An array of RadiCold cooling modules on the roof of a building on the University of Colorado Boulder campus. Source: Yang Lab / University of Colorado BoulderAn array of RadiCold cooling modules on the roof of a building on the University of Colorado Boulder campus. Source: Yang Lab / University of Colorado BoulderIn 2017, engineers from CU Boulder developed an innovative, manufactured metamaterial that can cool objects, even in direct sun, with no energy or water consumption. Now, along with engineers from the University of Wyoming, they have taken the next big step by scaling up the project to a 140 square foot array that fits on most rooftops and functions as a natural air conditioner.

The low-cost hybrid organic-inorganic metamaterial uses natural radiative cooling principles to continuously cool structures and potentially make power generation plants more efficient in summer. This would, in turn, provide energy-efficient, environmentally friendly cooling for homes and businesses as well as for industries and utilities.

"You could place these panels on the roof of a single-family home and satisfy its cooling requirements," said Dongliang Zhao, lead author of the study and a postdoctoral researcher in CU Boulder's department of mechanical engineering.

The glass-polymer hybrid material, which is in the form of a film, reflects incoming sunlight and allows an object's stored heat to escape — keeping it cool even in midday sun. At 50 micrometers thick, it is about the thickness of standard kitchen aluminum foil. A roll manufacturing process makes it economical to produce on a large scale.

“We can now apply these materials on building roof tops, and even build large-scale water cooling systems like this one with significant advantages over the conventional air conditioning systems, which require high amounts of electricity to function,” said Associate Professor Gang Tan of the University of Wyoming’s department of civil and architectural engineering.

During tests of the proprietary module, known as RadiCold, conducted in August and September 2017, a container of water covered by the metamaterial remained 20° F cooler than the ambient air during intense sunlight from 12:30 p.m. to 3:00 p.m.

In order to make the technology even more efficient for buildings that have less or no cooling demand at night, the researchers incorporated dynamic scheduling. In addition, a cold storage unit could be used to capture the cold to be retrieved during peak demand periods during the following day.

“We have built a module that performs in real-world, practical situations,” said Professor Ronggui Yang of CU Boulder's department of mechanical engineering and lead author of the study. “We have moved quite far and fast from a materials level to a system level.”

The findings are published in Joule.