It takes a lot of force to keep the indoor space comfortable. About half of the energy Americans use in their homes goes to heating and cooling, which accounts for a significant portion of our utility bills and greenhouse gas emissions. Many buildings have insulation in their walls to maintain ideal temperatures, but older buildings in particular are surprisingly energy inefficient.
Scientists have been working on high-tech solutions to this problem for decades. Materials scientists have now developed a color-changing film that can switch between heating and cooling modes. Thinner than a credit card, this film works with very little energy and will someday envelop even the most wasteful buildings, radiating unwanted heat in the summer and helping to trap it in the winter. . The survey results recently natural sustainability.
“This is a really impressive result,” says Yao Zhai, a mechanical engineer at the University of Missouri, who was not involved in the study.
The new device takes advantage of a natural phenomenon called radiative cooling. Radiative cooling reduces outdoor temperatures at night and helps cool the entire planet. Everything around us, including our bodies and buildings, constantly emits heat in the form of mid-infrared radiation. This is an electromagnetic wave with a frequency lower than that of visible light. Po-Chun Hsu, a molecular engineer at the University of Chicago and senior author of the study, said:
If you point an infrared camera at Earth from orbit, you can also see the heat radiating from the planet into the cold vacuum of space. Our atmosphere, incidentally, allows more mid-infrared radiation to leave Earth than other wavelengths of light. Most of that heat leaves the Earth, but some is still trapped in greenhouse gases in the atmosphere. This is enough to throw the Earth’s thermal balance off and cause temperatures to rise, explains Peter Vermell, an electrical engineer at Purdue University Research. As global temperatures rise, scientists are developing solutions to maximize the heat released by radiative cooling. Among these technologies are films that can wrap structures to release more heat. However, many parts of the world experience not only sweltering summers, but also harsh winters. “You don’t need anything to quickly cool your home when it’s already below freezing,” says Barmel.
This dilemma was the inspiration for the new coating. This coating can switch between high and low heat emission with a simple electrical discharge. Similar tunable devices for visible light already exist. A so-called dynamic window can be switched from transparent to opaque to control the amount of light transmitted through it. However, until now, no architectural film has done the same for mid-infrared heat.
New material starts in cooling mode. Beneath the incredibly thin conductor are tiny puddles of dissolved copper ions. In this state, the device naturally dissipates heat and cools the building. Then, when a small electrical charge is placed on the conductor layer, molten copper deposits on its surface, forming a thin layer on the reservoir. Copper emits very little heat in the mid-infrared it absorbs, so the device traps heat. This change can be undone any number of times, but its effectiveness diminishes with repeated use. After 1,000 cycles, both cooling and heating modes become less efficient.
The authors estimate that applying this technology to the exterior of a building with a film could save 8.4% of the energy used for heating and cooling in climates with rapid temperature changes throughout the year. The building’s color also changes from dark white in summer to metallic copper in winter, but the film can be covered with a special paint that does not block the mid-infrared.
“Now this is just the first step in demonstrating the mechanism, and we are already seeing very good progress,” says Qiaoqiang Gan, a materials scientist and engineer at the King Abdullah University of Science and Technology in Saudi Arabia. . study. This new system looks particularly promising compared to previous attempts by other research groups to create tunable devices using aqueous solutions. Some of the materials considered for use in similar devices were highly flammable and clearly unsuitable for covering buildings. The new film will not catch fire, but it is not ready for use yet.
In addition to decreasing efficiency over time, the main drawback of newer devices is their high cost. The thin electrode covering the outer layer of the film is high-quality graphene, an expensive single-atom-thick array of carbon. Graphene’s exceptional thinness allows heat to pass through while the material conducts electricity. For these building envelopes to be feasible, researchers need to achieve the same results with materials that are inexpensive and can be manufactured on a large scale. Hsu and his team plan to experiment with lower-quality graphene and other materials to find more cost-effective alternatives. We also plan to try cheaper metals such as zinc instead of copper.
Balancing price and performance will take time, so your neighborhood may not be filled with color-changing eco-buildings for years to come. But it’s a “very hot topic” for research, says Gunn, and for good reason. Reducing energy use by about 8% may seem like a small thing, but “on a societal scale, it can have a huge impact,” says Vermell. “A few percent change in energy demand and supply can make a big difference.”
