An innovative small-scale two-stage axial turbine for low-temperature organic Rankine cycle

Abstract

High expander efficiency is required to achieve best performance for small-scale organic Rankine cycle (ORC) systems driven by low-temperature (<100°C) heat sources. In this paper a small-scale two-stage axial turbine is modelled and compared with a single-stage axial turbine, with the aim of enhancing the ORC performance by increasing its pressure ratio. The preliminary mean-line design approach is coupled with three-dimensional CFD modelling and ORC cycle analysis was used to assess the impact of two-stage axial turbine on the ORC cycle performance. Three-dimensional CFD analysis of the single and two-stage axial turbines was performed using ANSYS®17-CFX software. The RANS equations with a k-ω SST turbulence model were solved for three-dimensional viscous steady state flow. The real gas thermodynamic properties of three organic working fluids (n-pentane, R141b, R245fa) are used in modelling the flow with both turbine configurations. Results revealed that the two-stage axial turbine configuration exhibited a substantially higher turbine performance, with overall isentropic efficiency of 83.94% and power output of 16.037kW, compared to 78.30% and 11.06kW from the single-stage configuration, with n-pentane as working fluid and mean diameter of 64mm for the two-stage configuration. Also, results showed that the maximum ORC thermal efficiency was 14.19% compared with 10.5% for single-stage configuration using n-pentane as the working fluid. These results highlight the potential of using two-stage axial turbine in a small-scale ORC’s system for the conversion of low-temperature heat sources into electricity.