| Title | 3D modelling of borehole heat exchangers at hydrogeological conditions typical of the north European lowlands, sensitivity studies from Denmark |
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| Authors | Rasmussen, P; Højberg, A L; Ditlefsen, C |
| Year | 2016 |
| Conference | European Geothermal Congress |
| Keywords | Shallow geothermal energy, sensitivity studies, numerical modelling |
| Abstract | A recently completed Danish development and demonstration project has compiled knowledge, tools and best practices for closed loop boreholes combined with detailed mapping of thermal properties of Danish soils as well as 3D modelling of heat and groundwater flow. In Denmark the topmost 100-200 m are basically characterized by heterogeneous glacial and interglacial sediments on top of preglacial deposits and it thus comprises of a wide range of rock types with varying hydrogeological and thermal properties. In order to understand and quantify the effect of the present heterogeneous geology, it was necessary to establish complex and detailed numerical models in which these conditions could be described directly in 3D. The FEFLOW software package was used for the modelling studies. The 3D model scenarios illustrates the sensitivity of various parameters affecting the performances of BHEs under Danish and similar hydrogeological conditions. The present model sensitivity studies have focused on realistic variations in e.g. thermal conductivities, temperature gradients, and groundwater flow gradients. The natural variations in thermal conductivity of typical Danish sediments have been found to have significant impact on the possible yearly energy intake for a BHE. Thus the estimated energy extraction for the most unfavourable of the geological scenarios is found to be only about 60 % of the energy extraction for the most favourable conditions. Inclusion of ground water flow in the energy calculations have been found be of significant importance when estimating the possibly yearly energy extraction. The calculations also show that the groundwater flow besides ensuring higher yearly energy extraction helps maintaining an energy balance in the vicinity of the borehole during long term production when compared to situations with no groundwater flow. The importance of groundwater flow is often neglected in standard programs for calculating energy extraction. 3D energy modelling including groundwater flow is a useful tool for analysing and optimizing the energy extraction under heterogeneous hydrogeological conditions. Since it may not be economically feasible to model conditions at smaller plants with only a few boreholes the present results may instead be used as guideline for initial estimates of expected energy extraction at different conditions. |