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In Praise of Daytime Passive Cooling

Dec. 20, 2018
Chinese researchers at the Universities of Colorado and Wyoming have developed a new 'metamaterial' that has the ability to cool objects – even in direct sunlight – without consuming energy or water.

In 2015, the U.S. Dept. of Energy, through its Advanced Research Projects Agency-Energy (ARPA-E), awarded a $3-million grant to three researchers, two at the University of Colorado-Boulder and one at the University of Wyoming. The trio of professors, Xiaobo Yin, Ronggui Yang, and Gang Tan, used that grant to develop a metamaterial that has the ability to cool objects – even in direct sunlight – without consuming energy or water. 

According to Engheta and Ziolkowski’s Metamaterials: Physics and Engineering Explorations (John Wiley & Sons, 2006), metamaterials are “the fabricated structures and composite materials that either mimic known material responses, or qualitatively have new, physically realizable response functions that do not occur or may not be readily available in nature”.  In other words, they are engineered products that can be manufactured and have physical properties that are not naturally occurring.

The metamaterial developed by Professors Yin, Yang, and Tan, is a 50-micron thick, glass-polymer hybrid film that can be economically manufactured in rolls. When the film is applied to a surface, it reflects radiant solar energy and allows the underlying surface to radiate absorbed heat through infrared thermal emittance. Unlike typical passive radiative cooling that is effective only at night, this cooling can be used in daylight. In order to achieve both solar reflectance and thermal emittance, the metamaterials film contains IR-radiant glass particles over a reflective silver coating. 
In 2015, Prof. Tan predicted that 10-20 sq. m. (108-215 sq. ft.) of the metamaterials film, applied to a rooftop, could cool a home in the summer. It could also, according to Prof. Yin, be used to prevent solar panels from overheating, thereby improving the panel’s efficiency. Now the team’s newest research, published in October of this year, demonstrates that a 13-sq-m. (140-sq-ft.) array of the material installed on a roof could indeed satisfy the cooling requirement of a single-family home with no consumption of electric energy or water. The metamaterial was tested in a variety of weather conditions (rain, humidity, and wind) during the highest solar intensity period – 12:30 pm to 3:00 pm – during August and September, and the container of water covered with the film was 20 deg. F. (11 deg. C.) cooler than the ambient air temperature.Now that their patented film has proven itself to be commercially scalable, the researchers intend to apply their product to more than roof tops. They envision the possibility of eventually replacing vapor compression air conditioning systems, which are significant electric energy consumers, with water cooling systems using their metamaterial. However, to all the HVACR techs out there reading this, I suggest you not give up your EPA 608 card (refrigerant certification) quite yet.

A regular contributor to HPAC Engineering and a member of its editorial advisory board, the author is a principal at Sustainable Performance Solutions LLC, a south Florida-based engineering firm focusing on energy and sustainability.