Geothermische Simulation einer Energiepfahlanlage

Geothermal simulation of an energy pile system.

An energy pile system was installed in the foundation of a new office building in Böblingen in 2013. The building is founded on 114 reinforced concrete bored piles which are embedded in the hard silt-claystones containing gypsum. Since the risk of anhydrite/gypsum induced heaving precluded the geothermal use by deeper borehole heat exchangers, the geothermal pile system was chosen. To determine the allowable or possible exploitation of the geothermal potential of the underground in the 200 MWh/a plant, a coupled groundwater flow and heat transport FE model was used for a simulation period of 30 years. The results showed that the geothermal use for cooling alone results in a thermal overuse of the underground and in a temperature plume extending too far beyond the building site due to the low velocity of groundwater flow. In close cooperation with the HVAC engineers several forecasts consisting of different allocations of monthly basic cooling and heating loads were calculated. A largely balanced load with comparable cooling and heating loads proved to be the recommended project option that is computed to show hardly any effects outside of the building site. The calculations furthermore illustrated in all the options that a steady state is achieved after about ten years and that the temperature fields of closely spaced geothermal piles overlap so much that an efficient yield is no longer possible. The example shows that the FE calculations are an adequate tool for the planning and optimization of the operation of a geothermal energy pile system. The monitoring of underground temperatures via built-in fibre optic cables will allow the comparison between predicted and actual temperature development in the coming years.