Analysis of borehole heat exchanger in an existing ground-source heat pump installation

Abstract: Ground-source heat pumps systems (GSHP) are commonly used all over Sweden to supply heat and sometimes cool to different kinds of housings or commercial facilities. Many large installations are by now between 10 and 20 years old. Even when the design of such system has been tackled, rare are the studies that have dealt with following their performance throughout time in detail. Based on conductive heat transfer, the heat extraction process makes the ground temperature decrease when installations are only used for heating. This thesis aims at proposing a method to evaluate how the temperature in a borehole heat exchanger of a GSHP will evolve. The project is focusing on the heat transfer from the ground to the boreholes modelled using Finite Line Source (FLS) based generated g-functions. “g-functions” are non-dimensional parameters characterizing the evolution of the ground thermal resistance enduring variable heat extraction loads. A model using Matlab has been developed and validated against relevant publications. As a case study, the method is applied to an existing 15 years old GSHP installation, composed of 26 boreholes and 3 heat pumps, so as to compare the obtained results with data measured on site. Two sub-borehole fields compose this installation: 14 of them were drilled in 1998 and the remaining 12 in 2009. Measured variable heat extraction loads were superposed using dedicated site g-functions for the two boreholes fields. As a result, a comparison between modelled and calculated heat carrier fluid in the boreholes over the last 6 months is presented here, as well as a 20 years forecast of the ground temperature at the interface with the boreholes.