A thermoresponsive polymer sandwiched between a mechanical layer, for support, and a porous top membrane helps buildings remain cool. Rotzetter, ACC, et al./Advanced Materials/Wiley
Researchers in Switzerland have identified a hydrophilic, hydrophobic polymer that changes its water-absorbing qualities depending on its temperature, giving buildings the ability to ‘sweat.’
October 23, 2012—Researchers at Eidgenössische Technische Hochschule Zürich (ETH Zurich), have identified a polymer that changes its characteristics from hydrophilic to hydrophobic at 32 °C (89.6 °F). The real-world application of this technology is as unexpected as it is significant: the researchers have effectively discovered how to enable buildings to “sweat,” making significant energy savings possible.
A paper describing the research, “Thermoresponsive Polymer Induced Sweating Surfaces as an Efficient Way to Passively Cool Buildings,” was published in the journal Advanced Materials by Aline Rotzetter, a doctoral candidate at the university; Wendelin J. Stark, Ph.D., a professor at the Institute for Chemical and Bioengineering at the university; and five additional coauthors (Wiley, 2012). In response to written questions submitted by Civil Engineering online, Rotzetter, the primary author, said that the research was “bioinspired” by the “sweating action of humans,” and was driven by “the urgent need to reduce our energy consumption [and because] air-conditioning units are guzzlers.”
More than 40 percent of the energy consumed in the United States can be traced back to building utilities, according to the study, and heating and cooling are the main contributors to this energy consumption.
In the study, two mats—one a thermoresponsive hydrogel and one a conventional hydrogel—were created and placed atop model homes with roofs measuring 42 cm2 to test their ability to become saturated with water and then remain cool by “sweating.” The artificial “sunlight” source used in the tests operated at 1,000 W m-2, the equivalent of intense solar radiation at noontime in a midlatitude region. The thickness of the mats was adjusted in order to ensure that at full saturation both held the same amount of water.
The difference between the cooling protection provided by the
conventional mat, left, and the thermoresponsive hydrogel mat,
right, are visible in this infrared image taken 60 minutes after the
two buildings were irradiated with simulated sunlight. Rotzetter,
ACC, et al./Advanced Materials/Wiley
During the test, the mats absorbed water from simulated rainstorms and expelled the water as vapor when exposed to the simulated sunlight. Much like the process of sweating, the mats drew in heat to turn the water into vapor, allowing the interior of the building to maintain a cooler temperature. However, while the conventional hydrogel stopped providing cooling benefits relatively quickly and began heating up after 20 minutes, the thermoresponsive hydrogel continued working until all its absorbed water had evaporated, some three hours after continuous irradiation.
The importance of the thermoresponsive hydrogel used in the study —known as poly(N-isopropylacrylamide, or PNIPAM—was its ability to change its chemical property from hydrophilic to hydrophobic at 32 °C (89.6 °F), Rotzetter explained. This means that it will eagerly absorb water below that temperature and actively work to expel water above that temperature. Once the hydrogel begins to repel its stored water, “the water is pressed to the surface and available for evaporation,” she explained.
The conventional hydrogel used in the study differs because it does not change characteristics in response to thermal changes. The researchers found that because the conventional mat does not react to temperature changes, the stored water evaporates “constantly and at the same ratio,” Rotzetter said. “So if it gets really hot—above 32°C—the conventional hydrogel just dries out from the top and therefore it builds up a dense top layer, which further retards the evaporation.
“The thermoresponsivity of the PNIPAM makes [all] the difference,” Rotzetter said. Its ability to change its properties from hydrophilic to hydrophobic as it heated “is the reason for the good cooling performance.”
The mats were put through various cycles of simulated rain and direct sunlight to test their effectiveness at helping buildings maintain a lower internal temperature, their longevity in the face of multiple extreme rain/heating cycles, and their ability to withstand intense, direct, heat or quickly changing conditions—all while continuing to function.
The researchers estimated that the savings in electrical energy and its associated reduction in carbon dioxide emissions for a midsized, detached house would be at least 60 percent—all while maintaining a constant interior temperature of 20°C (68°F) in direct sunlight.
The next steps, according to Rotzetter, include “upscaling” the research, installing a thermoresponsive hydrogel mat on the roof of a full-size building. Researchers could then begin to “deal with new challenges, like harsh weather conditions,” she said. The study also noted that future material developments could target polymers with the ability to “reload” their water levels by absorbing humidity at night, enabling the mats to work even without precipitation.