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Publication Title | Mesoscale Electric Power Generation From Pressurized Gas Flow

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Mesoscale Electric Power Generation From Pressurized Gas Flow

D. Krähenbühl1, C. Zwyssig1, H. Weser2 and J. W. Kolar1

1Power Electronic Systems Laboratory, ETH Zurich, Switzerland 2High Speed Turbomaschinen GmbH, Wolfsburg, Germany

Abstract: New ultra compact energy generation systems require increased speeds for higher power densities of the electrical machine and the turbomachinery. This paper presents a miniature compressed-air-to-electric-power system, based on a radial turbine with a rated rotational speed of 490 000 rpm and a rated electric power output of 150 W. A comprehensive description including turbine and diffuser, permanent magnet (PM) generator and power and control electronics is given and steps of future research are described.

Key Words: radial turbine, turbomachinery, ultra high speed 1. INTRODUCTION

In pressure reduction devices, such as valves, conventional throttles or turbo expanders, the excess process energy is usually wasted as heat. However this energy could be recovered by a system that removes the energy from pressurized gas flow and converts it into electrical energy. One example is the replacing of the throttle in automotive applications. There, a turbine with generator can replace the function of a conventional throttle and thereby produce a maximum electrical power of 1.5 kW [1]. In gas pipelines, energy can be recovered at pressure reduction stations if throttling valves are replaced by expanders driving electrical generators [2]. The power levels can be up to 1 MW, however, the pressure reduction process is usually done in several stages. Also, the turbo expanders nowadays used in cryogenics plants transfer the excess power (in the kW range) to a brake compressor where the energy is finally dissipated into cooling water. If the turbo expander was braked by a generator, the energy would be recovered, and therefore the efficiency of such plants could be increased.

Several of these applications, such as in automobiles, need ultra compact power generation systems. Power density in both turbomachinery and electrical machines increases with increasing rotational speed [3]. Therefore, these systems can have a speed between 100 000 rpm and 1 Mrpm at power levels of up to several kilowatts.

Besides higher power applications micro-turbines with less then 100 W power output and very high speeds have been reported in the literature. In [4], a modular system consisting of an air-turbine, a permanent- magnet (PM) generator and the power electronic controller has been designed, with a maximal power output of 1.1 W and a maximal speed of 200 000 rpm.

Fig. 1.

Solid model of the turbo compressor system. Dimensions:33 x 43mm.

Fig. 2.

PM Generator with stator guide vanes and the radial-flow turbine (d = 10.5 mm).

The off-the-shelf air turbine is taken from a dental drill. Drawbacks of this system are the poor power density (0.02 W/cm3) and the large inlet flow rate of 45 l/min at maximum power output. In [5], a micro gas turbine coupled to a commercial electric machine with a maximal electric power output of 16 W at 160 000 rpm has been introduced for later use in a gas turbine system. This system achieves a power density of 1.6 W/cm3, excluding power and control electronics.

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