Summertime blue

It is a paradox of modern, transparent glass buildings that as soon as the sun comes up, the blinds go down. Now that everyone has a computer on their desk, the occupants of many offices are forced to sit in artificial gloom to stop the glare on their screen.

But a new form of glazing from Pilkington is about to make blinds a thing of the past. Called electrochromic glass, it gradually turns from clear to dark blue at the flick of a switch, cutting the amount of light transmitted and putting an end to glare.

The implications are immense. Electrochromic glazing could bring about a whole new approach to facade design as architects and designers are freed from facades swathed in bands of louvres and brises-soleil. As for services engineers responsible for maintaining the environment within buildings, reducing the solar load will mean smaller and possibly more efficient air-conditioning systems – and lower operating costs.

Glaringly obvious?

The glazing is being used commercially for the first time on a bank in Dresden, Germany. From the outside, the Stadtsparkasse Savings Bank, designed by architect Bauer & Keller, does not look like a pioneering building. On the contrary, its scale, local sandstone cladding and restrained appearance ensure that it sits comfortably among the other buildings in Dresden’s old town – a vital requirement, since one of those buildings is the city’s historic town hall.

The challenge for the architect was to design a building for a narrow site that was pleasant to work in. To achieve this, large openings were needed in the prominent south-facing facade to allow light to penetrate into the building’s core and create the illusion of openness on the restricted plot.

The architect’s solution was to put a cavernous, rectangular atrium in the centre of the building, stretching up for five of its seven storeys. This would open up the office interiors to natural daylighting, but would also face south, into the sun. With external shading ruled out by the building’s prestigious location, the project team had to find some other way to prevent glare and keep the air-conditioning cooling loads to a minimum.

The obvious solution would have been to fit internal blinds, but installing and maintaining them in a five-storey atrium would have been difficult and expensive. More importantly, they would have restricted the occupants’ view of the square opposite. Electrochromic glazing seemed to offer an alternative, and, even though it had not been tried or tested, the architect decided the risk was worth taking.

Tech spec

Electrochromic glazing works by passing a low electrical voltage across a sandwich of microscopically thin coatings, causing them to change colour. The transformation from clear to blue takes place gradually over 12 minutes and is barely perceptible from inside the building. Throughout the transformation, the view to the outside is uninterrupted, with the glazing remaining transparent even when it is at its darkest.

“The chemistry behind the colour transformation is analogous to a battery being charged and discharged,” says John Siddle of Pilkington’s research and development centre.

The electrochromic film is sandwiched between two 4 mm layers of Pilkington’s K-glass, a low-emissivity glass designed to reduce heat loss through windows. One sheet has a tungsten oxide coating applied to it, the other a coating of vanadium-based oxide. Holding the sheets together is a polymer that is conductive to ions. An electrical voltage applied across the film causes ions to be transferred out of the vanadium layer, through the polymer, to settle on the tungsten oxide film, which causes the oxide to turn blue.

Uniform appearance

The electrochromic glazing units are manufactured to the same width as standard double-glazed windows, comprising a 9 mm thick electrochromic outer pane, a 16 mm argon-filled gap to provide a thermal insulating layer and a 4 mm inner pane. As for dimensions, current technology limits the glazing to a maximum size of 900 × 2000 mm.

Each glazed unit needs its own power supply, making it possible to alter each panel’s colour individually. For the bank in Dresden, Bauer & Keller connected all 80 of the glazed units and controlled them as one to give a uniform appearance rather than a variegated facade of blue and clear glass. A control unit switches the glass to any one of five modes depending on the cooling and light requirements of the building.

A huge benefit of the glazing is that controlling the amount of light entering a building also affects the amount of heat that can pass through. Studies at the Frauhofer Institute for Building Physics in Holzkirchen, Germany, have shown that savings of 12% in a building’s cooling load are possible over a year. However, although the amount of heat that enters is reduced compared with normal glass, electrochromic glass allows more heat to escape, and increases a building’s heating load by about 6%. “But since cooling a building is more expensive than heating a building, the glazing still has massive energy-saving potential,” says Siddle.

The Dresden bank is due to be completed by Christmas. But, while the response of the air-conditioning system to the electrochromic glazing can be calculated fairly accurately, how the occupants will react to working behind a giant pair of sunglasses is not so easily gauged.