Energy prices around the world have been rising alarmingly since 2021, with many of us now really starting to feel the pinch. Some have shared advice on how to heat the person rather than the home, but researchers at KTH are working on a building material that could help regulate indoor temperatures.

The new composite is the work of a research team from the Department of Biocomposites at KTH Royal Institute of Technology in Stockholm, Sweden, and makes use of three renewable material sources: coconuts, lemons and wood.

The researchers first created an open pore structure in the wood by removing the lignin, which also strips away its color. The gaps were then filled in with limonene acrylate (which can be sourced from peel waste in the juice industry) and a coconut-based molecule.

As the composite heats up, say from exposure to sunlight or a rise in ambient temperature, the limonene acrylate becomes a polymer, trapping the coconut molecules within. The temperature at which the transition occurs can be tailored to requirements, but was set at a comfortable 24 °C (75 °F) for this project. And the process is reversed as the material cools.

"The elegance is that the coconut molecules can transition from a solid-to-liquid which absorbs energy; or from liquid-to-solid which releases energy, in much the same way that water freezes and melts," said KTH researcher Céline Montanari. "Through this transition, we can heat or cool our surroundings, whichever is needed," added team member Peter Olsén.

Though not yet ready for use in construction, a possible initial application for the "wood composite thermal battery" could be as interior dividing walls or, since there is a degree of transparency to the material, as some sort of screen. However, the team says that more work will be needed before it's ready for use as an exterior building material.

It is estimated that a saving of around 2.5-kWh per day can be had for every 100 kg (220 lb) of material used in building construction – assuming an ambient temperature of 24 °C – though it may also find use in the garden.

"Why not as a future material in greenhouses?” asked Olsén. "When the sun shines, the wood becomes transparent and stores more energy, while at night it becomes cloudy and releases the heat stored during the day. That would help reduce energy consumption for heating and at the same time provide improved growth."

The research was published in the journal Small.