Researchers at Stanford University have shown that a system involving mirror-like optical surfaces can cool flowing water to a temperature below that of the surrounding air. The entire cooling process is done without electricity.

They say the research builds on previous work with radiative sky cooling and provides a high-fidelity technology demonstration of how the concept can be used to passively cool a fluid. A paper detailing this research is published in Nature Energy.

Panels used as part of the Stanford study.The Stanford researchers have founded SkyCool Systems, which is working on further testing and commercializing the technology.

Radiative sky cooling is a natural process that can be seen in the heat that comes off a road as it cools after sunset.

"If you have something that is very cold -- like space -- and you can dissipate heat into it, then you can do cooling without any electricity or work. The heat just flows," says Shanhui Fan, professor of electrical engineering and senior author of the paper.

Although the human body releases heat through radiative cooling to both the sky and the surrounding environment, on a hot, sunny day, radiative sky cooling is less noticeable. This is because the sunlight warms the body more than radiative sky cooling will cool. To overcome this problem, the research team's surface uses a multilayer optical film that reflects about 97% of the sunlight while also being able to emit the surface's thermal energy through the atmosphere. Without heat from sunlight, the radiative sky cooling effect can enable cooling below the air temperature even on a sunny day.

The researchers created a system in which panels covered in the specialized optical surfaces sat atop pipes of running water and tested it on the roof of a university building. These panels were slightly more than 2 feet in length on each side and the researchers ran as many as four at a time. With the water moving at a relatively fast rate, they found the panels were able to consistently reduce the temperature of the water 3 to 5 degrees Celsius below ambient air temperature over a period of three days.

The researchers also applied data from this experiment to a simulation where their panels covered the roof of a two-story commercial office building in Las Vegas, Nev. They calculated how much electricity they could save if, in place of a conventional air-cooled chiller, they used vapor-compression system with a condenser cooled by their panels. They found that, in the summer months, the panel-cooled system would save 14.3 megawatt-hours of electricity, a 21% reduction in the electricity used to cool the building. Over the entire period, the daily electricity savings fluctuated from 18-50%.

The Stanford research team is not alone in its work. A team of University of Colorado Boulder engineers developed a scalable manufactured metamaterial to act as a kind of air conditioning system for structures. It too has the ability to cool objects even under direct sunlight with no energy or water consumption.

When applied to a surface, the Colorado team's metamaterial film cools the object underneath by reflecting incoming solar energy back into space while also allowing the surface to shed its own heat in the form of infrared thermal radiation.