Rachana Vidhi Clean Energy Research Center, University ot South Fiorida, 4202 E. Fowier Avenue, Tampa, FL 33620 e-mail: rachana@maii.ust.edu Sarada Kuravi Department ot iViechanicai and Aerospaoe Engineering, Fiorida institute of Technology, 150 W. University Bouievard, Melbourne, FL 32901 e-mail: sküravi@til.edu D. Yogi Goswami^ e-mail: goswami@ust.edu Elias Stefanakos e-maii: estefana@usf.edu Ciean Energy Research Center, University ot South Fiorida, 4202 E. Fowier Avenue, Tampa, FL 33620 Adrian S. Sabau Oai< Ridge Nationai Laboratory, Materiais Science and Technoiogy Division, P.O. Box 2008, Oai< Ridge, TN 37831 e-mail: sabaua@orni.gov Organic Fluids in a Supercriticai Rankine Cycle for Low Temperature Power Generation This paper presents a performance analysis of a supercritical organic Rankine cycle (SORC) with various working fluids with thermal energy provided from a geothermal energy source. In the present study, a number of pure fluids (R23, R32, R125, R143a, RI34a, R218, and R170) are analyzed to identify the most suitable fluids for different operating conditions. The source temperature is varied between 125°C and 200°C, to study its effect on the efficiency of the cycle for flxed and variable pressure ratios. The energy and exergy efficiencies for each working fluid are obtained and the optimum fluid is selected. It is found that thermal efficiencies as high as 21% can be obtained with 200 °C source temperature and 10 °C cooling water temperature considered in this study. For medium source temperatures (125-150°C), thermal effciencies higher than 12% are obtained. [DOI: 10.1115/1.4023513] 1 Introduction Traditional methods of power generation require high tempera- ture heat sources and generally use nonrenewable sources of energy. Geothermal sources are available all over the Earth and the amount of energy contained in those sources is high enough to fulfill all the energy requirements of the world for several millen- nia [1]. However, the geothermal temperatures are, in general, lower than normal power generation temperatures. Hence, newer thermodynamic power cycles are continuously investigated for efficiently converting these low-grade heat sources into electrical power. Different types of thermodynamic cycles have been proposed to efficiently utilize the low temperature heat sources, such as or- ganic Rankine cycle (ORC), Kalina cycle, Goswami cycle, trilat- eral flash cycle, and the supercritical Rankine cycle (SRC) [2-11]. ORC have been used more often to convert low-grade heat into power [11-13]. Conventional ORCs work under subcritical condi- tions and generally use pure working fluids. Since a pure fluid boils at a specific temperature at a fixed pressure, it causes a mis- match with the temperature profile of the heat source [7]. This mismatch can be reduced by using a SRC [7]. In a SRC, as shown in the T-S diagram in Fig. 1, the working fluid is pressurized Corresponding author. Contributed by the Advanced Energy Systems Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received November 6, 2012; final manuscript received January 23, 2013; published online May 27, 2013. Assoc. Editor: Kau-Fui Wong. beyond its critical pressure and then heated isobarically to convert it into the vapor phase. The superheated vapor then expands in the turbine and mechanical work is extracted. The turbine exhaust is then cooled to the liquid state in a condenser and the condensed liquid is pumped back to the high pressure to complete the cycle. The choice of a working fluid depends on its physical and chemical properties as well as its environmental and economic aspects. Carbon dioxide is one such choice, being abundant, non- toxic, nonflammable, and inexpensive. Chen et al. [14] compared the efficiency of an R-32 based ORC with a CO2-based cycle under similar source and sink conditions and concluded that an R- 32 based cycle gives higher efficiency than a CO2 based cycle and Entropy Fig. 1 A supercritical Rankine cycle on a T-S diagram Journal of Energy Resources Technology Copyright © 2013 by ASME DECEMBER2013, Vol. 135 / 042002-1