Hi Sharp,
Air driven motors (also known as expanders) use the energy of a compressed gas to do useful work. The power output is simply the inlet enthalpy minus the discharge enthalpy times your mass flow rate.
The inlet enthalpy is a known state which you can calculate knowing the inlet pressure (100 psig) and temperature (70 F). The discharge enthalpy isn't as easy to determine. The way this is done is to consider a perfect expander or air motor which is operating at 100% isentropic efficiency (ie: no
change in entropy of the gas as it expands) and then compare that to what it actually gets. That value is provided by the manufacturer as Russ eludes to. It will typically be well below 100%, and I'd go along with the rough values Russ has provided (30% - 70%) but the manufacturer will give that as an efficiency.
For a perfectly isentropic expansion (100% efficiency), I created a function of the difference in enthalpy between some inlet pressure and a discharge pressure of 14.7 psia (atmospheric pressure) using a properties database. That function is:
dh(theoretical) = 16.8 Ln (p) - 21.8
where dh(theoretical) = change in enthalpy (Btu/lbm)
p = inlet pressure (psig)
Assumptions: Inlet temperature is 70 F. Discharge pressure is 14.7 psia. Inlet pressure between 10 psig and 100 psig.
The power output is a function of your air motor efficiency.
P = dh(actual) * mdot
Where P = power output (Btu/second)
dh(actual) = actual enthalpy difference (Btu/lbm)
mdot = flow rate (lbm/second)
I'll leave it to you to change Btu/s to Watts.
To determine dh(actual) you need efficiency.
dh(actual) / dh(theoretical) = Eff(%)
You can generally obtain the isentropic efficiency from the motor manufacturer.
Once you know the output power from the air motor, you can determine how much of that power will be converted into electricity from the efficiency of the generator, and including any losses you have in transmitting that power from the motor to the generator.
Try and Google "air motor" to give you an idea of efficiency. They may also provide the power output on a chart instead of having to go through the above calculation.
What you'll find is that trying to use compressed air as a source of power is highly inefficient. This has to do with the loss of energy during compression more than anything. In general, using compressed air as a source of energy is only worthwhile if you aren't worried about wasting huge amounts of energy.