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Session: Poster Session and Sponsors Exhibition
Session starts: Thursday 22 September, 14:00

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Renzo Molinari (University of Padova)
Ernesto Benini (University of Padova)

Abstract:
In this paper, an in-depth comparative analysis between the Organic Rankine Cycle (ORC) and the classical Water Rankine Cycle (WRC) technologies is carried out. For this purpose, the behavior of a cogenerative power plant fed by the output gases of a micro-gas turbine at a temperature of 300°C was considered where the net electrical power level requested was 30kW. Through this comparative analysis we have picked out the advantages of ORC technology in terms of thermal efficiency (i.e. more electric power can be produced from a given heat source) and feasibility of the turbine. In order to deepen the characterization of the ORC we have simulated the plant performance for three different organic fluids: N-pentane, Cyclohexane and Toluene. Thanks to this approach we have parameterized the main subjects of the power plant as a function of the thermophysical properties of the organic fluid. The work is divided into two parts. In the first part we have investigated, after an appropriate choice of evaporation and condensation temperatures, the cycle thermal efficiency. Great attention has been given towards the heat exchange behavior and the volume flow rates of the plant. From this first analysis we have demonstrated that the ORC technology makes it possible to obtain higher thermal efficiencies when low enthalpy heat sources are available. In this way, we have shown the dependence of efficiency from the properties of fluid and then important guidelines to choose the suitable fluid are given. In the second part, a preliminary design of the turbine is carried out. From the temperature-enthalpy diagrams for organic fluids you can see, contrarily to water, a positive slope of satur-vapour curve. This fundamental characteristic allows organic fluids an expansion in a superheated vapor region. Otherwise, if you spare a superheating process, water expands in a two phase region whit a great disturbance of the flow field and the consequent decrease of the turbine expansion efficiency. With the purpose to realize a preliminary study of the main geometrical and functional parameters of the turbine we have developed a dedicated computational code. In this case we have parameterized, as a function of the fluid, the dimension of the rotor inlet blade and other main turbine parameters. Great attention has been given towards the behavior of the flow field at the exit of the turbine nozzle. In the last part of this paper we have matched, for each fluid, thermodynamic efficiency and feasibility of the turbine. In this way some considerations arose about the selection of suitable organic fluid in accordance with plant power level and heat source temperature. As a consequence you can understand the reason why some companies are looking for new organic fluids or mixtures of them. The importance of this work is related to the general approach utilized and the results obtained represent an important preliminary guideline to design ORC systems.