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Ultra micro gas turbine generator

1. Introduction

The current trend towards miniaturization, portability and more in general ubiquitous intelligence, has led to the development of a wide range of new products such as laptops, cellular phones, PDAs, etc. However, the power requirements of such systems have received much less attention: typically, traditional battery-operated electronic systems are used. Nevertheless, the energy density of most fuel types is still 100 times more than that of the most performing batteries, which makes the use of a fuel-based micro power unit interesting. Such power units can be based on a wide range of operating principles, ranging from fuel cells and thermo-electric devices, to combustion engines and gas turbines. While fuel cells are expected to offer the highest efficiency, micro gas turbines are expected to offer the highest power density.

A first prototype of a turbine driven by compressed air shows that speed is the limiting factor for both power and efficiency. The next step, the development of a complete gas turbine, is many times more difficult, and is not simply the scaling down of larger gas turbines. Major problems are the high rotational speed (> 500,000 rpm) and temperature (> 1200 K), and the efficiency of the components.

2. Micro gas turbine

Gas turbines are amongst the most advanced systems as they combine extreme conditions in terms of rotational speed with elevated gas temperatures (up to 2100 K for military engines). Miniaturisation of such a system poses tremendous technical problems as it leads to extremely high rotational speeds (e.g. 10⁶ rpm). Moreover scaling down the system unfavourably influences the flow and combustion process. Fabricating such devices requires new materials to be explored (such as Si₃N₄ and SiC) and also requires three-dimensional micromanufacturing processes.

The micro gas turbine developed by the Belgian PowerMEMS project has a rotor diameter of 20 mm and will produce a power output of about 1000 W. The system basically consists of a compressor, regenerator, combustion chamber, turbine and electrical generator, as illustrated in figure 1.

Figure 1 shows the general layout of the microturbine generator. The system basically consists of a compressor, recuperator, combustion chamber, turbine and electrical generator. In total it has a diameter of around 100 mm and a length of 110 mm. The compressor and turbine impellers are 20 mm in diameter.

In order to accomodate the relatively large volume of both the combustion chamber and the recuperator in a compact way, an annular design was chosen for both components. As a consequence of the adopted layout, the hottest part - the combustion chamber - is enclosed by the recuperator on the outside and by the exhaust diffuser on the inside. This allows to recycle heat losses from the combustion chamber. An exhaust diffuser is added to create a sub-ambient pressure at the turbine exit, such that more power can be extracted.

To avoid demagnetisation of the magnets, the generator is located away from the hot parts and the inlet air is aspirated through cooling channels in the generator stator. Generator, compressor and turbine are mounted on a single shaft for simplicity and reliability.

3. Thermodynamic cycle

The thermodynamic cycle has been determined and optimised in an iterative way. Fixed values are the compressor diameter (20 mm), nominal shaft speed and max. turbine inlet temperature (TIT). The max. TIT is set by material limits to 1200 K. The nominal shaft speed was set to 500,000 rpm as models predicted that with the given compressor diameter, a pressure ratio of 3 is achievable. Below this value efficiency drops sharply, while higher values offer smaller efficiency improvements.

A detailed gas turbine model was built containing compressor and turbine maps, and models for the combustion chamber and recuperator. An iterative process was used to optimise the efficiency of the individual components as well as the global cycle. The following parameters were obtained:

Nominal mass flow: 20 g/s Pressure ratio: 3.0 Power Compressor: 3800 W Turbine: 5083 W Net mechanical output: 1180 W t-s polytropic efficiency Compressor: 66 % Turbine: 78 % Turbine inlet temperature: 1200 K Cycle efficiency Without recuperation: 11 % With recuperation: 20 %

Fig. 2: Mechanical power vs. speed and turbine inlet temperature (TIT) (without recuperator).

While the primary goal of the optimisation was the maximisation of the cycle efficiency, a major result was an enlargement of mass flow and power, this way reducing thermal and flow losses in a relative sense.

An off-design analysis has been performed to investigate stability, transient behaviour and start-up. Figure 2 shows the mechanical power as function of speed and TIT, indicating that a minimal TIT of 600 K is required for start-up.

4. Bearings

The bearings must operate throughout the whole domain of possible temperature conditions during startup and in steady state operation. Maximum temperatures between 100ºC and 1000ºC can be expected depending on the exact location of the bearings.

Rotor imbalance can result in dangerously high dynamic radial loading, and therefore, the eccentricity of the mass centre should be balanced within a few micrometer.

