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Session: Prototypes II
Session starts: Tuesday 08 October, 09:00
Presentation starts: 09:40
Room: Van Weelde Zaal

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Antonio Rubino (Ecole Polytechnique Fédérale de Lausanne (EPFL))
Jürg Schiffmann (Ecole Polytechnique Fédérale de Lausanne (EPFL))

Abstract:
In the last decade, distributed power generation has gained renewed interest. Two main driving forces are contributing towards this tendency: deregulation of electricity markets and environmental concerns. For selected applications, distributed generation has been proven to be more cost-efictive than centralized generation, showing a high potential in terms of reliability and efficiency. Small-scale high-speed turbomachinery, combined with oil-free bearing technology, has been identified as a key enabler for enhancing the performance of distributed energy conversion systems. Typical applications range from gas turbine generators, compressors for domestic heat pumps, expanders for waste heat recovery using Organic Rankine Cycles (ORC). Prior work at EPFL has proven the feasibility of an electrically driven turbocompressor (1.8 kWel) supported on dynamic gas lubricated bearings. Pressure ratios in excess of 3.3 and isentropic eefficiencies up to 80% were achieved with a 20mm tip diameter impeller rotating at 210krpm while processing R134a. Key challenges in the design of small scale turbomachinery have been identified as being : 1) a thorough understanding of gas bearing supported rotor behaviour and 2) new integrated design and optimization approaches. An investigation has shown that a holistic design method allows to yield signicant performance improvement as compared to more traditional design approaches. The optimum design was found by linking a previously validated reduced order system model to a multi-objective optimizer based on evolutionary algorithms. The integrated design approach will be adopted to further improve the performance of an oil-free heat pump, thermally driven by an ORC. A first proof-of-concept prototype has been tested successfully in our laboratory. An exergy analysis indicated that special focus is required on the mechanical and aerodynamic design of the oil-free compressor-turbine unit, made up of the heat pump single stage centrifugal compressor directly connected by a shaft to the ORC radial turbine. Both compressor and turbine are characterized by having tip diameters of about 20mm and rotational speeds in excess of 200 krpm.