Air compressor that doubles as a pneumatic motor?

Question

I am working on a compressed air energy storage system. The size and weight of the system are heavily constrained; nothing should (ideally) exceed a few pounds. For this reason, I would like to store the energy (compress the gas) and extract the energy (make the gas do work) with the same mechanism in a rotary fashion.

Essentially what I need is a rotary air compressor that, when air is forced through in the opposite direction, doubles as a pneumatic motor. I'm working with fairly high pressures (I'm estimating a few hundred psi) but low volume. In my search, I have found a plethora of compact rotary air compressors and rotary pneumatic motors, but there is hardly comment on what systems would work as both.

To me, it seems very intuitive that an air compressor could have these properties, but I don't want to jump to any conclusions. I have looked at several compressors, and the most applicable to my situation seem to be:

• Centrifugal compressor
• Axial flow compressor
• Rotary screw compressor
• Rotary vane compressor

The centrifugal compressor is ideal, but it seems the least likely in my eyes to be reversible, at least with any efficiency. I also looked at pneumatic motors, of which there were fewer available. Most applicable seemed to be the:

• Rotary vane motor

Other systems, such as the pietro motor, were obviously not applicable in my light weight, compact application. The correlation between the rotary vane compressor and the rotary vane motor is promising, but I would like to know about any options I have.

What rotary gas compression systems can double as motors powered by the gas they compress?

EDIT The answer most likely lies in the similarity between a radial (centripetal) turbine and a centrifugal compressor.

Answer 1

I would recommend a forward inclined centrifugal system, such as a forward curved fan.

The power input/output of any device, where fluid comes in/leaves with fluid rate $Q$ at $V$ and enters/exits at an angle $\theta$ at velocity $U$, would be:

$$\mathcal P = (V-U)(1-\cos(\theta))\rho QU$$.

If you have this device compress the gas, the power input runs in reverse. In both cases the angle helps. See the velocity triangle.

The real heart of this will comes down to putting some good valves on the openings. $U$ is a double edged sword - while it ups your power, if $V$ isn't very high compared to $U$ nothing is really happening. Don't forget $Q$ depends on $V$ or $U$, depending on how you look at it. The key to modifying this is to throttle your inlet opening down (whichever way you run) to a very small opening to have the highest $V$ possible, while keeping the outlet carefully controlled to not constrict $Q$ or $U$ beyond what is necessary to keep $V/U$ decent.

Perhaps using this as a first stage in a two stage rotary compressor could also help - the second stage is a true rotary compressor to really boost the pressure, but this assists the second stage to increase pressure beyond atmospheric.

Ultimately no device on the market will be built to this strange service - but by having a fairly symmetric rotary system with carefully controlled inputs should yield some decent results. I would definitely consult with a custom fan manufacturer.

Answer 2

I'm not an expert on air compressors or motors, but from my limited knowledge I think as you say that the centrifugal compressor would be the best for compression and a Tesla Turbine would be ideal as the motor. I think it should be possible to mount them on the same shaft but in separate airtight chambers, with some valves such that when the turbine is in operation the air is pumped out of the compressor chamber, and vice versa, so as not to cause undue resistance from the other impeller. Alternatively a clutch/grip mechanism that selects which one turns with the shaft.

Such a device could be considered a both-ways compressor/motor. To try to do it both ways with an impeller optimised for one of those scenarios seems like you will always be inefficient in the other.