Hannover fare

It is debatable whether the staging of an Expo is a fundamentally sustainable act. With up to two million people travelling from all over the world to the site by air, rail and car, Expo’s are essentially a licence to emit carbon. No matter how energy efficient the Expo buildings, they can’t mitigate the fuel consumption of a 747.

Still, there is no denying that the buildings that make up Expo 2000 and its environs have been built to uncommonly high standards, even for this part of Europe. One such site is a development of 142 low-rise flats for 500 people, called Wohnen 2000, near Kronsberg.

Kronsberg is a new residential district on the outskirts of Hannover adjacent to the Expo 2000 site. The district was established using the “Hannover Principles” – a design philosophy which, among other things, demands energy efficient buildings. Materials were required to be certified from sustainable sources, and materials such as pvc were not permitted, even for for cabling. The materials selection on this project is a story in itself.

All designs were required to demonstrate that the heating energy consumption will not exceed 50 kWh/m²/y, a difficult target given the cold winters in northern Germany.

All designs were independently scrutinised and numerically checked by the Institut für Bauphysik, Hannover. No building permit could be achieved without this independent approval. The Institut was also active on-site, checking and signing off the contracting works.

The sites at Kronsberg were developed by a variety of investors and house builders for rent and purchase. The client was Deutsche Bau-Becon, a major investor in apartments in Germany with a portfolio of over 30 000 units in the rental sector. The project was a design and build venture with BauBecon’s subsidiary BekoBau GmbH from Leipzig.

Concept design

The project was won following a limited integrated design competition, which the UK consulting firm Atelier Ten entered with architect Jürgen Willen of Willen Associates from Wiesbaden.

The site is broadly square in plan (110 m x 110 m) which could accommodate either a north-south or an east-west oriented scheme. Sketch modelling at the concept design stage suggested that the logical south aspect arrangement of living spaces with single dwelling units was unsuitable. It was inefficient in surface area to volume terms, and fraught with solar access problems in winter, particularly given the density of the required dwellings.

In urban design terms, the southerly aspect arrangement also left the spaces for communal gardens between the buildings in the shade for much of the year, which would have been undesirable.

The relatively simple move made at this point was to effectively double-load the living spaces by locating blocks back-to-back, facing east and west and separating them by a covered atrium.

This atrium became known as the MikroKlimaZone (MKZ). The architects organised living rooms and bedrooms so that they opened to the outside, while circulation, bathrooms and kitchens were planned to open into the MKZ.

To ensure all rooms receive adequate external air throughout the year, each of the eight apartment blocks is connected to a mechanical background ventilation system with heat recovery. The ventilation operates 24 h/day, with a basic ventilation rate at night and a boosted rate during the day when toilets and kitchens are used more frequently. This is controlled by light switches and/or humidity sensors. Variable speed drives control the air volume, while heat recovery efficiency is estimated at 71%.

The apartments are designed to be very airtight when the windows are closed. They were pressure-tested prior to completion to ensure that infiltration was virtually eliminated. Blower door tests (ISO 9972) carried out before hand-over achieved 0.4 m3/h/m² of envelope at 50 Pa, which indicates excellent detailing and construction quality. This compares to the Institute target of 1 m³/h/m². (UK good practice would be less than 5 m³/h/m² and the low energy standard, less than 2 m³/h/m²).

Reducing the exposed external wall area of each apartment allows a greater area of glass on the external walls to increase daylighting levels in living spaces and maximise passive solar gains when available. External movable shutters were provided so the occupants can keep out unwanted gain in summer, but move them out of the way in the winter when the gains are beneficial. The scheme is easy to differentiate from others in the development, which tend to have smaller areas of punched windows in the facades.

The roof of the MKZ between each of the blocks is aligned north-south. The walls of the apartments which open into the MKZ have their thermal insulation on the inside, while the outside walls have external insulation. The thermal mass presented into the MKZ is designed to act as a heat store during winter.

MikroKlimaZone roof structure

The roof structure is particularly unusual. It is made up of three skins of etfe foil formed into a series of inflated cushions running across the length of the roof. The outer and middle membranes are screen printed with a regular matrix of reflective dots. These are set up in manufacture to align into a continuous block when the surfaces are juxtaposed. The bottom membrane is clear.

