When the weather gets hot, everyone wants to stay cool. That often means turning on the air conditioning, which consumes vast amounts of energy – and money – in developed countries. But that could change, thanks to a new photonic device that can cool to below the ambient temperature while consuming no energy.

Cooling accounts for around 15% of the energy used in buildings in the US and contributes heavily to greenhouse-gas emissions. Worldwide, energy consumption related to cooling is expected to surpass that used for heating by 2070.

Open window

Objects can cool themselves without consuming energy by radiating energy in the form of infrared light. This process is not normally very efficient because objects can also be warmed by convective air currents and by absorbing radiation emitted by other objects and by the air. However, air absorbs and emits very little infrared radiation at wavelengths of around 8–13 μm. It is through this "window" that the Earth lowers its temperature at night – especially when the sky is clear – by sending radiation out into space.

To make practical use of this effect during the day, the surface of an object must emit radiation within this window, while at the same time reflecting sunlight to minimize the amount of heat it absorbs. The problem is that no known naturally occurring material can do both of these things.

Layered material

Now, physicist Shanhui Fan and colleagues at Stanford University have made a device that fits the bill. The team designed a structure of seven alternating layers of silicon dioxide – essentially glass – and hafnium dioxide. Both materials are transparent to visible light but emit radiation strongly at wavelengths of around 10 μm. These layers are stacked on a layer of silver to create a mirror that reflects visible light. Fan and his team used a computer simulation to choose thicknesses for the different layers that would maximize both how much sunlight the combined device reflects and how much infrared radiation it emits.

The researchers then mounted the device, which was just under 2 μm thick, onto a 20 cm-diameter circular silicon wafer, added a plastic sheet to block convective air flows, and placed the apparatus on the rooftop of a building at Stanford. They found that on a sunny day, the device cooled to between 4 and 5 degrees below the surrounding air temperature. The device therefore appears to be the first object known to achieve such cooling under direct sunlight without consuming energy.

Rooftop coverage

The researchers are now planning to test their invention over a larger section of rooftop. They say a device like theirs could someday cool a building through direct contact, or by cooling water that is then pumped through the building. While hafnium dioxide is a relatively expensive material, the team says that it could be substituted with cheaper titanium dioxide. The device can be made using commercial fabrication techniques, so under the right conditions it could be combined with conventional air conditioning driven by solar power to create low-cost, low-carbon cooling systems. It could also work alongside solar panels, the researchers say, because these panels are typically installed on rooftops facing the Sun, whereas the new device operates best when not facing the Sun.

In addition to cooling buildings, such devices could help solar panels work better, says engineer Min Gu of Swinburne University of Technology in Australia. Solar panels become less efficient as they heat up, so a way to keep them cool without consuming energy would be a huge boon. "It's very encouraging for us. Now we can design something to integrate into a solar cell," Gu says.

Physicist Claes-Göran Granqvist of Uppsala University in Sweden says that Fan's team has demonstrated an "interesting effect". But he notes that the researchers face additional challenges in creating a practical device. For instance, the device will not work well on cloudy days, when water vapour largely blocks the 8–13 μm atmospheric window. In addition, the thin plastic sheet the researchers used to block convective air currents may not stand up to high winds. "It's a step forward, but there are many more steps to be taken," says Granqvist.

The research is described in Nature.

  • The challenges of keeping cities cool are discussed by Roland Ennos of the University of Manchester in the feature article "Urban cool".