New plastic wrap might keep buildings cool even when the sun is shining
It sounds crazy in the headlines: Newly engineered material can cool roofs, structures with zero energy consumption. Some are more effusive, suggesting that This film might just save the planet. After all, we use a lot of energy for air conditioning and the way things are going, will probably be using a lot more in the future.
The film was developed by a team of University of Colorado Boulder engineers, and works on the principle of passive radiative cooling. As one article titled “it’s not rocket science” explains, radiant cooling has been used for centuries; the Persians used it to make ice at night 2,000 years ago.
Engineer Robert Bean has explained how sun warms us up in the daytime with short wave infrared radiation. We cool off at night as our buildings radiate long wave radiation out to the coldness of space. Our buildings do this in the daytime too, but the effect is overwhelmed by the incoming short wave infrared from the sun.
What the University of Colorado team has apparently done is to develop a film that is highly reflective to the incoming radiation from the sun, but that is transparent to the long wave radiation being given off by the building underneath; the building is radiating and space is cold day and night.
More technically, it is a thin layer of polymethylpentene plastic with a thin silver coating and “embedded visibly-scattering but infrared-radiant glass microspheres” which, according to the Economist, are just the right diameter to emit “wavelengths which pass straight through the infrared “window” in the atmosphere. Since the source of the heat that turns into this infrared is, in part, the building below, the effect is to cool the building.”
According to The University of Colorado website, the material is cheap enough that we can shrink-wrap our buildings in it, at about 50 cents per yard;
“Both the glass-polymer metamaterial formation and the silver coating are manufactured at scale on roll-to-roll processes,” added Ronggui Yang, also a professor of mechanical engineering and a Fellow of the American Society of Mechanical Engineers. “Just 10 to 20 square meters of this material on the rooftop could nicely cool down a single-family house in summer,” said Gang Tan, an associate professor in the University of Wyoming’s Department of Civil and Architectural Engineering and a co-author of the paper.
This probably won't totally replace conventional air conditioning; it won't work on cloudy days. It is not going to be totally zero energy consumption; if the film wraps an insulated building, how does it get the heat that has to be radiated? There would have to be some kind of system to move the heat to the surface of the building.
But as Robert Bean wrote last year:
There will come a time when we won’t use compressors for the cooling of people and buildings. It is simply not necessary. The heat sinks we need to reject heat to, or absorb heat from, are literally within our reach and there are some very smart people who will show us how to get very good at accessing them.
He concludes by quoting Randy Bachman: “B-b-b-baby, you just ain’t seen n-n-nothin’ yet.” Perhaps we are beginning to.