Control Studies of Organic Rankine Cycles with Different Working Fluid Mixtures

Abstract

Organic Rankine cycles operate under continuously changing conditions due to enthalpy variations in the hot source, uncertainty in the thermodynamic properties of novel working fluid mixtures, variable efficiency in the expander and the heat exchanger units, and malfunctions in the pumping system. An optimal model based control system can effectively alleviate the influence of exogenous disturbances and uncertainty from the changes in the operating specifications. The performance of such a controller applied on an ORC system that uses as a working fluid a mixture of two components is investigated in the present work. Multi-component mixtures as working fluids in ORC systems can substantially improve the overall efficiency and reduce the exergy losses due to better proximity between the hot source and the working mixture temperature profiles compared to a system with a pure component as the working medium. Previously identified as potentially good working fluids in terms of thermal and exergy efficiency under steady state conditions are being investigated for their dynamic performance under closed loop conditions. The study provides an additional criterion for the ultimate selection of the most suitable working fluid candidate.