We left off last time mentioning two research reports, both of which discuss a functional material that might—someday—serve as a cooling device. No moving parts and no energy required to operate this device. Science fiction? Definitely not, but there is still much to be done before such a device becomes a commercial reality. But, for the moment, let’s not worry about such details.
At the 2015 CRRC Membership meeting, Aaswath Raman, Ph.D., from the Ginzton Laboratory at Stanford University, presented his work on sub-ambient cooling of sky-facing surfaces. To understand Dr. Raman’s work, picture a sheet of material with an exceptionally high solar reflectance (around 97%), mounted in a fixture on a roof in Phoenix. It has a shiny metal appearance. (Super-high reflectance is only possible with mirror-like materials.) Because the material has such a high reflectance, it will not absorb much incoming IR radiation, but it certainly will be heated by the surrounding hot desert air (convective heating). Since this shiny material is hot because of the desert heat, it is emitting plenty of IR radiation. Here is the actual installation.
Typical building material would be radiatively emitting IR radiation at various levels between 0.7 µm and 1,000 µm. But not Dr. Raman’s shiny material. It only emits between 8 and 13 µm, and because there is not much moisture in the air in Phoenix, the IR passes through the sky window with very little absorption and reflection. (The sky window was mentioned in the last post. It is that part of the IR spectrum where little IR radiation is absorbed by the atmosphere.) You’d expect this material to be cooler than the surrounding material (such as cool metal roofing) because of this selective emittance. But not only is Dr. Raman’s membrane cooler than the surrounding material, it is also cooler than the surrounding ambient air, thus the term “sub-ambient cooling.” Think about it: Here is a material sitting in the open air, during the day, with the sun beating down on it, that is cooler than the surrounding air temperature. The hope is to someday harness the coolness characteristic of this material to reduce the air-conditioning demand of a building. When Dr. Raman presented his work to CRRC in June 2015, his material was operating at about 15 °F to 20 °F below the ambient air temperature.
Whew! Let’s stop for a minute and take a few steps backward. Many roofing materials will emit IR and cool down below the ambient air temperature after sunset. But most of these materials—during the day—will get hot, sometimes very hot! Even the best IR-reflective coil coating will heat up to higher than the ambient air temperature during the day. That’s normal. Who would have thought that some material could—literally—keep its cool during the heat of the day? Chalk one up to modern-day R&D efforts. I need to reconsider my cynicism.
You may have guessed that Dr. Raman’s shiny film is no ordinary material. That’s an understatement. Through a process called electron-beam evaporation, titanium is deposited onto a silicon wafer. Then silver is deposited on top of the titanium, and then a number of alternating layers of silicon dioxide and hafnium dioxide are applied. (See Figure 1 below.) This is clearly not your typical coil coating. I do not profess to fully understand how this material works, but I can tell you that it falls under the category of photonics. A simple definition of this complex field is making light (photons) do what you want it to do!
Dr. Raman and his colleagues did not stop with this interesting shiny material. Their recent work (December 2016) describes a new technique that takes a different material down to about 104°F below the ambient air temperature. To achieve this, they had to build a device (see Figure 2 below), complete with a vacuum chamber and other considerations that, frankly, I don’t fully understand. But the point remains the same: These researchers are making steady headway to finding ways to cool buildings.
Radiative cooling to deep sub-freezing temperatures through a 24-h day–night cycle, Zhen Chen, Linxiao Zhu, Aaswath Raman & Shanhui Fan, Nature Communications, Article number: 13729 (2016)
What does this heavy-duty photonic science have to do with coil coatings? It’s all about functionality that reduces the demand for energy. As we think about metal roofing, we know that the metallic substrate does a great job of minimizing corrosion, which prolongs the life expectancy of the prepainted product. Pretreatment contributes to the overall corrosion resistance, while at the same time it enhances the adhesion between the substrate and the organic coatings. These organic coatings need to provide a lifetime of aesthetics, and indeed do a splendid job. The longer the lifetime of a metal roof, the longer the period of time before it needs to be replaced (and this saves energy). The higher the solar reflectance of this metal roof, the less demand there will be for air conditioning in the building. But what if we could add some further functionality? I’ll admit that selective IR reflectance may sound like science fiction right now for a coil coating, but we did hear about pollution-killing technology a few years ago at an NCCA meeting that can be—and, in Germany, is being—applied to building panels as a functional offering. We strive for self-cleaning coatings, which maintain aesthetics and the enviable long-term solar reflectance of metal roofing. We consider the possibility that some coatings—on their own—could repair damage, such as a scratch in the coating that might occur during installation of the building panels.
This post, therefore, is not really about IR reflectance, photonics, and sky windows as much as it is a request for the reader to continually wonder how we might add greater functionality to coil coatings. Corrosion resistance and aesthetics are of paramount importance, but are there other opportunities? Remember that we take “cool roofing” as the new normal, but 20 years ago—had someone asked me how to achieve these IR reflective properties—I would likely have said, “Don’t know,” and I might have even cynically said, “Can’t be done!” Now it is commonplace.
Our challenge, therefore, is to stay aware of developing technologies. Sometimes these new technologies represent solutions to problems that don’t even exist yet. I will be on the lookout for these future possibilities and keep you informed about new ideas and concepts. Science explores the unknown, so keep an open mind.
David Cocuzzi, NCCA Technical Director