Could adding see-through wood make windows better at retaining heat?
A new study found that replacing the air in double-glazed windows with a transparent wood aerogel could make them airtight as walls. If true, this could significantly impact the construction and energy efficiency/sustainability industries.
For several reasons, windows are often considered the "weakest link" when saving energy in a building. Chief among them is that they typically have a lower thermal resistance than other building components, such as walls and roofs. This means they are less effective at insulating a building and are more likely to allow heat to escape during colder or enter warmer months. As a result, buildings with large windows or poorly insulated windows can experience significant heat loss, leading to increased energy consumption and higher heating and cooling costs.
The number of glass panels, which can affect the view's quality or the air layer's width, can be increased to make windows with air between the panes of glass better insulators. However, anything wider than 1.5 centimeters hurts the insulation effect because it makes it easier for convection currents to move. Ivan Smalyukh and his colleagues at the University of Colorado Boulder solved this problem by using cellulose nanofibres to make an aerogel, a solid gel with gas-filled pockets that could replace the air in double glazing.
“We have a very unusual combination of properties, which is a very high transparency aerogel that also has very high thermal insulation,” says Smalyukh. “You could think [of] it as a pillow that keeps heat where you need it, and at the same time, you can see through it [to] use it in a window.”
The research team achieved this by first hanging cellulose nanofibres from wood in water before switching the water out for ethanol to create the aerogel. They then used higher temperatures and pressure to dry the aerogel, replacing the ethanol used to fill the material's pockets with air. Finally, they added silicon compounds to the surface to make the aerogel water-repellent, stopping condensation when applied to windows.
Aerogel can fill a larger area without the convection effects you would get from air alone, thanks to the tiny air pockets embedded in it. A window that is 2.5 centimeters broad and filled with aerogel could be made to be as insulating as a wall.
“This is a nice development that could be easily employed as a retrofit to existing windows,” says Steve Eichhorn at the University of Bristol in the UK. “The reduction in heat transfer, with the added benefit of maintained transparency and low haze, make this material truly remarkable, and all with a sustainable material, cellulose.”
Even though there will be difficulties, it is already possible to produce cellulose nanofibres at a large size, according to Eichhorn.
You can view the study for yourself in the journal Nature Energy.
"To maintain comfortable indoor conditions, buildings consume ~40% of the energy generated globally. In terms of passively isolating building interiors from cold or hot outdoors, windows and skylights are the least-efficient parts of the building envelope because achieving simultaneously high transparency and thermal insulation of glazing remains a challenge. Here we describe highly transparent aerogels fabricated from cellulose, an Earth-abundant biopolymer, by utilizing approaches such as colloidal self assembly and procedures compatible with roll-to-roll processing. The aerogels have visible-range light transmission of 97–99% (better than glass), haze of ~1% and thermal conductivity lower than that of still air. These lightweight materials can be used as panes inside multi-pane insulating glass units and to retrofit existing windows. We demonstrate how aerogels boost energy efficiency and may enable advanced technical solutions for insulating glass units, skylights, daylighting and facade glazing, potentially increasing the role of glazing in building envelopes."
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