Rooftop Cooling System Reflects Sunlight and Sends Excess Heat Out Into Space

©. Aaswath Raman / Stanford University

There are a few different approaches to cooling buildings with less energy. There are smart building techniques that allow for more passive cooling, geothermal cooling that pumps liquid underground where it gets cooled and then back up to cool the building and now researchers at Stanford University are developing a technology that takes advantage of a natural process called radiative sky cooling to slash a building's energy consumption.

Radiative sky cooling is an ever present process that results from molecules releasing heat. Everything and everyone on Earth is constantly releasing heat and that heat eventually makes its way through the atmosphere and into the cold, dark depths of space. Because space is so incredibly cold, the heat from Earth just dissipates into it.

On a hot, sunny day, the effects of radiative sky cooling down at ground level are outweighed by the heat of sunlight, but researchers figured out how to reflect that sunlight so that the natural cooling process can take over. Shanhui Fan, an electrical engineering professor, and his team developed rooftop panels that are made up of mirror-like optical surfaces capable of reflecting 97 percent of sunlight and emitting the surface's thermal energy into the atmosphere.

“With this technology, we’re no longer limited by what the air temperature is, we’re limited by something much colder: the sky and space,” said Eli Goldstein, a member of the research team.

The cooling system consists of panels with the reflective optical surfaces resting pipes carrying running water. In testing, the panels are able to cool the water to 3 to 5 degrees Celsius below the air temperature. The team ran computer simulations where the panels covered an entire roof of a commercial office building in Las Vegas and found that if their panels were tied into a vapor compression cooling system where the condenser was cooled by the panels, the office building would save 14.3 megawatt-hours of electricity during the summer months, which would amount to a 21 percent reduction in electricity use for cooling.

The team is looking to integrate the panels into building cooling systems as well as refrigeration systems with a particular focus on data centers which require a lot of energy to cool the servers and keep them from overheating.