Households in the U.S. Southwest spend about $400 per year to cool their homes, about twice the national average. Now, a new type of wood that radiates heat into space could offer some relief. If used on a building’s exterior, such as in siding and roofs, the material could drop a building’s temperature as much as 10°C and reduce cooling costs as much as 60%.
“This is just brilliant work,” says John Simonsen, a chemist who specializes in wood science at Oregon State University in Corvallis. However, he says, the new wood could be expensive, and potential energy savings may not offset the price.
When most materials heat up, they emit that heat as photons of near infrared (IR) light. The light is readily absorbed by molecules in the surrounding air, trapping the heat—and keeping houses, for example, hot. But in the past 2 years, researchers have devised plastic films and paints that absorb heat and re-emit that energy at longer mid-IR wavelengths, which air doesn’t absorb. If emitted toward the sky, these photons pass unimpeded and dump their energy into deep space. But to use these materials in buildings, engineers need to laminate rooftop or siding materials with the plastics or apply the heat-emitting paints.
To see if the same attributes could be engineered into a building’s structure, Liangbing Hu, a materials scientist at the University of Maryland in College Park, looked at wood. Wood consists of three main components: cellulose and hemicellulose, which form long strawlike structures, and lignin, which acts as a glue holding the straw strands together. Lignin is a strong emitter of IR light, so Hu and his colleagues knew they had to ditch it.
The researchers hit upon a simple chemical procedure. They soaked basswood in a solution of hydrogen peroxide, which chops normally long lignin molecules into small fragments. The fragments diffuse out of the solution and can be washed away. The team then used a hot press, an industrial vise for making wood composites, to compress the remaining cellulose and hemicellulose components together. The result was an engineered wood with eight times the strength of natural wood.
The new wood is more than just strong. Stripped of lignin, it turns white, reflecting virtually all incoming light. The new composite also absorbs heat from its surroundings and reradiates it as mid-IR light. That allows the material to cool surfaces to which it is attached by up to 10°C, the researchers report today in Science. The wood doesn’t radiate heat away quite as well as the plastic films. But it’s still cool to the touch, and could make a big difference, Hu says.
If this material were applied to the outsides of buildings in the desert Southwest of the United States or other similarly warm climates, the passive cooling effect could reduce air conditioning costs by as much as 60%, the team calculates. The material could also provide welcome relief in developing countries where air conditioning is less common.
As promising as that sounds, Simonsen cautions that this relief may not come easily. For starters, wood isn’t regularly used in roofing. It’s flammable and isn’t as durable as asphalt shingles or other standard roofing materials. It also wouldn’t make sense to use cooling wood to side buildings in cooler climates, where it would increase the cost of heating homes in winter.