A reflective, ultra-thin panel, which can cool buildings without air-conditioning, is being hailed as an energy-saving breakthrough.

Invented by engineers at Stanford University, the multi-layered material deals with both visible and invisible light. It reflects incoming sunlight and sends heat from inside the building directly into space as infrared radiation.

The result is what the Stanford team calls photonic radiative cooling.

With as much as 15 per cent of the energy used in buildings in the United States spent powering air conditioning systems, this cost-effective solution could be deployed on a large scale across rooftops to passively cool structures by simply radiating heat into the cold darkness of space.

The ultrathin coating – just 1.8 microns thick and thinner than the thinnest aluminum foil – has been carefully constructed to send infrared light away from buildings at the precise frequency that allows it to pass through the atmosphere without warming the air; a key feature given the dangers of global warming.

Made of seven layers of silicon dioxide and hafnium oxide on top of a thin layer of silver, the multi-layered material also acts as a highly efficient mirror, preventing 97 per cent of sunlight from striking the building and heating it up.

Together, the radiation and reflection make the photonic radiative cooler nearly nine degrees Fahrenheit cooler than the surrounding air during the day.

A graphic representation

A graphic representation

“This photonic approach gives us the ability to finely tune both solar reflection and infrared thermal radiation,” said Linxiao Zhu, a doctoral candidate in applied physics and a co-author of the paper.

A warming world needs cooling technologies that don’t require power, according to research associate Aaswath Raman.

“Across the developing world, photonic radiative cooling makes off-grid cooling a possibility in rural regions, in addition to meeting skyrocketing demand for air conditioning in urban areas,” he said.

“This is very novel and an extraordinarily simple idea,” said Eli Yablonovitch, a professor of engineering at the University of California, Berkeley, and a pioneer of photonics who directs the Center for Energy Efficient Electronics Science. “We can now use radiative cooling not only at night but counter-intuitively in the daytime as well.”

The next stage of the research is making the technology practical.

The current prototype is the size of a personal pizza. Cooling buildings will require fabrication of much larger panels.

The other challenge is conducting the heat inside the building to the exterior coating. The coating solves the problem of directing the heat away into space but the engineers need a more efficient solution to first get the heat onto the coating.

Beyond this, the researchers see this project as a first step toward using the cold of space as a resource. In the same way that sunlight provides a renewable source of solar energy, the cold universe supplies a nearly unlimited expanse to dump heat.