24 June 2014

Modeling of short geothermal boreholes in series with experimental validation

Bore fields composed of short boreholes offer several modeling challenges. For example, ground temperatures variations in the first few meters below the ground surface are larger than the ones experienced by deep boreholes. Also, boreholes need to be piped in series to maintain
a relatively high flow rate which implies that the inlet temperature to each borehole changes from one borehole to the next. In the first part of this study a methodology to model short boreholes is proposed. First, a
detailed single borehole model which includes borehole thermal capacity is introduced. Then, the entire bore field is modeled using analytical solutions to ground heat transfer and spatial and
temporal superposition to predict the outlet fluid temperature from each borehole at each time step. Finally, the model is validated against experimental data obtained on a test facility composed of 16 short boreholes (9 m deep), with 4 rows of 4 boreholes in series. It is shown that the results from the proposed model are in close agreement with the experimental results if the ground temperature variation with depth is taken into account.