Integrating working fluid design into the thermo-economic design of ORC processes using PC-SAFT

To exploit the full thermo-economic potential of an Organic Rankine Cycle (ORC), the process, equipment and working fluid have to be optimized simultaneously. Today, working fluid selection and thermo-economic process optimization are commonly separated. This separation leads to suboptimal solutions if the prior working fluid selection fails. In this work, we present an approach for the integrated thermo-economic design of ORC process, equipment and working fluid using consistent thermodynamic modeling. The approach is based on the Continuous-Molecular Targeting-Computer-aided Molecular Design (CoMT-CAMD) approach. In CoMT-CAMD, the properties of the working fluid are modeled by the physically-based Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) equation of state. A CAMD formulation allows the design of novel working fluids during the process optimization. So far, CoMT-CAMD was limited to equilibrium thermodynamics. Some of the authors recently developed models for the transport properties viscosity and thermal conductivity based on entropy scaling and PC-SAFT. The integration of these models allows designing the equipment within the CoMT-CAMD approach. In particular, the heat exchanger of the ORC can be designed using detailed correlations for single phase, evaporation and condensation heat transfer. Based on the equipment sizing, a thermo-economic objective function can be considered in the resulting mixed-integer nonlinear optimization problem. Thereby, the thermo-economically optimal working fluid is identified in a single optimization problem jointly with the corresponding optimal process and equipment. The resulting approach is illustrated for the design of a subcritical ORC for waste heat recovery. We show that the predicted specific purchased-equipment costs are in good accordance with real ORC applications.