Researchers at the University of Chicago’s Pritzker School of Molecular Engineering (PME) have designed a chameleon-like 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 a mere 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,” says asstistant professor Po-Chun Hsu, who led the research, which has been published in Nature Sustainability. “This kind of smart material lets us maintain the temperature in a building without huge amounts of energy.”

Driven by climate change

According to a study, buildings consume almost 151 EJ energy, equivalent to 36% of the world's final energy consumption. And about 30% of the global energy consumption is used for the operation of buildings. They emit 10% of all global greenhouse gas. Half of this energy footprint can be traced 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,” says 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, the invisible heat that radiates from people and objects. Materials also exist that prevent the emission of infrared in cold climates. 

The material contains a layer that can take on two conformations: solid copper that retains most infrared heat, which helps keep the building warm; or a watery solution that emits infrared, which can help cool the building. Image: Courtesy of Hsu Group

“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,” says PME graduate student Chenxi Sui, the first author of the new manuscript.

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.

From metal to liquid and back

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. 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 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 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,” 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.”

Scaling up

So far, Hsu’s group has only created pieces of the material that measure about six centimeters across. However, they imagine that many such patches of the material could be assembled like shingles 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.

Hsu Group created models of how their material could cut energy costs in typical buildings in 15 different US cities, finding that, on average, the material would use less than 0.2% of the building’s total electricity, but could save 8.4% of the building’s annual HVAC energy consumption. Image: Courtesy of Hsu Group

"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." 

Study abstract

Radiative thermoregulation can reduce the energy consumption for heating, ventilation and air-conditioning (HVAC) in buildings, and therefore contribute substantially to climate change mitigation.

Electrochromism, a phenomenon in which a material exhibits reversible colour changes under an external electrical stimulus, can help control the heat balance of buildings in response to fluctuating weather conditions; however, its implementation has been largely limited to visible and near-infrared wavelength regimes.

Here we develop an aqueous flexible electrochromic design for use as a building envelop based on graphene ultra-wideband transparent conductive electrode and reversible copper electrodeposition, in which the thermal emissivity can be tailored to vary between 0.07 and 0.92 with excellent long-term durability.

Building energy simulations show that our design as building envelopes can save on year-round operational HVAC energy consumption across the United States by up to 43.1 MBtu on average in specific zones.

Such dynamic emissivity tunability can further serve as a non-destructive technological solution to retrofit poorly insulated or historic buildings. Our work suggests a feasible pathway to radiative thermoregulation for more energy-efficient HVAC and solving some of the global climate change issues.