The Reichstag in Berlin is a very public place for pursuing an environmental agenda, especially considering the strong green movement found in Germany. Its energy-conserving design is inevitably a compromise for the designers, Foster and Partners, Kaiser Bautechnik and Kuhn Bauer und Partner, who are seeking to make best use of a strong, established building form. Besides, energy-saving is only one of several design concerns, what with the significance of the Reichstag as a new democratic forum, aiming to make the process of government more accessible, as well as the building's own history.
The most dramatic change to the existing structure, whether viewed from afar, near or from within, will be the main debating chamber with its glass cupola at roof level. Twin helical ramps within the cupola will provide visitors with panoramic views across the city. Stephan Behling, a Foster partner, believes this will become a 'must-visit' Berlin landmark like the Eiffel Tower for Paris. He sees it not only as part of the energy agenda but also as 'communicating externally the themes of lightness, transparency, permeability and public access'.
The cupola is a separate, unheated, single-glazed space above the chamber, divided from it by an insulated roof and glazing. Penetrating down into the chamber is a central, concave cone-like form (see section) clad in angled mirrors. Designed with the lighting consultant Claude Engel, the mirrors reflect diffuse skylight from the horizon down into the chamber, reducing the requirement for artificial lighting. Seen from below will be a reflected sky view. Sun-tracking shading outside the cupola can block direct sunlight. Where sun angles are low enough for the sunlight not to interfere with proceedings, the shading can be moved away to let it through. When the chamber is in use after nightfall, artificial light from the chamber will reflect upwards, illuminating the cupola.
The cone also houses air-extract and heat-exchange equipment for the chamber. Both this and the motorised shading are powered by 40kWp of photovoltaic cells.
The chamber uses displacement ventilation with tempered air entering through the floors. These floors comprise 3 x 3cm mesh, perforated panels, and a porous carpet making the whole area of floor the air inlet, an approach rarely used before.
This floor air inlet makes use of enormous exiting airways in the building - a storey-height void beneath the chamber and 3 x 3m masonry feeder ducts. Only the air inlet to the building has been changed - it was moved away from the ground for security reasons. Air now enters above the north portico. This large-scale air-handling route allows low air velocities and thus relatively low fan-energy consumption.
The spaces around the building perimeter are diverse, ranging from small offices to large meeting rooms. Opening perimeter windows can provide some single-sided ventilation at best, with mechanical extract. Laminated glass panels are set 0.5m outside the opening windows. An edge gap of 30-40cm between the glass panel and the reveal is filled with a metal grille, the whole providing ventilation paths while maintaining security.
Chilled ceilings provide cooling. Many spaces have solid floors, so ventilation is via a conventional ducted system. The new accommodation on the top floor has raised floors providing displacement ventilation But because it is behind the parapet it has no windows in the walls. Instead, rooflights are clad with Siemens' microlouvre glass. This has narrow triangular-section ridges across it, acting as prisms, allowing only north light into these spaces.
The building's large thermal mass will dampen peak temperatures. Some rooms have mechanised openings, and with the window security approach there is some potential for night cooling. Lighting is generally fluorescent, with central and local control.
A cogeneration plant will provide heat, electricity and cooling (via heat-using absorption chillers), both to the Reichstag and to the nearby parliamentary offices (which are about five times the size of those being designed by Hopkins for Westminster). Some electrical power will be drawn from the grid. Sizing the cogeneration plant to meet demand peaks for electricity would lead to cogeneration of very large heat surpluses. Even now there will be surpluses, which can be stored inter-seasonally in aquifers 400m down. There is also a small buffer 'cold store' in another aquifer layer 40m down.
The presence of these aquifers of still water is fortuitous. In principle, moving groundwater could be tapped by heat pump for heating and cooling. But permission would not have been granted to use the groundwater because of 'thermal contamination' of neighbouring sites.
The designers have proposed fuelling the cogeneration plant with rapeseed oil, a renewable oil traded as a commodity alongside conventional fuel oils, but the plant can also be fuelled by oil or by gas. It is for the client to choose between the lower CO2 output of the renewable oil and the lower cost of gas. (The histogram illustrates the CO2 impact of this choice.)
More significant is the predicted energy performance compared with the typical figures shown for 1960-1995 - not to mention the greater levels of comfort promised. Although this is far from a one-dimensional building, aiming to save energy at the expense of all other aspects of design, the energy-conserving promise is high.