| Abstract |
For geothermal heat pumps (GHPs) the issue of sustainability concerns the long-term production stability of the various heat sources. In the horizontal systems the heat exchanger pipes are buried at shal-low depth; for combined heating/cooling by GHPs the heat balance (in/out) is given by the system de-sign itself. In the case of groundwater-coupled GHPs the resupply of fluid is secured by the hydrologic cycle. The situation with borehole heat exchanger (BHE)-coupled GHP systems is different. BHE based geothermal heat pump(HP) systems oper-ate, if properly designed reliably on the long term. This has been proven by experimental and theoretical investigations (numerical modeling). The results of modeling for a single BHE show that the long-term performance of the BHE/HP system stabilizes, rela-tively to initial conditions, at a somewhat lower but practically constant level after the first few years. Thus sustainable operation can be achieved. The BHE operation creates a local heat sink and thus strong temperature gradients in the BHE vicinity which in turn leads to heat inflow, directed radially towards the BHE, to replenish the deficit created by the heat extraction. This heat flow density attains, compared to the terrestrial heat flow (80 - 100 mW/m2), rather high values (up to several W/m2). After shut-down of BHE operation thermal recovery begins, strong in the beginning and decreasing as-ymptotically afterwards. Model simulations with different operation/recovery periods show that the recovery duration roughly equals that of operation: e.g. for 30 years of BHE operation the thermal recov-ery of the ground needs 30 years. . |