Initial Notes

3kW hr of energy storage at 8 bar requires 65 cubic meters of volume - [1]. Low Tech Magazine on Compressed Air Storage.
- This is 510 cubic meters. But air is free.
But - if you use high pressure, you can reduce that volume down to 300 bar simply using a $300, 1800W Scuba compressor. (can this handle methane or hydrogen? )
Scuba compressor on AliExpress - $300 - [2]
80 cu ft tanks cost $200. That is 2 cubic meters. They are rated for 3000 psi. Standard is 200 bar [3]
For DIY - take schedule 80 steel pipe.
510 cubic meters reduced by 200x is 2.5 cubic meters or 90 cu ft
Take 12" Schedule 160 pipe at 2700 psi rating [4].
A 20' stick of this pipe gets us 15 cubic feet. Need 6 of these pipes to provide 3 kWhr of energy. That pipe is over 100 lb/foot!
Would need 3kWhr PV array to generate the pumping power if we assume 16% overall storage efficiency. This storage efficiency is brute force - would need to look for more efficient expanders. But 6 of these are 12000 lb of steel!
Solution: (1) go to slightly lower pressures. (2) Possibly 3D print plastic pipe. (3) Use automotive high pressure tanks. (4) Use more efficient systems - improve from 16% officiency to 50% efficiency or up to 85% using methods discussed in Low Tech Magazine link above
Schedule 80, 8" pipe may be better- [5] - weight of 43 lb/foot. 1/2" wall.
Or Schedule 40 8" pipe - 1000 psi - 30 lbs/ft [6] - $600 in cost per 20 foot stick.

Storage Vessels

Cost per 1kWhr of storage compared to $300/kW/3 years.

$159 for 10 feet of 4" Sch 40 pipe - [7]. Pipe Chart shows 660 PSI working pressure, 5000 PSI burst. Volume - 0.9 cubic foot.
$207 for 10 feet of 6" sch 40 - [8]. 4000/530 PSI. 2 cubic feet vol. 20% better than the last.

6" PVC - schedule 40 - $40 for 10 feet. PVC Pipe. 180 PSI working. Volume - 2 cu ft.
1" polyethylene rolls - 200 PSI -

Tesla warranties cars for 8 years or 150k miles, with 70% of battery life left. [10]. That is pretty amazing.