New material like a jacket for buildings

While air conditioning was a marvel of its time, concerns over the Earth's climate has led to the pursuit of more efficient energy consumption and management. Now, researchers at the University of Chicago's Pritzker School of Molecular Engineering have designed a chameleon-like building material made with copper that changes its infrared color – and how much heat it absorbs or emits based on outside temperature.

"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 Po-Chun Hsu, an assistant professor 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."

Efforts to reduce the carbon footprint of commercial buildings have been a long-term goal for many conscientious architects, as metropolitan areas are crammed with colossal buildings of steel and concrete.

Water use, electricity, HVAC (heating, ventilation, air conditioning) – the logistics of a 300- to 500-foot-tall building is generally not in consideration for those that work in it day to day. However, upkeep is a significant portion of overhead.

According to some estimates, buildings account for roughly 30 to 40% of global energy consumption and emit approximately 10% of all global greenhouse gas. Half of that is attributed to the heating and cooling of the interior spaces alone.

"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."

Exploring the diversity, the 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.

"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 PME graduate student Chenxi Sui, first author of the published paper.

While the color black is mostly well-known for absorbing heat, sunscreen works by applying "black" in the infrared spectrum, thereby protecting from harmful UV rays in the sun while not necessarily overheating in direct sunlight.

Essentially, the researchers have created a controllable "sunscreen" for buildings.

Solid to liquid and back

In their paper, the team detailed how their new material can switch rapidly and reversibly between a metal and liquid state.

While being called a material, it functions more like a device. Hsu and colleagues designed a non-flammable "electrochromic" building material that contains a layer that can take on two forms – 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 the copper off entirely.

In their tests, the team showed that the ability to switch between the two states remained efficient even after 1,800 cycles.

"Once you switch between states, you don't need to apply any more energy 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 negligible amount of electricity."

Creating models of how the material could cut energy costs in typical buildings, the team chose 15 different U.S. cities.

In an average commercial building, the electricity used to induce electrochromic changes in the material would be less than 0.2% of the total electricity usage of the building. This meager electricity draw offers a major upshot – a nearly 10% reduction of the building's annual HVAC energy consumption.

Future prospects

On hot days, the researchers found that the material can emit up to 92% of the infrared heat it contains, helping cool the inside of a building, while on colder days, the material emits only 7% of its infrared, helping keep a building warm.

So far, Hsu's group has only created pieces of the material measuring six centimeters across. However, they imagine that many such patches of the material could be assembled like shingles into larger sheets.

Furthermore, they report that the material could also be tweaked to use different, custom colors as the liquid phase is transparent, and nearly any color can be used behind it without impacting its ability to absorb infrared.

Currently, the team is 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."

While the prospect of having shielding against infrared to deter and retain heat in buildings is an admirable goal, perhaps there may be other unforeseen uses of such an interesting technology. Possibly, this energy-saving discovery could be used for battery insulation, solar panels, electric vehicles, aerospace, or other applications that would benefit from better thermal control.

Nevertheless, this small step is just the beginning of what could be the HVAC of the future.