In my Applied Math and Science 320 course at GBC we perform an interesting and fun experiment that investigates the transfer of thermal energy by photons. Normally there are three processes that can account for the flow of heat: conduction, convection and electromagnetic radiation. It is the last one that we examine and as we do students learn that without the continual flow of photons from the Sun, our planet would be a frozen orb of ice without life or activity as we know it.
The physics behind the emission of thermal energy from heated objects was worked out over a hundred and thirty years ago by Josef Stefan on the basis of experimental measurements made by John Tyndall which showed that the number of joules per second leaving a surface was proportional to the fourth power of its temperature. Certainly one of the largest power relationships found in nature, the process also depends, as most of us know, upon the color of the object. This factor, called the emissivity of the material, is why portable radiators that you plug in are mostly painted black. Darker colors usually have higher emissivities than shiny or bright objects.
The absorption of heat photons also depends upon the color of the surface as well. That is why a black jacket will get warmer on a sunny winter day than a white jacket. Because this thermal transfer process works in both directions it may be possible to devise a surface that has a low emissivity for incoming photons but a high value for radiating energy outward.
Last week, it was announced that engineers at the University of Colorado Boulder developed a new meta-material with extraordinary properties not found in nature that can act sort of like an air conditioning system. It has the ability to cool objects even under direct sunlight using zero energy and water. It is a thin film that has the ability to reflect incoming solar energy back into space while at the same time allowing the warm surface to radiate outward as well in the form of infrared thermal radiation. The new material, which is described in the journal Science, could provide a green-friendly means of reducing the temperatures found on building roofs and in turn reducing the electricity needed to cool such structures.
The challenge for the U of C team was to create a material that could reflect any incoming solar rays while still providing a means of escape for infrared radiation. To solve this, the researchers embedded visibly-scattering but infrared-radiant glass microspheres into a polymer film of thickness 50 micrometers. They then added a thin silver coating underneath in order to achieve maximum spectral reflectance. In a sense they were able to selectively adjust the emissivities for different wavelengths of light and since the material is a film, it can be rolled out like plastic wrap onto places exposed to direct sunlight.
“We feel that this low-cost manufacturing process will be transformative for real-world applications of this radiative cooling technology,” said Xiaobo Yin, co-director of the research and an assistant professor for both Mechanical Engineering and Materials Science programs at Boulder. It is thought that the material can be manufactured economically on rolls, making it an ideal technology for both residential and commercial applications. It is mentioned in the article that just 10 to 20 square meters of this material on the rooftop could nicely cool down a single-family house in summer.
In addition to cooling homes, the material can be used to cool electrical solar panels. This is welcomed as good news because the slightest rise in temperature of a silicon PN junction lowers the band gap energy reducing the output voltages on solar arrays. “Just by applying this material to the surface of a solar panel, we can cool the panel and recover an additional one to two percent of solar efficiency,” said Yin.
The next plan is to create a 200-square-meter “cooling farm” prototype in Boulder this year.