Saqib Javed

Calculation of Borehole Thermal Resistance

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. Lower borehole thermal resistance leads to better system performance. Since the original identification of the concept 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 ground heat exchanger. We also compare a wide range of simple, relatively easy-to-calculate methods to a detailed reference method and recommend suitable simplified methods.

Evaluating the Applicability of the Radial Approximation for Pile Heat Exchangers

This paper appraises the efficacy of using an analytical radial approximation for different thermal pile heat exchanger geometries. Unsteady radial heat-flow from fluid in a pipe set within a grouted borehole into the external ground is well-documented and can be solved analytically very rapidly using Laplace Transforms (Javed and Claesson 2011). By comparing the radial model with finite-element simulations including explicit pile geometries, this paper provides a provisional analysis of the accuracy of this approach. Initial findings suggest that the radial model may provide an appropriate approximation to pile behaviour for certain pipe configurations, albeit with small ‘mid-time’ error.