This image shows how the novel electrochromic material can conduct both heating and cooling by switching between two different conformations. Image: University of Chicago PME/Hsu Group.
This image shows how the novel electrochromic material can conduct both heating and cooling by switching between two different conformations. Image: University of Chicago PME/Hsu Group.

Researchers at the University of Chicago’s Pritzker School of Molecular Engineering (PME) have designed a chameleon-like 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 to cool the inside of a building. On colder days, however, the material emits just 7% of its infrared heat, helping to keep the building warm.

“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 assistant professor Po-Chun Hsu, who led the research, which is reported in a paper in Nature Sustainability. “This kind of smart material lets us maintain the temperature in a building without huge amounts of energy.”

According to some estimates, buildings account for 30% of global energy consumption and emit 10% of all global greenhouse gases. About half of this energy footprint is attributed to the heating and cooling of interior spaces.

“For a long time, most of us have taken our indoor temperature control for granted, without thinking about how much energy it requires,” said Hsu. “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 boosting their ability to emit infrared heat, which radiates from people and objects. Materials also exist that prevent the emission of infrared heat in cold climates.

“A simple way to think about it is that if you have a completely black building facing the sun, it’s going to heat up more easily than other buildings,” said Chenxi Sui, a PME graduate student and first author of the paper. That kind of passive heating might be a good thing in the winter, but not in the summer.

As global warming causes increasingly frequent extreme weather events and variable weather, there is a need for buildings to be able to adapt; few climates require year-round heating or year-round air conditioning. To this end, Hsu and colleagues 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 heat. At any chosen trigger temperature, the device can use a tiny amount of electricity to induce a chemical shift between the two states by either depositing copper into a thin film, or stripping that copper off.

In the paper, the researchers detailed how the device can switch rapidly and reversibly between the metal and liquid states. They showed that this ability to switch between the two conformations remained efficient even after 1800 cycles.

Next, the team created models of how their material could cut energy costs in typical buildings in 15 different US cities. For an average commercial building, they calculated that 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 the material could save 8.4% of the building’s annual HVAC (heating, ventilation and air-conditioning) 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.”

So far, Hsu’s group has only created pieces of the material that measure about 6cm across. However, they imagine that many such patches of the material could be assembled like shingles into larger sheets. They also say the material could be tweaked to use different, custom colors – the watery phase is transparent and nearly any color can be placed 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.

“We demonstrated that radiative control can play a role in controlling a wide range of building temperatures throughout different seasons,” said Hsu. “We’re continuing to work with engineers and the building sector to look into how this can contribute to a more sustainable future.”

This story is adapted from material from the University of Chicago, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.