A systematic sensitivity analysis approach for the design of organic rankine cycles and the selection of efficient working fluids under operational variability
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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.
