A generalised assessment of working fluids and radial turbines for non-recuperated subcritical organic rankine cycles

Abstract

The aim of this paper is to conduct a generalised assessment of both optimal working fluids and radial turbine designs for small-scale organic Rankine cycle (ORC) systems across a range of heat-source temperatures. The former has been achieved by coupling a thermodynamic model of subcritical, non-recperated cycles with the Peng-Robinson equation of state, and optimising the working-fluid and cycle parameters for heat-source temperatures ranging between 80◦C and 360◦C. The critical temperature of the working fluid is found to be an important parameter governing working-fluid selection. Moreover, a linear correlation between heat-source temperature and the optimal critical temperature that achieves maximum power output has been found for heat-source temperatures below 300◦C (Tcr = 0.830Thi + 41.27). This correlation has been validated against cycle calculations completed for nine predefined working fluids using both the Peng-Robinson equation of state and using the REFPROP program. Ultimately, this simple correlation can be used to identify working-fluid candidates for a specific heat-source temperature. In the second half of this paper, the effect of the heat-source temperature on the optimal design of a radial-inflow turbine rotor for a 25 kW subcritical ORC system has been studied. As the heat-source temperature increases, the optimal blade-loading coefficient increases, whilst the optimal flow coefficient reduces. Furthermore, passage losses are dominant in turbines intended for low-temperature applications. However, at higher heat-source temperatures, clearance losses become more dominant owing to the reduced blade heights. This information can be used to identify the most direct route to efficiency improvements in these machines. Finally, it is observed that the transition from a conventional converging stator to a converging-diverging stator occurs at heat-source temperatures of approximately 165◦C, whilst radially-fibered turbines seem unsuitable as the heat-source temperature exceeds 250◦C; these conclusions can be used to inform expander design and selection at an early stage.