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Research Papers: Gas Turbines: Microturbines and Small Turbomachinery

# Demonstration of a Palm-Sized 30 W Air-to-Power Turbine Generator

[+] Author and Article Information
S. Sato, S. Jovanovic, J. Lang, Z. Spakovszky

Gas Turbine Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139

Power electronics were not part of the demonstration presented here.

The cost efficiency is defined as the cost of electrical power over the cost of pressurized air.

Given the short time frame of the project, the decision was made to use the ball bearings integral to the generator package in the concept demonstration. Future work includes the design of air journal and thrust bearings that are superior in performance and most suitable here since a pressurized air source is readily available. Note that an impulse turbine would not lead to axial rotor loads but has inferior performance compared with a reaction turbine.

As mentioned earlier, a thrust bearing could not be implemented without major changes to the generator assembly, introducing challenges in rotor axial alignment and stable operation.

Due to the small scale of the turbomachinery, particle contamination is a concern in real working environments and the use of an inlet filter is critical.

The O-ring recess in the casing was designed for thinner shims than was used in the experiments due to bearing compliance issues. This led to reduced O-ring compression and flow leakage at high supply pressure settings.

For shim thicknesses of $200 μm$ and $250 μm$, the rotor blades touched the casing at pressure ratios higher than 3 and 3.25, respectively. Excessive leakage was observed for a shim thickness of $325 μm$, which is also indicated by an overall pressure ratio greater than 6.0 at the highest loading condition.

J. Eng. Gas Turbines Power 133(10), 102301 (May 02, 2011) (10 pages) doi:10.1115/1.4002826 History: Received May 10, 2010; Revised July 16, 2010; Published May 02, 2011; Online May 02, 2011

## Abstract

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 from a plant pneumatic system or from 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 5–6 bars. 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 are not commercially available, a silicon-based micro electro mechanical systems (MEMS) turbine was designed using 2D and 3D computational fluid dynamics (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 475,000 rpm 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 turbo-generator of hybrid architecture where a MEMS turbine disk is joined with a precision machined titanium shaft and aluminum housing.

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Copyright © 2011 by American Society of Mechanical Engineers
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## Figures

Figure 1

Conceptual layout of A2P system (left) and thermodynamic model (right)

Figure 2

50 W turbo-generator design space with generator types and turbine blade manufacturing constraints

Figure 3

3D CFD rotor O-mesh (not all cells are shown)

Figure 4

High-pressure ratio, low-flow turbine design

Figure 5

3D CFD based turbine losses for design (top) and enlarged (bottom) blade tip clearances

Figure 6

Silicon turbine (left) and exploded/assembled view (right) of 35 mm diameter 50 W A2P turbo-generator

Figure 7

Axial alignment of rotor and stator assembly

Figure 8

Silicon turbine disk and rotor adapter

Figure 9

Rotor structural integrity test at 475,000 rpm

Figure 10

Experimental measurement of rotor critical speeds during slow acceleration transients

Figure 11

Computed modeshapes at critical speeds

Figure 12

A2P test setup and instrumentation

Figure 13

Measured generator power versus rotor speed for two different sets of load resistor banks

Figure 14

Turbine pressure ratio versus corrected mass flow for various rotor tip clearances (adjusted with shims)

Figure 15

Measured turbine pressure ratio versus physical mass flow and predicted design point (CFD)

Figure 16

Measured turbine adiabatic efficiency as a function of rotor speed and CFD predictions at design conditions for two blade tip clearances

Figure 17

A2P cost efficiency as a function of rotor speed for two different generator loadings

## Errata

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