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MME’02, The 13 Micromechanics Europe Workshop, October 6-8, 2002, Sinaia, Romania
A MICROTURBINE FOR ELECTRIC POWER GENERATION
Jan Peirs, Dominiek Reynaerts, Filip Verplaetsen, Michael Poesen, Pieterjan Renier
Katholieke Universiteit Leuven, Department of Mechanical Engineering, Celestijnenlaan 300B, 3001 Leuven, Belgium
A single-stage axial microturbine has been developed with a rotor diameter of 10 mm. This turbine is a first step in the development of a microgenerator that produces electrical energy from fuel. The turbine is made of stainless steel using die-sinking electro-discharge machining. It has been tested to speeds up to 160,000 rpm and generates a maximum mechanical power of 28 W with an efficiency of 18.4 %. When coupled to a small generator, it generates 16 W of electrical power, which corresponds to an efficiency for the total system of 10.5 %.
Keywords: Microturbine, microgenerator, powerMEMS, EDM
Most portable devices use batteries for their power supply. Current Li-ion batteries have energy densities up to 0.5 MJ/kg but still offer limited autonomy to for instance laptops and cellular phones. Charging times also pose problems. Fuel on the other hand offers a much higher energy density of about 45 MJ/kg, and the reservoir can easily be refilled. Therefore, several groups are working on the development of micro power generators based on fuel cells, thermo-electric devices , Stirling engines, Wankel motors , and gas turbines .
Specific about the microturbine presented in this paper is that it is an axial turbine produced with electro-discharge machining (EDM).
II. TURBINE DESIGN
In a first phase of the project, the problem has been scaled down to a turbine powered by compressed air. Compressor, combustion chamber, and generator have been left out and will be addressed in a later phase. The microturbine is a single-stage axial impulse turbine (Laval turbine). Expansion of the gas takes place in the stationary nozzles and not between the rotor blades. This type of turbine has been chosen because of its simple construction.
Figure 1 shows an exploded view and an assembly of the microturbine design. The
compressed air enters via a standard pneumatic connector (1) and expands over the stationary nozzles (3) where it is deflected in a direction tangential to the turbine rotor (5). After the air has passed the rotor blades, it leaves the device through the openings in the outlet disc (6). Screwing the pneumatic connector in the housing (8) presses the stationary nozzle disc against a shoulder in the housing. The rotor blades, wheel and axis are one monolithic part. The rotor is supported by two ball bearings (4), one mounted in the stationary nozzle disc and one mounted in the outlet disc. The outlet disc is locked in the housing by a circlip (7).
The diameter of the turbine rotor is 10 mm. The housing has a diameter of 15 mm and is 25 mm long. All parts, except the pneumatic connector and the circlip, are made of stainless steel.
The nozzles are designed for subsonic flow and therefore have a converging cross-section. Sonic speed is reached for a relative supply pressure of 1 bar. The exit losses (remaining kinetic energy in the exhaust) are minimal when the turbine is designed for a u/c1 ratio of 0.5, with u the circumferential speed and c1 the absolute speed at the nozzle exit. At 1 bar, c1 reaches sonic speed resulting in an optimal turbine speed of 420,000 rpm. As this is too high for the bearings, the turbine has been designed for a u/c1 ratio of 0.25, and is operated below its optimal speed of 210,000 rpm.
Fig. 1. Microturbine design.
Image | A MICROTURBINE FOR ELECTRIC POWER GENERATION
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