The thin film can switch from generating heat to trapping it. Such a coating can make buildings more energy efficient.
Maintaining a comfortable indoor environment takes a lot of effort. About half of the energy people use in their homes goes to heating and cooling, which accounts for a significant portion of both utility bills and greenhouse gas emissions. While the walls of many buildings are insulated to maintain the ideal temperature, others, especially older buildings, are startlingly energy inefficient, writes Scientific American. A new promising development of scientists in the future will bring us closer to significant savings in resources for heating and cooling buildings.
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Scientists have been working for decades on high-tech solutions for the energy efficiency of buildings. Now materials scientists have developed a color-changing film that can switch between heating and cooling modes. The film, which is thinner than a credit card, uses very little energy and could one day wrap even the most wasteful of buildings, helping to radiate unwanted heat in the summer and keep it out in the winter.
The new devices use a natural phenomenon called radiative cooling, which causes the temperature of the outside air to drop at night and helps to cool the Earth as a whole. Everything around us, including our bodies and buildings, is constantly radiating heat in the form of mid-infrared radiation: electromagnetic waves that have a lower frequency than the light you can see with your own eyes. “People can use a thermal imaging camera and see objects, people, buildings, meaning they radiate energy 24 hours a day, 7 days a week,” says Po-Chun Hsu, a molecular engineer at the University of Chicago and senior author of the study.
If you point a thermal imaging camera at Earth from orbit, you can also see the heat radiating from the planet into the cold vacuum of space. Fortunately, our atmosphere allows more mid-infrared radiation to radiate outward from the planet than other wavelengths of light. While most of that heat leaves the Earth, some is still trapped in the atmosphere by greenhouse gases—enough to upset the planet’s heat balance and cause temperatures to rise, explains Peter Bermel, an electrical engineer at Purdue University. As global temperatures rise, scientists are developing solutions to maximize the heat released through radiative cooling. Among these methods are films that can be wrapped around structures to radiate more heat. But many regions of the world have harsh winters and hot summers.
This dilemma inspired new coatings that can switch between high and low heat output with a simple discharge of electricity. Similar tunable devices already exist for visible light: so-called dynamic windows can switch from transparent to opaque to control the amount of light let through. But so far, no construction film has been able to do the same for mid-infrared heat.
New material is started in cooling mode. Under an incredibly thin electrical conductor is a small reservoir of water, inside which copper ions are dissolved. In this state, the device naturally radiates heat, cooling the room inside. Then, when the conductor layer applies a small electrical charge, the dissolved copper settles on its surface, forming a thin layer above the reservoir. Because copper radiates very little of the mid-infrared heat it absorbs, the device now traps heat. This change can be undone over and over again, although repeated use has diminishing returns: after 1000 cycles, both cooling and heating modes become less efficient.
The authors estimate that if this technology is applied in a film on the outside of a building, it could save 8.4 percent of the energy used for heating and cooling in climates where temperatures fluctuate dramatically throughout the year. The building also changes color, from dark white in summer to copper-metal in winter, although the film can be coated with a special paint that does not block mid-infrared radiation.
“Currently, this is only the very first step to demonstrate the mechanism, and we are already seeing very good progress,” says Qiaoqiang Gang, a materials scientist and engineer at the King Abdullah University of Science and Technology in Saudi Arabia. The new system seems promising, especially when compared to previous attempts by some other research groups to create a tunable device using aqueous solutions. Some of the materials intended for use in such devices were flammable and clearly not suitable for building cladding. The new film does not ignite, but this does not mean that it is ready for use.
Apart from the loss of efficiency over time, the main disadvantage of the new device is its high cost. The thin electrode covering the outer layer of the film is high-quality graphene, an expensive one-atom-thick carbon array. The extreme thinness of graphene allows heat to pass through while the material still conducts electricity. For such building envelopes to be feasible, researchers will have to achieve the same result with cheaper materials that can be produced on a large scale. Hsu and his team plan to experiment with lower quality graphene and other materials to find cheaper replacements. They also plan to replace copper with cheaper metals such as zinc.
Finding a balance between price and performance will take time, so neighborhoods may not fill up with color-changing eco-buildings in the coming years. “But it’s a very, very hot topic of research,” Gan said.
Previously, Focus wrote that the cold helps to work better. Scientists from the south coast of the United States say that stepping out of your thermal comfort zone straight into the cold and invigorating stream of water can have a positive effect on your health and brain function.
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