Analysis of heat transfer by pile and borehole ground heat exchangers: Composite-medium line-source approach

Abstract

Heat transfer by pile and borehole ground heat exchangers (GHEs) involves transient heat conduction in composite media together with complex geometry arrangements. This paper develops a composite-medium line-source approach to analyzing thermal responses of pile and borehole GHEs. The central idea of applying this new approach is that the downward and upward channels of U-shaped tubes (not the borehole) are approximated as line sources of heat placed in a composite medium. By the new method, effects of both heat capacity of grout/concrete and the difference between properties of soil and piles/grout on heat transfer can be fully addressed. More importantly, this approach is enough general and flexible to model various configurations of ground loops, including single and double U-shaped tubes, W-shaped channels, spiral-coils, etc. Therefore, the method developed here may contribute to providing a viable approach to modeling transient thermal processes of vertical GHEs. © Springer-Verlag Berlin Heidelberg 2014.