Energy 32 (2007) 1698–1706 Optimum design criteria for an Organic Rankine cycle using low-temperature geothermal heat sources H.D. Madhawa Hettiarachchi a , Mihajlo Golubovic a , William M. Worek a,Ã , Yasuyuki Ikegami b a Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, 842 W Taylor Street, Chicago, IL 60607, USA b Institute of Ocean Energy, Saga University, Honjomachi 1, Saga 840-8502, Japan Received 17 April 2006 Abstract A cost-effective optimum design criterion for Organic Rankine power cycles utilizing low-temperature geothermal heat sources is presented. The ratio of the total heat exchanger area to net power output is used as the objective function and was optimized using the steepest descent method. Evaporation and condensation temperatures, geothermal and cooling water velocities are varied in the optimization method. The optimum cycle performance is evaluated and compared for working fluids that include ammonia, HCFC123, n-Pentane and PF5050. The optimization method converges to a unique solution for specific values of evaporation and condensation temperatures and geothermal and cooling water velocities. The choice of working fluid can be greatly affect the objective function which is a measure of power plant cost and in some instances the difference could be more than twice. Ammonia has minimum objective function and maximum geothermal water utilization, but not necessarily maximum cycle efficiency. Exergy analysis shows that efficiency of the ammonia cycle has been largely compromised in the optimization process than that of other working fluids. The fluids, HCFC 123 and n-Pentane, have better performance than PF 5050, although the latter has most preferable physical and chemical characteristics compared to other fluids considered. r 2007 Published by Elsevier Ltd. Keywords: Organic Rankine cycle; Optimum design; Low-temperature; Power generation; Geothermal heat sources 1. Introduction Geothermal heat sources vary in temperature from 50 to 350  C, and can either be dry, mainly steam, a mixture of steam and water, or just liquid water. The temperature of the resource is a major determinant of the type of technologies required to extract the heat and the uses to which it can be applied [1,2]. Generally, the high-temperature reservoirs ð4220  CÞ are the ones most suitable for commercial production of electricity. Dry steam and flash steam systems are widely used to produce electricity from high-temperature resources [1–3]. Dry steam systems use the steam from geothermal reservoirs as it comes from wells, and route it directly through turbine/generator units to produce electricity. Flash steam plants are the most common type of geothermal power generation plants in operation today. In flash steam plants, hot water under very high pressure is suddenly released to a much lower pressure, allowing some of the water to convert into steam, which is then used to drive a turbine. Medium-temperature geothermal resources, where tem- peratures are typically in the range of 100–220  C, are by far the most commonly available resource. Binary cycle power plants are the most common technology for utilizing such resources for electricity generation. There are many different technical variations of binary plants including those known as Organic Rankine cycles (ORC) and proprietary systems known as Kalina cycles [4–12]. Binary cycle geothermal power generation plants differ from dry steam and flash steam systems in that the water or the steam from the geothermal reservoir never comes in ARTICLE IN PRESS www.elsevier.com/locate/energy 0360-5442/$ - see front matter r 2007 Published by Elsevier Ltd. doi:10.1016/j.energy.2007.01.005 Ã Corresponding author. Tel.: +1 312 996 5318; fax: +1 312 413 0447. E-mail address: wworek@uic.edu (W.M. Worek).