Standard Thermal: Energy Storage 500x Cheaper Than Batteries

Original Article ↗ Hacker News Discussion ↗

Concept and Intended Use

  • System: PV powers resistive heaters embedded in a large dirt/sand mound, heated to ~600 °C; pipes extract heat months later for space/process heat or steam.
  • Primary target: slow, seasonal storage (summer → winter) and steady industrial/district heat, not fast daily cycling or full electricity replacement.

Thermal Physics: Dirt as Storage Medium

  • Multiple comments note dirt/soil has modest R‑value per inch but becomes a strong insulator when used in large thicknesses (e.g., ~10 ft → R‑24–96).
  • Heat diffusion in large masses is very slow; seasonal ground temperature behavior is cited as an analogy.
  • Key design lever is scale: thermal time constant grows with the square of system size, so large piles can retain heat for months.
  • Some push back that dirt isn’t “very insulative” per unit thickness; viability depends on making the pile big enough and dry.

Round-Trip Efficiency and Economics

  • Article figure of 40–45% is clarified as electricity→heat→electricity, not end‑to‑end from PV; some commenters think 30% is more realistic.
  • Consensus: for seasonal storage, low capex can outweigh poor efficiency because there are few cycles per year and input energy can be very cheap or curtailed.
  • Comparisons are drawn to batteries: far higher efficiency but ~100–500× more expensive per kWh of capacity and unsuitable for seasonal storage.
  • Others compare to power‑to‑gas/methanol and hydrogen storage, arguing those may have similar efficiency but higher capex and more complex fuel cycles.

Use Cases and Scale

  • Strongest use cases discussed:
    • Industrial low/medium‑temperature process heat (~200–600 °C).
    • District or community heating with constant winter demand.
    • Repowering existing coal plants using stored heat instead of coal (reusing turbines and grid tie).
  • Poor fit for:
    • Individual homes lacking hydronic/district heating.
    • Fast daily arbitrage where batteries already pencil out economically.

Comparisons to Alternatives

  • Ground‑source heat pumps: excellent for heating/cooling, but can’t reach steam temperatures and involve substantial drilling costs; also fundamentally use the ground as a reservoir, not as high‑temperature storage.
  • Solar thermal / heliostats:
    • Direct solar‑to‑heat avoids PV conversion loss, but high‑temperature solar needs tracking concentrators, high radiative losses, and hot‑fluid transport.
    • PV + resistive heating is simpler, modular, and works with diffuse light.
  • Mechanical/gravity storage (blocks, pumped water): widely derided as having terrible energy density and poor economics vs batteries and thermal storage.

Engineering and Maintenance Concerns

  • Major open worry: how to inspect and repair embedded heat‑exchange pipes surrounded by 600 °C dirt; leaky tubes and fouling are known boiler issues.
  • Some expect systems to be designed for long life with minimal intervention, accepting gradual degradation or building new modules rather than repairing hot cores.
  • Corrosion and oxidation of resistive elements at high temperature are flagged as serious design challenges; material choice and atmosphere control matter.

Environmental and Practical Questions

  • Potential ground/wildlife impacts of large, hot mounds are raised; suggestions include placing them under parking lots or other already disturbed land.
  • Risk of unwanted snow melt/icing if heat leaks upward; ideally, well‑designed piles should have negligible surface temperature change.
  • Several commenters note similar concepts (sand batteries, seasonal borehole storage, PAHS, historic ice houses) already exist, reinforcing feasibility but also raising “if it’s so good, why isn’t it everywhere?” skepticism.

Skepticism and Open Questions

  • Calls for full thermodynamic and economic models: capacity, leakage, extraction rate, and full system capex (including turbines) are not rigorously quantified.
  • Some doubt the “500× cheaper than batteries” claim without detailed cost breakdown and field data.
  • Unclear at what minimum scale the approach remains viable and how close loads must be to storage to keep transport losses acceptable.