Implementation of the Distributed Thermal Response Test at Characteristic Geological Regions throughout Croatia

Energy Strategy of the Republic of Croatia by 2020 relies on renewable energy resources as the one of main priorities. The use of geothermal energy sources is specifically encouraged. The EU IPA (Instrument for pre-accession assistance) project entitled Research and the Promotion of the Use of Shallow Geothermal Potential in Croatia includes research in eight characteristic regions throughout the country. In this project a new method, the so called distributed thermal response test (DTRT) has been applied on a 100 m deep borehole heat exchanger (double U pipe) with 152 mm in the outer diameter of borehole. The fundamental difference from the TRT is the measurement of the carrier fluid temperature along the BHE using an optical fiber cable placed inside the BHE pipes. Hence, in DTRT vertical distribution of the ground thermal conductivity and borehole thermal resistance is determined along the borehole heat exchanger (BHE). With this method the undisturbed ground temperature profile along the BHE is also determined. The duration of the DTRT is about seven days. In the first three days the temperature is measured with no fluid circulation, followed by imposing a constant heat flux during the next 48 hours. Thermal properties of the ground are influenced by the soil and rock composition, porosity, moisture content and groundwater flow. This paper presents the results for the first region - the city of Osijek.