Undisturbed Ground Temperature in Urban Environment

Abstract

Undisturbed ground temperature is important design parameter for borehole heat exchangers in ground coupled heat pump systems. Ground temperature is commonly determined as an average value based on the measured temperature of circulating fluid in the borehole prior to the thermal response test. Still, the shape of temperature profile along borehole depth is not uniform and is affected by geothermal and surface heat flux characteristic for urban areas. Temperature profile obtained by fiber optic measurement on 130-meter deep borehole is numerically replicated. Analysis has been conducted to evaluate temporal and spatial influence of surface heat source on resulting temperature profiles and estimated average underground temperature. Noticeable temperature increase diminishes after 30 years of surface heat flux while the spatial effects can be traced on distance larger than 10 meters. Not accounting for a local increase of temperature can lead to the erroneous sizing of borehole field, especially for shallow boreholes.