Michael Bodmann, M. Norbert Fisch

Institute for Building Services and Solar Technology, Technical University of

Braunschweig,

Muehlenpfordtstr. 23, D-38106 Braunschweig, Germany

As part of the World Exposition EXPO 2000, a solar assisted district heating system for 106 residential units was put into operation in Hanover-Kronsberg in June 2000. Roof-integrated collectors with an area of 1,473 m2 were installed and a hot water storage with a volume of 2,750 m3 was erected. For the construction of the storage a nearly water diffusion tight concrete was used. In comparison with former storage constructions no inner stainless steel liner was needed.

Project description

Figure 1 shows a ground plan of solar housing estate. Residential buildings with a total living area of 7,250 m2 and a common room area of 110 m2 are connected to the solar assisted district heating. Solar collectors were mounted on the south-western and south­eastern orientated roofs. The seasonal storage was erected on a public playground in 120 m distance from the heating central, which is placed in the basement.

A simplified scheme of the solar assisted district heating is shown in Figure 2. The collected solar heat is transported by a piping network to the heating central. A heat exchanger is used to charge the storage circuit. Heat can either be directly used for covering the heating demand or be stored in the seasonal storage. If the solar heat does not have a sufficient temperature, additional heat is supplied by the heat distribution network of the entire estate Kronsberg, which is connected to a cogeneration plant.

Compared with systems of the first generation, the plant was

optimised. The storage can be charged and discharged at three levels, which allow simultaneous loading and unloading of the storage. The flow in the collector and the secondary circuit can be variably controlled by pumps and will be adjusted to the required temperature.

Hot water preparation is ensured by domestic hot water storage systems in substations. The dwellings are equipped with a low temperature heating system with radiators at 65/39°C (supply/return temperature). The heat distribution network with a maximum supply temperature of 70°C was mainly installed in the basement of the buildings.

Collector area

The collector plant is distributed in 13 partial areas. The size of the different collector fields varies between 40 and 290 m2.

The used solar roof system allows an exact adaptation to the substructure. The utilisation of the roof area was almost 90%. Therefore, the solar roof system replaced the conventional roofing. Figure 3 shows an example of collector integration in the detached buildings.

Seasonal storage

The seasonal storage was constructed as a concrete cylinder with a roof which is formed as a conical shell, see Figure 4. The internal diameter comes to about 19 m, and the largest inside height amounts to approximately 11 m.

For the construction of the storage in this project a high performance nearly water steam diffusion tight concrete was used for the first time (Reineck and Lichtenfels,

2000). Thus, it does not only carry the load but also has the function of waterproofing. In this case there was no interior stainless steel liner needed as used at the storage constructions in

Friedrichshafen-Wiggenhausen and Hamburg-Bramfeld (Fisch et al., 2001). Due to the water loss (by water steam diffusion) a water resistant insulation was necessary. Therefore, wall and
roof of the storage were insulated on the outer side with pressure resistant granulated recycling glass. The insulation thickness in the wall area increases from 30 cm at the bottom to 70 cm at the top. The thickness of the roof insulation reaches 70 cm. Above the roof insulation a protective concrete layer and an earthcover were applied. Figure 5 shows the storage after completion of the work.