When writing New plastic wrap might keep buildings cool even when the sun is shining I was a little unsure about the science; the Stefan-Boltzmann Law was not on my architecture curriculum. I consulted with two experts I know: Robert Bean of Healthy Heating (who is quoted in the previous story) and Allison Bailes of Energy Vanguard. Now Allison has picked up on the story and explains it in greater detail.
Allison has a gift for making the complex comprehensible and fun; he explained the basic concept of radiative cooling a few years ago in a provocative post titled Naked People Need Building Science. He reminds us that people have been playing with the basic concept for a while:
Back in the 1970s, when the energy crises spurred lots of energy innovation, radiative cooling was one of the ways people tried to save energy. In the solar energy class I took in grad school, I learned about one method that required a pool of water on the roof. The water was thermally connected to the indoors during the day while being covered and insulated from direct solar gain and ambient heat. At night, the cover came off and the water could radiate the heat it picked up from inside the house during the day, sending it out to the cold night sky. Nice idea, if you don't mind having all that water overhead. But it was limited by being able to get rid of that heat only at night. So we have a day and night problem. Light and dark.
He then goes on to describe in greater detail than I did (and he read the paywalled paper) about how "the researchers have developed a material that can radiate heat away at night and during the daytime. And it does so with an impressive cooling capacity." Allison is impressed:
What could turn out to be a really exciting discovery for the building community is their measured cooling capacity. ...What it means is that you could get a ton of cooling capacity (12,000 BTU per hour) with about 400 square feet of this film at their noon conditions. At night, you'd get more than a ton from those same 400 square feet. A typical new house these days has a cooling load of about one ton per 1,000 square feet of conditioned floor area. It shouldn't be hard to find enough roof area to get all your cooling needs met if this material turns out to be as good as it looks now.
Read the full post and explanation at Energy Vanguard.