GeoStar – a Scalable Borehole Heat Exchanger System

Abstract

The implementation of low-temperature district heating systems with central heat pump stations in large and growing urban infrastructures is a difficult investment. Generally the development of new city quarters lasts several – often more than ten - years. Centralized heating stations with ground coupled heat exchangers may serve the district heating system; this is normally constructed together with road works and with the subsurface infrastructure like waste- /freshwater pipes and electricity / communication cables. However, investors want to pay the expensive bills for the last necessary boreholes not earlier as the last buildings are constructed and the last consumer is connected to the grid. Therefore a scalable heat pump (HP) system in combination with a growing borehole heat exchanger (BHE) system over time is required. While central HP-stations may easily be enlarged by just adding unit by unit on request, new boreholes and ditches are complicated to be constructed and connected in an already built environment without reopening of roads, pavements, disturbance of traffic etc.. When additional consumers are added to the grid it is most likely desirable to drill more boreholes just from one existing spot. Such growing “hot-spots” are already connected to the heating station. This concept requires a new design of BHE fields leading to deviated wellbores that are drilled from one site in a radial arrangement – a GeoStar. This design is also applicable for heating and cooling of already existing large building complexes in constricted urban areas with limited space. A first GeoStar prototype has been realized in a research project in Bochum, Germany. The aim was a) to develop the drilling technology for deviated and stable boreholes in larger depths, b) to install a set of up to 20 BHE (200 – 500 m) in a starshaped manner for heating and cooling with a set of 4*35kW HP and c) to understand the performance and possible thermal interferences of BHE with different cementations. As an outcome of the R&D program a first reference unit was taken into operation at the new GZB campus. In total 17 inclined BHE were installed plus 3 monitoring wells; all of them running from a central spot to depths of 200 m in angles of up to 15°. All wells are completely equipped with optical fibers for distributed in-situ temperature sensing and for investigations on the thermal and mechanical influences on the performance of the fibers plus on the thermal rock and cement properties during operation. From beginning of the construction an extensive and unique scientific measuring program was carried out. It involved logging and analysis of drilling parameters, borehole deviations and rock structure (borehole geophysics), thermal conductivity (Enhanced Geothermal Response Tests) and temperature (Fiber Optic Measurement Techniques). Based on the measured data, a three-dimensional geological model was created which is also used for numerical heat transfer simulations. Currently a longterm monitoring is being carried out during the operation of the BHE-system. That allows a calibration and validation of the simulation model and will give an insight to the energetic performance of the whole BHE-DH-HP system at GZB..