To reduce the size and weight of power generation machines for portable devices, several systems to replace the currently used heavy batteries are being investigated worldwide. As micro gas turbines are expected to offer the highest power density, several research groups launched programs to develop ultra micro gas turbines: IHI firm (Japan), PowerMEMS Consortium (Belgium).

At Onera, a research program called DecaWatt is under development in order to realize a demonstrator of a micro gas turbine engine in the 50 to 100 Watts electrical power range. A single-stage gas turbine is currently being studied. First of all, a calculation of the overall efficiency of the micro gas turbine engine has been carried out according to the pressure ratio, the turbine inlet temperature and the compressor and turbine efficiencies. With realistic hypotheses, we could obtain an overall efficiency of about 5% to 10% which leads to around 200 W/kg when taking into account the mass of the micro gas turbine engine, its electronics, fuel and packaging. Moreover, the specific energy could be in the range 300 to 600 Wh/kg which exceeds largely the performance of secondary batteries.

To develop such a micro gas turbine engine, experimental and computational work focused on:

• a 10 mm in diameter centrifugal compressor, with the objective to obtain a pressure ratio of about 2.5

• a radial inflow turbine

• journal and thrust gas bearings (lobe bearings and spiral grooves) and their manufacturing

• a small combustor working with hydrogen or hydrocarbon gaseous fuel (propane)

• a high rotation speed micro-generator

• the choice of materials

Components of this tiny engine were tested prior to the test with all the parts assembled together. Tests of the generator at 700,000 rpm showed a very good efficiency of this component. In the same way, compressor testing has been performed up to 500,000 rpm and has shown that the nominal compression rate at the 840,000 rpm nominal speed should be nearly reached.

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