Vertical distribution of shallow ground thermal properties in different geological settings in Croatia

Abstract

Heat pumps combined with vertical borehole heat exchangers utilize the ground as a heat source or sink for the purpose of heating or cooling. The proper dimensioning of the borehole heat exchanger requires understanding of the thermophysical properties of the ground surrounding boreholes.Presented research was carried out in two geologically different regions with varying thermal parameters relevant for the utilization of ground-coupled heat pumps: Pannonian Basin System and the Dinarides. Eight different sites were selected based on characteristic geological settings and climate in order to include inland, seaside and mountainous regions. In boreholes, 100 and 130 m deep, double U-pipe borehole heat exchangers were installed.The improved method for evaluation of the ground thermal conductivity and thermal resistance of a borehole heat exchanger known as distributed thermal response test was used. This method employs optic fiber cables for the measurement of temperature profiles along the borehole heat exchangers. Measurements conducted at different locations show great variability of thermal properties and undisturbed ground temperatures. Obtained results indicate that the average thermal conductivity of sediments in the Pannonian part varies between 1.62 and 2.10 W/(m K), compared to Dinaridic carbonate and evaporite rocks' values between 2.01 and 3.19 W/(m K).