In winter the upper cavity is inflated and the middle membrane is flush to the lower surface. Solar gains are admitted to the MKZ and are absorbed by the surfaces. The space is naturally ventilated but high and low level vents are kept closed. Air is drawn into the apartment ventilation systems from high level in the MKZ, the buffer zone effect reducing the load on the reheater batteries. Dynamic simulation of the conditions in the MKZ in winter suggest that the air temperature will only drop to freezing when the outside air temperature falls to between -10°C and -15°C (about 14 days/y).

In summer the lower cavity is inflated and the middle membrane is flush to the upper surface. The continuous printed surface reflects solar gains, reducing heat gains to the space below. Vents at high and low level are opened automatically to vent hot air.

The etfe has the benefit of being very light, therefore using little material in the manufacture and the supporting structure. It also has a lower U-value (1.8 W/m²) than conventional double-glazing, at a fraction of the weight.

ETFE has been described as a “millennium material” because of its adaptability. It does, however, have a significant drawback, namely the noise created by rain drumming on the tensioned surface, which can be extremely loud in the space beneath. Here it is not a problem because the living spaces are insulated by well- sealed double-glazing from the MKZ and the noise is barely audible internally, even in a heavy storm.

The concept design phase was followed by an extremely rigorous phase of detailed thermal modelling to prove the concept to the satisfaction of the checking authority. This was accompanied by the inevitable pincer movement on building fabric costs, which were significantly increased by the very high insulation standards (the external wall insulation is 290 mm thick).

Modelling extended beyond the surface modelling that is normal in the UK into an appraisal of numerous building element intersections and construction joints, to eliminate cold bridges. A number of intersections, particularly at wall/floor junctions and at balcony level, had to be redesigned several times to incorporate surface insulation before the cold bridges were eliminated.

Energy targeting

There was considerable pressure to optimise the model to achieve precisely 50 kWh/m²/y. This led to the manipulation of material area and thickness. In practice this was considerably more complicated than it sounds, particularly because the MKZ, a key element of the scheme, did not fit the standardised calculation model and required painstaking validation simulations. Each of these had to be validated by the checking engineer, who was at times less than understanding of the client’s commercial programme and objectives.

Ultimately, the energy targets were agreed as an estimated annual energy demand of 36-42 kWh/m²/y (depending upon orientation) or 49.83 kWh/m²/y based on the standardised model used under the Hannover Principles.

Building services systems

Heating and hws come from a central plantroom that is connected to the district heating network via a heat exchanger. Variable speed drive pumps circulate weather compensated heating water to all apartments.

The heat metering arrangement is quite unusual, and involves a surface-mounted heat meter on each radiator. This has an analogue read-out and can be interrogated through a bi-directional radio link. It is therefore possible to identify the heat consumption of individual rooms.

The dhw is supplied from centralised storage calorifiers at a constant temperature of 60oC to a volume flow meter for each apartment. This is also metered by a radio signal as above. The cold water and electricity meters are connected to the same transmitter for remote monitoring and metering of each apartment.

Services are distributed at basement level, through the (naturally ventilated) car park and rise within each staircase to serve the flats.

The electrical services system is relatively straightforward. Each apartment has a metered single-phase supply and lighting throughout the development uses low energy light sources. Lighting to common areas is switched using a combination of time switches and movement detectors with time delay.

The majority of power is delivered by a combined heat and power unit in the energy centre next to the site, and an aerogenerator that also feeds into the local grid. With these highly efficient sources, no case could be made for the application of photovoltaics.

The scheme has a very modern architectural feel. No doubt it could have been even more efficient had less glazing been used, but as built it meets the most rigorous of environmental standards while retaining a strong architectural character. The building has proved popular with potential tenants, who have up to 30 other sites on the Kronsberg to choose from. Many of the flats are already occupied.

The very high quality of building construction has been achieved solely as a result of the regulatory framework established for the area. Although the capital cost is indubitably greater than a building constructed to the normal German building regulations.