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

asme-orc2015 Tracking Number 31

Organic Rankine Cycles adopting turbine technology have been commercially successful in large-scale ORC application. In small power output systems (about 10-50 kW) a volumetric expander is usually preferred instead of a turbine. However, the limitations of using volumetric expanders are related to the lower achievable expansion ratios over the expanders when compared to the use of turbines and thus, disabling the use of high molecular weight and high critical temperature fluids characterized by large expansion ratios in the cycle. In this paper the design of small capacity ORC turbines is studied and discussed. The selected application is exhaust gas recovery of a small-scale gas turbine. A turbine design tool was coupled with a thermodynamic analysis 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. The studied fluids can be classified as hydrocarbons, siloxanes, and fluorocarbons and the studied turbine type is a radial inflow turbine, since radial turbines represent relatively simple geometries when compared to multistage configurations and can have a high expansion ratio over a single stage. The results indicate that the main difficulties in the design of small capacity ORC turbines are related to high rotational speeds, small turbine dimensions, and relatively large variations in the blade height between the turbine rotor inlet and outlet. In addition, the use of single stage turbines leads to highly supersonic flow in the turbine stator even when adopting low or moderate flow velocities caused by the low speed of sound of the organic fluids. The results highlight the importance of combining both the thermodynamic process design and the turbine design when evaluating suitable working fluids and operational parameters for the cycle.