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The Comparision of a Basic and a Dual‐Pressure Organic Rankine Cycles (ORC): Geothermal Power Plant Velika Ciglena Case Study

In the Republic of Croatia there is some medium temperature geothermal fields (between 100 and 180 degrees Celsius) by means of which it is possible to produce electricity. However, only recently concrete initiatives for the construction of geothermal power plants have been started. In previous papers, the possible cycles for geothermal fields in the Republic of Croatia are proposed: Organic Rankine Cycle (ORC) and Kalina cycle. Also for the most prospective geothermal fields, energy and exergy analysis for the proposed cycles are performed, on the basis of which the most suitable cycle is proposed. It is ORC which in all cases has better both the thermal efficiency (the First Law efficiency) and the exergy efficiency (the Second Law efficiency). With aim to further improving of geothermal energy utilization in this paper the replacement of a basic ORC with a dual-pressure ORC is analyzed. A dual-pressure cycle reduces the thermodynamic losses incurred in the geothermal water heat exchangers of the basic ORC, which arise through the heat transfer process across a large temperature difference. The dual-pressure cycle maintains a closer match between the geothermal water cooling curve and the working fluid heating/boiling curve and these losses can be reduced. Now, on the example of the most prospective geothermal field, Velika Ciglena - 175 degrees Celsius, energy and exergy analysis for the proposed the dual-pressure cycles are performed. In previous analysis the basic ORC in case of geothermal field Velika Ciglena has thermal efficiency (the First Law efficiency) 14.1 % and the exergy efficiency (the Second Law efficiency 52 %. Now, in analysis presented in this paper for geothermal field Velika Ciglena, a dual-pressure ORC has thermal efficiency (the First Law efficiency) 13.96 % and exergy efficiency (the Second Law efficiency 65.0 %. Simultaneously, a dual-pressure ORC achieves net power 6371 kW while a basic ORC 5270 kW. As a conclusion, in case of Geothermal Power Plant Velika Ciglena, a dual-pressure ORC has slightly lower thermal efficiency but considerably higher both exergy efficiency and net power.

Geothermal energy; Geothermal power plant; Organic Rankine Cycle (ORC); Dual-pressure ORC; Velika Ciglena - Croatia

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