Colors ‘n Curves: A new bank headquarters in Frankfurt may well be the world's most energy-efficient office tower.
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Shrouded in shades of many colors, it is a building that claims to be green. And so it is. It is not often that a midsize bank building has good reason to make that claim, but the new 400,000-square-foot, $85 million expansion of the headquarters for the KfW Bank in Frankfurt, designed by Sauerbruch Hutton Architects of Berlin, does. If it performs as intended, the building will consume about 7 kWh (24,000 Btu) per square foot per year, making it one of the world's most energy-efficient office towers.
KfW, an abbreviation for Kreditanstalt für Wiederaufbau, or Credit Buro for Reconstruction, was founded in 1948, largely with money from the post-World War II European Recovery Program, also known as the Marshall Plan. Today, it is owned by the German government and is one of the 10 largest banks in the country, employing 3,500 people. Among its many initiatives is the funding of Germany's ambitious energy-conservation programs, including existing building retrofits and photovoltaic panel installation. Since 2006, KfW has distributed $1.4 billion for CO2reduction programs that stem from the Kyoto Protocol. The bank also defined KfW-40 and KfW-60—two widely cited energy standards used as credit criteria.
Being at the forefront of Germany's tough energy policies, it was only logical that the bank would want a green building when the need to expand its existing headquarters arose. The German-British architecture duo of Matthias Sauerbruch and Louisa Hutton, selected in 2004 as the result of a design competition, were no strangers to this task. Among their many ambitious green projects is Berlin's GSW building, completed in 1999 and widely regarded as one of the world's first environmentally friendly high-rise towers.
The recently completed KfW fits remarkably well into the context. Situated in Frankfurt's affluent Westend neighborhood, the building, known as the Westarkade, provides office space for 700 employees and includes a conference center. At its base, the building has a curvy four-story podium that reinforces the street edge and defines a small green space to the rear. It also serves as a backdrop to the nearby Palmengarten or Palmtree Garden, a public botanical garden. The podium connects to the adjacent KfW buildings on several levels, so the building forms an extension to the KfW ensemble of buildings from the seventies, eighties, and nineties.
The 10-story tower that rises from the podium has a flowing form that responds to prevailing wind directions and the sun's daily and yearly path. Yet it is also reminiscent of the jazzy architecture of the late forties and early fifties, the era when the bank was founded. According to Sauerbruch Hutton Project Architect Tom Geister, the tower is shaped like a wing in order to maintain access to daylight and the best possible views for the occupants of the neighboring KfW buildings.
Energy models predict that the new building's primary or source energy consumption for building operations (excluding the data center and other process loads) will be only 9.1 kWh per square foot. Monitoring by researchers from the University of Karlsruhe, to be completed this summer, will determine if the model was accurate. The simulation was conducted according to the parameters of the German EnEV 2004, the country's strict guidelines for building insulation and energy conservation.
A number of tightly coordinated strategies should help the building meet its ambitious targets. These include thermal activation of the slabs and a recovery system that captures heat from the data processing center and from exhaust air. A supplemental raised floor ventilation system, used only when outside temperatures are below 50°F or above 77°F, supplies fresh air drawn through a duct buried beneath a below-grade parking garage. The duct carries the air from an intake louver located at the site's edge near the botanical garden, modulating it with the constant temperature of the earth.
The building's most unusual feature is a specially devised double-skin facade, dubbed a “pressure ring” by the design team. “Originally we wanted to call it a ‘gauge-pressure ring,' but we thought that would sound intimidating,” says Bjoern Roehle, a physicist in the Munich office of Transsolar KlimaEngineering, the firm responsible for the building's climate-control concept.
The envelope consists of an encircling sawtooth-shaped cavity, 28 inches wide at its deepest point. It encloses automated blinds that help block solar gain and control glare. This “ring” is defined on the exterior by a skin made up of fixed, tempered-glass panels and colorful ventilation flaps, and on the interior by alternating operable and fixed argon-filled insulated glazing units incorporating a low-E coating. The dynamic system negates the effects of variable pressure around the building, enabling natural ventilation much of the year. It also allows occupants to open windows in the inner skin, regardless of the season, without drafts or heat loss. The system reduces detrimental cross ventilation—a typical problem in high-rise buildings with operable windows—to a “convenient minimum,” explains Geister.
The building has a roof-mounted weather station that monitors wind direction and speed, among other factors, and controls the outer skin's ventilation flaps. Depending on conditions, the building management system opens or closes the flaps to introduce fresh air and create a zone of consistent pressure surrounding the curtain wall's inner skin, while also producing a slight pressure differential between the cavity and the building's interior. The air is then drawn into offices through floor vents near the perimeter, or through the occupant-controlled windows, and subsequently exhausted naturally to the negatively pressurized corridor, and ultimately through the building core.
Colorful facade panels, also deployed at GSW and by now a signature Sauerbruch Hutton device, animate the elevations. In the Frankfurt building, the architects combined red, blue, and green panels, with a different hue dominating each elevation. This colorful and innovative envelope, along with the building's highly coordinated climate-control systems, should help KfW establish a new benchmark for red, blue—and, of course, green—design in Europe.