Possibilities of electricity generation in the Republic of Croatia from medium-temperature geothermal sources

In the Republic of Croatia, there are several sources of medium-temperature geothermal water in the range of 90–140 °C, with which it is possible to produce electricity in binary plants, either with the Organic Rankine Cycle (ORC) or with the Kalina cycle. In the literature, the Kalina cycle is viewed as more thermodynamically favourable than the ORC; i.e., the Kalina cycle has a higher thermal efficiency and produces more power. However, the Croatian experience with a medium-temperature geothermal source with the relatively high temperature of geothermal water (Velika Ciglena, 175 °C) is that the ORC is thermodynamically better than the Kalina cycle. In this paper, a comparison between the ORC and the Kalina cycle is performed based on energy analysis results for a medium-temperature geothermal source with a relatively lower temperature (Lunjkovec–Kutnjak, 140 °C). Additionally, in this case the ORC has better thermal efficiency (First Law efficiency) – 13.5% vs. 12.8% – and accordingly higher net power – 2225.5 kW vs. 2101.4 kW. This difference is explained by the relatively high average annual temperature of the cooling air in the condenser (15 °C), which has a more unfavourable influence on the condensing pressure in the Kalina cycle than in the ORC (6.35 bar vs. 0.68 bar).