FRANÇAIS
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Fundamental and Applied Energetics
ONERA and micro energy sources : the micro gas turbine engine
Introduction
Thermal micro machines, micro scale engines: a particularly difficult field. Which concepts are potentially viable? A typical research-type theme.
For 15 years, a great research effort has been dedicated to thermal micro machines, at first in the US, then in Japan, Canada, China, Singapore, Corea and Europe (France, England, Switzerland, Italy, Belgium…). These micro scale thermal machines, which represent a real challenge, have a very wide range of power and various applications.
Power range and examples of application for the micro sources of energy
MEMS sources weight only a few tenth of gram, their power is within 1 µWatt and 0.1 Watt, and they supply “distributed systems” such as micro actuators, medical implants or micro sensors called “smart dust”…
Miniature sources weight between 1 gram and a few tens of grams and must be able to produce a power within 1 Watt and 100 Watts, during an hour. The applications of theses very small engines are various: supply of micro drones, supply of nomadic battery chargers, supply of portable systems such as laptops, supply of robots…
Columbia University, the universities of Berkeley, Seoul, Tokyo, Sendai, Sherbrooke and industrial companies such as IHI or Honda develop different types of micro machines: micro gas turbine engine, micro Wankel engine, micro piston engine, thermoelectric effect machine, thermo-photovoltaic effect machine, thermo-acoustic effect machine, etc…
ONERA has dedicated its efforts to one main application: energy supply of micro drones, by developing the micro gas turbine engine. Indeed, this system has very high specific power and specific energy, which means that the ratios power/mass and energy/mass are high. This feature is actually crucial for micro energy sources, because the latter are to be implanted in some nomadic systems.
Hence, the features of the ONERA micro gas turbine engine are as follows:
- electric output power: 50 to 100 Watt;
- dimensions: 2-3 centimetres in each direction;
- opertaing time: 30 minutes to one hour.
These features also fit to the high power applications of micro energy sources, such as: supply of nomadic battery chargers, supply of robots, etc...
The micro gas turbine engine entails many challenges:
- To revise thermodynamic cycles, while trying to maintain the thermal gradients between hot parts and cold parts;
- To limit the thermal leakages, which are negligible at common scale and become predominant at micro scale;
- To develop the source of thermal energy: to manage micro combustion, which involves specific difficulties linked to micro scale;
- Select a fuel, which is easy to stock and favourable to combustion;
- To conceive small scale compressor impeller and turbine impeller (about 1 cm in diameter), while keeping acceptable performances;
- To develop a guiding system, which is compatible with very high rotation speeds, involved by the downscaling of the compressor and turbine impellers (in the range of 900 000 rpm);
- To select materials, which are adapted to the specific constraints of the system: isolating materials, materials with high mechanic characteristics, etc...
- To select and develop, if necessary, fabrication methods, which makes it possible to manufacture small parts with very high machining precisions (in the range of the micrometer for some parts);
- To develop a micro electric engine/generator, with a high efficiency, which is compatible with very high rotating speeds and high temperatures.
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