Compressed Air Energy Storage

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HintLightbulb.png 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 TLDR; air storage is 5x cheaper than any other due to cycle life.

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Initial Notes

  • Bottom line - considering lifetime design - current air storage energy costs are lower than any battery technology. If we go mass thermal + PV, then our system can handle all loads with a 12kW PV system even in winter, provided simply ample thermal storage. Therefore, it is now possible to kill all energy issues, since 2016, when wind and solar attained grid parity with coal.
  • 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 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 - [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.

Industry Standards

  • 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 [7]. Website - [8]

Storage Vessels

Cost per 1kWhr of storage compared to $300/kW/2 years. 18650 Battery Life.

Steel Pipe

  • $159 for 10 feet of 4" Sch 40 pipe - [9]. Pipe Chart shows 660 PSI working pressure, 5000 PSI burst. Volume - 0.9 cubic foot.
  • $207 for 10 feet of 6" sch 40 - [10]. 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) - [11]. Would need 24 of these to get 6000 cu ft. That is $4800. Almost makes sense, definitely if lifetime design, which it is.
    • Next think of 700 bar or 10ksi - and we have 3.5 kW hr.

Scuba Tanks

  • 3000 PSI rating -

Class 1 or 2 Compressed Gas Tanks for Cars

Plastic Pipe

  • 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 - [12]. 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. [13]. Say 10 atmospheres. 1330 cu ft STP gas. This makes it possible to store 1/4 kWhr. $10k overall cost.

3D Printed

  • 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?

Compressors

  • 70% efficient liquid piston compressor - [14]

Conclusions

  • 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.

Overall Conclusions

  • 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.

Notes

Reference 7

  • "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." - [16]. 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.

Links

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