In 2020, Stanford's radiative cooling technology reached a California supermarket roof; panels were estimated to save 15% to 20% energy, while a Tempe bus shelter film was reported 30°F cooler

Picture a substance that can cool itself down just by sitting in the sun, without a fan or an electrical compressor. This isn't science fiction; it traces back to a 2014 paper in Nature, "Passive radiative cooling below ambient air temperature under direct sunlight", by a team of Stanford University researchers. The cooling technology that grew out of that research is now used in a California grocery store's rooftop system and in a separate field test on bus shelters in Arizona. With heat waves growing longer and more brutal every year, this quiet piece of engineering could be one of the more pragmatic tools we have.

A coating that bounces off the sun and beams heat into space

The concept was developed by Stanford engineer Shanhui Fan and his graduate student Aaswath Raman, who is now a professor at UCLA. Together, they developed a film with two functions. First, it reflects most incoming sunlight, helping keep the surface cooler. Second, it radiates heat as infrared radiation at a wavelength that passes through the Earth’s atmosphere and escapes into outer space. Normally, heat from the roof is absorbed by the atmosphere and then radiates back toward the roof. The film avoids this heat-trapping process. Moreover, if this film were placed on a car hood, it would cool the hood without using any energy.


From a lab bench to a supermarket rooftop

The concept stayed mostly in the research world until 2020, when Raman set up panels using this technology on the rooftop of a supermarket in California to help power its refrigeration system. These panels were installed to help power the supermarket’s refrigeration system. According to the California Energy Commission, the panels are estimated to save 15% to 20% energy and reduce power demand on hot days. Since Fan and Raman's original paper, other researchers have developed radiative cooling paints, gels and even engineered wood panels that stay cool in direct sunlight. It has become a growing area of materials science.

Bus shelters that stay 30 degrees cooler in the Arizona sun

Another radiative-cooling film from 3M is being field-tested on bus shelters in Tempe, Arizona, one of the hottest areas in the nation. According to initial findings reported by National Geographic, the film-covered roof is about 30 degrees Fahrenheit cooler than an ordinary painted steel roof under the same conditions. It is a product from another company that uses radiative-cooling technology developed by Fan and Raman. That kind of temperature drop can make a real difference for anyone waiting at a metal bus shelter in desert heat.

Going beyond shade

Raman isn't stopping at bus shelters. He is now involved in a UCLA project called Heat Resilient L.A., which brings together engineers, urban planners, and public health researchers to rethink how cities respond to extreme heat. The project has built prototype canopy structures that combine radiative and evaporative cooling to cool outdoor public spaces quickly and cheaply. This matters because traditional cooling solutions, like planting shade trees or building parks, can take years to deliver benefits and often miss lower-income neighborhoods that need them most. A lightweight canopy can be erected in a fraction of that time and at a fraction of the cost.

Why any of this is urgent

Heat is no longer a background nuisance, but one of the deadliest weather-related threats we face. Based on data from 2000 to 2019, the World Health Organization estimates that around 489,000 people around the world die each year due to heat-related causes, with people over 65 among the most vulnerable. The heat death rate among that risk group has skyrocketed over the last two decades, and exposure to extreme heat is growing as the planet heats up.

In that context, a film that cools a roof or a bus stop without plugging into the grid no longer looks like a novelty but like infrastructure. It won't take the place of the bigger, harder work of cutting emissions. But for people standing in the sun today waiting for a bus, or worrying about their next electricity bill, it can provide real relief. This kind of technology alone probably won't solve the climate crisis, but in a California supermarket parking lot or on a bus stop in Tempe, it is already showing that some cooling can be achieved with far less energy use.