In the arena of university R&D, one often thinks, who cares? I certainly do. But in this and the next post, you will read about some fascinating work coming out of Stanford University. As a self-professed cynic, I would never mention “the next great thing” if I did not see some very real potential. There is, of course, a big valley between R&D and commercial reality, but from what I have heard and read … well, you be the judge.
One of these ideas that could fall into the “too good to be true” category is radiative cooling. Before you fear that I’ve taken leave of my senses, yes, I do understand that all objects radiate energy and will cool as a result. All cool-roofing codes contain an emittance requirement for this reason. Objects (such as painted metal roofing) with a high thermal emittance (>75%; a perfect emitter would be 100%) cool down faster than low-emittance materials such as shiny metal. When the sun sets, you want the roof to radiate as much heat as possible to the atmosphere as fast as possible. Of course, when the sun is beating down on the roof, you also want the roof to reflect as much heat as possible in the form of infrared radiation (IR), keeping the roof as cool as possible. That’s why both parameters—reflectance and emittance—are important in the cool-roofing arena.
Let’s take a critical look at the above phrase “ … radiate as much heat as possible into the atmosphere.” This simple phrase has much complexity built it into it. Consider the Northern Hemisphere when winter is upon us. On nights when the moon is brilliant, you can bet that it’s going to get really cold. With no cloud cover, the IR merrily leaves Earth and travels to outer space. Some of it gets absorbed by moisture and gases in the atmosphere, and, if there were a heavy cloud cover, a portion of the IR emitted from Earth would be reflected back. But here’s where it gets fun.
The IR wavelength range begins at 700 nanometers (nm)—just a little bit longer than the wavelength of a red color—and extends out to a million nanometers. That’s quite a range, and it is easier to discuss IR radiation in terms of microns (µm). The beginning of the IR range is 0.7 µm (700 nm), and it extends to 1,000 µm. Any object with a temperature above absolute zero—minus 273 °C or minus 459 °F (that’s outer-space cold)—emits IR radiation throughout the IR range (0.7 µm through 1,000 µm), and the amount of radiation emitted at these wavelengths depends upon the temperature of the object. Shown below are the wavelength distributions at two temperatures. The amber area shows the distribution of incoming IR radiation from the sun (which glows at around 10,000 °F). The blue area shows the wavelengths emitted by objects at 80 °F (normal “room temperature” objects).
Something interesting about our atmosphere happens between 8 and 13 µm. In this narrow IR wavelength range, the atmosphere does not absorb IR radiation. The y-axis scale on the graph below is transmittance. The higher the blue lines, the greater the transmittance. A value of 1.0 would be complete transmittance (i.e., no absorption).
Our atmosphere will absorb some of the IR radiation below 8 µm and above 13 µm, but between 8 and 13 µm, it’s as if there is an “IR window,” or “sky window,” where the IR is not absorbed at all by the atmosphere. Imagine if you could find a way to have all IR radiation from, say, a roof emit in the 8-13 µm range, no matter the temperature of the roof. And that’s where The Next Great Thing comes in.
My healthy cynicism has been assuaged by the reality of some fine work and real science, which—in my definition—is research being done by those who are willing to share with you the obstacles to their approach (cost, practicality, etc.) and not just gloating over the remote possibilities that their work might demonstrate. Two recent reports have produced some great work showing how a material might take advantage of the IR window. My next post will dig into both reports and what they mean for the coil coating industry.
David Cocuzzi, NCCA Technical Director