The comparision of a basic and a dual-pressure ORC (Organic Rankine Cycle): Geothermal Power Plant Velika Ciglena case study Zvonimir Guzovi  c a, * , Predrag Ra  skovi  c b , Zoran Blatari  c a a University of Zagreb, Faculty of Mechanical Engineering and Naval Architecture, Zagreb, Croatia b University of Ni  s, Faculty of Technology, Leskovac, Serbia article info Article history: Received 11 December 2013 Received in revised form 28 May 2014 Accepted 1 June 2014 Available online xxx Keywords: Geothermal energy Binary geothermal power plant Dual-pressure ORC (Organic Rankine Cycle) Energy and exergy analysis Velika Ciglena-Croatia abstract In the Republic of Croatia there is some medium temperature geothermal fields (between 100 and 180  C) 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: ORC (Organic Rankine Cycle) 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 analysed. A dual-pressure cycle reduces the thermodynamic losses incurred in the geothermal water-working fluid 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  C), energy and exergy analysis for the proposed the dual-pressure cycle are performed. As a conclusion, in case of Geothermal Power Plant Velika Ciglena, a dual-pressure ORC has slightly lower thermal efficiency (13.96% vs. 14.1%) but considerably higher both exergy effi- ciency (65% vs. 52%) and net power (6371 kW vs. 5270 kW). © 2014 Elsevier Ltd. All rights reserved. 1. Introduction In recent years, geothermal energy becomes more and more attractive energy source due to fluctuating oil prices and serious environment problems such as global warming, ozone layer destruction, acid rains and contamination of lands and seas, caused by accelerated consumption of fossil fuels. Geothermal energy provides power from a renewable energy source which is inde- pendent of season and time of day. Although geothermal energy represents important potential on a worldwide scale, only a small part of this huge potential is currently being used. Electricity generation from geothermal energy has of more than a century long history. In 1904, in Larderello e Italy, Prince P.G. Conti powered a 3/4-horsepower reciprocating engine by a geothermal steam well to drive a small generator. Ten years later, in 1914, the first commercial 250 kW geothermal power plant with turbo-alternator was the first commercial geothermal power sys- tem connected to the grid and in continuous operation there [1]. Over the past 25e35 years, worldwide electricity production based on different geothermal power plants has increased signifi- cantly [2,3]. The installed generating capacity in 1975 was 1.18 GW, 4.764 GW in 1985, 6.833 GW in 1995, 7.927 MW in 2000, 8.933 GW in 2005 and 10.715 in 2010. The short tem forecasting for year 2015 is 18.5 GW. An increase of about 1.8 GW in the five year period 2005e2010 has been achieved (about 20%), following the rough standard linear trend of approximately 350 MW/year, with an evident increment of the average value of about 200 MW/year in the preceding 2000e2005 period. Produced electricity in 1995 was 38,035 GWh, 49,261 GWh in 2000, 55,709 GWh in 2005 and 67,246 GWh in 2010. Although today it represents only small per- centage, less than 0.5% of the world's electricity needs, in some of the countries more than 10% of national power generation capacity is geothermal. Three major types of power plants are used today to produce electricity from geothermal resources, dry-steam, flash and binary plants, depending on the state of the fluid and its temperature, as it * Corresponding author. Tel.: þ385 1 6168 532; fax: þ385 1 6156 940. E-mail address: zvonimir.guzovic@fsb.hr (Z. Guzovi c). Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy http://dx.doi.org/10.1016/j.energy.2014.06.005 0360-5442/© 2014 Elsevier Ltd. All rights reserved. Energy xxx (2014) 1e12 Please cite this article in press as: Guzovi c Z, et al., The comparision of a basic and a dual-pressure ORC (Organic Rankine Cycle): Geothermal Power Plant Velika Ciglena case study, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.06.005