One of the world's oldest building materials may soon be getting a makeover. Wood is celebrated for being a good insulator, durable, and renewable, but despite its best qualities it's still opaque, and we need natural light—not only for our sanity but also to decrease our use of artificial light. Glass, however, is a really bad insulator and not energy-efficient. Looking for an alternative to drafty windows, a team of researchers at KTH Royal Institute of Technology in Sweden have developed a process of chemically treating wood to make it both transparent and capable of storing and releasing heat.
"We prepared a material that is multifunctional—it can transmit light very well and also it can store heat. We combined these two functions in a single material," expained Céline Montanari, one of the researchers on the project, when she presented their work at the American Chemical Society last month.
Since one-third of the world's energy consumption currently comes from the building sector—and largely due to our heating, cooling, and lighting systems—the material is being touted as a potential game-changer for energy-efficient construction.
The two-step process begins with removing the lignin—a polymer that makes wood rigid—from a piece of wood, which leaves behind a lot of porous space in the sample. This idea is not new and the resulting piece remains structurally sound and looks like frosted glass. The team took it a step further by filling the new microscopic holes they created with polyethylene glycol, a "phase-change material" that can be either solid or liquid depending on the temperature. As the temperature rises it takes in heat and melts to a liquid, and as it cools down it releases heat and becomes solid. The cell walls of the wood provide structure for the polymer so that it stays put even in its liquid form. As it undergoes this process it goes between different levels of opacity, from a more frosted look to almost fully clear.
During her presentation, Montanari showed a sample of the transparency that can be achieved (that's her very on-brand business card!).
This characteristic makes it a less-than-ideal window substitute, but it could easily be applied as a skylight or to increase the amount of natural light in a building. As Montanari says, the more you can incorporate into a design, the more it would reduce the energy footprint of the building.
"If you take 100g of this transparent wood material with the [polyethylene glycol] inside, it can absorb up to 8,000J of heat, which corresponds to basically what a 1W [bulb] could produce in two hours," said Montanari. Since different polymers melt at different temperatures, they would have to use different polymers depending on the application—a sheet being installed in Canada, for example, would have to work within a very different range of temperatures than one installed in Morocco.
Another pro is that it retains the mechanical strength of wood and wouldn't shatter on impact, so it's safer a safer option than glass.
Montanari and her team are currently looking at increasing the heat storage capacity of the transparent wood for greater energy savings. They're also working with a partner to figure out production at an industrial scale. They've already filed a patent and hope to have a commercially viable product within five years.
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