ON THE STEADY-STATE THERMAL RESPONSE OF SLENDER GEOTHERMAL BOREHOLES

Most theoretical models for the thermal response of vertical geothermal boreholes assume the mean azimuthal borehole wall temperature to be constant along the borehole. This fundamental simplification, closely related to the g-functions introduced by Eskilson in 1987, has dominated the research field for the past 30 years and has allowed the analysis of entire borehole fields in reasonable amounts of time. However, the assumption is not physically correct, which inevitably hinders the attainable accuracy. In the present work the steady-state thermal response of a single vertical geothermal borehole is analyzed using matched asymptotic expansion techniques, which exploit the extreme slenderness of real world boreholes. The obtained solutions do not require the aforementioned simplification and deliver accuracy levels that are on par with the best performing theoretical/numerical model developed so far, the superposition borehole model.