The volume of the heat store amounts to 12 000 m3 (height: 20 m, diameter:

32 m). Figure 5 shows the hot water heat store during construction. The charging and discharging behaviour for 2002 is shown in figure 6. Charging of the store mainly occurs from May to August, discharging in the autumn months. Approximately 20 % of the heat delivered by the solar collectors is used directly for preheating the district heating net. In figure 7 the variation of temperature in the store is depicted for the years 1997 to 2003. The seasonal variation of temperature can be seen easily. The highest temperatures reached in the store in the end of summer amount to 80 °C on the top and to about 60 °C at the bottom. This also means that the heat capacity of the store is not sufficiently used. The building development and the solar collector fields of the second extension are realised unexpectedly slow and different to the planning. The lowest temperatures reached in the store are about 40 to 45 °C at the bottom depending on the year, since the return temperatures of the district heating net, which are the lowest temperatures in the whole system, are higher than expected. The difference between the lowest temperature in the store and the start temperature in October 1996 (11 °C) represents the unusable heat content of the store which was needed to put the store into regular operation. Furthermore in figure 7 an increase of the temperature besides the store can be seen from November 2000 to April 2001. This is due to a defect of the drainage pump and the resulting wetting of the thermal insulation. The heat store is surrounded by a drainage system to protect the thermal insulation of the heat store from flooding by ground water. The water is collected in a drainage duct and delivered to a nearby lake by the drainage pump. After repairing of the pump the temperature around the store continuously decreased during the years 2001 and

2002. The thermal insulation seems to dry up.

The heat losses of the store are between 322 and 360 MWh/a, corresponding to a moderate efficiency of 60 % for the seasonal heat storage. The calculated value amounts to 220 MWh/a. This difference is due to the operation of the heat store at higher temperatures than expected caused by the high net return temperature. It generates an offset of the store temperature of at least 10 K at the bottom and increases the heat losses significantly since the lower third of the heat store is not thermally insulated. It is also assumed that the thermal insulation of the heat store is partly wet due to insufficient drainage of ground water. Approximately calculations to take into consideration the wetness of the thermal insulation yield additional heat losses of 200 MWh/a.

The contribution of the connecting pipes between heating plant and heat store is also not negligible. The pipes have an overall length of 55 m. These pipes were used for charging and discharging operations for 6 600 h in 2002. Assuming a temperature drop of 0.5 K between in — and outlet of the pipes the resulting heat losses amount to 50 MWh/a.