Centrifugal Turbines for Mini Organic Rankine Cycle Power Systems

Abstract

Organic Rankine cycle (ORC) power systems are rapidly diffusing as a technology for the conversion of thermal energy sources in the smalltomedium power range, e.g., from 150 kWe up to several MWe. The most critical component is arguably the expander, especially if the power capacity is small or very small, as it is the case for innovative highpotential applications such as waste heat recovery from truck engines, or distributed conversion of concentrated solar radiation. In these socalled hightemperature applications, the expansion ratio is very high; therefore, turbines are the expanders of choice. Recently, multistage radialoutflow turbines (ROT), a nonconventional turbine configuration, have been studied, and first commercial implementations in the MWe power range have been successful. The objective of this work is the evaluation of the radialoutflow arrangement for the turbine of hightemperature miniORC power systems, with power output of the order of 10 kWe. To this end, a method for the preliminary fluiddynamic design is presented. It consists of an automated optimization procedure based on an inhouse meanline code for the onedimensional preliminary design and efficiency estimation of turbines. It is first shown that usually adopted simplified design procedures, such as that of the socalled repeatingstage, cannot be extended to minicentrifugal turbines. The novel methodology is applied to the exemplary case of the 10 kWe turbine of an ORC power system for truck engine heat recovery documented in the literature. The expansion ratio is 45. The preliminary fluiddynamic design of two miniturbines is presented, namely, a fivestage transonic and a threestage slightly supersonic turbine. The outcome of the preliminary design leads to two turbine configurations whose fluiddynamic efficiency exceeds 79% and 77%, respectively. The speed of revolution is around 12,400 and 15,400 RPM for the fivestage and the threestage machine, respectively. These results show that the ROT configuration may allow for compact and efficient expanders for low power output applications.