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|a A compact, high power-density turbo-generator system was conceived, designed and experimentally tested. The air-to-power (A2P) device with a nominal design point of 50 W electric power output operates on high pressure air such as for example from a plant pneumatic system or a portable bottle of pressurized air. A concept design study was first carried out to explore the design space for a range of output power at cost efficiency levels specified in collaboration with industry. The cost efficiency is defined as the cost of electrical power over the cost of pressurized air. The key challenge in the design is the relatively low power demand of 50 W while operating at high supply pressures of nominally 5 to 6 bar. To meet the cost efficiency goal under these conditions, a high-speed turbine and generator (∼450,000 rpm) are required with small blade span (∼200 μm) minimizing the mass flow while achieving the highest possible turbine performance. Since turbines with such small turbomachinery blading aren't commercially available, a silicon-based MEMS turbine was designed using 2-D and 3-D CFD computations. To reduce the development time, existing and previously demonstrated custom-made generator and ceramic ball-bearing technology were used, resulting in a compact A2P proof-of-concept demonstration. The cylindrical device of 35 mm diameter resembles a tube fitting with a standard M24 adapter. Without load, a top turbine speed of 475,000rpm was demonstrated exceeding the design specification. Using load resistors, the proof-of-concept A2P device achieved 30 W of electrical power at 360,000 rpm and a turbine efficiency of 47%, meeting the cost efficiency goal. Higher speeds under load could not be achieved due to thrust load limitations of the off-shelf ball bearings. The demonstrated performance is in good agreement with the projected CFD based predictions. To the authors' knowledge, this is the first successful demonstration of a self-contained, 50 W-class turbogenerator of hybrid architecture where a MEMS turbine disk is joined with a precision machined titanium shaft and aluminum housing.
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