- This one says 70% efficient - really? 
- CFM of 20 gets 0.5 hp about. 1 cu m is 35 cf. 1 cu m at 190 atm is 7000 cf, at 20 cfm gives 350 minutes or almost 6 hours.
- Piston type mixer motor - 7 cfm, 0.2 hp - $250 - .
- One k-type cylinder gets me 35 minutes of this power. That's at least a start, but too short duration for practical use. Adiabatic.
- 14 cfm gets me the right power - 0.4 hp or 300W.
- 17 cfm - 0.33 hp - 250 W - 
- 1 cubic meter of air storage - at 200atm - gets you 7000 cfm at stp. Since we need to save 10 atm to run, have 190 atm available - so 5% negligible loss - most is usable air. This is 500 minutes of air - 8 hours - of 300W - or 2.4kW hrs! This is just plain adiabatic.
- One Type K tank gets 120Whrs.
- Thus, having access to 1 cu m of stored air - is key - for an inefficient storage system.
- Going to isothermal - we are 2.5x better
- Compressed air with heat would be the way to go - where we store the thermal energy of compression via a heat exchanger.
- 2" pipe - 10' - is $33. It is about 6l volume for 10'. 8 of these equal a Type K cylinder in volume. Same price.
- Metals depot - 12" schedule 40 pipe, 6' long, is $726. That is 1/2 cu meter. Getting there for storage - but working pressure is 370 psi and burst is 3000 psi. Needs reinforcement.
- How about 12" PVC - 3D printed, reinforced with fiberglass winding? That sounds like a possible idea for 2000 PSI air tanks on the cheap. $100 for pipe, then winding after that.
- Or - Type IV pressure vessels, all composite, from HDPE liner and wound with 500,000 psi+ tensile strength fiber (glass or carbon)