When the Stuttgart, Germany–based firm Behnisch Architekten began designing what would become Hannover’s Norddeutsch Landesbank am Friedrichswall in 1997, materials such as triple-pane or low-E glass were uncommon, and existing technologies and methods such as double-skin façades and natural ventilation had never been used on a large-scale office building. But because such glassy towers have traditionally consumed more energy per square foot than smaller buildings (due to increased elevator and air-conditioning usage, and a lack of shading from surrounding buildings), the project represented an opportunity to green the globally ubiquitous glass skyscraper.
Occupying an entire city block, the 807,300-square-foot building’s ground-floor base stretches to the property’s rectangular perimeter to meet the street, providing a public courtyard inside. As it rises to a height of 20 stories, the building twists like a DNA double helix to face north to downtown. Although it looks different from the traditional rectangular monolithic office tower, Behnisch’s design transformed the building’s energy performance by harnessing its height: a chimney effect moves air from the building’s courtyard up through the building’s double-skin façade, which acts like a duct bringing cool air to the upper floors (while also limiting noise from the busy thoroughfare outside). “At that time, the double skin had been around since the ’60s in Germany, but what was innovative, very much so, was trying to create a very comfortable climate at that scale in a commercial building with mostly passive means,” firm leader Stefan Behnisch explains. “Given the fact that 50 percent of the energy you use for air conditioning is fan power, this makes a big difference.”
Going without air conditioning has not decreased occupant comfort. “Even on very hot summer days, the room temperature within the office rooms is agreeable and comparatively cool,” says Catrin Kuhlmann of Norddeutsche Landesbank. “We have to deal with considerably less complaints concerning the climate control in this building than our other, traditionally air-conditioned buildings.”
Norddeutsche Landesbank is heated with a geothermal heat exchanger that uses approximately 120 piles driven beneath the foundation, coupled with radiant slabs in the floor—also never previously achieved at this scale. “At first, the energy consumption levels were almost like a regular building,” Behnisch says. “We found out two-thirds of the piles had never been activated. We turned them on and [they] reduced energy consumption.”
The building’s façade has a large amount of glass, and Behnisch says today that he would use less of the material—but not by much. “The insulation could be better with a bit less glass, but 80 percent of the building is naturally daylit, which brings a lot of energy savings,” he says. “You’re searching for the right balance.” He notes that if designed today, the building could take advantage of more-efficient glass, but Behnisch also decries the amount of tinting that occurs, particularly in North American office towers. “They are so dark. You have all glass buildings and still need 24 hours of artificial lighting,” he says. “Outside shading can actually work with lighting. For this project we have sun shading where the top third is tilted to not close but reflect the daylight in. The lower part is micro-perforated so you can still see through. I think that was the right design.”
The biggest regret about the Norddeutsche Landesbank project, its designers say, is its lack of monitoring. “I think the problem at the time was that monitoring was not considered being valuable enough, at least for the clients,” says Thomas Auer of Transsolar, the project’s energy consultant.
Even so, Behnisch adds: “Today, we could do far better. But the building is still better than your average benchmark office building built to code.”