Coconuts and lemons sound like the start of a tropical drink or accoutrements to a fancy dish. To the scientists from the KTH Royal Institute of Technology in Stockholm, they're the key ingredients of a sustainable building material with insulative properties.
According to a study published in the scientific journal Small, the fruits are combined with a wood-based structure to create a "thermal energy battery" that can alter its transparency and regulate temperatures.
During the day, the wood becomes clear, allowing the material to store heat to keep indoor temperatures cool while promoting natural lighting. Céline Montanari, a researcher for the Department of Fiber and Polymer Technology at KTH, noted in the university's news release that any heat source, in addition to sunlight, can charge the battery.
At night, the material releases its stored heat and returns to an opaque state, reducing energy costs for climate control and providing privacy. The team approximated that 100 kilograms (220 pounds) of the material in an ambient temperature of 24 degrees Celcius (75.2 degrees Fahrenheit) can save 2.5 kilowatt-hours per day in heating and cooling.
To create the battery, the researchers extracted lignin from sustainably harvested wood to create open pores in the cell walls of the wood and filled the pores with citrus peel and coconut oil extracts.
The citrus-based molecule restores the strength of the wood but maintains transparency when heated. Meanwhile, the coconut-based molecule gets trapped in the structure and can store or release energy, depending on its state.
"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," Montanari said.
"Through this transition, we can heat or cool our surroundings, whichever is needed," Peter Olsén, a researcher in the division of biocomposites at KTH, added.
With the International Energy Agency estimating that the operations of buildings accounted for 30% of worldwide energy consumption and 26% of emissions from the global energy sector in 2022, the need for environmentally conscious options in this industry is paramount.
Though the researchers have only made structures about a centimeter wide with the material, Montanari believes there will be no issues for larger projects.
"By laminating thin layers together, thick and large panels can be prepared for scalable applications," she said. "In addition, the raw materials used are low-cost and renewable. Sustainably harvested wood is a lightweight, cost-effective, and eco-friendly material. This makes a cost-effective and sustainable solution for high-end applications."
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