Air in double-glazed windows could replace transparent material made of cellulose nanofibers and gas pockets. Scientific work “Highly transparent silanized cellulose aerogels for increasing the energy efficiency of glazed areas in buildings”, published this week in the journal Nature Energyclaims that this can improve the thermal insulation of the windows up to the level of the walls.
Ivan Smalyukh and his colleagues from the University of Colorado at Boulder used cellulose nanofibers to create a solid gel containing pockets of gas that can perform better than air as an insulator. In this way, they solved the problem where it is not practical to leave a gap of more than 1.5 centimeters between the two glass panes of the window.
Solving the convection effect
Windows with air in the gap between the glass panes can be better insulators either by increasing the number of panes of glass, which can affect visual quality, or by increasing the width of the air layer. However, the gap can be increased to a maximum thickness of 1.5 centimeters, when the convection effect begins to manifest.
Convection effect in windows, it refers to the transfer of heat through the air that circulates in the space between two panes of glass. When there is a temperature difference between the panes – for example, if the sun heats the outer glass, the air inside starts to heat up and expand.
Heat is then transferred by convection, where warm air rises while cold air descends. This air movement can create a flow that results in heat loss from the room or an increase in the temperature inside the room if the sun-heated air flow enters the interior.
“We got a very unusual combination of properties in the form of very high transparency airgel, which has at the same time extremely high thermal insulation,“ says Smalyukh. “You can think of it as a cushion that keeps the heat where you need it, and you can see through it, so you can use it in a window.”
To make the airgel, the researchers first suspended cellulose nanofibers from wood in water and then replaced the water with ethanol. During this process, the fibers are placed in a liquid where they are then allowed to float freely in space without settling on the bottom or walls of the container. In this way, the nanofibers are evenly dispersed in the environment.
The airgel was then dried by increasing the temperature and pressure, while ethanol they replaced it with air that filled the pockets in the material. They then added silicon compounds to the surface to make the airgel repel water and prevent condensation when used in windows.
Windows with insulation
These lightweight materials can be used as panels between glasses and to modernize existing windows. Aerogels increase energy efficiency and can enable advanced technical solutions for glass insulating panels, skylights, conventional windows and facade glazing, which can increase the role of glazing in building envelopes.
Material made of cellulose nanofibers and gas pockets
The tiny air pockets embedded in the airgel mean it can be used to fill a wider space without the convection effect of using air alone. Window with airgel filling about 2.5 centimeters wide it can insulate practically as well as a wall.
“This is a really interesting discovery that could easily be used as a retrofit to existing windows,” says Steve Eichhorn of the University of Bristol in the UK. “Reducing heat transfer with the added benefit of maintained transparency and low fogging makes this a truly remarkable solution, all using a sustainable material – cellulose.”