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Publication Title | Working fluids for low-temperature organic Rankine cycles

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ARTICLE IN PRESS

Energy 32 (2007) 1210–1221

Working fluids for low-temperature organic Rankine cycles Bahaa Saleh1, Gerald Koglbauer, Martin Wendland, Johann Fischer

Institut fu ̈r Verfahrens- und Energietechnik, Universita ̈t fu ̈r Bodenkultur, Muthgasse 107, A-1190 Wien, Austria

Received 19 October 2005

www.elsevier.com/locate/energy

Abstract

A thermodynamic screening of 31 pure component working fluids for organic Rankine cycles (ORC) is given using BACKONE equation of state. The fluids are alkanes, fluorinated alkanes, ethers and fluorinated ethers. The ORC cycles operate between 100 and 301C typical for geothermal power plants at pressures mostly limited to 20bar, but in some cases supercritical pressures are also considered. Thermal efficiencies Zth are presented for cycles of different types. In case of subcritical pressure processes one has to distinguish (1) whether the shape of the saturated vapour line in the T,s-diagram is bell-shaped or overhanging, and (2) whether the vapour entering the turbine is saturated or superheated. Moreover, in case that the vapour leaving the turbine is superheated, an internal heat exchanger (IHE) may be used. The highest Zth-values are obtained for the high boiling substances with overhanging saturated vapour line in subcritical processes with an IHE, e.g., for n-butane Zth 1⁄4 0.130. On the other hand, a pinch analysis for the heat transfer from the heat carrier with maximum temperature of 120 1C to the working fluid shows that the largest amount of heat can be transferred to a supercritical fluid and the least to a high-boiling subcritical fluid.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Organic Rankine cycle; Working fluids; Low temperature heat; Geothermal power plant

1. Introduction

Presently, there is much effort in using renewable energies like solar energy, wind energy, biomass and geothermal heat as well as in using waste heat for the production of electricity. The organic Rankine cycle (ORC) is a promising process for conversion of low and medium temperature heat to electricity. The ORC process works like a Clausius–Rankine steam power plant but uses an organic working fluid instead of water. A certain challenge is the choice of the organic working fluid and of the particular design of the cycle. The process should have a high thermal efficiency Zth and allow a high utilization of the available heat source. Moreover, the working fluid should fulfil safety criteria, it should be environmentally friendly, and allow low cost for the power plant. An

Corresponding author. Tel.: +43 1 3709726 201; fax: +43 1 3709726 210.

E-mail address: johann.fischer@boku.ac.at (J. Fischer).

1Present address: Mechanical Engineering Department, Faculty of

Engineering, Assiut University, Assiut, Egypt.

0360-5442/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2006.07.001

important aspect for the choice of the working fluid is the temperature of the available heat source, which can range from low temperatures of about 1001C to medium temperatures of about 350 1C. A question, which also has to be considered in discussing ORC-processes, is whether an organic substance is really better than water as working fluid for a given task. This question arises, in particular, for medium temperature heat sources. For low temperature heat sources the advantage of organic fluids is obvious because of the volume ratio of the working fluid at the turbine outlet and inlet. This can be smaller by an order of magnitude for organic fluids than for water and hence allows the use of simpler and cheaper turbines [1].

Here, we concentrate on the production of electricity from low temperature heat sources as, e.g., geothermal heat with a temperature of about 1001C or somewhat higher. We want to emphasize that geothermal power plants already exist. Examples are the plants in Altheim, Austria, with a power production of 1MWel [2,3] and in Neustadt-Glewe, Germany, with a power production of 0.2MWel [4] both of which used initially n-perfluoropen- tane as working fluid [3,4]. This substance, however, bears

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