250MWh 'Sand Battery' to start construction in Finland

Original Article ↗ Hacker News Discussion ↗

What powers the sand battery and what does it replace?

  • Battery will be “charged” mostly with surplus grid electricity, especially from wind (and some hydro/solar), when prices are low.
  • It feeds an existing district heating network and is expected to sharply cut natural gas use and reduce reliance on wood chips.
  • It is not designed to generate electricity, only heat.

How the sand battery works (and why sand/air, not water)

  • Resistive heaters warm air using cheap electricity; hot air circulates through a sand-filled silo, heating sand to ~600°C.
  • When needed, hot air is run through an air–water heat exchanger to supply 65–120°C water for district heating.
  • Air is used instead of water because of much higher operating temperatures (water would boil); high temperature increases energy density and potential thermodynamic efficiency if ever used for power cycles.

Thermal storage scale, geometry, and networks

  • Large thermal masses lose proportionally less heat (surface/volume and thermal time constants scale favorably), making big centralized stores efficient.
  • However, aggregating heat centrally requires long, insulated pipe networks, which are costly and lossy compared to electric wires; viable mainly where district heating already exists.
  • Nordic and some Central European cities and even villages already operate extensive district heating, often fed by waste heat, CHP, or large heat pumps.

Comparison with batteries, heat pumps, and other storage

  • For electricity-to-heat, heat pumps (COP ~2–4) beat resistive heating, but for storing heat cheaply at scale, sand/water reservoirs can win on capex and longevity.
  • Round-tripping via heat→electricity→heat is discussed as theoretically possible but very inefficient (~25%) and machinery-heavy; this project explicitly avoids that.
  • Participants contrast sand/thermal storage with:
    • Chemical batteries (good for short-term, degrade over time, expensive for week-scale)
    • Pumped hydro (cheap where geography, politics, and environment allow, but siting is hard)
    • Long-term hydrogen or other seasonal storage (still expensive and immature).

Nordic grid and policy context

  • Finland has large winter peaks driven by heating, little solar in winter, and increasingly relies on wind plus nuclear and imports.
  • There is debate over how much can be covered by renewables plus storage versus needing more nuclear or gas peakers.
  • Broader arguments touch on interconnectors, hydro’s limits, failed deep geothermal attempts, and politically contentious choices (nuclear cancellations, peat, imports of waste fuel).