Langzeiteffekt von Mehrfach-Erdwärmesonden

Pahud, Daniel and Kohl, Thomas and Megel, Thomas and Brenni, Renzo (2002) Langzeiteffekt von Mehrfach-Erdwärmesonden. Project Report UNSPECIFIED

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Due to the increasing density of borehole heat exchanger (BHE) systems in Switzerland, their mutual influence needs to be investigated. Depending on the number of systems involved, it may take from several tens up to several hundred of years for typical BHE lengths of ~100m before they start influencing each other. There is however a need to account for their long term influence since this situation is supposed to occur more often in future, as the number of ground coupled heat pump systems (GCHP system) steadily increases with time. Since it is difficult to collect measurements of thermally interacting BHE's in these conditions, only simulation programs will provide a solution to this problem. This topic needs also to be stud-ied to discuss themes such as “neighbour rights” or sustainability related to GCHP system. The first phase of the project is to check the often-used "g-function" calculation. Therefore, the programs SBM being used in g-function mode and the fully numerical finite element pro-gram FRACTure are tested. The comparison between the two programs is performed for a three-borehole configuration of an equilateral triangle. Under the assumption of steady-state conditions and a homogeneous, purely diffusive subsurface, a good agreement of the pro-grams could be established. In these conditions the g-function analysis provides fast and suf-ficiently accurate results. Further calculations, performed with FRACTure, investigated the possible effect of a subsurface aquifer. Depending on the aquifer thickness and flow rate, the results indicated a performance increase of up to 70%. Although a time constant was roughly considered, further calculations need to be performed. Especially, the transient behaviour, ef-fects of heterogeneous subsurface or temperature-dependant material behaviour should be in-vestigated in further steps. This first comparison has demonstrated that both programs SBM and FRACTure working with completely different algorithms can reliably be used for simula-tions of BHE systems. In the second phase of the project, the g-function concept is used to calculate the long term influence of the two borehole configurations (two-boreholes and three-boreholes forming an equilateral triangle). The results are presented in a graphical form in order to quickly assess a particular problem. The ground thermal conductivity may vary between 1 and 4 Wm-1K-1, the borehole length between 50 and 200 m, the borehole spacing between 5 and 100 m and the time horizon between 5 and 200 years. The long term influence is estimated for any mean an-nual heat extraction rate, which includes an eventual thermal recharge of the ground. The in-fluence of a regional ground water flow is not taken into account. As calculated with FRAC-Ture, such an influence may significantly improve the situation by decreasing the long term temperature drop at the borehole wall.

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