Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.
A thermodynamic limitation of single-component working fluids in organic Rankine cycles (ORCs) is the large exergy destruction (and, consequently, useful power loss) associated with evaporation and condensation. Due to their non-isothermal phase-change behaviour, non-azeotropic working-fluid mixtures have shown reduced exergy losses, leading to improved cycle efficiencies and power outputs. These benefits are exclusively observed from a thermodynamic perspective. The present paper considers the effects of selecting such working-fluid mixtures on heat transfer performance, component sizing and system costs compared with those of pure fluids; a mixture of n-pentane and n-hexane is selected. While the fluid-mixture cycles do indeed allow higher efficiencies and the generation of higher power outputs, they require larger evaporators, condensers and expanders; thus, the resulting ORC systems are more expensive than those based on the pure fluids. While a working-fluid mixture (60% n-pentane + 40% n-hexane) leads to the thermodynamically optimal cycle, a pure n-pentane ORC system has reduced costs of 37% per unit power output over the thermodynamic optimum.