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Stronger than aluminum, a heavily altered wood cools passively

Enlarge / A look at the lignin-free compressed wood.

University of Maryland

Most of our building practices aren’t especially sustainable. Concrete production is a major source of carbon emissions, and steel production is very resource intensive. Once completed, heating and cooling buildings becomes a major energy sink. There are various ideas on how to handle each of these issues, like variations on concrete’s chemical formula or passive cooling schemes.

But now, a large team of US researchers has found a single solution that appears to manage everything using a sustainable material that both reflects sunlight and radiates away excess heat. The miracle material? Wood. Or a form of wood that’s been treated to remove one of its two main components.

With the grain

Wood is mostly a composite of two polymers. One of these, cellulose, is made by linking sugars together into long chains. That cellulose is mixed with a polymer called lignin, which is not really a single polymer. The precise chemical formula of its starting material can vary among species, and it typically contains multiple places where chemical bonds can form, turning the polymer into a chaotic but extremely robust mesh.

The complicated structure of lignin.
Enlarge / The complicated structure of lignin.

Lignin creates a lot of problems for biofuels production, since its variability makes it difficult to digest (cellulose, in contrast, can be broken down into simple sugars given time). But lignin provides a toughness to wood that cellulose alone wouldn’t.

Or at least cellulose wouldn’t in its native form in wood. The new chemical treatment essentially removes the lignin from wood. The precise nature of the process isn’t mentioned in the paper, which suggested it might be nightmarishly complex or involve extremely toxic chemicals. But a check of the supplemental material shows that the process involves dumping the wood in concentrated hydrogen peroxide and boiling it. While I wouldn’t want to drink boiling, concentrated hydrogen peroxide, it’s not an especially difficult chemical to handle safely.

Based on the description of lignin, you’d expect the resulting wood would be weaker. But the chemically treated wood is then compressed. With no cells or lignin to keep them apart, the many oxygen/hydrogen groups that hang off sugars are free to interact with each other, creating a dense hydrogen bonding mesh. This ultimately makes the material much stronger than wood (though the researchers don’t compare its strength to that of pressure-treated lumber, which is also stronger than untreated wood).

There are myriad ways to measure toughness: resistance to bending, stretching, impacts, etc. The researchers measured a number of these, and the modified wood came out ahead of untreated wood by large margins—anywhere from three to 10 times wood’s value. Strikingly, for at least one of these measures (tensile strength), it edged out some types of steel and titanium. All of which means that it should be possible to use this material in places where wood wouldn’t normally be considered.

It’s cool stuff

But rather than simply being structurally useful, the wood has some properties that could make it extremely useful as cladding, covering the exterior of a building. While most of the cellulose fibers are aligned along the grain of the wood, that alignment is very rough—there’s plenty of variability in their orientation. That means light that strikes the processed wood will bounce around within a dense mesh of cellulose fibers, scattering widely in the process. The end result is a material that looks remarkably white, in the same way a sugar cube looks white even though each sugar crystal in it is transparent.

As a result, the material is really bad at absorbing sunlight, and thus it doesn’t capture the heat in the same way regular wood does.

But it gets better. The sugars in cellulose are effective emitters of infrared radiation, and they do so in two areas of the spectrum where none of our atmospheric gasses is able to reabsorb it. The end result is that, if the treated wood absorbs some of the heat of a structure, wood can radiate it away so that it leaves the planet entirely. And the wood is able to do so even while it’s being blasted by direct sunlight; the researchers confirmed this by putting a small heater inside a box made of the treated wood and then sticking it in the sunlight in Arizona.

In the heat of the day, a square meter of the wood could radiate away about 16W of power. At night, that figure shot up to 63W, for a 24-hour average of 53 Watts per square meter. At mid-day, if there was no source of heat in the box, its ambient temperature was over 4°C lower than the surrounding air. This is all the result of the fact that the treated wood emits energy in the infrared more efficiently than it absorbs energy in thee visible wavelengths.

Around the country

What could this do for a building? To find out, the researchers used a model of a typical apartment building that included sources of heat like lights and occupants and tracked radiative heat transfer using ray tracing. They then placed the building in 16 different US cities and tracked its energy balance over the course of a year using historic weather data. In cities in the West and South, like Atlanta, Las Vegas, and Phoenix, the material cut down on the amount of energy needed for cooling considerably.

The researchers estimate that covering an apartment building with the treated wood could save about 35 percent of the energy used for cooling. In a dense urban setting, that number goes up to over half. Plus, it also has the strength to handle some of the internal structure of the building. And while forestry can create environmental issues, it is certainly possible to manage it in a way that is sustainable.

All of which makes the wood—the researchers refer to it as “cooling wood”—a very promising looking material. So it’s no surprise to see that three of the team members behind the new paper have a patent out to commercialize the tech.

Science, 2019. DOI: 10.1126/science.aau9101  (About DOIs).

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About Alfred Jackson

Alfred R. Jackson writes for Technology section in AmericaRichest.

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