AN ASSESSMENT OF WORKING-FLUID MIXTURES USING SAFT-VR MIE FOR USE IN ORGANIC RANKINE CYCLE SYSTEMS FOR WASTE-HEAT RECOVERY

摘要

Working-fluid mixtures offer an improved thermal match to heat source streams in organic Rankine cycles (ORCs) over pure (single) fluids. In the present work we investigate the selection of working-fluid mixtures and component mixing ratios for an ORC system from a thermodynamic and economic point of view. A mathematical model of a subcritical, nonregenerative ORC is constructed. We employ the SAFT-VR Mie equation of state, a state-of-the-art version of the statistical associating fluid theory (SAFT), to predict the thermodynamic state properties and phase behavior of the fluid mixtures. The effect of the working-fluid mixture selection on the efficiency and power output from the cycle is investigated, as is its effect on the sizes of the various components of the ORC engine. This is done in order to appreciate the role that the fluid mixtures have on the investment/capital costs attributed to the installation of such a unit, intended for waste-heat recovery and conversion to power. Results of an ORC using a binary decane-butane mixture as the working fluid demonstrate a significant improvement in the cost per unit power output compared to the two pure fluid components. Specifically, the added costs of the four main ORC system components (pump, expander, and two heat exchangers) were found to be as low as 120-130 ￡/kW, 20-30% lower compared to the pure fluids.