Left: the new wood-derived white foam. Right: the foam demonstrating its cooling nature in direct sunlight
Adapted from Nano Letters 2022
A huge chunk of energy usage goes towards heating and cooling homes and buildings, so it’s important to find new ways to regulate temperature passively. Engineers in China and Germany have developed a new wood-based foam that can drastically cool buildings, to the degree that can be easily tuned.
Air conditioning is effective, but it’s not the most environmentally friendly way to cool off. In recent years scientists have been developing techniques that don’t guzzle so much electricity, including super-white paints or mirror coatings that reflect sunlight, and radiative cooling systems that emit a building’s heat as infrared waves into the sky, where it passes straight through the atmosphere and vents into the cold of outer space.
But the effectiveness of these systems can vary under different conditions. Reflective coatings, for instance, can still let in heat if it’s particularly hot or humid, while radiative cooling doesn’t work as well if it’s cloudy. The level of cooling can also be hard to tune.
Now, scientists at the University of Göttingen and Nanjing Forestry University have created a new material that they say solves some of these problems. It’s essentially a foamy substance made out of wood that reflects sunlight, acts as thermal insulation, and emits absorbed heat back into the atmosphere.
It’s made out of cellulose nanocrystals, connected with a bridge of silane, which is then freeze-dried into a white, lightweight foam. The final product reflects 96 percent of visible light from the Sun and emits 92 percent of the infrared radiation it absorbs.
To test the material’s cooling abilities, the researchers placed it over a box lined on the inside with aluminum foil and set it in direct sunlight at noon. The material kept the inside of the box 16.5 °F (9.2 °C) cooler than the air outside, and even in humid weather it still kept the interior 13.3 °F (7.4 °C) cooler. If scaled up to line the roof and walls of a building, the researchers estimate that it could cut energy requirements for cooling by 35 percent on average.
Most intriguingly, the foam’s cooling power decreased if it was compressed. That might sound like a disadvantage, but the team suggests that squeezing the foam could be a useful way to fine-tune the amount of cooling as required by the weather or environment.
The researchers say that the new cooling foam could eventually pave the way for thermal-regulating materials that are not only environmentally friendly themselves, but also cut the cost of energy used for cooling.