Thermodynamic Analysis and Quality Mapping of A Geothermal Resource at the Ďurkov Hydrogeothermal Structure, Košice Depression, Eastern Slovakia

Abstract

Geothermal energy in Slovakia is under a systematic research since 70’s of the last Century. Out of 27 identified prospective geothermal fields within a territory of the West Carpathians, the Ďurkov depression hydrogeothermal structure in the Košice Depression is repeatedly accented as of the most enormous potential for a heat (and in some, rather wishful, plans for power) production. Unlike a conventional evaluation of thermal potential and temperature, this study aims at description of the reservoir under an exergetic concept; quantifying reservoir enthalpy, exergy and specific exergy index prior classifying a system according to a thermodynamic quality. The geothermal resource is a geothermal water with only a minor equilibrated steam fraction (XS less than 0.18), questionable in natural occurrence. A reservoir associates Mid Triassic carbonates, occupying depths of ztop = 1,600-2,600 m and zbtm = 1,600-2,600 m. At given geothermic conditions, a mean reservoir enthalpy reaches h = 640 kJ.kg-1 for the entire body, however, only 10 % exceed a h = 800 kJ.kg-1, a limit to classify a resource of high enthalpy. Specific exergy index has been calculated within an interval of SExI = 0.04-0.25, with a mean of SExI = 0.11. Only 10 % of the reservoir at depths of z = 3,100-3,900 m can target a SExI more than 0.2 with more than 90 % probability of success. A geothermal resource and the entire system can be classified thus as of a moderate-low exergy (thermodynamic quality) for SExI = 0.05-0.2 or of a moderate low exergy for SExI = 0.05-0.5. Thermodynamic data show the resource is rather suitable for a large-scale heat (district heating) production – supply.