Fluid Selection for Small-Scale Rankine Cycle Plants: Can You Draw Some Lines in the Sand?

The aim of this paper is to define general guidelines for fluid and cycle selection for small-scale Rankine cycle power systems based on heat-source temperature, heat-source temperature drop and heat sink availability. This is developed through optimisation studies for subcritical and supercritical cycles, which includes a model to estimate the achievable efficiency for a single-stage radial-inflow turbine, and the introduction of a fluid ranking procedure. The method is applied to 20 potential working fluids including hydrocarbons, hydrofluoroolefins, and siloxanes, alongside water, CO2, Novec 649 and Novec 774. The results indicate that the top five working fluids are isobutane, isopentane, n-propane, R1233zd and n-pentane. Moreover, fluid selection is not significantly affected by heat-sink availability, whilst subcritical cycles are preferred for lower heat-source temperatures and heat-source temperature drops, whilst supercritical cycles are better for higher heat-source temperatures and are most suitable when trying to maximise power output