IGSHPA Research Conference 2018 - Research Conference Proceedings

142 Research Conference Proceedings - IGSHPA Research Conference 2018 IGSHPA Research Track Stockholm September 18-20, 2018 Malin Malmberg (malin.malmberg@bengtdahlgren.se ) is a civil engineer at Bengt Dahlgren Geoenergi, Jasmin Raymond and Erwan Glaugen are proffessors at Institut national de la recherche scientifique, Lorenzo Perozzi is a postdoctoral fellow at Institut national de la recheche scientifique, Claes Mellqvist is geologist and Gerhard Schwarz is a geophysicist at the Geological Survey of Sweden and José Acuña is GSHP specialist at Bengt Dahlgren Geoenergi and researcher at KTH Royal Institute of Technology. Development of a thermal conductivity map of Stockholm Malin Malmberg Jasmin Raymond Lorenzo Perozzi Erwan Gloaguen Claes Mellqvist Gerhard Schwarz José Acuña ABSTRACT New methods have been suggested to spatially extend in situ thermal response test (TRT) assessments based on geostatistical analysis. These methods can be used to determine a stochastic distribution of the subsurface thermal conductivity beyond the test borehole on larger scales by interpolating the data with geostatistics, including sequential Gaussian simulations (SGS) used in the present study. This paper presents a simulated thermal conductivity map for Greater Stockholm in Sweden, based on the SGS method with input data from in situ measurements (TRT and DTRT). The geology of Stockholm is used as a background raster in the simulations, based on bedrock maps from the Geological Survey of Sweden (SGU). The resulting maps are compared with a point map of punctual ground thermal conductivity of Greater Stockholm earlier derived by SGU, compiled from laboratory data that were obtained by thermal conductivity scanning and modal analysis of surface rock specimens of the area. INTRODUCTION All ground source heat pump (GSHP) installations in Sweden deliver together around 15 TWh of heat per year and constitute the third largest renewable energy source in the country. When designing larger GSHP systems, consisting of multiple boreholes, it is today a usual practice to perform an in situ Thermal Response Test (TRT) to assess the thermal conditions of the ground. TRT assessments give, among others, a measure of the effective thermal conductivity of the subsurface. A research group in Japan (Fujii et.al. 2006) suggested a method to improve TRT and measure the thermal conductivity of the rock along the borehole depth using optical fiber cables. The method was further used in Sweden by KTH researchers (Acuña, 2013), within the Effsys2 and Effsys+ research program and was named Distributed Thermal Response Test (DTRT). Knowing the thermal conductivity allows for a more detailed evaluation of parameters such as the number of boreholes, their depth and the distance between the boreholes, and thereby facilitating to optimize the system. However, TRTs are spatially limited to the location of the borehole with a test radius of about 1 to 2 m (3.3 to 6.6 ft). The use of geostatistical methods was suggested by Raymond et.al . (2017) with the objective of spatially extending TRT assessments beyond the test borehole on an urban district scale. The method, relying on stochastic simulations, has been performed in Greater Stockholm, Sweden, to map the subsurface thermal conductivity of the region based on the TRT and DTRT measurements from the company Bengt Dahlgren Geoenergi. Similar methodology has been used as for the first application of the method made in the St. Lawrence Lowlands geological provinces of Canada (Lorenzo et. al. 2016; Raymond et.al . 2017), with a few improvements. The bedrock map to be found in the 2018 Research Conference DOI: 10.22488/okstate.18.000037

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