Photograph of Aerogel-AB (a). Scale bar: 10 mm.
Photograph of Aerogel-AB (a). Scale bar: 10 mm.

A team of Norwegian researchers have shown that the thermal and optical properties of aerogels depend on their particle size – useful in the design of insulating windows.

We’ve all seen images of the ghostly-looking material aerogel. Famously, in 2006, panels of it were used on NASA’s Stardust mission to capture tiny samples of interstellar dust. But here on Earth, its low density and thermal conductivity have attracted the interest of a much more ‘urban’ research effort – in the development of insulating windows.

Windows have a huge impact on a building’s energy efficiency, with some figures suggesting that ~50% of the total energy loss from a standard office building happens through its windows. As global efforts to produce ‘green’ buildings become ever more ambitious, we’re seeing a growth in research programmes on windows. So far, there have been several window innovations which have shown potential to meet the requirement of energy efficient buildings – multi-layered, vacuum, and silica aerogel windows.

Arild Gustavsen and his team at the Norwegian University of Science and Technology are focused on the use of silica aerogel granules as the “filler” in double-glazed windows [Applied Energy 128 (2014) 27-34 DOI: 10.1016/j.apenergy.2014.04.037]. Because aerogel is mechanically very weak, much of the current research on aerogel glazing units (AGUs) focuses on the synthesis of the aerogel. But Gustavsen and his team specifically looked at the effect that aerogel granule size and layer thickness have on the thermal and optical properties of standard double-glazings.

Both AGUs show improved thermal insulation performance when compared to double glazings - AGUs containing ‘large’ aerogel granules (diameter 3–5 mm) showed a 58% reduction in heat loss. Smaller particles (<0.5 mm) had an even larger effect on the thermal conductivity of the window unit – there, the team saw a 63% reduction in heat losses. However, the introduction of these granules did have an effect on the optical transmittance of the windows – Gustavsen showed that the smaller the particle, the more diffuse the transmitted light. The team believe that this property may be useful in situations where glare and/or privacy need to be considered.

Highly insulating glazing units are defined as those with U-values of about 0.5–0.7 W/(m2K) – so far, results on these AGUs fall short. But this work has opened the debate on how to optimise not only the aerogel, but the design of the final glazing units for a range of building applications.

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