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The figure shows a blue-switching 1.2 square metre sample at the Glasstec 2016 trade fair.
© Fraunhofer IAP
Electrochromic windows
02.03.2017

Schematic diagram of the production of electrochromic casting resin laminated glass.
© Fraunhofer IAP

Darken windows in seconds

When the sun shines, electrochromic windows keep the sun and heat out. However, until now, it has taken quite a long time for the windows to go completely dark. A new process lets windows react ten times faster.

When it is darker outside, the windows are transparent and let in light and heat. When the sun shines into offices, the electrochromic glass can be darkened to keep out most of the heat. Together with Tilse Formglas, researchers from the Potsdam Fraunhofer Institute for Applied Polymer Research developed a new manufacturing method for these electrochromic windows. They can produce glass panels with far faster switching times than the previous models.

The principle of electrochromic windows is simple: Glass coated with light-permeable indium tin oxide or the lower cost fluorine tin oxide is generally used. This coating makes the glass electrically conductive. Two of these panes are required to darken the window. Another layer – electrochromic wolfram oxide is most common – is vapour-deposited on one of the two panes. The panes are laid on top of one another with the coated sides facing one another, and connected by a gelatinous electrolyte. The wolfram oxide coating darkens when voltage is applied to the glass. Reversing the polarity brightens them again. However, that takes a long time. Large windows, two to three square metres in size, can take 15 to 20 minutes to darken fully.

Faster darkening effect

Researchers at Fraunhofer IAP are focusing on a different technology to darken the windows. They use organic monomers mixed in a specially developed casting resin. As with the previous process, the scientists use tin oxide-coated window panes as the initial substrate. However, they do not apply any additional coatings. Instead, they place the panes with tin oxide coating on top of one another, pour the casting resin with the electrochromic molecules into the resulting cavity and harden the resin using heat or UV radiation.

Afterwards, the scientists apply direct current: This causes the monomers to connect to an electrochromic polymer on one of the electrodes. The window can then be switched at a far lower voltage. Advantage of the organic colourant: The glass pane reacts far faster. It only takes approximately 20 to 30 seconds to darken a 1.2 square metre window. “The switching range is not as great as with wolfram oxide yet. We can achieve a darkening effect of roughly 15 to 20 per cent. With smaller cells on a laboratory scale, values of roughly 30 per cent are currently possible,” says project manager Dr Volker Eberhardt.

Wide range of dark colours

Until now, classic metal oxide-based windows have always had a blue tint. However, this process also allows other colours to be produced. There is already a prototype of the electrochromic casting resin glazing, which currently also switches to a blue tone. Other monomers can turn the windows red or purple.

The stability of the windows is another argument in favour of the new process. Researchers also tested the stability of the electrochromic windows in accordance with the current DIN standard: Combining just two layers is sufficient for overhead glazing or walk-on glass floors. This also saves on material costs, as multi-layer composite glazing was required prior to this.
The scientists are currently testing the stability of the effect under the extreme conditions of continuous sunshine in an endurance test with a solar simulator.

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Addresses

Project management
Fraunhofer IAP

Industry partner
TILSE FORMGLAS GmbH

Link

Switchable architectural glazing with electrochromic layers
Project of the EnOB research initiative. Dimmable, EControl solar protection glass is now available in double and triple glazing versions based on electrochromic laminated glass.

EnOB research initiative
Projects, reports, news and analysis from the energy-optimised construction research field