24 June 2014
The preferred case when designing a ground-source heat pump (GSHP) system for a single-family house in Sweden is to drill a single borehole to the depth required for the desired energy extraction. There are however situations when the local geological,
hydrological or other conditions prohibit drilling to the required depth, and therefore two shorter boreholes are drilled. This paper analyses the thermal and economic consequences of drilling two shorter boreholes instead of one deeper borehole for a typical Swedish single family house. A series of simulation-based analyses are presented; the first accounts only for
the first order effects – the shorter borehole length and thermal interaction between the two boreholes. Then, second order effects – the geothermal gradient, extra horizontal piping in
the case of two vertical boreholes, and different hydraulic resistances – are analyzed. The analyses look at the thermal performance of the ground heat exchangers, the resulting heat
pump and circulating pump energy consumption, electrical energy costs, and different first costs due to differences in material usage, upper casing requirements and costs required for drilling and installation.