Energy Storage
Energy storage balances supply and demand across seconds to seasons. Compare pumped hydro, batteries, flywheels, compressed air, hydrogen, supercapacitors and thermal storage by power, energy, efficiency and cost, and learn their grid applications.
As more electricity comes from sun and wind, the grid needs ways to move energy in time: store surplus midday solar for the evening peak, smooth second-by-second fluctuations, and cover windless weeks.
No single technology does everything. Each is characterised by its power rating (MW), energy capacity (MWh), duration (MWh/MW), efficiency, response time, lifetime and cost.
An analogy
Energy storage is like a household's money: a wallet (supercapacitor) for instant small payments, a current account (batteries) for daily bills, and a savings account (hydrogen, pumped hydro) for long-term needs.
Why store energy
Storage performs several grid services.
- Time-shifting (arbitrage): charge when cheap, discharge when expensive.
- Peak shaving: cap a site's maximum demand to cut demand charges.
- Frequency regulation and reserves: respond in milliseconds to seconds.
- Backup power and black start.
- Deferring grid upgrades by relieving congested lines.
Technology comparison
Technologies differ widely in duration and efficiency.
- Pumped hydro: GW scale, 6–20 h, ~75–80 %.
- Li-ion BESS: MW scale, 1–4 h, ~85–92 %, fast response.
- Flywheels: seconds to minutes, very high cycle life.
- Supercapacitors: seconds, ~95 %, millions of cycles.
- Compressed air (CAES): hours, ~50–70 %.
- Hydrogen: days to seasons, ~30–40 % round trip.
- Thermal (molten salt, heat pumps): hours, for CSP and heating.
Sizing and economics
Sizing starts from the application's power and duration, then adds depth-of-discharge, efficiency, degradation and augmentation over life. LCOS divides lifetime cost by lifetime energy delivered. Revenue stacking — combining arbitrage, capacity payments and ancillary services — is usually needed for projects to pay back.
Grid-forming storage
Battery inverters with grid-forming control provide synthetic inertia, voltage and frequency references, enabling grids to run with very high renewable shares and to restart after blackouts.
Formulas
- Duration:
t = E / P - Flywheel:
E = ½ I ω² - Supercapacitor:
E = ½ C V² - Arbitrage profit per cycle:
E·p_high − (E/η_rt)·p_low
Worked example: Supercapacitor vs flywheel
Given: Supercap: 3000 F, 2.7 V; Flywheel: I = 50 kg·m² at 10 000 rpm
Find: Stored energy of each.
- Supercap: ½ × 3000 × 2.7² = 10 935 J = 3.04 Wh
- Flywheel: ω = 1047 rad/s → ½ × 50 × 1047² = 27.4 MJ = 7.6 kWh
Answer: 3 Wh vs 7.6 kWh
Worked example: Arbitrage revenue
Given: 100 MWh delivered per cycle; η_rt = 88 %; Buy at 30, sell at 80 per MWh; 300 cycles/year
Find: Profit per cycle and per year.
- Energy bought = 100/0.88 = 113.6 MWh → cost 3409
- Revenue = 100 × 80 = 8000
- Profit = 4591 per cycle → ≈ 1.38 million per year
Answer: ≈ 4600 per cycle, 1.38 M per year
Common mistakes
- Wrong: Quoting a battery only in MW. — Right: Specify both power (MW) and energy (MWh); duration determines which services it can provide.
- Wrong: Ignoring round-trip losses in revenue estimates. — Right: Buy E/η_rt to sell E — efficiency directly reduces arbitrage margins.
Applications
- Solar + storage: Utility PV plants pair with 2–4 h batteries to deliver firm evening power.
- Frequency response: Batteries respond in under a second, outperforming thermal plants for fast frequency services.
- Long-duration storage: Iron-air, flow batteries and hydrogen target multi-day balancing for deep decarbonisation.
Key takeaways
- Specify power (MW), energy (MWh) and duration.
- Match technology to timescale: supercaps/flywheels (s), batteries (h), pumped hydro (h–days), hydrogen (seasons).
- Round-trip efficiency and degradation drive economics.
- Revenue stacking and grid-forming control are key for modern storage.
Going further
- Flow batteries: Vanadium and other flow batteries store energy in liquid electrolytes in tanks, decoupling power (stack size) from energy (tank size) — attractive for long durations and deep cycling.
- Degradation-aware dispatch: Optimisers include the cost of battery wear per MWh cycled, so the battery only trades when the spread exceeds its degradation cost.
- Sector coupling: Heat pumps with thermal stores, EV smart charging and electrolysers turn heat, transport and industry into flexible 'storage' for the power grid.