COMBINED THERMODYNAMIC AND TURBINE DESIGN ANALYSIS OF SMALL CAPACITY WASTE HEAT RECOVERY ORC

In this paper, the design of small-capacity ORC turbines for a waste heat recovery application is studied and discussed. A turbine design tool was coupled with a thermodynamic analysis tool in order to evaluate the effect of different working fluids and process parameters, not only by taking into account the thermodynamic aspects of the process design, but also evaluating the availability to design turbines with a relatively high efficiency and feasible geometry. The studied turbine type is a radial inflow turbine since radial turbines represent relatively simple geometries when compared to multistage configurations, can have a high expansion ratio over a single stage, and have a better efficiency at a low power capacity than the axial counterparts. The results indicate that the main difficulties in the design of small capacity ORC turbines are related to high rotational speeds, small dimensions, and large blade height ratios. In addition, the use of single stage turbines leads to highly supersonic flow in the stator even when adopting low or moderate flow velocities. The results of this study highlight the importance of combining both the thermodynamic process design and the turbine design when evaluating suitable working fluids and operational parameters.