Coconuts and lemons make for the ingredients of a refreshing cold drink. But researchers have now used them to make a material that could cool down or heat up homes. They combined these renewable materials with modified wood to make a what they call a “thermal energy battery” and report in the journal Small.
Besides being good at storing heat, the material changes its transparency depending on the environment. When the sun is shining and it is warm outside, the material absorbs heat and becomes clear. At night, when temperatures drop, it releases the heat that it stored during the day and becomes cloudy. This makes it suitable for privacy-providing windows or panels that help regulate indoor temperatures, reducing the energy consumed for heating and cooling.
“This material was designed using renewable and green chemistry approaches,” says Céline Montanari of the department of fibre and polymer technology at KTH Royal Institute of Technology in Stockholm. Around 100 kilograms of the material can save about 2.5 kilowatt-hours of energy per day in heating or cooling when the ambient temperature is 24°C, Montanari, Peter Olsén and their colleagues estimate.
The operation of buildings accounted for about a third of all energy used around the world and 27 percent of energy sector emissions, according to the International Energy Agency. This immense carbon footprint of our built environment has prompted a push to increase the energy efficiency of buildings and homes.
Many researchers are developing sustainable solutions for energy-efficient buildings. This includes see-through insulating materials made from wood pulp. Some researchers have also found creative ways to modify wood so that it blocks heat like Styrofoam or reflects and radiates the sun’s heat into space.
Similar to previous work on engineered wood, the KTH team start by removing lignin from sustainably harvested wood. This is the material that forms the cell walls in wood, adding strength and also imparting color. Removing lignin gives transparent wood with open pores that the researchers fill with chemical extracts derived from citrus peel waste generated by the juice industry and from coconut oil.
Heating the material converts the lemon-based molecule into a long polymer that restores strength but keeps the material transparent. It also traps the coconut extract in the material. This coconut-based molecule can switch from solid to liquid and vice versa, absorbing or releasing energy in the process, similar to how water freezes and melts.
The team has so far made centimeter-thick structures. “By laminating thin layers together, thick and large panels can be prepared for scalable applications,” Montanari says. “In addition, the raw materials used are low-cost and renewables. Sustainably harvested wood is a lightweight, cost effective and eco-friendly material. This makes a cost effective and sustainable solution for high-end applications.”
Source: Céline Montanari et al. Sustainable Thermal Energy Batteries from Fully Bio-Based Transparent Wood. Small, 2023.