  Borehole heat exchanger arrays have become a common implement for the
utilization of thermal energy in the soil. Building these facilities is
expensive, especially the drilling of boreholes, into which closed-pipe heat
exchangers are inserted. Therefore, cost-reducing drilling methods are common
practice, which can produce inaccuracies of varying degree. This brings into
question how much these inaccuracies could potentially affect the performance
of a planned system. In the presented case study, an uncertainty quantification
for seasonally operated borehole heat exchanger arrays is performed to analyze
the bore paths' deviations impact. We introduce an adaptive, anisotropic
stochastic collocation method, known as the generalized Smolyak algorithm,
which was previously unused in this context and apply it to a numerical model
of the borehole heat exchanger array. Our results show that the borehole heat
exchanger array performance is surprisingly reliable even with potentially
severe implementation errors during their construction. This, coupled with the
potential uses of the presented method in similar applications gives planners
and investors valuable information regarding the viability of borehole heat
exchanger arrays in the face of uncertainty. With this paper, we hope to
provide a powerful statistical tool to the field of geothermal energy, in which
uncertainty quantification methods are still rarely used at this point. The
discussed case study represents a jumping-off point for further investigations
on the effects of uncertainty on borehole heat exchanger arrays and borehole
thermal energy storage systems.
