(463f) A Systematic Sensitivity Analysis Approach for the Design of Organic Rankine Cycles and the Selection of Efficient Working Fluids Under Operational Variability
Organic Rankine Cycles (ORCs) have received increased attention in recent years, spanning a wide range of applications including power generation from industrial waste heat recovery, geothermal energy and solar irradiation, to name but a few. Their operation is based on the extraction of heat, which is used to evaporate an appropriate organic working fluid subsequently expanded in a turbine to produce work. ORC process flowsheets involve various complex options including multiple pressure loops, expansion stages etc. to increase the energetic and exergetic efficiencies and improve economics. Their use with intermittent heat sources implies that ORCs are often required to operate under variable heat input conditions, while as in every other process system internal variability is always important due to leaks, malfunctions, fouling and so forth. Unless such variability is accounted for, the ORC ability to perform satisfactorily under conditions different from the nominal design settings, namely on the ORC operability, will be significantly jeopardized. This is because potential unexpected changes in the system operation may result in deviations from the performance for which the system was initially designed. To ensure efficient operability, ORC systems need to be designed so that they are sufficiently capable to handle operating variations. Working fluids are inherent components of ORC operation performance, hence their impact on operability should also be accounted for together with ORC design decisions for a wide range of operating conditions. Whereas some working fluids or ORC system designs and operating characteristics may be less sensitive to such changes, others may significantly deviate from their expected performance, eventually failing to meet the desired operating specifications. Published research considers such issues as an afterthought to the selection of working fluids and determination of optimum ORC features under nominal operating conditions. The employed methods are often heuristic as design or operating system parameters are selected within an arbitrarily defined range and with very limited consideration of their combined effects into the system performance. Such approaches fail to provide assurances regarding the validity of the resulting insights or that they will not be affected by the consideration of additional parameters or more extensive design and operating ranges.
In our current work, we propose a systematic method which explicitly considers the impacts of working fluid and ORC design/operating decisions on ORC operability. The proposed method is based on a systematic sensitivity analysis procedure aiming to identify working fluids and ORC operating/design characteristics that optimize the overall performance under nominal operation while simultaneously minimizing the system sensitivity under variability. The method facilitates a) the identification of design parameters with high influence on the overall ORC-working fluid performance, b) the quantification of the overall system sensitivity with respect to these parameters and c) the incorporation of sensitivity metrics to the decision-making process of optimum working fluid and ORC design characteristics. It is based on the development of a sensitivity matrix that incorporates the impact of ORC operating conditions and working fluids on a set of relevant ORC performance measures (e.g., energetic or exergetic efficiencies, irreversibility and so forth). The singular value decomposition of the sensitivity matrix enables the calculation of the major directions in the operating condition space that cause the largest variability in the performance indicators. The entries in such dominant directions determine the impact of each operational condition in these directions. It is therefore possible to explore the behavior of the ORC system along a small number of directions of variability that encompass the combined effect of multiple operating conditions and obtain clear knowledge about the system behavior under perturbed conditions. The variation is not limited to small in magnitude variations but allows a comprehensive outlook of the system attitude for large deviations from the nominal operating point. The investigation is capable of identifying critical process constraint violations and performance features for a wide range around the region where the system exhibits the greatest sensitivity. Through such a procedure a quick but efficient screening and rank-ordering of alternative ORC-working fluid systems can be achieved utilizing steady-state simulations. The proposed aggregate sensitivity metric encapsulates the system variability and acts as an additional objective function in the multicriteria assessment problem, along with all performance indices calculated under nominal operation. The proposed method is presented within a formal mathematical analysis framework which avoids arbitrary assumptions regarding the effects of the selected design and operating parameters and subjective interpretations of the obtained results. It also remains generic and independent of the proposed application.
The proposed developments are illustrated considering solar collectors connected with a storage tank to provide the necessary heat for ORC operation. Multiple working fluid mixtures  are evaluated together with several operating and design parameters (e.g., mass flowrates and temperatures, collector area, tank volume and so forth) considering multiple performance indices (e.g., net work output, energetic efficiency, irreversibility, volume ratio across the turbine and so forth). Mixtures are considered as working media because they facilitate increased exergy efficiency while maintaining relatively high energy efficiency due to the thermodynamic characteristics of phase change, but result in a decision making problem of increased complexity due to the need to assess different types of molecules at various concentrations . The obtained results indicate that in order to avoid significant performance losses, variability should be systematically considered as part of the working fluid and ORC design and selection process.
 Papadopoulos, A.I., Stijepovic, M., Linke, P., Seferlis, P., Voutetakis, S., Toward optimum working fluid mixtures for Organic Rankine Cycles using molecular design and sensitivity analysis, Industrial and Engineering Chemistry Research 52 (34) (2013) 12116-12133.
 P. Mavrou, A. I. Papadopoulos, M. Z. Stijepovic, P. Seferlis, P. Linke, S. Voutetakis, Novel and conventional working fluid mixtures for solar Rankine cycles: Performance assessment and multi-criteria selection, Applied Thermal Engineering 75 (2015) 384-396.