“The experiment offers the potential of creating buildings that do such a good job of passive cooling that energy costs will be cut in half.“
In a paper published by Science, the team writes that the best way to save energy on a building is not to expend it in the first place, and that one way to do that is simply to reflect away heat. That way buildings, especially those in hot and dry areas, don’t have to expend even more energy on air conditioning. And their delignification process for wood seems to create a material that might be widely used, one that holds genuinely remarkable properties.
Wood is composed of a number of materials, with cellulose in lignin at the top of the list. Most of the time, when people are talking about what gives wood its remarkable strength, the credit goes to the complex structure of lignin. But the team making the heat-reflecting wood went the other way. It processed it to break down the usually more resistant lignin, leaving the cellulose behind. Then it compressed the resulting cellulose into a sandwich of layers that create a “reflective, hazy surface” that’s very effective at scattering light.
Many materials have been tested that can radiate away heat at night, but daytime radiative cooling is many times more difficult, as usually materials that can reflect infrared light (heat) absorb visible light and end up turning that light into heat. But the treated wood is very efficient at reflecting infrared light and visible light, with properties that match some heavily engineered metamaterials. But unlike lab-grown metamaterials, the compressed wood may be able to be produced on a scale needed for nationwide construction.
One other result of the treatment is that the material is considerably stronger than the original wood. Its tensile strength (resistance to breaking) was not just 8.7 times that of natural wood, but greater than that of many forms of steel.
Tensile strength isn’t the only important mechanical property, but the “cooling wood” seems to score high on all factors. It’s also 10 times tougher and more resistant to scratches, dents, and cracks than natural wood. That kind of number suggests that the cooling wood could be useful in roofing applications.
The team studied older midrise apartment buildings in all climate zones across the United States and projects cooling-cost savings of approximately 35% by replacing some of the external structures with cooling wood. This kind of replacement strategy is one of the goals of the Green New Deal, which calls for retrofitting existing structures with energy-saving advances.
From the information provided in the article, it appears that the lignin in the wood was broken down by boiling the wood in a solution of highly concentrated hydrogen peroxide. While this material is certainly reactive (you would not want to get it on your skin, or anywhere else), it’s also a chemical that’s readily produced and doesn’t leave any lasting noxious residue. If that represents the compete process—something that is not clear from the published article—and the process is as scalable as the authors suggest, there would seem to be extensive possibilities for a material that has both the cooling properties and strength they describe.
The cooling properties of the material discussed in the publication would mean that buildings using it in cold climates would capture less heat. However, generating heat from electricity is far more efficient than cooling.
Ordinary pressure-treated lumber has a tensile strength and compression comparable to that of untreated lumber, but is more resistant to breakdown by fungi or bacteria. Older treated lumber sometimes contained compounds that included arsenic or lead. Most treated lumber today contains copper chromate. The cooling wood, at least as described in the publication, seems to contain none of these chemicals.