one day could make energy and save energy, in the same place and at the same time, by controlling two very different properties of light,” Fan said writing in the journal Joule.

The next step to solar powered air conditioning

The device, tested on the roof of a Stanford University building, comprised traditional semiconductor materials used in solar panels and an innovative multi-layered material that Fan, and two other academic colleagues, had been working on for years.

During their tests it was found the top layer of the device – traditional solar panel material – was hotter than the roof. However, as hoped, the bottom layer was much cooler, confirming their expectations. Celebrating the news then postdoctoral scholar Zhen Chen said: “This shows that heat radiated up from the bottom, through the top layer and into space.”

Welcoming the news of a successful experiment of the device prototype, which was the size of a pie plate, professor of electrical engineering at the University of California’s Berkeley campus and director of the Center for Energy Efficient Electronic Science, Eli Yablonovitch, says: “This is a further development in harvesting free energy from the sun. The Shanhui Fan team has made significant strides in harvesting the free energy implicit in the temperature difference between space, which is very cold, and the Earth. That resource has an advantage over solar, in that it’s on for 24 hours a day.”

This work would not have been possible if it weren’t for years of research conducted by Fan, students and university colleagues, in developing a new material that is critical to the new device they’re hoping to produce.

Can radiative cooling work for buildings?

In 2013, Fan, Professor Eli Goldstein, and Aaswath Raman, research assistant at the time but now assistant professor at the University of California, Los Angeles, announced they had successfully developed a panel that could, in years to come, have a dual purpose.

Speaking at the time, Fan said: “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. We’ve developed a new type of structure that reflects the vast majority of sunlight, while at the same time sends heat into that coldness, which cools manmade structures even in the daytime.”

The team developed a material that would not only reflect heat coming from the sun, but also use radiative cooling. Using engineered nanostructured photonic materials and mirror-like panels, they were able to send excessive heat out into space. The material – ultrathin but multi-layered – converts heat coming off a building into an infrared wavelength of invisible light. That wavelength avoids being trapped by Earth’s atmosphere by taking advantage of what the group says are holes in the atmosphere the heat can escape through.

The material is an optical film just 1.8 microns thick, made of quartz and silicon carbide, which can reflect as much as 97% of sunlight while emitting the thermal energy of the building it is placed on. Radiative sky cooling can cut heat by as much as 5˚C, even when the sun is shining all day, a one-two punch said the team. All buildings shed their heat as an invisible infrared light but the atmosphere prevents it from dispersing.

Fan explained: “If you have something that is very cold – like space – and you can dissipate heat into it, then you can do cooling without any electricity,” a huge bonus as it allows building cooling without the need for a power source.

Their work was significant because although a lot of research had been carried out into understanding after-dark radiative cooling, the ability to cool during the day was a far greater challenge. “No one had yet been able to surmount the challenges of daytime radiative cooling, of cooling when the sun is shining,” explained doctoral candidate Eden Rephaeli, “it’s a big hurdle.”

Raman added: “We’ve taken a very different approach compared with previous efforts in this field. 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.”

By 2017, Fan and colleagues had been able to prove the material could not only reflect sunlight and radiate heat into space, and that it could cool flowing water to a level below the ambient temperature. Applied correctly, this could be used to cool a building without any power supply at all.

“This research builds on our previous work with radiative sky cooling but takes it to the next level. It provides, for the first time, a high-fidelity technology demonstration of how you can use radiative sky cooling to passively cool a fluid and, in doing so, connect it with cooling systems to save electricity,” Raman remarked.

Recognising the breakthrough, Yablonovitch continues: “The authors deserve a lot of credit for pointing out that both functions can be combined in one device. This complements well the previous discovery that solar cells achieve a substantially higher voltage with a good rear reflector which helps internal luminescence to escape.”

SkyCool: from radiative cooling to solar power

Today one of the few remaining challenges for Fan and his students is to use the solar material to produce electricity via new device they’re working on. During the experiment, the top layer – the solar material and cells – did not have the required foil needed to produce energy. The decision was taken because the foils are known to block the infrared beam, which would have resulted in the radiative heat being trapped.

Research continues with the aim of developing cells that don’t need the foil, something Fan is confident of achieving. “We think we can build a practical device that does both things,” he said.

While he seeks the next big innovation, Fan has already had success. Together with Goldstein and Raman, they launched SkyCool Systems in 2017. “SkyCool is focused on improving the efficiency of air conditioning and refrigeration systems,” Goldstein explains. “We have an incredible film that stays cool when it’s outside and exposed to the sky. Even when under direct sunlight, the film will be below ambient temperatures.”

The product has its foundations in the work the trio did more than five years ago. Their panels are made using a thin layer of silver covered by silicon dioxide and hafnium oxide and can circulate water-glycol. They were first installed as a prototype on a building in Las Vegas in 2014, where they were able to remain 4.9˚C below the ambient air temperatures even under direct sunlight, delivering cooling power of 40.1 watts per square metre.

“Since then we’ve done several commercial installations, and are collecting data now to demonstrate energy savings,” Goldstein concludes.

Only time will tell what more Fan and his current research team will achieve, but the signs are promising. Should they make the breakthrough they’re hoping for it really could be a revolution. For now, however, his previous work and the ongoing commitment to SkyCool is already having a big impact.