Design maps for axial turbines operating with non-ideal organic compounds

Axial turbines are arguably the most common configuration of expanders employed in organic Rankine cycle (ORC) power plants of large power capacity. In most of these power plants, the working fluid enters the turbine with a few degrees of superheating, and its thermodynamic properties do not obey the ideal gas law in the first part of the expansion. To date, there is a lack of fast but reliable methods to perform the preliminary design of the first turbine stage while taking into account non-ideal thermodynamic effects. For this reason, an extensive database of efficiency maps as a function of the typical stage design parameters has been derived in the present study. The idea is to use the maps to quickly identify the optimal stage configuration in the preliminary design phase of the ORC system. Results demonstrate that nonideal thermodynamic effects can be quantified for an extensive range of compounds by solely relying on the compressibility factor and fluid molecular complexity. It is also found that, depending on the stage configuration, non-ideal thermodynamic effects can increase or decrease the stage efficiency compared to the stage operating with the same fluid in ideal thermodynamic conditions.