The duct heat store, which is used for seasonal heat storage, is described in detail in [1], [2] and [3]. In figure 2 temperatures in the centre of the first and second extension of the store are shown for different depths. The store was heated up from about 10 °C and has not yet reached the designed maximum temperature of about 85 °C due to its long heating-up time of 5-8 years. Nevertheless discharging started in 2003 and a steady state operation with storage efficiencies of about 70 % (for the completed store) is expected within the next 3-4 years.

In figure 3 the temperatures in the centre of the first extension in different depths are shown for 2003. The highest temperature in this part was approx. 58 °C and was reached in October. At the end of October discharging started and the temperatures decreased. In the second extension, see figure 4, the maximum temperature was approx. 65 °C. The temperatures in the first extension are lower since this part was not charged in 2002 in order to reach a fast temperature adaptation of the various store parts. The temperatures below the store (depth 30 to 40 m) slightly increased due to heat losses of the store. For economical and constructional reasons the store is only insulated on top.

Until now some minor problems related to the duct heat store occurred. In 2003 a sludge trap was installed since corrosion deposits were found in distribution pipes. Since the duct heat store is directly connected to the heat distribution system the borehole heat exchangers must be prevented from clogging.

The temperatures in the various parts of the store vary (referring to same depth). The temperatures between the first and second extension vary because of different operating times. The analyses of the measured data also show different temperatures in the north and south part of the second extension of the store. Assuming equal hydraulic transmissivity in the ground, no groundwater flow and equal amounts of charging heat, temperatures should be the same. The most likely explanation for this observation is a poor hydraulic adjustment of the double-U-pipes.

Figure 4: Temperatures in the seasonal duct heat store in 2003 (second extension)