Power production from a moderate temperature geothermal resource with regenerative Organic Rankine Cycles Alessandro Franco Department of Energy and System Engineering (DESE), University of Pisa, Largo Lucio Lazzarino, 56122 Pisa, Italy abstract article info Article history: Received 7 September 2010 Revised 4 June 2011 Accepted 4 June 2011 Available online 7 July 2011 Keywords: Moderate-temperature geothermal sources Organic Rankine Cycle Binary cycle Recuperated Rankine cycle Rejection temperature Optimum design Much remains to be done in binary geothermal power plant technology, especially for exploiting low- enthalpy resources. Due to the great variability of available resources (temperature, pressure, chemical composition), it is really difficult to “standardize the technology”.The problem involves many different variables: working fluid selection, heat recovery system definition, heat transfer surfaces sizing and auxiliary systems consumption. Electricity generation from geothermal resources is convenient if temperature of geothermal resources is higher than 130 °C. Extension of binary power technology to use low-temperature geothermal resources has received much attention in the last years. This paper analyzes and discusses the exploitation of low temperature, water-dominated geothermal fields with a specific attention to regenerative Organic Rankine Cycles (ORC). The geothermal fluid inlet temperatures considered are in the 100–130 °C range, while the return temperature of the brine is assumed to be between 70 and 100 °C. The performances of different configurations, two basic cycle configurations and two recuperated cycles are analyzed and compared using dry organic fluids as the working fluids. The dry organic fluids for this study are R134a, isobutane, n-pentane and R245fa. Effects of the operating parameters such as turbine inlet temperature and pressure on the thermal efficiency, exergy destruction rate and Second Law efficiency are evaluated. The possible advantages of recuperated configurations in comparison with basic configurations are analyzed, showing that in a lot of cases the advantage in terms of performance increase is minimal but significant reductions in cooling systems surface area can be obtained (up to 20%). © 2011 International Energy Initiative. Elsevier Inc. All rights reserved. Introduction Moderate-temperature water-dominated systems, with tempera- tures below 130 °C, account for about 70% of the world's geothermal energy potential (Barbier, 2002). The distribution of geothermal energy as function of the resources temperature and the technical resource potential has been evaluated recently by Stefansson (2005), starting from a general correlation between the existing geothermal high temperature resources inferring a total geothermal potential of 200 GWe. Binary technology allows the use of low temperature water dominant reservoirs and makes geothermal power production feasible even for countries lacking high enthalpy resources at shallow depth. For binary plants two different systems currently are state of the art, the Organic Rankine Cycle (ORC) and the Kalina cycle. The binary power plants have the least environmental impact due to the “confinement” of the geofluid. In a binary cycle power plant the heat of the geothermal water is transferred to a secondary working fluid, usually an organic fluid that has a low boiling point and high vapor pressure when compared to water at a given temperature. The cooled geothermal water is then returned to the ground by the re- injection well to recharge the reservoir (DiPippo, 2008). Such a geothermal plant has no emissions to the atmosphere except for water vapor from the cooling towers (only in case of wet cooling) and any losses of working fluid. Thus, environmental problems that may be associated with the exploitation of higher temperature geothermal resources, like the release of greenhouse gases (e.g. CO 2 and CH 4 ) and the discharge of toxic elements (e.g. Hg and As) are avoided. Another advantage of the binary technology is that the geothermal fluids (or brines) do not contact the moving mechanical components of the plant (e.g. the turbine), assuring a longer life for the equipment. Binary plants have allowed the exploitation of a large number of fields that may have been very difficult (or uneconomic) using other energy conversion technologies (Schochet, 1997; DiPippo, 2004; Bronicki, 2007). Of the about 10,700 MW of geothermal plants installed worldwide, more than 1170 MW use ORC or steam/ORC combined cycles (Bertani, 2010). There exist a great number of studies addressing both the different characteristics of geothermal fields and the various types of power plants that could be used in their exploitation for electricity production; Barbier (2002), Bertani (2005), Lund (2007) and DiPippo Energy for Sustainable Development 15 (2011) 411–419 E-mail address: alessandro.franco@ing.unipi.it. 0973-0826/$ – see front matter © 2011 International Energy Initiative. Elsevier Inc. All rights reserved. doi:10.1016/j.esd.2011.06.002 Contents lists available at ScienceDirect Energy for Sustainable Development