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Multi-objective optimization of a bottoming Organic Rankine Cycle (ORC) of gasoline engine using swash-plate expander

J. Galindo, H. Climent, V. Dolz, L. Royo-Pascual

CHALLENGE - This paper presents a mathematical model of a bottoming Organic Rankine Cycle coupled to a 2l turbocharged gasoline engine to optimize the cycle from a thermo-economic and sizing point of view. These criteria were optimized with different cycle values. A methodology to optimize the ORC coupled to Waste Heat Recovery systems in vehicle applications is presented using a multiobjective optimization algorithm.

SOLUTION - Three objective functions have been considered in order to simplify the optimization of the system: SIC in €/kW, heat exchangers area (sum of Atot,b and Atot,c) in m2 and expander size (VC) in MJ/m3. The Specific Investment Cost has been chosen as a global parameter of the thermo-economic behavior of the system. The remainder economic parameters (NPV and PB) are characterized by high degree of uncertainty due to the estimation of the fuel price (Cf) and the number of ORC operating hours during a year (hryear). Therefore, in order to know the influence of these parameters two parametric studies are presented after the optimization of the system. In addition to SIC, two more sizing parameters were chosen to take into account both the size of the heat exchangers and the expander. The multi-objective optimization problem of the ORC system is performed by using a genetic algorithm in ModeFRONTIER. In order to select a single-solution from the Pareto frontier, a multiple attribute decision-making method (TOPSIS) was implemented in order to take into account the preferences of the Decision Maker.

BENEFITS - Considering the weight factors 0.5 for Specific Investment Cost (SIC), 0.3 for the area of the heat exchangers (Atot) and 0.2 for Volume Coefficient (VC) and the boundaries of this particular application, the result is optimized with values of 0.48 m2 (Atot), 2515 €/kW (SIC) and 2.62 MJ/m3 (VC). Moreover, the profitability of the project by means of the Net Present Value and the Payback has been estimated.

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