It is clear that conventional ball bearings are not feasible regarding speed and temperature. Magnetic bearings could offer a solution regarding speed, but the high temperature dissuades the use of permanent magnets as these could demagnetise. Consequently, such bearings should be constructed with electromagnets which consume a considerable amount of electrical energy.

Air bearings seem most suited for this application. Aerostatic as well as aerodynamic ones can be used. Aerodynamic bearings are preferred as they need no external supply of pressurised gas (e.g. tapped from the compressor) as they are self-pressurising. The main drawback is the short phase of dry friction during startup and stop.

The speed of air bearings is usually limited by self-excited instabilities (fractional-speed whirl). However, we succeeded in suppressing this instability and realised a record speed of 1 200 000 rpm on aerodynamic bearings.

5. Compressor and turbine

As stated before, the efficiency of all components is critical. This is especially true for the compressor and turbine, requiring efficiencies of at least 60-70%, values lying far above the numbers obtained for the former air driven turbine. Thus, it is clearly a challenge to obtain the required efficiency despite the low Reynolds numbers, increased heat transfer, and lower relative geometric accuracy of the components. The compressor and turbine impellers have been optimized by a multi-disciplinary method, optimizing simultaneously stress and aerodynamic performance. The resulting geometries are shown in figure 3.

6. Recuperator

Heat recuperation is often used to improve the overall cycle efficiency of standard gas turbines. In small-sized gas turbines this improvement is however much more questionable. Indeed, both achievable compressor pressure ratios and turbine inlet temperatures are significantly lower and pressure drops are much larger compared to conventionally sized gas turbines. The additional pressure drop introduced by the small channels in the recuperator should not undo the benefits of heat recuperation.

In conventionally sized recuperators, complex, well designed fin configurations are used in order to improve the gas-air heat transfer. In order to avoid these costly and difficult to machine fin configurations, alternative recuperator designs are needed for microscale applications.

The recuperator consists of 6 identical blocks positioned around the gas turbine. The design is determined by a multi-dimensional optimization in which cold and hot side recuperator pressure drops are used as optimization parameters [9]. The optimal design has a heat exchanger effectiveness of 74.5 % for relative pressure drops at cold and hot side of 8.5 kPa and 5.5 kPa respectively. The recuperator blocks consist of alternating hot and cold plates (52 in total), with longitudinal channels in counterflow (see fig. 6). Channels and collectors are etched with a uniform depth in stainless steel plates, 63 by 25 mm in size. Total stack height is 34 mm.

Fig. 6: Stacked recuperator block and individual hot and cold plates.

7. Generator

The generator operates at much higher speeds and temperatures than conventionally. For reasons of mechanical strength (centrifugal load), a switched reluctance machine is chosen for the generator. Permanent magnets and coils are placed on the stator to avoid damage resulting from high stresses. The temperature load is minimised as the generator is located at the lowest temperature side of the device (the compressor side) and by extra cooling with the inlet air.

The high speed results also in high operating frequencies which introduce skin effects in the electrical circuit and eddy currents in the magnetic circuit. To reduce the magnetic losses, both the generator’s rotor and stator core have a laminated structure from nanocrystalline foils. The generator also serves as a startup motor.

8. Fabrication

The rotor and bearing geometries are the most critical components for production. Bearing surfaces have to be produced and aligned with micrometer accuracy. Especially the bearing surfaces on the rotor are critical as this rotor consists of 4 assembled components. Compressor and turbine impellers have a complex 3D blade geometry due to their axial-radial design. Unigraphics NX 3.0 CAD/CAM software is used for modelling and tool path generation.

The titanium (Ti-6Al-4V) compressor is produced by 5-axis milling on a Kern micromilling machine, with tools down to 0.5 mm in diameter. The blank including the precise bearing and mating surfaces is machined on a Hembrug lathe.

The ceramic turbine is produced by die-sinking electrical discharge machining (EDM). An electrically conductive ceramic composite is chosen with good mechanical, thermal and machining properties: Si3N4-TiN. The graphite EDM electrodes are machined by 3-axis micromilling. The roughness after EDM is 2.3 µm Ra, such that postprocessing by grinding or abrasive flow machining is required. For future large series production ceramic powder injection moulding is envisaged. The ceramic bearing surfaces on the turbine´s backside are finished with diamond grinding tools to micrometer tolerances and 0.10 µm Ra.

9. Testing

To test the performance of the most critical components without the overhead of the complete system, a special set-up is built, containing only compressor, turbine, diffuser and bearings: a so-called turbo-shaft set-up.