Wärmeleitfähigkeit der Wärmedämmung von unterirdischen Langzeit-Wärmespeichern am Beispiel des Erdsonden-Wärmespeichers in Neckarsulm

The solar assisted district heating system with borehole thermal energy store (BTES) in Neckarsulm is being realized since 1997. Solar heat is stored in the borehole thermal energy store from summer to winter. So far 5263 m² of solar thermal collectors are installed. The BTES presently contains a volume of 63 360 m³ with 528 borehole heat exchangers (double-U-pipes, 30 m deep) for charging and discharging. The BTES was extended twice; the operation of the first and second extension started in 1999 and in 2002, respectively. For economic and constructional reasons the BTES can only be insulated on the top surface. The BTES in Neckarsulm is insulated by two layers of XPS insulation sheets. The XPS insulation is packed between two liners: the lower one to protect the insulation from getting wet by diffusion from soil moisture, the upper one to protect against rain and surface water. Detailed information about construction and operation can be found in [1] and [2]. The predicted annual storage utilization factor assumed for the case the BTES is fully charged by the solar collectors to temperatures up to 90 °C has not been realized for various reasons, see [3]. This paper focuses on the effect of humidity and temperature on the thermal conductivity of the insulation. Measured data from heat flux and temperature sensors are analyzed; results are compared with data obtained from indoor measurements. It is observed that despite the diffusible construction in the second extension the insulation is partially moist resulting in an increased thermal conductivity. This may be one of several reasons for the higher thermal losses with respect to the predictions.