Integrated design of ORC process and working fluid using PC-SAFT and Modelica
Dominik Tillmanns, Christoph Gertig, Johannes Schilling, Andrej Gibelhaus, Uwe Bau, Franz Lanzerath, André Bardow
Session: Poster session & Welcome drinks
Session starts: Wednesday 13 September, 17:30
Dominik Tillmanns ()
Christoph Gertig ()
Johannes Schilling ()
Andrej Gibelhaus ()
Uwe Bau ()
Franz Lanzerath ()
André Bardow ()
Organic Rankine Cycles (ORC) use low-temperature heat for electrical power generation. To use the full potential of a heat source, the ORC has to be tailored to the specific application. Tailoring a cycle means an integrated design of both process and working fluid. This integrated design leads to complex mixed-integer nonlinear program (MINLP) optimization problems. Today, working fluid candidates are commonly selected using heuristic guidelines; subsequently, the process is optimized for the set of preselected working fluids. However, heuristic guidelines cannot capture the strong interdependence between working fluid properties and process conditions sufficiently. Thus, the preselection can fail easily leading to suboptimal solutions.
An approach for the integrated design of ORC process and working fluid is the Continuous-Molecular Targeting–Computer-aided Molecular Design (CoMT-CAMD) framework . In CoMT-CAMD, the physically-based Perturbed-chain Statistical Associating Fluid Theory (PC-SAFT) equation of state  is used as thermodynamic model of the working fluid. In PC-SAFT, each working fluid is described by a set of pure component parameters. In a first step, the pure component parameters are relaxed during the integrated design of process and working fluid transforming the MINLP into a nonlinear program (NLP). This step is called Continuous-Molecular Targeting. The result of the CoMT step is a hypothetical optimal working fluid and the corresponding process. Real working fluids with similar properties are identified in a second step, the so-called structure-mapping. For this purpose, the objective function values of real working fluids are estimated using a second-degree Taylor approximation of the objective function around the hypothetical optimal working fluid. A Computer-aided Molecular Design formulation allows designing novel working fluids by solving the resulting mixed-integer quadratic program (MIQP). So far, the process models in CoMT-CAMD were implemented in a procedural programming language, which hinders the reusability, the use for more complex processes and dynamic simulations.
In this work, we have integrated CoMT-CAMD into the object-oriented modelling language Modelica. For this purpose, Modelica is directly linked to PC-SAFT. Thereby, already existing model libraries for Modelica can be included and the programming effort for studying process variations can be decreased. The resulting design approach is applied to the integrated design of an ORC process and working fluid for a geothermal power station.
We thank the Deutsche Forschungsgemeinschaft (DFG) for funding this work (BA2884/4-1).
 Lampe, M.; Stavrou, M.; Schilling, J.; Sauer, E.; Gross, J.; Bardow, A. (2015): Computer-aided molecular design in the continuous-molecular targeting framework using group-contribution PC-SAFT. Computers & Chemical Engineering 81, pp. 278–287.
 Gross, J.; Sadowski, G. (2001): Perturbed-Chain SAFT. An Equation of State Based on a Perturbation Theory for Chain Molecules. Ind. Eng. Chem. Res. 40 (4), pp. 1244–1260.