Figure 7 shows the heat balance of the seasonal storage in 2003. The storage was charged predominantly in months May to August. Discharging occurred mainly in autumn. 220 200 180 160 140 120 100

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The development of the temperatures at different levels inside the storage are represented in Figure 8. The largest temperature difference between top and bottom amounts to 31 K. To reduce stress of the concrete shell by too large temperature differences over the storage high, the storage was charged at the medium level in the first month of operation. The highest temperature in the storage so far was measured in 2003 with almost 90°C. Starting from a level of about 8°C, the soil temperatures outside the storage increased distinctly, see Figure 8. At the end of 2003 soil temperatures of almost 30°C were reached 4 m below the storage.

Since starting operation no extraordinary water losses were recorded. In 2000 and 2001 a loss of about 10 m3 was calculated. After sealing a flange in the lid of the heat storage the water loss in 2002 and 2003 could be reduced to 8 m3. The water loss remains within the dimension of the predicted loss of approximately 4,5 m3 per year. To balance the losses water was refilled in a 2-year-cycle.

In 2000 and 2002 water samples were taken to examine the chemical and microbiological quality of the storage water. According to the results of analysis, the requirements on drinking water could be satisfied in each case. The microbiological quality is considered to be completely harmless.

SHAPE * MERGEFORMAT

Figure 8: Temperatures inside and outside the seasonal storage at different levels Control engineering

The collector circuit is regulated on a target temperature by revolution adaptation of the collector and the secondary circuit pump. The control strategy has shown a good behaviour with a high practical suitability. The slow-action characteristic of revolution adaptation caused a steady and non-fluctuating development of the temperature.

The collector circuit pump is switched on in accordance with an ambient temperature — dependent characteristic. Therefore, the running time of the pump during the winter months could be reduced and optimised. The necessary frost protection operation only worked on a few winter days for about 5-10 minutes immediately after start of the collector circuit pump.

The realised connection of the storage with three levels of charging and discharging showed good operation results. By programming the control algorithm it should be taken into account that every motor driven valve gets a definite off position. The running time through the storage connecting pipes has to be considered by programming the pre­heating algorithm.

Return temperature in heat distribution net

In the hot water network of the buildings highly varying temperatures were registered. As a result the hot water comfort of the user was reduced. The flow of hot water circulation through hot water store was dedicated to be the cause of the varying temperatures.

As a result the charging of the store took place only, when the upper range had already reached temperatures clearly below the regulation switching value of 55°C. In order to guarantee the hot water comfort, but avoiding a continuous charging of the store and thus a rising return temperature, a

separate heating of the circulation net was suggested (Figure 9). All storage charging systems were modified as suggested in autumn 2002.

The additional heat exchanger was designed for a minimal return temperature difference between the primary and the secondary side. After heating up the circulation network the net water can be cooled down in the heating circuit if the temperature level is still high enough. Furthermore, the circulation network was levelled out and the oversized charging pumps were exchanged.

Measuring results after the modification display, that a continuous temperature level is reached. In addition the return temperature of the heat distribution network could be reduced.

01

Conclusions

The solar assisted district heating in Hanover-Kronsberg has been in operation since June

2000. It has been working reliably except for the mentioned leaking in the collector area piping. For the construction of the storage a nearly water diffusion tight concrete was used successfully. Extraordinary water losses of the hot water storage could not be recorded yet. The results of the first operational years achieve the range of past pilot projects.

After focussing the investigations on subjects concerning the range of building engineering in the past, the main points of research work in the future will be the optimisation (increase of the solar fraction) of the solar system.

Acknowledgement

The monitoring of the plant is supported by the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) under contract No. 0329607 F. The authors gratefully acknowledge the support of the BMU and of the operating company, Avacon AG, Helmstedt. The authors themselves carry the responsibility for the content of this paper.

References

Bodmann, M. and M. N. Fisch (2001). Solarcity „Hanover-Kronsberg". Proceedings Northsun Conference 2001, The Netherlands

Fisch M. N. et al. (2001). Solarstadt — Konzepte, Technologien, Projekte (Solarcity — concepts, technologies, projects). Kohlhammer-Verlag, Stuttgart, ISBN 3-17-015418-4 (In German)

Reineck, K.-H. and A. Lichtenfels (2000). High performance concrete hot-water tanks for the seasonal storage of solar energy. Proceedings of Terrastock 2000, August 28 — September 1, 2000, Stuttgart, Germany, pp 263 -266