Design of a 50-watt air supplied turbogenerator

Abstract

This thesis presents the design of a high-pressure-ratio, low-flow turbogenerator with 50 W electrical power output, designed to operate from a 5-bar air supply. The research shows that a MEMS-based silicon turbine in combination with a micro-machined generator can meet the design objectives of the turbogenerator. The turbogenerator architecture comprises a single-stage radial inflow turbine and a direct coupled cylindrical permanent-magnet synchronous generator. To address its key design trade-offs and fundamental engineering challenges, the design space is first explored in terms of the key design variables: device diameter and rotor shaft speed. To guide the selection of the two variables, a simplified model for the scaling of turbine and generator power is developed. Next, the integrated turbine and generator design is analyzed and designed in detail. The minimum manufacturable turbine blade span is identified as the key challenge imposed by the low flow requirement. Furthermore, the small scale of the blades results in high rotational speeds. Since the turbine and generator are integrated, the high speed makes the generator design challenging because of manufacturing tolerances, material stress limitations and losses. Thus, the design trade-off is between generator complexity and turbine blade manufacturability. The analysis shows that the viable low-flow 50 W turbogenerator design space is narrowly constrained by the minimum blade span and the 5-bar pressure supply, demanding MEMS fabrication for the short turbine blades. A single-stage MEMS radial turbine is designed in detail, and assessed for performance and manufacturability. The rotor speed at design conditions is 450,000 rpm with a rotor radius of 5 mm, a rotor blade span of 200 um, and a blade tip clearance of 20 um.

(cont.) Based on 3D RANS simulations, the turbine is predicted to achieve 48% total-to-static adiabatic efficiency and to produce 77 W of shaft power at a turbine mass flow of 1.45 g/s. Assuming a generator efficiency of 80% and a power electronics efficiency of 90% yields a net electrical turbogenerator power output of 50 W. End-wall losses are dominant in the planar turbine and a diffuser is included to reduce the pressure losses in the right exit turn. The final high-speed, low flow design integrates a MEMS turbine and a meso-scale permanent-magnet synchronous generator combined with hybrid ball bearings.