Examples of the assessment of temperatures on the surface of solid ground in the design of the shallow geothermal energy extractions

Abstract

The thermal conductivity of rocks and soils and the mean annual temperature of the ground have the biggest impact on the dimensioning of the extraction of geothermal energy with closed ground-water systems. The method of determining the ground temperature is presented in four ways according to the available data at a given location: 1) we have data on the mean annual air temperature, 2) we only have information about the altitude of the place, 3) nearby is a meteorological station with soil (ground) temperature measurements, and 4) a borehole with a thermogram is in the vicinity. The use of these four methods and the differences between them are illustrated by five examples in different parts of Slovenia (Cerkno, Lucija, Brnik, Babno Polje and Maribor). It has been shown that the ground temperature measured at meteorological stations is on average higher than the temperature calculated from the borehole thermograms. The ground temperature can be well estimated with a regression line between the altitude and the measured ground temperatures at meteorological stations only for the continental part. In the coastal part of Slovenia, such an assessment was not feasible, as only two stations with ground temperature measurements are available. There are significantly more boreholes with thermograms (as much as 458) than meteorological stations with measurements of ground temperature (only 9). For this reason, the use of borehole thermograms makes sense. In addition, the borehole thermograms allow us to calculate the heat-flow density, which is also needed in the dimensioning of geothermal energy extractions. For more GEOLOGIJA 62/1, 103-122, Ljubljana 2019 https://doi.org/10.5474/geologija.2019.005 104 Dušan RAJVER, Simona PESTOTNIK & Joerg PRESTOR comparable assessment of the ground temperature from the thermograms, several thermograms from the recent period 1981-2010 should be available, because this period already contains the effect of global warming of the atmosphere. Since this was not the case, we obtained at all locations according to the method of calculation the lowest value from the thermograms. On the other hand, in most boreholes, the temperature record in the upper 20 m is missing, so in the correct extrapolation of the T-z profile from the deeper section of the profile, we mainly covered sections between 20 and 100 m depth. With this we captured such a course of the T-z profile, which still contains in itself a memory of usually slightly lower temperatures on the surface in the past.