S. Raab, Institute for Thermodynamics and Thermal Engineering (ITW), University of Stuttgart

D. Mangold, Solar — und Warmetechnik Stuttgart (SWT), ein Forschungsinstitut der Steinbeis-Stiftung

W. Heidemann, Institute for Thermodynamics and Thermal Engineering (ITW), University of Stuttgart

H. Mtiller-Steinhagen, Institute for Thermodynamics and Thermal Engineering (ITW), University of Stuttgart, Solar — und Warmetechnik Stuttgart, German Aerospace Centre (DLR) Institute for Technical Thermodynamics

In Friedrichshafen, close to the Lake of Constance, a solar assisted district heating system with seasonal hot water heat store was put into operation in 1996. The seasonal storage is realised by a hot water heat store constructed from reinforced concrete. At present the solar assisted district heating system is extended and additional collector fields and buildings are connected to the system. In this paper the system heat balances of the last seven years and the most important experiences are reported.

System description

Figure 1 shows the planned development of the settlement in Friedrichshafen. It is divided into three phases of extension. In the course of the realisation of the 1st and 2nd extension solar collector fields are integrated in the district heating system. When realising the 3rd extension additional buildings will be connected to the system, but no additional collector fields will be integrated.

After completion of the first extension in 1996 280 apartments (in multi­family houses) and one kindergarten were connected to the district heating system. The heated area amounts to around 23 000 m2. On top of the buildings of the first extension 2 700 m2 of flat plate collectors have been mounted, divided into seven fields. The inclination angle varies between 20 and 25 °, the orientation deviates from ideal south orientation up to 110 ° to the north-west. The energetic optimum inclination angle range of the collector fields is between 30 and 45 ° facing to south. The seasonal hot water heat store, which was also built in 1996, has a water volume of 12 000 m3 and is energetically designed for the integration of an additional extension with 2 700 2 of solar collectors. The hot water heat store transfers the heat collected in summer by the solar collectors to winter when the heat demand in the district heating net is comparatively high. In figure 2 the schematic layout of the system is shown. In 2002 the realisation of the second extension started. Contrary to previous planning of approx. 280 apartments (in multi-family houses) around 110 accommodation units mainly in terraced houses are being built. Originally the integration of 2 700 m2 of solar collectors was planned for the year 1998. Due to the decreased roof area only around 1 700 m2 of solar collectors will be integrated in the system after the completion of the second

extension. In 2002 and 2003 two collector fields of 413 m2 and 400 m2 respectively have been installed as roof integrated field or mounted on a subconstruction. In figure 3 an example of a roof integrated collector field (multi-family house of the first extension) is shown.

Heat balances

In table 1 the system heat balances for the years 1997 to 2003 are shown. The achieved solar fraction (based on total heat demand) varies between 21 and 30 %. The design solar fraction related to the 1st phase of extension was calculated to 43 %. This value has not yet been reached due to several reasons. The heat consumption of the buildings (1st extension) is approx. 10 % higher than expected. Furthermore the design return temperatures of the district heating net were supposed to be lower than 40 °C (yearly average weighted by volume flow). In 2003 this value rose to 51.5 °C. The heat losses of the seasonal heat store are in the range between 322 and 360 MWh/a and are significantly higher than the calculated value of 220 MWh/a.

The efficiency factor of the gas condensing boiler during the last years of operation amounts from 94 % to 100 % (based on lower heating value Hu). The solar collectors’ heat gain of the 1st extension amounted to 941 MWh in 2003 (gross solar heat gain including heat losses caused by collector pipes; as measured at the solar heat exchanger). This is around 5 % less than in 2002 whereas the irradiation on the collector plane is about 13 % higher. This is caused by stagnation of the collector field from 7/21/03 to 8/18/03 (corresponding to an irradiation on collector plane of 175 kWh/mF) because of necessary work in the heating plant.

The degree day value for 2003 amounts to 3 931 Kd and is high compared to the years 1997 to 2002 owing to relatively cold winter months. The overall heat delivery to the district

heating net in 2003 (1st and 2nd extension) was 3 325 MWh. This is significantly higher than in the previous years, mainly due to the connection of new buildings in the supply area of the 2nd extension. In figure 4 the heat flow of the plant is depicted in a Sankey diagram for the year 2002.

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