A company is building a giant compressed-air battery in the Australian outback

Original Article ↗ Hacker News Discussion ↗

Comparison to Pumped Hydro

  • Many ask why not just use conventional pumped hydro instead of air–water systems.
  • Advantages proposed for A-CAES:
    • All major machinery can stay at the surface, simplifying access and safety.
    • No need for large surface reservoirs or dams, which are slow to permit and can be environmentally contentious.
    • Works in flat or low-relief regions (e.g., outback, deserts) where suitable elevation differences for hydro are scarce.
  • Counterpoints:
    • Pumped hydro is mature, efficient (claimed 80–90% round-trip), and still has many viable sites in some countries.
    • Suitable sites with good geology and low environmental impact may be rarer than advocates suggest.

Physics & Thermodynamics

  • Key insight: The water column acts as a “piston” giving high pressure from depth; energy is stored as:
    • Gravitational potential of the lifted water, plus
    • Compressed air (“air spring”), plus
    • Stored heat from compression.
  • Debate on how much extra energy the compressed air adds vs pure water head; rough intuitions suggest around 2× water-only for the same head and displaced volume, but this is not rigorously settled in the thread.
  • Several comments stress that:
    • Traditional CAES is inefficient mainly because compression heat is discarded.
    • A-CAES stores that heat and feeds it back, with claimed ~70% round-trip efficiency (from commenters, not the article).
    • One view: compressed air is best seen as “stored negative entropy”; the real energy store is the hot/cold thermal reservoirs.

Engineering Details

  • Questions on how air avoids bubbling up the water shaft:
    • Answer: separate inlets at top (air) and bottom (water); water pipe remains submerged so air can’t escape through it.
  • Leak-tightness and long-term cavern integrity are raised but not answered in detail.
  • Some discuss how heat might be stored (e.g., packed beds, liquid thermal stores) but details for this project remain unclear.

Costs, Lifetimes, and Alternatives

  • Quoted capital costs (~$260–375/kWh) appear higher than turnkey lithium-ion systems cited from other sources.
  • Supporters note:
    • Very long projected lifetimes (~50 years), essentially unlimited cycles, and primarily local civil works.
    • Potential to benefit when solar/wind overbuild makes input energy very cheap, making efficiency less critical than cost per stored kWh.
  • Skeptics argue lithium-ion costs and deployment speed may undercut such large, slow-to-build projects.
  • Other storage ideas briefly mentioned: flywheels, rock/dirt gravity systems, pumped thermal storage.