Building material offers low-energy solution to maintaining temperature

The team from the University of Chicago’s Pritzker School of Molecular Engineering (PME) said that on hot days the material can emit up to 92 per cent of the infrared heat it contains, and on colder days the material emits seven per cent of its infrared.

“We’ve essentially figured out a low-energy way to treat a building like a person; you add a layer when you’re cold and take off a layer when you’re hot,” said Asst. Prof. Po-Chun Hsu, who led the research published in Nature Sustainability. “This kind of smart material lets us maintain the temperature in a building without huge amounts of energy.”

Some estimates suggest that buildings account for 30 per cent of global energy consumption and emit 10 per cent of all global greenhouse gas, with about half of this energy footprint attributed to heating and cooling.

“For a long time, most of us have taken our indoor temperature control for granted, without thinking about how much energy it requires,” Hsu said in a statement. “If we want a carbon-negative future, I think we have to consider diverse ways to control building temperature in a more energy-efficient way.”

Researchers have previously developed radiative cooling materials that help keep buildings cool by increasing their ability to emit infrared. Materials also exist that prevent the emission of infrared in cold climates.

Now, Hsu and colleagues have 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 small amount of electricity to induce the chemical shift between the states by either depositing copper into a thin film, or stripping the copper off.

In their paper, the researchers detailed how the device can switch rapidly and reversibly between the metal and liquid states. They showed that the ability to switch between the two conformations remained efficient after 1,800 cycles.

The team then produced models of how their material could cut energy costs in typical buildings in 15 different US cities. They reported that in an average commercial building the electricity used to induce electrochromic changes in the material would be less than 0.2 per cent of the total electricity usage of the building but could save 8.4 per cent 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,” said 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.”
Hsu’s group has so far created pieces of the material that measure about 6cm across, but they imagine that many patches of the material could be assembled into larger sheets. They said the material could also be adjusted to use different, custom colours as the watery phase is transparent and nearly any colour can be put behind it without impacting its ability to absorb infrared.

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.