SUSTAINABLE PRODUCTION FROM BOREHOLE HEAT EXCHANGER SYSTEMS

Heat pump-coupled borehole heat exchanger (BHE) systems (= "Ground coupled" or "Geothermal heat pumps") are ideally suited to tap the ubiquitous shallow geothermal resources. Traditionally, geothermal direct use aims to utilise the heat content of formation fluids, if present. The heat content of the rock matrix is generally higher; this heat is the target of shallow resource utilisation. The thermal load of the BHE depends mainly on the thermal conductivity of the surrounding ground. The heat transfer from the ground to the BHE is by conduction, i.e. there are no formation fluids produced. Experimental and theoretical investigations prove that BHE/HP systems, if properly designed, operate reliably on the long term without negative environmental effects. An extensive measurement campaign has been performed at a commercially installed BHE/HP installation in Elgg near Zurich. Object of this study is a single, coaxial, 105 m long BHE, in use since its installation in a single family house. The measurement results were used to calibrate a 2D numerical code. Ground temperatures over the first five years of measurement were fitted to within one or two tenths of a degree Celsius. Additionally the formation temperature was predicted for several further years using assumed load profiles. The immediate surroundings of the BHE cool down in the first years and do not fully recover during the system lifetime. The long-term performance stabilizes, albeit at a somewhat lower but constant level. Thus sustainable production can be achieved on the long term. But how renewable are BHE/HP systems? Long-term heat extraction causes heat depletion/temperature decrease in a certain volume of the ground. After termination of BHE operation thermal recovery beings. Different simulation runs have been performed with different operation durations (constant or intermittent heat extraction, different operation/recovery time ratios). The results show that the duration of ground thermal recovery for the analysed simple case (single BHE) roughly equals the duration of the heat extracting operation (e.g. for 30 years of BHE operation: ground recovery in about 30 years).