I have been working on it since I posted the question.

At the moment on the testbench is now an air powered generator delivering 12W 1W running :)

  It was really easy, actually:
I found a 12cm x 12 cm fan, 12V 5W, ball bearing, I selected this one because it felt like it had strong magnet (i turned different fans using the finger)
This was a pretty expensive one, Papst brand.
What the heck...  Really easy to open.  
And very easy to change bearings if ever needed.
Disconnected the driver electronics.
Reconnected the windngs in series, to full bridge schottky rectifier.
(tested with scope they were in phase)
Covered both sides of fan housing, air inlet tangentially, outlet center.
Nozzle 0,7mm diam and 8bar pressure give 12V 1W out at approx original fan rpm.

At bigger nozzle, double rpm, I get close to 4W output, but what about lifetime?
double rpm=4 times the radial force (mis-balance)
that and double speed make 1/8 original lifetime... I guess??
So double rpm is too high, but I can increase 50% or so to get 2W out :)


The fan has 70 000 hr L10 @40¡C when running as fan.  If I keep to fan mode rpm it will have *at least* that life.  

(L10 is the time when 10% of the units have died, contrary to mtbf which is the time where 50% have died)


So, I have reached the goal of both Service life, output power, cost, and size.

Remainding is noise level...  rubber suspension, encapsulate,...

And it consumes pretty lot of air, but the customer have not specified that... yet...

To have a turbine running efficient would requiore very high rpm = high cost, and still maybe not long enough life.

Other possibilities is using a long-life permanetn-lubricated cylinder, to drive a permanent magnet throught a coil.  But that, the big magnet, and the valves are all pretty expensive and make vibration, and I suspect it don«t have enough lifetime.

Membrane something oscillator to oscillate the magnet in r4esonance with a spring?

Air motor running very slow (for lifetime), driving a stepper motor as generator (they work at low rpm).  But how to lubricate, and how to make sure no oil is escaping in exhaust (customers customer requirement)

Piston motors; steam engine, hobby size:  lubrication, lifetime?

Ideas welcome.


  ** Some more project spec **
Air: dry, oil free, filtered, 7bar
Output: 12V min 1W
Size: max about half litre
Weight: not spec but say max 1kg
Lifetime: 10yr, 8hr/day, 5day/week, 46 week/year = 18400 hr.
NO SERVICE: no batt change, no oiling, no part switch...
No oil: not in input air, not in exhaust
Production need: about 200/yr
Air-electric conversion budget: max about $140US/unit



I know it sound strange, but not needing to electrically connect, and not needing to open the tool give valuable properties.

No, I«m not allowed to tell what it is.


Thank you for your input.

Regards
/Morgan