Stanford ran water beneath mirror-like rooftop panels; they cooled it 3–5°C below the surrounding air by sending heat into space

Somewhere on a Stanford rooftop, water got colder than the air around it, and nothing was plugged in to make that happen. No compressor hum, no electricity-meter spike, nothing. It was just pipes, sunlight, and a very clever mirror. In 2017, Stanford researchers Eli Goldstein, Aaswath Raman and Shanhui Fan published a study titled ‘Sub-ambient non-evaporative fluid cooling with the sky’ in Nature Energy describing rooftop panels that cool flowing water without using electricity. The panels were tested on the roof of Stanford’s Packard Electrical Engineering Building, and they lowered the water temperature 3 to 5 degrees Celsius below the outside air temperature, with no compressor, no refrigerant and no power cord. They just sent the water's excess heat outward into space.

How do you send heat into space?

Everything on Earth emits heat in the form of infrared radiation that passes through the atmosphere into the cold vacuum of space. Roads often lose heat rapidly after sunset, even before the air cools. This is called radiative cooling of the sky. Sunlight has always been the catch, heating things faster than radiative cooling can cool them, the study said. Fan, the paper's senior author, described space as a giant cold reservoir: if you dump heat into it, it cools without requiring electricity.


The mirror trick that beats the sun

That daytime obstacle is exactly what the panel's coating is designed to cancel out. The panels use a multilayer optical film that reflects about 97 percent of incoming sunlight, which keeps the surface from absorbing solar heat in the first place, while a narrow atmospheric window still lets the panel's own infrared radiation escape outward. So even on a bright afternoon, the surface stays cool enough that heat continues flowing out faster than the sun can add it back in. A 2014 study titled ‘Passive radiative cooling below ambient air temperature under direct sunlight,’ by the same team, demonstrated the underlying idea using small optical wafers about 8 inches across, which cooled only the surface and did not use fluids. The 2017 study scaled this up into an actual fluid-cooling system.

Testing it on Stanford's rooftop

Since 2013, Fan and his team had been using the Packard Building rooftop to test this technology. In this new experiment, panels just over 2 feet in each dimension were set up atop pipes of running water, and testing was done over three days in September 2015. At a fairly high flow rate, the panels cooled the water 3 to 5 degrees Celsius below the surrounding air temperature. The researchers recorded a flow rate of 0.2 L/min/m² and a heat rejection rate of 70 W/m², with no power consumption or water evaporation.

Modeling the savings in Las Vegas

To gauge real-world potential, the team modeled a two-story office building in Las Vegas, a hot, dry city where the panels would perform best, comparing a standard air-cooled chiller to one whose condenser was cooled by their panels. The simulation showed summer electricity savings of around 14.3 megawatt-hours, a 21 percent decrease in cooling power, with daily savings ranging from 18 to 50 percent. These figures are simulated from test data, not measured in an occupied building.

Why cooling demand is a growing problem

At the time of the study, the cooling industry was consuming 15 percent of electricity produced globally and generating 10 percent of the world's greenhouse gas emissions, with demand projected to climb sharply through 2050. The International Energy Agency's 2017-era estimate projected that air-conditioner energy use could triple by 2050 as unit count rose from 1.6 billion to 5.6 billion, roughly ten new units installed every second for thirty years. Nearly a decade on, the underlying technology has moved from lab projection to commercial deployment: SkyCool reported $3.33 million in 2025 revenue, a tenfold year-over-year increase, with panels running at Target locations across six states and a spot as a 2026 BNEF Pioneers Finalist in data-center sustainability.

From a rooftop test to a startup

Fan, Goldstein, and Raman co-founded SkyCool Systems to test and commercialize the panels, and the company measured savings at a facility that pairs them with standard air-conditioning and refrigeration equipment. They were especially interested in cooling data centers, which run around the clock and use large amounts of energy. Fan has applied the same idea in other places, too, including a coating for solar cells developed with Raman, and a cooling fabric developed with Stanford materials scientist Yi Cui. The research was supported by the Advanced Research Projects Agency-Energy (ARPA-E), part of the U.S. Department of Energy.

A cool idea with real limits

It’s important to keep things in perspective here. The technology works best in hot, dry climates like Las Vegas and performs poorly in humid environments because moisture in the air reduces heat loss. It is also built to complement current air conditioners and not to replace them, at least for now. Still, it is a concept that lets heat escape into the cold, dark sky. As Fan described it, the universe itself is one such remarkable resource for cooling purposes.