Use of thermal conductivity from thermal response test for estimating steady-state temperatures in rock and stratified soil near a line source of heat

Abstract

This article addresses the influence of anisotropy of the ground on steady-state temperature in the surroundings of vertical borehole and effective thermal conductivity measured by a field thermal response test. This is a key parameter in the design of ground coupled heat pumps to heat and cool buildings. First, this article provides a brief overview of the current technique of estimating thermal conductivity from data obtained in a thermal response test based on predictions for temperature from a line source of heat in an isotropic ground. Then, the analytical solutions to the isotropic model for the ground are used to obtain the solutions to the anisotropic model for stratified medium. In addition, the article provides a new analytical exact solution for temperatures around a finite line source of heat penetrating anisotropic semi-infinite medium, in which the angle between the ground surface and the sedimentary strata is arbitrary. Approximate expressions for the temperature evolution during the test and for the steady-state temperature are presented. Such approximate expressions are also given for integral mean temperature for two special orientations of the strata. The limitations of the finite line source method in stratified medium and recommendations for layout of multiple vertical or horizontal ground coupled heat exchangers or waste canisters in repository rock are discussed. Copyright © 2011 American Society of Heating, Refrigerating and Air-Conditioning Engineers.