Temp-sensing smart building material changes color to save energy
Researchers at the University of Chicago’s Pritzker School of Molecular Engineering (PME) say they have designed a building material that changes its infrared color — and how much heat it absorbs or emits — based on the outside temperature. On hot days, the material can emit up to 92% of the infrared heat it contains, helping cool the inside of a building; on colder days, however, the material emits just 7% of its infrared, helping keep a building warm.
“We’ve essentially figured out a low-energy way to treat a building like a person,” says Asst. Prof. Po-Chun Hsu, who led the research. “You add a layer when you’re cold and take off a layer when you’re hot. This kind of smart material lets us maintain the temperature in a building without huge amounts of energy.”
Buildings account for 30% of global energy consumption and emit 10% of all global greenhouse gas according to some estimates. About half of this energy footprint is attributed to the heating and cooling of interior spaces.
Previously developed radiative cooling materials help keep buildings cool by boosting their ability to emit infrared, the heat that radiates from people and objects. Materials also exist that prevent the emission of infrared in cold climates.
However, say the researchers, there is a need for buildings to be able to adapt; few climates require year-round heating or year-round air conditioning. To address this, the researchers designed a non-flammable “electrochromic” building material that contains a layer that can take on two conformations: solid copper that retains most infrared heat, or a watery solution that emits infrared.
At any chosen trigger temperature, the device can use a tiny amount of electricity to induce the chemical shift between the states by either depositing copper into a thin film, or stripping that copper off. The device, say the researchers, can switch rapidly and reversibly between the metal and liquid states, with the ability to switch between the two conformations remaining efficient even after 1,800 cycles.
The researchers then created models of how their material could cut energy costs in typical buildings in 15 different U.S. cities. In an average commercial building, say the researchers, the electricity used to induce electrochromic changes in the material would be less than 0.2% of the total electricity usage of the building, but could save 8.4% of the building’s annual HVAC energy consumption.
“Once you switch between states, you don’t need to apply any more energy to stay in either state,” says Hsu. “So for buildings where you don’t need to switch between these states very frequently, it’s really using a very negligible amount of electricity.”
So far, the researchers have only created pieces of the material that measure about six centimeters across. However, say the researchers, many such patches of the material could be assembled like shingles into larger sheets. The material could also be tweaked to use different, custom colors—the watery phase is transparent and nearly any color can be put behind it without impacting its ability to absorb infrared.
Looking ahead, the researchers are now investigating different ways of fabricating the material. They also plan to probe how intermediate states of the material could be useful.
“We demonstrated that radiative control can play a role in controlling a wide range of building temperatures throughout different seasons,” says Hsu. “We’re continuing to work with engineers and the building sector to look into how this can contribute to a more sustainable future.”
For more, see “Radiative electrochromism for energy-efficient buildings.”