One-stage approach for the integrated design of ORC processes and working fluid using PC-SAFT

Organic Rankine Cycles (ORC) can transform low-temperature heat into electrical power. To ensure optimal use of a heat source, process and working fluid need to be tailored to the specific application. We present a one-stage approach for the integrated design of ORC process and working fluid, which identifies the optimal working fluid and the corresponding optimal process in a single optimization problem. For this purpose, a process model is combined with a modern thermodynamic model of the working fluid. The process model is based on equilibrium thermodynamics. The perturbed-chain statistical associating fluid theory (PC-SAFT) is used as physically-based thermodynamic model of the working fluid. The fluid model is extended by a group-contribution method based on PC-SAFT to enable Computer-aided molecular design (CAMD) of novel working fluids within the optimization. The full model enables the integrated design of process and working fluid. The optimization is an MINLP problem depending on two kinds of design variables: continuous process variables and integer variables representing the molecular structure of the working fluid. The one-stage approach is exemplified in a case study for a subcritical ORC process. The approach is shown to efficiently identify the optimal working fluid and the corresponding optimal process parameters. Integer cuts are employed to generate a ranked list of candidates.