Heat Flow Determination with Geothermal Heat Pump Data

The basic, indispensable ingredients to calculate heat flow are temperature gradient and thermal conductivity (λ). Ground-coupled heat pumps systems are often installed in many countries for space heating and cooling, most of them with borehole heat exchangers (BHE). The higher λ of the surrounding ground, the higher the heat exchange efficiency (W per m borehole length). Therefore, special measuring set-ups and procedures are commonly used in construction areas to determine the average thermal conductivity λ (TRT method) or the thermal conductivity profile (λ(z), ETRT method). These data are needed for design calculations (BHE number, depth, spacing for a given object to be heated and/or cooled). At the same time, the borehole temperature profile T(z) is also measured. Data from several BHE sites have been acquired, analyzed, and processed. First, the borehole temperature data for z > 100 are plotted to display the T(z) trends, and then average λ or λ(z) are taken from the TRT or ETRT records and reports at a given site. Finally, the local heat flow is calculated either with the average λ and the average gradient or with the Bullard plot technique, which uses λ(z). In the latter, layer-wise integrated thermal resistivities (Δzi/λi) are plotted against measured temperatures at corresponding depths. The method is demonstrated by examples from BHE sites in the Zurich region/Switzerland. The “shallow” heat flow values elaborated by these means are in the range 80 to 100 mW/m2 and fit reasonably well with the general Swiss heat flow trends, determined from deep (> 1 km) borehole data.