by Roland Wengenmayr
Large buildings full of people and machines that are sources of heat are a challenge to air-conditioning engineers. This is especially true of alternative designs, which dispense with energy-consuming air conditioning and make the most of natural resources such as winds and the chimney effect.
arge buildings often present a challenge for providing a comfortable climate to the inhabitants. Their extensive glass facades turn them into greenhouses on sunny days. The heat output of human beings is also a factor which is not to be underestimated when many people occupy a building; furthermore, one must not forget the energy output from technical equipment such as computers. In conventional high-rise buildings, air conditioning machinery thus takes up about every 20th floor, and on all the other floors there are voluminous air ducts above the false ceilings. This air conditioning eats up money and energy for its construction and operation, and it causes health problems for many of the occupants of the buildings.
Architects have been looking for a way to solve these problems with intelligent architecture for some years. They want to make expert use of sources of heating and cooling from the environment, as well as various physical effects which can contribute to providing a basic air conditioning. Additional equipment can then manage the fine tuning’ and compensate for peak loads; it can be much smaller and more energy economical. Such an alternative construction however represents an enormous technical challenge: It must provide comfortable conditions in hundreds of rooms reliably during every season of the year.
For this reason, architects work closely together with air conditioning and climate experts, for example with the firm Transsolar Energietechnik GmbH in Stuttgart, Germany and New York. This pioneering venture into sustainable airconditioning technology was founded by the mechanical engineer Matthias Schuler and some of his research colleagues in 1992, coming from the University of Stuttgart. In the meantime, Transsolar can exhibit an impressive list of reference projects, ranging from the Mercedes Museum in
This cross-section through one floor of the Post Office Tower shows the path taken by the fresh air supply through the double facade into the offices (red arrows). The air first flows into the intermediate space between the inner and outer facades through the air shutters (left). From there, it enters the office spaces either through opened windows or through the subcorridor convectors. Via the exhaust ducts and the corridors it then flows on into the sky gardens, which transport it out of the tower using the chimney effect. During rare weather conditions, fans support the air flow. Water pipes in the concrete ceilings provide additional cooling for the rooms in summer (blue arrows); in winter, hot water is pumped through them for space heating (graphics: R. Wengenmayr).
Stuttgart, the main rail station in Strasbourg, to the international airport in Bangkok.
Every large building has to function like a single organism: Glass facade, roof, atria and stairwells, offices, conference rooms, cafeterias and the basement all become elements of an architectural air-conditioning system. Its task is to maintain the air in the building at a comfortable temperature and humidity and circulate it without creating unpleasant drafts. Unusual methods are employed, for example large channels in the ground which precool the entering air.
Beginning with the earliest planning phase, it must be guaranteed that all the components of the building will work together as planned. The engineers from Stuttgart employ elaborate computer models, which simulate the complete structure with all of its physical properties in detail. The software, developed by the firm, models the interior climate during the day and night, for every weather condition and in all seasons. The structural and physical properties of the windows, walls and ceilings and even the behavior of the people in the building are taken into account, insofar as they influence the interior climate. For example, the less effectively the architects make use of daylight, the more investment and power must be expended for illumination. Artificial lighting can with unsuitable planning become an important heat source within a building. The simulation of the interior illumination is therefore closely keyed to interior climate modeling.
In constructing very large buildings, the engineers are often breaking new technological ground, and computer simulations alone are not able to describe the situation correctly. In such cases, Transsolar builds real models of the planned building or its critical sections and tests them under varying weather conditions. If necessary, the engineers even construct a 1:1 mockup of a complete office with a
section of the glass facade and let it be “occupied” for several months by monitoring equipment.
The Post Office Tower in Bonn (Figure 1) gives an impressive example of what modern climate engineering can accomplish. The architects Murphy and Jahn in Chicago designed this new administration building for the German Post Office, a 160 meter high structure of 41 stories, and Helmut Jahn brought Transsolar on board. The result is summarized by Transsolar’s Thomas Lechner: “It is the first high-rise building with decentral venti-lation”. This dry statement sums up a technical sensation: Instead of a central airconditioning plant with its enormous air-supply and exhaust shafts, the rooms themselves, supplemented by many small air channels, secure the ventilation of the building.
Two physical forces keep the air for ventilation and basic air conditioning in motion: The wind that almost always blows around such a high building that stands alone, and the chimney effect, which causes warm air to rise in the interior of buildings.
In addition, they designed “activated” concrete ceilings: These contain thin water pipes, which carry cooling ground water from two wells below the building in summer, and hot water for heating in winter.
