Object-oriented modeling of micro-ORC systems for low-grade waste heat recovery applications

Organic Rankine Cycle (ORC) systems are one of the most suitable technologies to produce electricity from low-temperature sources. A comprehensive understanding of these systems is necessary for their further deployment especially in micro scales, in which the intrinsic issues such as low isentropic efficiency and unreliability arise more severely. In this thesis, the components of a non-regenerative, micro-scale ORC unit are modeled empirically using the experimental data. The components models are used as functions in the system-level solver. The system solver uses a novel approach, in which no assumption is made for the modeling and only the components specifications and the real system boundaries that an operator has during the system run are put as inputs. This assumption-free, object-oriented model follows the two fundamental conservation laws of thermodynamics that are the conservation of energy and the conservation of mass. The developed mass-sensitive model is a robust solver that leads to a full understanding of the system limitations and losses using the empirical models of the main system components. In addition to the system modeling, the thesis theoretically studies the impact of the expander lubricant oil on the system performance using the empirical lubricant-refrigerant miscibility model and a data reconciliation method. The impact of the expander’s lubricant is found significantly effective on the performances of the heat exchangers, the expander, and the overall system especially in micro-scales, despite its influence has been mostly neglected in the literature so far. Since the use of micro-scale ORC systems for low-temperature waste heat recovery (WHR) applications is investigated, a novel kind of expander for such systems has been here analyzed using computational fluid dynamics (CFD). The regenerative flow turbine (RFT) is found particularly interesting due to its good and reliable performance in very low-pressure ratios making the machine suitable as an alternative expansion. Finally, a biomass-fed integrated system is studied in detail. The integrated system consists of a dual-fluidized-bed gasifier, a hot gas conditioning unit, a steam-injected-micro gas turbine, and an ORC as the bottom cycle. This system-level study shows the performance of the integrated system when full conservation of energy and mass is applied. The results show that the overall system efficiency improvement by the ORC is limited to 1-2 % using the wet gas turbine compared to 5-8% without the steam injection. Nevertheless, the produced thermal power in the condenser of the ORC unit shows the ability of the integrated system to meet higher users’ thermal demands in small-scale CHP applications.