Transparent wood now also stores and releases heat

Transparent wood now also stor...
The material is clear when storing heat (left) and turns cloudier upon releasing it (right)
The material is clear when storing heat (left) and turns cloudier upon releasing it (right)
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The material is clear when storing heat (left) and turns cloudier upon releasing it (right)
The material is clear when storing heat (left) and turns cloudier upon releasing it (right)

Three years ago, we heard how scientists from Sweden's KTH Royal Institute of Technology had created transparent wood – it could serve as a cheaper alternative to the silica-based glass currently used in windows and solar cells. Now, the material is additionally able to store heat and later release it.

In order to produce the initial version of the transparent wood, a KTH team led by Prof. Lars Berglund started by chemically removing light-absorbing lignin from natural balsa wood fibers – lignin is one component of wood cell walls.

The white-colored fibers were then combined with prepolymerized methyl methacrylate (PMMA), more commonly known as plexiglass. This altered the refractive index of the fibers, turning them transparent. The resulting material was stronger and tougher than regular clear plexiglass panels.

More recently, Berglund and PhD student Céline Montanari added a "phase-change" polymer called polyethylene glycol (PEG) to the treated wood fibers and PMMA. Ordinarily taking the form of a solid, PEG has a high affinity for wood, plus it's also able to contain and disperse heat. It does so by melting when it reaches a temperature of 80 ºF (27 ºC), storing energy in the process. When it later cools back down, it reverts to a translucent solid, releasing that stored energy.

"During a sunny day, the material will absorb heat before it reaches the indoor space, and the indoors will be cooler than outside," says Montanari. "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."

As an added bonus, the wood fibers and PEG are both biodegradable. Although the PMMA is not, it could likely be replaced with a more eco-friendly polymer, allowing the material to completely biodegrade when discarded.

It is hoped that a commercialized version of the technology may be on the market within five years.

Source: American Chemical Society

"... the wood fibers and PEG are both biodegradable..." The article describes the use of this material as something like that of exterior windows. Doesn't seem like being biodegradable would be a good quality for an exterior window. How is the material going to know when it has been discarded and should degrade as opposed to when it is just outside acting as a window and should not degrade?
How does the material know which way to transmit the heat? It says it will release the heat inside? I can perhaps understand how it would absorb energy in a phase change but it is not clear as to why heat would not merely be released to a lower ambient temperature - unless they are saying the inside would be at 27 degree C - which seems very hot