3 - Calculation of borehole thermal resistance

Abstract

The borehole thermal resistance—that is, the thermal resistance between the fluid in the U-tube and the borehole wall—is a key performance characteristic of a closed-loop borehole ground heat exchanger (GHE). Lower borehole thermal resistance leads to better system performance. Since the original identification of borehole thermal resistance by Mogensen (1983), there have been numerous methods proposed for calculating this. In this chapter, we present methods for calculating local borehole thermal resistance and effective borehole thermal resistance which account for short-circuiting between the upward- and downward-flowing legs of the GHE. We also compare a wide range of simple, relatively easy-to-calculate methods to a detailed reference method and recommend suitable simplified methods.