Chameleon-like Smart Material

A new chameleon-like building material can change its colour – and how much heat it absorbs or emits – based on the outside temperature, its creators have said.

Designed to provide a low-energy way of maintaining building temperature, the material was developed by researchers at the University of Chicago in the US.  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 it emits just 7%, 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. “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 gas, the researchers said. About half of that 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, which radiates heat from people and objects. Materials also exist to prevent the emission of infrared in cold climates.

As global warming causes increasingly frequent extreme weather events and variable weather, buildings need to be able to adapt – few climates require year-round heating or year-round air conditioning.

Hsu and colleagues designed a non-flammable electrochromic building material that contains a layer that can switch between two ‘conformations’: solid copper, which retains most infrared heat, and a watery solution, which 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 a new research paper, the team detailed how the device can switch rapidly and reversibly between the metal and liquid states. The ability to switch between the two conformations reportedly remained efficient after 1,800 cycles.

The researchers also created models of how the material could cut energy costs in typical buildings in 15 different US cities. In an average commercial building, they found 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 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 6cm-wide pieces of the material. “Many such patches of the material could be assembled like shingles into larger sheets,” the research announcement said. “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 team is now investigating different ways of fabricating the material, and studying if intermediate states could be useful.