In-situ measurements of ground thermal properties around borehole heat exchangers in Plovdiv, Bulgaria

The knowledge of the geodata (subsurface characteristics) is important for the design and construction of the Underground Thermal Energy Storage (UTES) and to design an installation using Borehole Thermal Energy Storage (BTES) - it is a big advantage during the project calculations and the construction of the geothermal systems. In situ determination of ground thermal conductivity, borehole thermal resistance and undisturbed soil temperature can be done by installing a vertical borehole heat exchanger (BHE) and performing the so-called Thermal Response Test (TRT). This paper describes the determination of ground thermal properties by a research group of the Technical University of Sofia, Plovdiv Branch. A mobile system for conducting Thermal Response Test has been created recently in Plovdiv, Bulgaria. The first Bulgarian TRT was carried out in January 2009 using a 41 m deep BHE, constructed in November 2008. Later tests are carried out on other two constructed BHEs in Plovdiv (single and double, 50 m depth and 32 mm diameters of the U-tubes). The tests were realized while the ambient temperature, the inlet and outlet fluid borehole temperatures and the temperatures of the BHEs at different depths were measured every minute. A large quantity of experimental data was gathered and analyzed by two parameters curve fitting based on the analytical formula of the Line Source Model for temperature distribution in the borehole (determining the ground thermal conductivity and the borehole thermal resistance). The detailed study of ground properties in different regions of Bulgaria is a good precondition for future application of the geothermal technology in the region.