Designing a Small 5W Generator from Compressed Air

[Mike_In_Plano]

I'm looking to drive a small, 5 watt generator from compressed air (psig>60) and while being fairly frugal with the air (<.25 ft^3/min).
I hope to have the base and spindle cut using a common three axis mill.

Hears where I need help - I don't know how the control surfaces should be shaped or how this thing operates.

Any advice is welcome,

Thanks
Mike

 

[Phrak]

That's very small. The power available would depend on the pressure as well as the cubic feet per minute. Have you looked up various types of pumps? In your case, you would be operating a pump in reverse; deriving power by moving air.

 

[Mike_In_Plano]

Hello, Phrak,

Yes, 5 watts is a tiny output, but that, longevity, and the ability to tolerate "dirty" air lines is part of the requirements. And wouldn't you guess, it needs to be fairly inexpensive to fabricate. (i.e. machined, mounted, and balanced for ~$300 ea)

I have looked at a number of designs.

One which is truly enticing utilizes a series of coaxial rings with alternate rings serving as stators (pinned to the housing) and rotors (engaging the shaft). Each stator ring vectors the air flow into the next set of rotors, while each rotor, captures the momentum of the stream and redirects it to the next set of stators. The problem is that this design requires 4-axis machining, and from my quotes, I can't get it done for $250. Besides, it's a lot more than I need for this application.

If I can get 30% efficiency from a radial design and mill the housing and rotor from "hocky pucks," I'd be delighted. I'm almost frustrated enough to start paying the model shops to attempt random designs in hopes that something will come close..
.
- Mike

 

[Q_Goest]

Hi Mike. For 60 psi, 0.25 SCFM air flow, isentropic efficiency is roughly 32%. From what I've seen, air motors peak at around 40%, so that may work out for you. These are generally vane type motors that don't need lubrication. Look for miniture ones. http://www.terhorst.com/PDF%20Website/P1V-S.pdf" .

 

[Mike_In_Plano]

Thank you Q_Goest for the leads. By the way, I like the Bill the Cat.

I found some interesting motors at the Parker site. Unfortunately, they're supposed to be serviced after 500 hours. My customers would expect something like 20,000 hours between servicing. Perhaps my expectations for this thing are out of line...


-Mike

 

[Q_Goest]

Hi Mike. I wonder why Parker suggests 500 hours. Is it the bearings? Vanes? Try giving them a call and ask about it. Bearings can be made to last 20,000 hours or even longer. The vanes should be able to do the service too - there are carbon filled Teflons that will do the job. Wear on the ID where the vanes are moving is also an issue, but there are a number of very hard coatings that can make it work. Maybe they're concerned about contamination, but you can always filter the incoming air. In short, whatever the concern is, there's probably a way to improve it.

20,000 hours is common for larger, industrial duty machines. Turbine expanders for example. But the service you're looking at is a bit too small for a turbine expander. The wheel would be insanely small and fast. Until a better technology comes along, the only way I know of to do what you want is to use a positive displacement expander such as the air motor.

 

[Mike_In_Plano]

Q_Goest,

They were concerned about the wear of the vanes. I think your right; I should call. 500 hours is an insanely short time between servicings.

As for the rotor speed, I don't mind 20,000 RPM, maybe even a tad more. As long as I can work the coupling out, there are readily available brushless DC motors that are designed for these speeds, and I can use one as the generator.

One of the attractions of the radial turbine was that the motors already have bearings that
were designed for considerable loading and coupling isn't an issue. Even the issue of runaway isn't an issue as I can easily limit the upper speed through loading of the generator.