Compressed Air Energy Storage
Hint: While inefficient, compressed air is a cheap and accessible energy storage medium, which has one of the highest ratios of any energy storage medium for Energy Stored on Energy Invested - along with Gravity Storage
- 3kW hr of energy storage at 8 bar requires 65 cubic meters of volume - . Low Tech Magazine on Compressed Air Storage.
- This is 510 cubic meters STP (18,000 cu ft) . But air is free.
- To make this manageable, do 1kW - or 170 cu m or 6000 cu ft
- 1 cubic meter at 200 ATM does it (35 cu ft). XXH pipe does it here for 3000 PSI.
- 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 - 
- 80 cu ft tanks cost $200. That is 2 cubic meters. They are rated for 3000 psi. Standard is 200 bar 
- 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 .
- 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-  - weight of 43 lb/foot. 1/2" wall.
- Or Schedule 40 8" pipe - 1000 psi - 30 lbs/ft  - $600 in cost per 20 foot stick.
- Isothermal compression - LiGE system - This ability to ramp up compressed air pressures also allows for the energy density of compressed-air energy storage to exceed those of typical stationary batteries. Compressed air stored at 250 bar has a potential energy density of approximately 0,16 MJ/l whereas stationary batteries offer approximately 0,006 MJ/l. - hydro-pneumatic compressor . Website - 
Cost per 1kWhr of storage compared to $300/kW/2 years. 18650 Battery Life.
- $159 for 10 feet of 4" Sch 40 pipe - . Pipe Chart shows 660 PSI working pressure, 5000 PSI burst. Volume - 0.9 cubic foot.
- $207 for 10 feet of 6" sch 40 - . 4000/530 PSI. 2 cubic feet vol. 20% better than the last.
Shop Gas Containers
- $200 each - 200 bar - 250 cu ft (7 cu m) - . Would need 24 of these to get 6000 cu ft. That is $2400 - begins to make sense.
- 3000 PSI rating -
Class 1 or 2 Compressed Gas Tanks for Cars
- 6" PVC - schedule 40 - $40 for 10 feet. PVC Pipe. 180 PSI working. Volume - 2 cu ft.
- 1" polyethylene rolls - 200 PSI -
- 12" - $12/ft - . 8 Cubic feet per 10ft section. Compare to 2300 cu ft - needs more than 200 of these pipes at $120 each - $24,000 in PVC off the shelf. Not practical.
1000 gallon propane tank
- 133 cu ft - $2500. . Say 10 atmospheres. 1330 cu ft STP gas. This makes it possible to store 1/4 kWhr. $10k overall cost.
- 12" pipe at 10 atm - weighs 11 lb/ft. 110 lb/10'. 5 day prints with Supervolcano nozzle.
- For 6000 cu ft, 10 atm would mean 600 cu ft. 900' long! 90 sections of 10' pipe. 9900 lb of pipe.
- This is where composite fiber tanks begin to make sense. 450 supervolcano printer days - or one month with a 15 printer cluster. Not a small job.
- Maybe smaller diameter with higher pressure may optimize it. But still not likely for more than 1000 PSI?
- 70% efficient liquid piston compressor - 
- Off-the-shefl PVC is too expensive. It begins to make sense if 100 year life is considered (20x better) - so $24,000 cost turns to $1200 equivalent cost of 5-year batteries). 9900 lb of pipe. That is a lot. But it's still attractive if the storage container lasts 100 years. And - it's about 50 years of per capita plasic a person consumes, so recycling to your own energy storage is a good idea.
- If we print with waste plastic - say for 10 cents/lb - print costs $1000 from waste. Doable. (Would layers burst/leak at pressure? Perhaps a Precious plastic extruder for making pipe? Again needs research to be found and/or in house testing)
- If we are at 5% efficiency, the only feasible way is 10x the efficiency to 50% full cycle - and need 90' of 12" PVC pipe. That is 45 days of printing with 1 Supervolcano, and 5 days with a cluster of 9 printers.
- If we are talking about compressed air storage volumes being prohibitive, then maybe shift to 140 PSI storage of hydrogen.
- Gas Storage in PVC Pipe is an attractive option ($500/kWhr for 33 atm hydrogen)
- 50% efficient compressed air storage at 8 atm is an option
- Chemical fuel - plants to charcoal - is a good option - via CHP where charcoal is made from pellets used for space heating.
- Synthesis of liquid fuels from wood is a good option - biomass is abundant.
- Gravity water storage is a good option if low cost means of earthworks exist.
- Volume Calculator
- Baseline for air storage: 20 cubic meters (700 cu ft) per 1kWhr storage.
- Actual experimental data shown in Ref  of Low Tech Magazine on Compressed Air Storage - 18 cu m at 8 ATM for 0.4 kWhr.
- Apparently a $1000 vane motor - 100W was used - (https://www.radwell.com/Shop?source=GoogleShopping&IgnoreRedirect=true&ItemSingleId=140895809&utm_source=google&utm_medium=cpc&adpos=&scid=scplp140895809&sc_intid=140895809&gclid=Cj0KCQjwjo2JBhCRARIsAFG667VPmN-YShE5bg0jEaWQ8jNOTPF73ETdFcs3WAstYCXlTuETmII1RKUaAqm2EALw_wcB)
- Tesla warranties cars for 8 years or 150k miles, with 70% of battery life left. . That is pretty amazing.
- Cubic meter = 35 cubic feet
- "load of 29.65W for 12 hours requires a tank size of 18 m3, with an initial pressure of 8 bar and regulator setting 3.511 bar." - . Ie, 0.4 kWhr requires a huge tank at 8 bar.
- Above with 200 bar would indicate a 18/25 cu meter (0.7 cu m) tank.