Glass windows have been around since ancient Rome. Modern windows, made of two glass panes of glass with air in between, have come a long way since those drafty windows of old. But they still let some heat and cold through, wasting energy.
Replacing the air in today’s double-pane windows with a new see-through gel made from wood could make them as insulating as walls, a new study shows. The insulating aerogel film, reported in the journal Nature Energy could be made for cheap on large scale, is more transparent than glass, and it could be used to retrofit existing windows, its developers say.
Buildings consume around 40 percent of the world’s energy. Most of that is used for heating, cooling and ventilation. Windows and skylights, by allowing heat to enter or escape out of a building, lead to much of this heating and cooling being wasted. Single-pane windows, which are still common, are especially energy-inefficient.
So Ivan Smalyukh, a physics professor at the University of Colorado, Boulder and his colleagues made flexible films of aerogels—sponge-like solids composed mostly of air—using cellulose nanofibers derived from wood pulp.
They make a network of these fibers in water to make a hydrogel. Then they swap the water with ethanol solvent, stick the gel film on a plastic layer and roll it, before drying the gel to remove the solvent, giving an aerogel film. Finally, the team coated the cellulose nanofibers in the aerogel with a silicon-based chemical that made the film water-repellant to prevent condensation.
The aerogels films are 97–99 percent transparent, more than glass, which has a transparency of 92 percent. Millimeter to centimeter-thick slabs of the aerogels could be bent and rolled without loss in transparency, cracks or any deterioration in performance.
To test the materials, the researchers placed a film on the inside of single-pane window glass, and put a 2.5 cm-thick slab in the gap between two panes of a modern window. They placed these test panels in the openings of hot and cold boxes with inside temperatures of 40°C and -20°C, mimicking summer and winter temperatures.
Aerogel-containing windows more efficiently blocked heat flow between the inside and outside of the boxes. And the 2.5-inch layer made the double-pane window as insulating as a wall. In addition to making the windows more insulating, the aerogels also increased their resistance to condensation.
Cost and scale-up have been two critical challenges for widely deploying conventional aerogels made from silica, the researchers say. For this new aerogel, raw material cost should be minimal since the cellulose nanofibers can be derived from waste wood pulp, or from food and beverage waste with the help of bacteria, as the researchers have reported before. The team estimates the raw materials to cost about $1 a square foot.
The special rolling and drying process they use should reduce manufacturing costs, Smalyukh writes. “Nevertheless, further research is needed to reduce the haze of thick aerogel slabs and to lower manufacturing costs to drive widespread deployment.”
Source: Abraham, E., Cherpak, V., Senyuk, B. et al. Highly transparent silanized cellulose aerogels for boosting energy efficiency of glazing in buildings. Nat Energy, 2023.