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Architect: Ingenhoven Overdiek und Partner, Dusseldorf. Project Development: Hochtief AG. Building Envelope: Josef Gartner and Co. Completion Date: March 1997. Building Use Summary: Mid-rise commercial (31 floors over approximately 120 meters). Useable Floor Area: 20,000 m² (36,000 m² gross). Ventilation Strategy: Natural ventilation using double skin façade. Supporting air-conditioning available (chilled celings).
Tour on August 4, 2000 with Jana Hula and Frank Reineke, architects of Ingenhoven Overdiek und Partner, as well as with Franz-Wener Kandora, Building Manager of the RWE tower.
The RWE tower is the home of primarily, mid- and upper-level management of RWE, one of Germany’s largest power companies. The 120 meter tall circular tower was designed by Ingenhoven, Overdiek, und Partner and built at a cost of approximately 150 million US dollars. Ingenhoven was selected to design the tower as a result of an intense competition to create an environmentally friendly high-rise. During the RWE competition, Ingenhoven’s firm had a simultaneous entry in the Commerzbank tower competition. Norman Foster’s firm won the Commerzbank competition (Ingenhoven’s firm came in second place), but the RWE tower was completed first, earning it the right to be called the first ecological high-rise in Europe. The tower was ready for occupancy in March of 1997.
Upon entering the tower, visitors are greeted by a three-story lobby that features a significant amount of exposed structural concrete. Heating in the lobby occurs at the base of the windows, while fresh air is directed through vents aimed directly at the windows, rather than into the interior space. The vents are incorporated in the façade frame.
The RWE tower, the 5th highest building in Germany, features a double-skin façade engineered by the Gartner Company. The top-level conference room is fully air-conditioned using a displacement ventilation system. Source air is channeled through perforations in the metal floor and then through a loose-mesh carpet. Exhaust vents are located at the top of the room, surrounding a large skylight. The upper five levels of the tower are reserved for upper-level management.
The BMS controls lighting and ventilation. When windows are open, all systems are shut off. When windows are closed, air conditioning is available through the use of chilled ceilings. When wind speeds are in excess of 8 m/s, an audible warning to close the windows is issued in each office. During the summer, the maximum temperature allowed is 27 °C. During the winter, heating can be controlled in each room to ± 3 °C around the building set point. The blinds, located in the channel of the double-skin façade, are lowered automatically during inclement weather.
Creating as transparent a building as possible is based on a desire to use daylight as much as possible in order to increase the quality of the working environment. A critical requirement of using daylight is to have very transparent glazing. In addition to day lighting, RWE specified that natural ventilation was to be used. Having operable windows in a skyscraper, at the time, was unprecedented. A third demand was to provide occupants with adequate sun protection without using interior-mounted devices.
The demands specified were fulfilled without compromise by using a double skin façade with a 50-cm wide airflow gap. The exterior wall of the RWE tower is made of flint glass that is fastened in eight locations; specialists from Gartner note that the exterior wall is “practically” invisible from the interior. In the façade channel, metal panels in the shape of a fish mouth form a transition from the inner to outer glass surfaces. Window cleaners can raise the top flap of the fish mouth to reach a flat walking platform.
The inlet and outlet vents on the façade include louvers designed to prevent rain infiltration without the use of electronically controlled flaps. Arranging the inlet and outlet vents on top of each other was decided to be unacceptable because exhaust air would take the shortest path up to the floor above and enter it in the place of fresh environmental air. If this happened, air quality would decrease with every subsequent floor. Another concept was to have air flow from the bottom to the top of the façade; this was found to also be problematic. The final solution was to create diagonal air streams in the façade cavity. This required that supply and extract air vents were placed next to each other. This was achieved by alternately perforating the bottom and topsides of the double-paneled fish mouth platforms connecting the inner and outer glass walls. The final vent width was 120 mm.
The slatted blinds in the façade corridor have virtually the same effect as exterior sun shading. The slats absorb solar radiation, which in turn causes them to heat up. The secondary heat transmitted by the slats remains within the infrared spectrum and is primarily deflected by the interior layer of glass. The exterior glass layer protects the blinds from wind, humidity, and other weather.
