Material that changes colour to save energy

USA: Researchers at Chicago University have designed a building material that changes its infrared colour 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; 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 at the University’s Pritzker School of Molecular Engineering (PME). “This kind of smart material lets us maintain the temperature in a building without huge amounts of energy.”

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.

In the new paper, published in Nature Sustainability, 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 even after 1,800 cycles.

Then, the team created models of how their material could cut energy costs in typical buildings in 15 different US cities. In an average commercial building, they reported, 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,” 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 into larger sheets. They say the material could also be tweaked to use different, custom colours—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.