Environmentally friendly transparent wood that can help heat a home

Materials World magazine
28 May 2019

A transparent wood that can store and release heat could be an eco-friendly alternative for construction. Shardell Joseph finds out more.

An ecological transparent wood has been invented, with the potential to replace materials such as plastic or glass, for building energy-efficient homes. Using timber, researchers from KTH Royal Institute of Technology, Sweden, said the transparent wood is not only sturdy, but is capable of storing and releasing heat – making it a candidate for lightweight, low-cost, light transmitting buildings and see-through solar cell windows.

Funded by the European Research Council, the KTH project aims to introduce a way to use the material on a large scale. According to KTH Head of Biocomposites Division, Lars Berglund, the optically transparent wood (TW) was developed for easier microscopic tests in the study of wood anatomy, but the material has proven to be suitable for mass production.

‘Transparent wood is a good material for solar cells, since it’s a low-cost, readily available and renewable resource,’ said Berglund. ‘This becomes particularly important in covering large surfaces with solar cells.’

According to Berglund, this wood is a realistic environmentally friendly option, as it is just as effective in achieving transparency as well as having the capacity to store carbon dioxide in the material during a long service time. ‘The wood tissue is manufactured by nature, so that the carbon footprint of the material in the tree is even negative,’ he told Materials World.

When in panel format, the wood allows light to pass through but is not 100% sheer so maintains privacy, and could be used for windows and semi-transparent facades. The material keeps a wood-grain texture, making it aesthetically unique for internal and decorative applications.

‘From professionals in interior design, we have learnt that the most desirable surface is where the wood grain provides topology to the material, and this we can produce,’ said Berglund.

The transparent effect

The findings, published in the American Chemical Society Journal, Biomacromolecules, 2016, stated that the TW reached transmittance as high as 85% and haze of 71%. This was achieved by using a delignified nanoporous wood template – prepared by removing the light-absorbing lignin component and creating nanoporosity in the wood cell wall. Naming the process delignification, the team claimed they used an eco-friendly process using peracetic acid.

‘Lignin serves as a glue for cellulose in the wood tissue, and removing it reduces mechanical integrity of the wood template,’ said Berglund. ‘For this reason, we have developed a new method where we are only removing the fraction of lignin which is absorbing light.’

Acrylic, which is non-biodegradable and water-repellent, is then introduced into the remaining tissues filling both the tiny pores left by the removal of lignin and the hollow vessels, xylem, that carried water in the tree. According to the team, this is a necessary process in order to obtain the transparency effect, as the wood porosity scatters light.

In the next stage, the team mixed the acrylic with polyethylene glycol (PEG). As well as permeating the wood, the PEG also makes it harder for heat to cross through, which can be useful for either insulating a building against the cold, or trying to keep out summer heat.

‘PEG is a phase change material, which absorbs heat as the material is heated, and releases heat as the material is cooled,’ Berglund said. ‘One can then use this to preserve the indoor temperature at suitable level, and reduce the need for air-conditioning.’

PEG is solid at room temperature but melts at 30°C, and still remains locked within the wood structure. According to the team, it enables the material to capture energy from the sun during the day and release it into the interior, making buildings more energy-efficient.

‘If you take 100g of this transparent wood material with the [PEG] inside, it can absorb up to 8kJ of heat, which corresponds to basically what a 1W bulb could produce in two hours,’ said KTH Royal Institute of Technology PhD student, Céline Montanari. Different types of PEG melt at different temperatures, so the team could tweak the transparent wood’s properties to suit its application.

The future is clear   

Integrating the material into construction, the researchers plan to use the wood for specific functions where either optical transmittance is important, like frosted glass, where the new heat control mechanism can be used, or where LED lighting function is integrated. ‘For instance, we envision load-bearing ceiling panels with LED lighting function,’ said Berglund.

He said the team is aware of the amount of work left to develop the wood technology further. ‘There is potential [for mass production], but the processing technology needs improvement and scaling,’ said Berglund. ‘Like any nanotechnology, it is likely that high-tech applications will come first, where profit is high, material cost and high-speed production are less important.’

The project will run for three more years and will continue on novel functionalisation methods, including a wood laser, solar cells, LED panels and electrochromic – colour-changing – windows.