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A sample of new wood that could reduce cooling costs.
University of Maryland
By Robert F. Service
Households in the southwestern United States spend about $ 400 a year to cool their homes, about double the national average. Now, a new type of wood that gives off heat in the space could offer some relief. If they are used outside a building, such as coatings and roofs, the material can lower the temperature of a building up to 10 ° C and reduce cooling costs up to 60%.
"It's a brilliant job," says John Simonsen, a wood science chemist at Oregon State University in Corvallis. However, he says, the new wood could be expensive and the potential energy savings might not offset the price.
When most materials heat, they emit this heat as photons of near infrared light. The light is easily absorbed by the molecules of the ambient air, thus capturing the heat and keeping the houses warm, for example. But over the last two years, researchers have developed plastic films and heat-absorbing paints that re-emit that energy at longer wavelengths in the average IR, than the air does not absorb. If they are emitted to the sky, these photons pass unimpeded and pour their energy into the depths of space. But to use these materials in buildings, engineers must laminate roofing or cladding materials with plastics or apply heat-emitting paints.
To see if the same attributes could be incorporated into the structure of a building, Liangbing Hu, a materials scientist at the University of Maryland at College Park, examined the wood. The wood is composed of three main components: cellulose and hemicellulose, which form long structures resembling straw, and lignin, which acts as a glue holding the straw strands together. Lignin is a powerful emitter of infrared light. Hu and his colleagues therefore knew that they had to abandon it.
The researchers used a simple chemical procedure. They soaked American linden in a solution of hydrogen peroxide, which cuts into small fragments of normally long lignin molecules. The fragments diffuse out of the solution and can be washed. The team then used a hot press, an industrial vice for the manufacture of wood-based composites, to compress the remaining components of cellulose and hemicellulose. The result was an engineered wood eight times stronger than natural wood.
The new wood is more than solid. Devoid of lignin, it becomes white and reflects almost all incoming light. The new composite also absorbs the heat of its environment and redirects it in the form of average IR light. This allows the material to cool surfaces to which it is attached up to 10 ° C, researchers report today. Science. Wood does not release heat as well as plastic films. But it's cool to the touch, and it could make a big difference, Hu said.
If this material was applied to the outside of buildings in the Southwest Desert of the United States or in other similar hot climates, the passive cooling effect could reduce air conditioning costs by 60%, calculated the team. This material could also provide welcome relief in developing countries where air conditioning is less common.
As promising as it may sound, Simonsen warns that this relief may not come easily. For starters, wood is not used regularly in roofs. It is flammable and is not as durable as asphalt shingles or other standard roofing materials. It is also not wise to use cooling wood for buildings in colder climates, which would increase the cost of heating homes in winter.
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