Since 1997 the first solar assisted district heating system with duct heat store in Germany is being realised in Neckarsulm-Amorbach. The solar assisted district heating system presently supplies about 200 accommodation units with heat. For the final extension stage approximately 1,300 accommodation units were planned.

Presently 5,263 m2 solar thermal collectors are installed on different buildings as well as on a carport and a noise protection wall. The heat from the solar collectors is delivered to the heating plant and collected in buffer tanks which are used for short-term heat storage to balance peaks in heat delivery from the solar collectors, see figure 1.

The buildings are connected to the district heating system by a 3-pipe heat distribution net. The heat distribution net is supplied either by the buffer tanks or the duct heat store, depending on the temperature level. A gas condensing boiler supplies additional heat if none of the stores is able to deliver heat at the requested temperature level.

The duct heat store was extended twice and presently contains a volume of 63,360 m3 with 528 borehole heat exchangers (double-U-pipes, 30 m deep) for charging and discharging.

In table 1 an overview about the project history and the extension stages of the collector area and duct heat store volume is given.