Flywheel systems provide very fast response, high power density, and long cycle life, but generally only store energy for seconds to minutes. As such, they are best treated as high-power assets for frequency regulation, inertia emulation, and short bridging, rather than as long-duration energy storage.
- Modern flywheel systems can reach round-trip efficiencies of 80–90% over short durations, with response times in the tens of milliseconds.
- Typical storage durations are 15 seconds to 15 minutes, depending on design.
- Cycle lifetimes are extremely high—hundreds of thousands to millions of cycles—making them well suited to fast, frequent dispatch.
- Economics should be evaluated per kW and per service delivered (e.g., frequency regulation), not per kWh alone.
1. Technology benchmarks: flywheels vs. batteries and supercapacitors
Flywheel systems consist of a rotor spinning in a low-friction environment (often vacuum), coupled to a motor-generator and power electronics. Key performance metrics are power density, energy density, efficiency, and response time.
| Parameter | Flywheel systems | Supercapacitors | Li-ion batteries |
|---|---|---|---|
| Typical duration | 15 sec – 15 min | sec – minutes | 0.5–4 hours |
| Response time | <100 ms | <10 ms | ms – seconds |
| Cycle life | 100,000 – 1,000,000+ | 100,000 – 1,000,000+ | 3,000 – 7,000 |
| Round-trip efficiency (short duration) | 80–90% | 85–98% | 85–92% |
In many grid applications, flywheels and supercapacitors compete for similar roles; local vendor ecosystem, footprint, and integration preferences often drive the final choice.
2. Economics: frequency regulation revenue vs. capex and O&M
Because flywheels are power-oriented, economics must be tied to the value of regulation and other fast services:
| Metric | Flywheel systems | Li-ion batteries (regulation-optimized) |
|---|---|---|
| Installed cost (USD/kW) | 300–800 | 300–700 |
| Installed cost (USD/kWh) | 800–2,500 | 400–900 |
| Useful life (years) | 15–20 | 10–15 |
| Cycle life | 100,000+ | 3,000–7,000 |
Flywheels can generate attractive returns in markets with robust frequency regulation products and where lifecycle degradation costs for batteries would be high. Conversely, in markets without strong ancillary service revenues or with limited need for ultra-fast response, batteries may be more cost-effective.
Use Energy Solutions tools to compare flywheels and batteries
Our EMS and ancillary services tools let you simulate different regulation markets, duty cycles, and technology mixes to assess whether flywheels, batteries, or hybrids deliver the best lifecycle value.
3. Use cases: where flywheels are a strong fit
Flywheels are particularly compelling when:
- Grid codes require fast frequency response and high cycling, such as in systems with high renewable penetration.
- Industrial plants need ride-through during short sags and swells.
- Microgrids and islands require inertia-like behaviour and fast power balancing.
Design tip: consider flywheels as part of a stack of assets: they stabilize power quality and frequency, while other assets manage energy over longer durations.
4. Constraints and risk factors
Flywheels also carry risks and limitations:
- Mechanical integrity of rotors and bearings is critical; containment for failure events is essential.
- Standby losses can be significant due to friction and auxiliary loads.
- Site-specific engineering often required, especially for large systems.
Risk note: failure modes are mechanical and can be dramatic; robust containment and conservative design margins are non-negotiable.
5. FAQ: common questions on flywheel storage
Can flywheels store energy for hours like batteries?
Most commercial flywheel systems are designed for seconds to minutes. While longer durations are technically possible, they are rarely economical compared with batteries for multi-hour storage.
Are flywheels and supercapacitors interchangeable?
They overlap in some high-power use cases, but each has specific integration and footprint considerations. The best choice depends on vendor ecosystem, space, maintenance, and desired form factor.
Where do flywheels sit in long-term planning?
In planning studies, flywheels are usually categorized as high-power, short-duration assets used for ancillary services rather than bulk energy shifting. They complement, rather than replace, batteries and other LDES.
What are realistic deployment timelines?
For grid-scale projects, 1–3 years from concept to commissioning is typical, depending on permitting and civil works. Smaller industrial projects can often be executed faster.
Can flywheels provide inertia-like benefits?
Yes. Because energy is stored in rotating mass, flywheels can provide physical inertia and fast response, especially when integrated with suitable power electronics.