ORLANDO, Fla., April 3, 2019 -- Wood may seem more at home in log cabins than modern architecture, but a specially treated type of timber could be tomorrow's trendy building material. Today, scientists report a new kind of transparent wood that not only transmits light, but also absorbs and releases heat, potentially saving on energy costs. The material can bear heavy loads and is biodegradable, opening the door for its eventual use in eco-friendly homes and other buildings.
The researchers will present their results today at the American Chemical Society (ACS) Spring 2019 National Meeting & Exposition. ACS, the world's largest scientific society, is holding the meeting here through Thursday. It features nearly 13,000 presentations on a wide range of science topics.
A brand-new video on the research is available at http://bit.ly/HLS_Transparent_Wood.
"Back in 2016, we showed that transparent wood has excellent thermal-insulating properties compared with glass, combined with high optical transmittance," says Céline Montanari, a Ph.D. student who is presenting the research at the meeting. "In this work, we tried to reduce the building energy consumption even more by incorporating a material that can absorb, store and release heat."
As economic development progresses worldwide, energy consumption has soared. Much of this energy is used to light, heat and cool homes, offices and other buildings. Glass windows can transmit light, helping to brighten and heat homes, but they don't store energy for use when the sun goes down.
Three years ago, lead investigator Lars Berglund, Ph.D., and colleagues at KTH Royal Institute of Technology in Stockholm, Sweden, reported an optically transparent wood in the ACS journal Biomacromolecules. The researchers made the material by removing a light-absorbing component called lignin from the cell walls of balsa wood. To reduce light scattering, they incorporated acrylic into the porous wood scaffold. The team could see through the material, yet it was hazy enough to provide privacy if used as a major building material. The transparent wood also had favorable mechanical properties, enabling it to bear heavy loads.
Building on this work, Montanari and Berglund added a polymer called polyethylene glycol (PEG) to the de-lignified wood. "We chose PEG because of its ability to store heat, but also because of its high affinity for wood," Montanari says. "In Stockholm, there's a really old ship called Vasa, and the scientists used PEG to stabilize the wood. So we knew that PEG can go really deep into the wood cells."
Known as a "phase-change material," PEG is a solid that melts at a temperature of 80 F, storing energy in the process. The melting temperature can be adjusted by using different types of PEGs. "During a sunny day, the material will absorb heat before it reaches the indoor space, and the indoors will be cooler than outside," Montanari explains. "And at night, the reverse occurs -- the PEG becomes solid and releases heat indoors so that you can maintain a constant temperature in the house."
The team encapsulated PEG within the de-lignified wood scaffold, which prevented leakage of the polymer during phase transitions. They also incorporated acrylic into the material to protect it from humidity. Like their earlier version, the modified wood was transparent, though slightly hazy, and strong, but had the added bonus of storing heat.
The researchers point out that the transparent wood has the potential to be more environmentally friendly than other building materials such as plastic, concrete and glass. In addition to its thermal-storage capabilities, the transparent wood could be easier to dispose of after it has served its purpose. "The PEG and wood are both bio-based and biodegradable," Berglund notes. "The only part that is not biodegradable is the acrylic, but this could be replaced by another bio-based polymer."
Now, the focus turns to scaling up the production process to be industrially feasible. The researchers estimate that transparent wood could be available for niche applications in interior design in as little as five years. They are also trying to increase the storage capacity of the material to make it even more energy-efficient.
A press conference on this topic will be held Wednesday, April 3, at 9 a.m. Eastern time in the Orange County Convention Center. Reporters may check-in at the press center, Room W231B, or watch live on YouTube http://bit.ly/ACSLive_Orlando2019 ("ACSLive_Orlando2019" is case sensitive). To ask questions online, sign in with a Google account.
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Multifunctional transparent wood for thermal energy storage applications
Recent research on transparent wood composite highlighted its potential to be used in thermal insulating buildings thanks to the combination of low thermal conductivity, unique optical features, and favorable mechanical properties. In this study, a multifunctional transparent wood composite was designed, extending the applications of transparent wood to thermal energy storage systems in passive buildings. Organic phase-change materials (PCMs), such as polyethylene glycol (PEG), can store and release latent heat energy during a reversible phase change process. PEG is an environmentally friendly and recyclable organic PCM, which exhibits large latent heat and undergoes solid-liquid phase transitions at suitable temperature range. However, the utilization of solid-liquid PCMs is limited due to leakage issues occurring during the melting process, hence decreasing their long-term thermal reliability. In this work, the wood scaffold was used to encapsulate PEG to prevent leakage during phase transitions. A multifunctional transparent wood composite was designed with functionalities including optical transparency and latent heat energy storage. During the experimental procedure, PEG was firstly shape-stabilized into a polymethyl methacrylate (PMMA) matrix, and further encapsulated into a delignified wood scaffold. Wood delignification was performed to remove the light absorbing components (lignin), and transparency was eventually achieved after impregnation of the shape-stabilized PEG into the porous wood cells. The prepared multifunctional transparent wood combines excellent thermal properties with large latent heat, and tunable optical properties with unique high optical transmittance and haze. The multifunctional transparent wood composite can be used as a load-bearing material in glazing structures with reduced heat energy consumption.