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Session: Working fluids II
Session starts: Tuesday 08 October, 14:00
Presentation starts: 14:20
Room: Willem Burger Zaal

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Athanasios Papadopoulos (Centre for Research and Technology-Hellas, Chemical Process and Energy Reseources Institute)
Mirko Stijepovic (Texas A&M University at Qatar, Chemical Engineering Department)
Patrick Linke (Texas A&M University at Qatar, Chemical Engineering Department)
Panos Seferlis (Aristotle University of Thessaloniki, Department of Mechanical Engineering)
Spyros Voutetakis (Centre for Research and Technology-Hellas, Chemical Process and Energy Resources Institute)

Abstract:
Organic Rankine Cycle (ORC) systems commonly utilize single working fluids to support power generation, although reports [1] indicate that mixtures enable a significant increase of the ORC performance. Despite the enormous importance of both single and mixed working fluids for ORCs, their selection is largely based on trial-and-error approaches applied in repositories containing conventional options. The authors have been the first to address the above limitation through the use of a Computer-Aided Molecular Design (CAMD) method applied in the design and selection of single ORC working fluids [2, 3]. The considerable advantages of the designed fluids compared to conventional choices constitute a significant motivation for the development and implementation of a CAMD-based method for the design of ORC mixtures. The challenges associated with such an effort are due to the need to simultaneously determine the optimum mixture composition (chemical structure of all participating working fluids) and concentration (amount of each fluid in the mixture). Furthermore, uncertainty associated with the predictive capabilities of the employed models may impact both on the type of the designed fluids and their operating features. While CAMD-based mixture design has yet to be considered in ORC research, it has been reported for few other applications. This work presents a systemic method for the simultaneous determination of the composition and concentration of binary ORC working fluid mixtures using CAMD-based optimization, implemented in three interacting stages. The first stage aims to explore and identify the highest possible economic, operating, environmental and safety performance limits of a wide set of mixtures in an ORC system. This is approached by searching for chemically feasible molecular structures only for one of the two components of a binary mixture, while emulating the mixture behaviour of the remaining component within a much wider structural design space. The second stage serves to determine the optimum and chemically feasible structure of the second component for each one of the molecules already obtained in the first stage, together with the optimum mixture composition. Subsequently, a non-linear sensitivity analysis method is presented to address model-related uncertainties in the mixture selection procedure. The identification of multiple optimum mixture candidates is accomplished through a multi-objective formulation of the CAMD-optimization problem The merits of the proposed approach are illustrated through a case study on ORC systems. The considered design indices employed in CAMD reflect important ORC performance measures such as thermodynamic efficiency and exergy through an ORC mathematical model utilized in the course of CAMD-optimization. Important mixture properties such as flammability, toxicity and azeotropic mixture behaviour are also considered. [1] Angelino, G., Di Paliano, P. C., Multicomponent working fluids for organic rankine cycles (ORCs), Energy 1998; 23(6): 449. [2] Papadopoulos A.I., Stijepovic M and Linke P., On the systematic design and selection of optimal working fluids for Organic Rankine Cycles, Applied Thermal Engineering 2010, 30, 760. [3] Stijepovic M.Z., Linke P., Papadopoulos A.I., Grujic A.S., On the role of working fluid properties in Organic Rankine Cycle performance, Applied Thermal Engineering 2012, 36, 406-413