The architectural design accommodated this ventilation concept from the outset. The floor plan of the tower, which is supported by a pedestal structure, corresponds to two slightly shifted circular segments. Between them is a transitional area which contains very high spaces. These so-called sky gardens extend over nine stories each and form chimneys. The warm exhaust air from the offices rises up through them and then leaves the tower via exhaust vents on the sides. This gentle chimney effect is one of the two natural air-conditioning motors in the high-rise building (Figure 2).
The other natural driving force works at the air inlet side of the tower. There, the wind pushes fresh air into the build-
ing through some thousands of openings. In order to be able to use this pumping effect even under extreme weather conditions, the exterior facade consists of a double construction; the air first flows through ventilation shutters into the intermediate space between the facades (Figure 2). The north and south facades are separately regulated by a control system, which opens or closes the shutters depending on the wind velocity, the wind direction and the temperature.
The essential element for air conditioning is the many thousands of windows in the interior facade, which can be opened in order to adjust the interior climate. The occupants of the building can control the air conditions in each room individually. The windows are one of the openings for cross ventilation of the tower; if they are closed, then the air enters through a second opening (Figure 2): In the floor under each window is one of the 2000 “subcorridor convectors”. These convectors are themselves small, individually adjustable air conditioning systems, which can heat or cool the air entering the rooms when the windows are closed. They have only a supplementary function. Ducts conduct the exhaust air into the neighboring corridor, where it passes through ventilation slits and finally into a sky garden.
Normally, the wind pressure and the chimney effect are sufficient to provide good ventilation within the whole building. On an average of thirty days each year, however, a lull in the wind combined with minimal temperature differences between the inside and outside of the building make additional ventilation necessary, and it is provided by fans in the exhaust air ducts.
In spite of this elaborate design, which was completely new, with the subcorridor convectors designed especially for this project, the contractors were able to construct the building at lower cost than with a conventional air conditioning system. The decisive point was that the decentral climate concept saves about 15 % in the interior volume of the building. This space is saved because the floors for airconditioning machinery are not needed, and air ducts and false ceilings can be dispensed with. Of course, a portion of the cost savings went into the control systems.
The basic principles of the ventilation system are indeed surprisingly simple, but their realization in a high-rise building was demanding. The greatest problem is the enormous wind pressure. If the pressure difference between the windward and the leeward side were allowed to punch through into the interior of the building during an autumn storm, then many of the office doors would be pressed shut
Transsolar projects and further reading www. transsolar. com
Cloud Spaces installation, Architecture Biennale 2010 blog. cloudscap. es/category/blog-tags/transsolar
and desks would be swept clear of papers. The climate engineers overcame this problem with a trick: They permit powerful air flows through the building in order to equalize the pressure differences. But these strong airflows lose their power in the intermediate space between the two facades, damped by the inlet shutters and the ventilation slits. This damping is so effective that the windows in the inner faqade can be opened without causing problems even during a hurricane.
Transsolar had to demonstrate with certainty that their refined concept would be functional in practice. The air flow and temperature relationships in such a high building are much too complex to be able to trust computer models alone. The engineers tested physical models in a wind tunnel; the measurements were carried out by the Institute for Industrial Aerodynamics of the Technical University in Aachen. A decisive point was the demonstration that the air shutters would function as planned.
Since mid-2003, the Post Office Tower has been in use. Architectural psychologists from the University of Koblenz have investigated whether the roughly 2000 persons who occupy it every day are content with their working environment. The climate in the tower has been highly praised by all. In traditional Arabian houses, the chimney effect has been used for millennia to cool the interior; now it has also proved itself in a modern high-rise building.
Large modern buildings with glass facades, full of equipment and people, are a challenge to architectural climate engineers. This is particularly true of alternative concepts which dispense with enormous, energy-devouring air conditioning machinery and instead make use of natural resources. The Post Office Tower in Bonn (Germany) is the first high-rise building worldwide to utilize such a decentral air-conditioning concept. The chimney effect in its sky gardens and the pressure of the wind outside of the building provide passive air conditioning. Its concrete ceilings contain thin water pipes which carry cooling ground water from two wells below the building in summer, and hot water for heating in winter. Small, compact air conditioning units actively support the natural ventilation. The double facade even allows the office windows to be opened. Omitting the large-scale air conditioning machinery even allowed the 160 meter high building with its 41 floors to be built 15 % lower than a conventional structure with the same useful interior space.
About the Author
Roland Wengenmayr is the editor of the German physics magazine “Physik in unserer Zeit" and is a science journalist.
Roland Wengenmayr, Physik in unserer Zeit, Konrad-Glatt-Str. 17, 65929 Frankfurt, Germany. Roland@roland-wen genmayr. de
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