Surely an oxymoron? The orthodox 'green' would rule out anything above about 12 storeys since this is the height at which natural ventilation in the western European climate zone is said to become impracticable. Tower blocks usually require a heavy engineering services system. Also the construction energy costs rise significantly every five floors or so.
However, the ecological tower block has its advocates, most notably Ken Yeang from Kuala Lumpur. He pioneered the idea of gardens in the sky coupled with natural ventilation. To cope with the wind speeds (up to 40 metres per second at 18 storeys) he uses wing wind walls and wind scoops which deflect the wind into the centre of the building.
The first manifestation of these principles in the west was the Commerzbank in Frankfurt (Figure 12.16). This began life as a limited competition for an office headquarters comprising 900 000 square feet of office space and 500 000 square feet of other uses. The brief was clear that it should be an ecological building in which energy efficiency and natural ventilation played a crucial role. At that time the Green Party was in control of the city. In the winning design by Norman Foster Associates, a 60-storey three-sided building wraps round an open central core ascending the full height of the building (Figure 12.17). The most remarkable feature of the design is the incorporation of open gardens. The nine gardens each occupy four storeys and rotate round the building at 120 degrees enabling all the offices to have contact with a garden.
The gardens are social spaces where people can have a coffee or lunch and each one 'belongs' to a segment of office space accommodating 240 people. As the architects put it: 'we're breaking the building down into a number of village units'. This is extremely important in reducing the scale of the place for its occupants. The gardens feature vegetation from North America, Japan and the Mediterranean according to their height above ground.
The natural ventilation enters through the top of the gardens passing into the central atrium. The atrium is subdivided into 12-storey units and within 12 floors there is cross-ventilation from the gardens in
the three directions (Figure 12.18). Air quality is good, enhanced as it is by the greenery. It is estimated that the natural ventilation system will be sufficient for 60 per cent of the year. When conditions are too cold, windy or hot, the building management system activates a backup
Natural ventilation paths in the Commerzbank
Natural ventilation paths in the Commerzbank ventilation system which is linked to a chilled ceiling system that operates throughout the building.
The curtain wall design is on Klimafassade (climate facade) principles. Air enters at each floor in the facade into a 200 mm cavity where it heats up and passes out through the top of the cavity, which is, in effect, a thermal chimney. The climate facade consists of a 12 mm glass outer skin that has been specially coated to absorb radar signals, presumably from the airport. The inner skin of the facade is Low E double glazing giving the overall system a high U-value. There are permanent vents in the outer skin whilst the inner double glazed element has openable vents which can be overriden by the BMS when circumstances demand it. Motorised aluminium blinds in the cavity provide solar shading. It is calculated that the ventilation system will use only 35 per cent of the energy of an air conditioned office.
This is a remarkable attempt to create an extremely high tower block which minimises its environmental impact whilst also providing optimum comfort and amenity for its occupants. It also demonstrates how bioclimatic architecture is subject to the vagaries of political fortune. If the Greens had not had their brief moment of glory it is likely that this building would never have happened.
In 2004 Number 30 St Mary Axe, the London headquarters of the international reinsurers Swiss Re, was completed (Figure 12.19). It is claimed by its architects Foster and Partners to be the first environmental skyscraper in the City. At 40 storeys its circular plan and conelike shape differentiate it from all other high buildings in London. The question is whether this is a piece of architectural whimsy or a form that arises from a logical functional brief. There is no doubt as to its genetic origin which is the Commerzbank in Frankfurt with its triangular plan and four-storey atria which rotate around the plan (Figures 12.19-12.21).
The idea of an atrium space easily accessible at all levels has now evolved into six spiral light wells that have a platform at every sixth floor. The spirals are accentuated in darker glass on the elevation. Triangular in plan they serve to provide both light and ventilation. The curved aerodynamic shape ensures then even high winds slide off the surfaces making minimum impact. This, in turn, has made it possible to incorporate motorised opening windows in the atria to assist natural ventilation. Floors between the break-out spaces have balconies to the atria. The 39th floor is a restaurant offering spectacular views for the privileged few.
According to the services engineers Hilson Moran, the ventilation system would be boosted by air pressure variation produced by the circular form driving the natural ventilation cycle. The atria/lightwells provide natural ventilation and act as 'lungs' for the building, providing natural ventilation for 40 per cent of the year. Overall the ventilation system is mixed mode employing air conditioning which is perhaps inevitable in a building of this height and location. However, the energy
impact of the air conditioning is reduced by a series of heat recovery units. Both natural and mechanical ventilation systems are controlled by an intelligent building management system.
The external skin is a climate facade consisting of an external double glazed external screen and single internal glazing. The space between serves as a ventilated cavity, removing warm air in summer and providing insulation in winter. Solar controlled blinds are positioned within the cavity.
A circular plan has the advantage of maximising daylight in the office floors which are situated around the perimeter with circulation taking up the core of the building.
Altogether the environmental attributes of the design result in an estimated energy consumption of 150kWh/m2 per year which represents a 50 per cent saving compared with a traditional, good practice design, fully serviced office development of similar size.
This building highlights one of the dilemmas of bioclimatic architecture, namely that a bespoke building may only be partly used by the building owner. In this case Swiss Re will undertake rigorous energy
Upper floors with triangular atria
Upper floors with triangular atria management. However, much of the tower will be let out with no guarantee of a similar quality of energy management. The worst case scenario is that the system will be allowed to default to air conditioning which will negate the energy efficiency targets of the designers.
Like the previous examples in the book this will maximise natural ventilation as part of a mixed-mode ventilation strategy within the constraints of a high rise building. The services engineers, Roger Preston and Partners, reckon that if the natural ventilation capacity is used to the full, it should produce a two thirds energy saving against a conventional sealed air conditioned equivalent. Again the design makes maximum use of the climate facade principle which adds about 3 per cent to the cost and reduces the floor plate. When the energy savings are capitalised it quickly becomes clear that the extra cost is soon recovered, offering considerable revenue benefits thereafter.
Up to the seventh floor occupants can open windows behind a protective glass screen. Above this level the climate facade comes into its own. This moderates the problems associated with high rise buildings: high wind velocity, pollution and noise. The natural ventilation works by allowing occupants to operate double glazed windows which open into a 650 mm space sealed from the outside by a single skin of
Minerva Tower glazing. Vents at the top and bottom of this void allow access for fresh air. This means that, even at a height of 200 m, air velocity can be moderated by vents, allowing it to enter the office space at an agreeable velocity. The designers are optimistic that the tower will be able to operate in natural mode for about two thirds of the year, with mechanical ventilation only necessary in extremely hot, cold or windy conditions.
The seasonal variations in the operation of the climate facade are shown in Figure 12.23.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.