In figure 7 the temperatures and volumetric flow rates of the heat distribution net are shown. In 2003 both supply and return temperatures increased in comparison to 2002. At the end of 2002 some new buildings were connected to the district heating system by an approx. 1000 m long pipe. Because of high circulation flow rates and a low heat demand this part of the net causes net return temperatures. The short-term increase of the volumetric flow rate in December 2002 was caused by a temporary supply of an adjacent district heating system.

It is also evident that net supply temperatures in summer 2002 / 2003 and in 2003 even in spring are higher than required. The reason therefore are the hydraulic conditions in the 3- pipe heat distribution net, see [3]. Because the collector volumetric flow rate is higher than the volumetric flow rate in the heat distribution net not enough cold water is available to cool down the supply temperature by admixing cold return flow to the hot supply flow.

Heat balances

In figure 8 a schematic heat balance for the district heating system in Neckarsulm-Amor — bach is depicted for 2003. About 70 % of the total solar heat delivery (2,121 MWh) was used to charge the duct heat store. 1,109 MWh heat were delivered by an auxilary gas boiler. The heat demand of the buildings amounts to 1,305 MWh and the heat losses in the heat distribution and solar net to 586 MWh. 153 MWh were discharged from the duct heat store and 548 MWh of thermal solar heat were directly used for heat supply in the heat distribution net. In 2003 a solar fraction of the total heat demand of 39 % was reached.

The heat losses in the heat distribution and solar net are high because the net is almost completely installed but less buildings are connected than expected. The discharge of the duct heat store is about 10 % of the charging heat amount because duct heat stores need a 5-8 years heating-up period to reach a quasi-steady-state behaviour. A significant

In table 2 some characteristic data for the heat distribution system in Neckarsulm-Amor- bach are listed. In 2003 the heat demand decreased eventhrough more buildings were connected to the district heating system. The solar fraction increased from 18 % in 1999 to 39 % in 2002 and 2003. The planned solar fraction of ~50 % is expected to be reached in the next years due to the heating-up period of the duct heat store.

Outlook

A further extension of the duct heat store is planned when the heat demand in the residential area increases. However instead of extending the duct heat store, the installation of a heat pump will be taken into consideration to increase the usable temperature level of the duct heat store.

References

[1]

J. NuBbicker, D. Mangold, W. Heidemann, H. MQller-Steinhagen: Erfahrungen aus Betrieb und Ausbau der solar unterstQtzten Nahwarmeversorgung mit Erdsonden-Warmespeicher in Neckarsulm-Amorbach. Proc. of 12. Symposium Thermische Solarenergie, Staffelstein, Germany 24.-26.04.2002, pp. 471-475

[2]

M. Benner, M. Bodmann, D. Mangold, J. NuBbicker, S. Raab, Th. Schmidt, H. Seiwald: Solar unterstQtzte Nahwarmeversorgung mit und ohne Langzeit-Warmespeicher (Nov. 98 bis Jan. 03), Forschungsbericht zum BMWi-Vorhaben 0329606 S, ISBN 3-9805274-2-5, Stuttgart, 2004

[3]

J. NuBbicker, D. Mangold, W. Heidemann, H. MQller-Steinhagen: Solar assisted district heating system with duct heat store in Neckarsulm-Amorbach (Germany), Proc. of ISES, Goteborg (Sweden), June 14-19, 2003

This project is being supported by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (Bundesministerium fur Umwelt, Naturschutz und Reaktorsicherheit), FKZ 0329607F. The authors gratefully acknowledge this support and carry the full responsibility for the content of this paper.