Experimental Investigation of a Small-Scale Organic Rankine Cycle Turbo-Generator Supported on Gas-Lubricated Bearings

Waste heat recovery (WHR) is expected to contribute to reducing CO2 emissions from trucks. Organic Rankine cycle (ORC) systems show the highest potential for this application, but still lack efficient small-scale expansion devices, in practice. A novel turbogenerator (TG) supported on gas-lubricated bearings is presented in this paper. The device combines a single-stage radial-inflow turbine and a permanent-magnet machine in a single rotating part supported on aerodynamic bearings, lubricated with the working fluid (R245fa). The oil-free expander was tested within a dedicated ORC test setup. It was driven up to its nominal speed of 100 krpm, generated up to 2.3 kW of electrical power, and reached a peak overall efficiency of 67%. Although the prototype was not actively cooled, the mechanical losses of the rotor shaft and the iron loss of the electrical machine reached their nominal levels. Only the copper loss was at a part-load level. The electromechanical efficiency of the TG reached 91% and is expected to increase while testing the device at higher load. This proof of concept confirms the high-speed and low-loss potential of gas-lubricated bearings for small-scale dynamic expanders.