Various aspects were considered in the ventilation design of the RWE tower. They included: natural ventilation in windy conditions, natural stack ventilation of the entire building, ventilation in the double-skin façade, ventilation of the elevator tower, and natural ventilation of the ventilation duct network.
It was found that cross ventilation at medium wind speeds would produce up to a 40-fold air change rate. Thus, the double-skin façade reduces cross-ventilation sufficiently to prevent papers from flying around, as long as outside wind speeds were not in excess of 8 m/s. At that speed, there is around a 200-fold air change rate. Using past weather data, it was found that the double-skin façade would be able to reduce door opening forces to levels around 40-60 N for the majority of the time.
Because of the stack effect in stairwells and the elevator shafts, special attention was paid to where certain air locks and vents should be placed throughout the building. Through extensive wind tunnel and computer modeling, a design consensus was reached that would allow natural ventilation to be used as long as outside wind speeds did not reach an excess of 8 m/s (300 hours per year) or a temperature below 2 °C (100-250 hours per year).
Based on personal observations, the RWE tower is an example of very bold and forward thinking. It appears that Gartner Company, the façade designers, as well as Ingenhoven, Overdiek, und Partner were very concerned with setting a good example for others to follow. It was also good to see that an energy company was visionary enough to specify that natural ventilation be used to reduce energy consumption and improve worker comfort. It was interesting to hear that security for this building is very tight, due to the fear of damage from environmental groups. Some groups have put up negative banners on the building site; perhaps without knowledge that the RWE tower has sparked a movement to the construction of greener skyscrapers. On a flip side, the RWE tower did not come to existence cheaply. Other companies may not be willing to erect high-rise towers with advanced facades, both due to their high cost, as well as the fact that very detailed analysis of airflow patterns must be performed. It will be necessary to find a compromise between the elegant façade design at RWE and something as effective, yet cheaper to design and manufacture.
Reference Picture 35. RWE Tower at night. The rectangular shaft on the left of the circular portion of the tower houses 4 elevators. Placing the elevators here allows for better use of space within the circular tower. The floor with circular openings houses mechanical equipment. (p. 9 RWE Book)
High-rise RWE AG Essen / Ingenhoven Overdiek und Partner, edited by Till Briegleb, 2000 (ISBN 3-7643-6108-5)
Reference Picture 36. Typical Floor Plan. The core of each floor consists of a conference room, bathroom facilities, storage, and a ventilation shaft. (p. 44 RWE Book)
Reference Pictures 37 & 38. (LEFT) Isometric view of façade element. (RIGHT) Actual view of façade element. From afar, the RWE tower appears to be a perfect circle, yet it is actually a 50-side polygon. The minimalist glass mounts (8 per panel) allow for a unobstructed and complete floor to ceiling glass façade in each room. (p.68)
Reference Picture 39. Cross-section view of “Fish-Mouth” façade assembly. While the apparatus appears complicated, this method of packaging components is unique. All components are carefully integrated to provide the maximum amount of unblocked view to the outside as possible. (p.64)
Reference Pictures 40 & 41. (LEFT) “Surfboard” chilled ceiling. Ingenhoven came up with the concept of a surfboard shaped set of heat exchanger fins. It was found that this would provide optimal temperature control using a fairly compact design. (RIGHT) Final Ceiling Covering. (p. 121)
Reference Picture 42. Schematic of top five floors. Note that curving stairs allow transit from floor to floor with ease. Fire codes were met by using special glass fire doors for zoning. The façade of the building continues to the top two floors. There is no roof over the outer ring of the upper two floors, thus creating an open-air garden space. (p. 50)
Reference Picture 43. General engineering requirements specified during the design phase of the RWE Tower. All requirements were met in the final design. (p. 74)
Reference Pictures 44 & 45. (LEFT) Individual Office Control Panel. The top block is for lighting control. The second block is for shade operation. The third block provides an audio and visual warning when the façade must be closed due to high wind pressures. The bottom block allows the user to select the desired temperature of the room. Efforts were made to make the panel as easy to use as possible. (RIGHT) Roof-top weather station.