Simultaneous Optimization of Working Fluid and Process for Organic Rankine Cycles Using PC-SAFT

Abstract

Organic Rankine Cycles (ORCs) generate power from low temperature heat. To make the best use of the diverse low temperature heat sources, the cycle is tailored to each application. The objective is to maximize process performance by optimizing both process parameters and the working fluid. Today, process optimization and working fluid selection are typically addressed separately in a two-step approach: working fluids are selected using heuristic knowledge; subsequently, the process is optimized. Such an approach can lead to suboptimal solutions, since the optimal fluid might be excluded by the heuristics. We therefore present a framework for the holistic design of ORCs enabling the simultaneous optimization of the process and the working fluid based on process performance. The simultaneous optimization is achieved by exploiting the rich molecular picture underlying the PC-SAFT equation of state in a continuous-molecular targeting approach (CoMT-CAMD). To allow for the prediction of caloric properties, a quantitative structure–property relationship (QSPR) for the ideal gas heat capacity is proposed that relies on pure component parameters of PC-SAFT. The framework is used for the optimization of a geothermal ORC in a case study. A sound holistic design of process and working fluid is achieved.

Systematic Methods for Working Fluid Selection and the Design, Integration and Control of Organic Rankine Cycles—A Review

“…As shown in Table 1 the optimum working fluids obtained from the integrated design approach of Papadopoulos et al [4,5] are very similar to those obtained by the simultaneous approach of Lampe et al [44,45]. Papadopoulos et al [4,5] were able to capture such working fluids without the use of an ORC model in the course of working fluid CAMD (hence the computations were fast) due to the use of multiple property objective functions.…”

“…Lampe et al [44,45] proposed an optimization-based method for the design of optimum ORC working fluids, namely the continuous molecular targeting (CoMT-CAMD) method (Table 1). Working fluids are designed based on a molecular model which allows the use of physical molecular characteristics as continuous decision parameters in the optimization problem.…”

“…It is worth noting here that an approach for working fluid design and selection which shares similar features to Lampe et al [44,45] has been recently proposed by Roskosch and Atakan [46]. The authors perform a reverse engineering design of the working fluid and a heat pump process (which has similarities with ORC) using a cubic EoS.…”

“…While deviations may be significant for particular fluids, others may exhibit similar property values regardless of the employed property prediction model. Lampe et al [45] illustrate the impact of the working fluid structure on the ORC net power output through their continuous molecular representation using the geometric and interaction parameters. The reported three-dimensional diagram indicates areas with very steep non-linear changes as well as areas with smoother changes.…”

“… An approach that supports the simultaneous working fluid and ORC design and/or selection (Palma-Flores et al [43]; Lampe et al [44,45]; Papadopoulos et al [18,19]; Molina-Thierry and Flores-Tlacuahuac [51]).  An approach that supports the integrated working fluid and ORC design and/or selection (Papadopoulos et al [4,5,37]).…”