KfW-Westarkade banking HQ, Frankfurt, by Sauerbruch Hutton
Sauerbruch Hutton’s glazed offices feature a technically innovative approach to passive ventilation
While aerodynamics often plays a large part in assessing the effect of a building on its context, particularly judging its impact on the street, rarely does it inform the design of interior conditions. But in Frankfurt, Sauerbruch Hutton has harnessed the power of wind to develop a model of passive ventilation, creating one of the first high-rise office buildings to consume less than 100KWh/sqm energy per year.
The 15-storey Westarkade tower provides a 39,000sq m extension to the KfW Banking Group’s headquarters in the city’s West End, joining an ensemble of buildings from the seventies, eighties and nineties. A four-storey plinth completes the western edge of the urban block, from which the tower rises, rotated 60 degrees and tapering to a point to face the prevailing westerly wind.
The facade is entirely clad in a sawtooth skin of angled glazing, with each window separated by a slender, coloured panel, alternately fixed and pivoting. This forms the outer layer of a 700mm-deep double-skin facade, which operates as a “pressure ring”. Unlike a conventional double-skin system, the cavity is not ventilated by means of a stack effect, but instead by exploiting wind pressure and suction at the lee side of the tower.
During warmer months, the motorised flaps on the south-west and north-east elevations open to allow air to flow through the cavity, while during cooler times of year the flaps are closed, allowing air to be preheated before entering the offices.
The pressure ring serves to neutralise the wind conditions at the window openings of the offices — conditions that would otherwise be too turbulent, especially at the building’s upper floors. The ventilation panels are designed to adjust to five different wind directions, outside temperature and solar radiation, as well as pressure differences on the windward and leeward sides of the building. They are controlled to maintain a constant and even pressure within the ring, with air flow regulated so that it never exceeds 6m/s.
The position and setting of the flaps results in a dynamic play of colour across the facade, which varies according to how the wind pressure is being regulated. Sometimes the facade appears in full colour while at other times it seems completely transparent.
As a result of this system, the offices can be ventilated naturally for eight months of the year without creating draughts or undesired heat loss, while mechanical ventilation is required for less than 50% of all working hours.
View of the 700mm-deep “pressure ring” cavity.
The building’s floor plate is designed as an aerodynamic “wing”, tapered to a point to face the prevailing westerly wind. This creates a zone of negative pressure at the lee side of the building to provide suction.
During warmer months, the motorised flaps on the south-west and north-east elevations open to allow air to flow through the cavity, while during cooler times of year the flaps are closed, allowing air to be preheated before entering the offices.
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