In figure 11 the temperatures of the 1st extension are shown for 2003. In the summer months (June to September) the net return temperature varies between 55 and 60 °C due to the indirect preparation of domestic hot water by storage charging systems. In the summer months normally there is no heating system required. In 2003 the installation of the 2nd extension results in a disconnection of the heat store from 7/21/03 to 8/06/03. In the winter months with high heating demand the return flow temperature is slightly below 50 °C. Since the net return temperature is the lowest temperature in the whole system, the seasonal heat store can not be discharged to temperatures below 50 °C. This results in higher heat losses to the environment compared to the design net return temperatures of below 40 °C. The integration of a heat pump into the system would lead to an improved discharging of the store increasing the efficiency of all components due to lower heat losses to the environment.

— net supply temperature —- net return temperature

— preheating temperature —- ambient temperature

Figure 11: Ambient temperature and temperatures in the district heating net in 2003

Summary and outlook

Between 1997 and 2003 solar fractions from 21 to 30 % were reached. The specific solar heat gains vary from 330 to 400 kWh/(m2 a) resp. 176 to 241 kWh/(m2 a) (gross resp. net). One major reason for not reaching higher solar fractions are the net return temperatures of around 50 °C which are more than 10 K higher than expected value of less than 40 °C (yearly average weighted by volume flow). In addition, the heat losses of the heat store are higher than expected, mainly due to wet thermal insulation, and the connecting pipes with a length of 55 m. Apart from this no major problems occurred during the last seven operational years.

References

[1] 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

[2] TRNSYS Version 15.0 — User Manual. Solar Energy Laboratory, University of Wisconsin, Madison und Transsolar, Stuttgart.

[3] DF — Dynamic Fitting Version 2.7, InSitu Scientific Software, c/o W. Spirkl, Germering, Germany.

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.