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Supercritical Fluid Parameters in Organic Rankine Cycle Applications

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Supercritical Fluid Parameters in Organic Rankine Cycle Applications ( supercritical-fluid-parameters-organic-rankine-cycle-applica )

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Int. J. of Thermodynamics ISSN 1301-9724 Vol. 11 (No. 3), pp. 101-108, September 2008 Supercritical Fluid Parameters in Organic Rankine Cycle Applications Sotirios Karellas, Andreas Schuster* National Technical University of Athens, Laboratory of steam boilers and thermal plants 9 Heroon Polytechniou, 15780 Athens Greece E-mail: sotokar@mail.ntua.gr Technische Universität München, Institute of Energy Systems Boltzmannstr. 15, 85748 Garching, Germany E-mail: schuster@es.mw.tum.de Abstract Nowadays, the use of Organic Rankine Cycle (ORC) in decentralised applications is linked with the fact that this process allows to use low temperature heat sources and offers an advantageous efficiency in small-scale applications. Many state of the art applications like geothermal and biomass fired power plants as well as new applications like solar desalination with reverse osmosis, waste heat recovery from biogas digestion plants or micro-Combined Heat and Power (micro-CHP) systems can successfully use the ORC process. The investigation of supercritical parameters in ORC applications seems to bring promising results in decentralised energy production. This paper presents the results from the simulation of the ORC process in normal and supercritical fluid parameters and discusses the efficiency variation in various applications. Keywords: Organic Rankine cycle (ORC), supercritical parameters, waste heat recovery. 1. Introduction The Organic Rankine cycle (ORC) is a Clausius Rankine cycle in which an organic working fluid is used instead of water-steam. In the last years it became quite popular in energy production processes, due to the fact that it gives the possibility to use heat of small supply rate and low temperature level. One of the main challenges when ORC is used in a process is the choice of the appropriate working fluid and of the particular cycle design with which maximum thermal efficiency can be achieved. The ORC process is similar to the Steam process, which uses water as working fluid. The difference between water and an exemplary organic fluid is shown in Figure 1. The diagram shows the saturation lines and three isobars with the same pressure for water and organic fluid. It can be clearly seen, that the Critical Point (C.P.) of organic fluids is reached at lower pressures and temperatures compared with water. For numerous organic fluids the vapour saturation line has a positive inclination. This allows the use of a recuperator for preheating the liquid working fluid by desuperheating the expanded vapour. In the state of the art applications which are discussed nowadays, saturated or slightly superheated vapour is expanded in the turbine. However, the investigation of supercritical fluid parameters is of high importance, since, as it will be discussed later, it leads to higher thermal efficiencies making these plants even more attractive for waste heat applications. The main advantage of the supercritical process is the fact that the average high temperature in which the heat input is taking place is higher than in the case of the subcritical fluid process. Therefore, according to Carnot, the efficiency is higher. Figure 2 shows the process of a sub- and supercritical ORC in a T-s-Diagram for a constant superheated vapour temperature. Even for constant superheated vapour temperatures, the heat input occurs at a higher * Author to whom correspondence should be addressed. schuster@es.mw.tum.de Int. J. of Thermodynamics, Vol. 11 (No. 3) 101

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