A cool feature if painted right polarica/Getty

The sun itself could soon become a low-cost air conditioner. A high-tech paint that actually cools when exposed to sunlight can provide a better way to chill buildings – and perhaps even solve the long-standing problem of cooling things in space.

In hot weather, electricity consumption soars as people turn on the air conditioning, pushing the grid to its limits and raising energy bills. Now Yaron Shenhav and his colleagues from SolCold, a firm based in Herzliya, Israel, have come up with an alternative that doesn’t require electricity. “It’s like putting a layer of ice on your rooftop which is thicker when there is more sun,” he says.

The technology is based on the counterintuitive principle of laser cooling, in which hitting specially designed materials with a laser can cool them by up to 150°C. It works because molecules in these materials absorb photons whose light is of one frequency while spontaneously re-emitting higher-frequency photons, which also carry more energy. Since energy is lost, the temperature of the material is reduced in the process.


Mounting lasers on your roof wouldn’t be very practical, though, so Shenhav wanted to see if he could tweak the technique to make it work with sunlight instead. “Heat from a building could be absorbed and re-emitted as light,” he says. “As long as the sun is shining on it, it would be continuously cooled.”

The problem is that the sun’s spectrum is much broader than that of laser light – a focused beam with a narrow range of frequencies. So the team had to create a material that could do the same trick using several frequencies of scattered light. They came up with a paint made up of two layers: an outer layer that filters out some of the sun’s rays and an inner one that does the heat-to-light conversion, cooling itself below the ambient temperature.

So far, the material has been successfully tested in the lab, where the researchers have found that the effect is more pronounced on metal roofs than on concrete, and works best over rooms with low ceilings. Simulations show that a room on the top floor will feel up to 10°C cooler than with the paint applied if applied to a roof of a house than without the coating. The team will conduct pilot tests on buildings within two years.

People already use white cooling paints to scatter and reduce the amount of heat buildings absorb. However, they can’t actively reduce the temperature in the building, whereas SolCold’s paint can, says Eran Zahavy at the Israel Institute of Biological Research, who was not involved in the research.

Cut the air conditioning

The new paint isn’t cheap, costing about $300 to coat 100 square metres. Shenhav and his team think the early adopters will be large commercial buildings like shopping malls and stadiums. There, the coating could lower energy consumption by up to 60 per cent, massively reducing bills and carbon emissions.

It could have other knock-on environmental benefits as well. The continuous use of air conditioning in already hot areas like Phoenix, Arizona, creates urban heat islands which have been linked to steadily rising temperatures in such cities. With the new paint, “buildings would be able to install much smaller air-conditioning units,” says Shenav.

The paint’s use isn’t limited to this planet: it could solve the significant challenge of how to cool objects in space. That might seem counterintuitive given the frigid temperatures there, but the problem is that there is no air to carry heat away from an object. Currently, the ISS uses reflective fabric to ward off radiation from the sun, and internal heat exchangers to get rid of excess heat produced by equipment. “With our technology, heat is transferred through light,” says Shenhav. “Space applications are a big market for us.”

The team will be presenting their work later this month at the Hello Tomorrow technology summit in Paris.