# Flywheels and small mechanical stores: Engineering and performance

Instant response, but only minutes of energy

Source page: https://www.materialinsights.org/energy/e_flywheels/sg-eng/

What this tab covers Engineering and operating performance: round-trip efficiency, whether power and energy can be scaled separately, footprint and siting, and safety.

Flywheels respond almost instantly and convert about 85 percent of energy round trip, but hold only minutes of energy at very high cost per kWh.

#### Round-trip efficiency
Favourable

Beacon Power reported about 85 percent terminal to terminal for its Stephentown plant (developer figure). Efficiency over a longer hold is lower because the rotor continually loses energy to bearing friction and residual drag.^[2](https://www.powermag.com/milestones-for-flywheel-lithium-battery-grid-scale-projects/)^[4](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2017/Oct/IRENA_Electricity_Storage_Costs_2017.pdf)
 Round-trip efficiency (Stephentown) about 85%^[2](https://www.powermag.com/milestones-for-flywheel-lithium-battery-grid-scale-projects/)

#### Power and energy decoupling
Unfavourable

Power and energy are tied in each unit: adding energy means more or heavier rotors, each with its own motor-generator. Stephentown delivers 20 MW for 15 minutes from 200 flywheels of 100 kW and 25 kWh each (5 MWh in total, calculated). The marginal cost of extra energy is therefore high, which is why IRENA's 2016 cost range was USD 1,500 to 6,000 per kWh.^[2](https://www.powermag.com/milestones-for-flywheel-lithium-battery-grid-scale-projects/)^[3](https://doi.org/10.1016/j.est.2021.103576)^[4](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2017/Oct/IRENA_Electricity_Storage_Costs_2017.pdf)
 Per-unit rating, Beacon Power 100 kW, 25 kWh^[3](https://doi.org/10.1016/j.est.2021.103576) Duration at full power 15 min^[2](https://www.powermag.com/milestones-for-flywheel-lithium-battery-grid-scale-projects/) Installed energy cost (2016) USD 1,500-6,000/kWh^[4](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2017/Oct/IRENA_Electricity_Storage_Costs_2017.pdf)

#### Footprint and siting flexibility
Favourable

Units are compact vacuum-enclosed cylinders that can sit on industrial land, at substations or inside buildings, with no geological requirement. The Chinese plant groups 12 units per array, connected at 110 kV.^[1](https://energy-storage.news/worlds-largest-30mw-flywheel-energy-storage-project-connects-to-grid-in-China)
 Units per array (Dinglun) 12, connected at 110 kV^[1](https://energy-storage.news/worlds-largest-30mw-flywheel-energy-storage-project-connects-to-grid-in-China)

#### Safety and thermal runaway risk
Mixed

There is no chemical fire or toxic gas risk. The hazard is rotor burst, where a failing rotor releases its energy suddenly, so units need strong containment and backup touchdown bearings in case magnetic bearings fail. A 2022 review notes that composite rotors carry more uncertainty about how they fail, because they spin faster and their material properties are less predictable.^[3](https://doi.org/10.1016/j.est.2021.103576)

#### Sources

1. ['World's largest' 30MW flywheel energy storage project connects to grid in China](https://energy-storage.news/worlds-largest-30mw-flywheel-energy-storage-project-connects-to-grid-in-China) Energy-Storage.news, 2024
2. [Milestones for flywheel, lithium battery grid-scale projects](https://www.powermag.com/milestones-for-flywheel-lithium-battery-grid-scale-projects/) POWER Magazine, 2011
3. [A review of flywheel energy storage systems: state of the art and opportunities (Li and Palazzolo, Journal of Energy Storage 46, 103576)](https://doi.org/10.1016/j.est.2021.103576) Elsevier (Journal of Energy Storage), 2022
4. [Electricity Storage and Renewables: Costs and Markets to 2030](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2017/Oct/IRENA_Electricity_Storage_Costs_2017.pdf) IRENA, 2017

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_flywheels/index.md)
- [Overview](https://www.materialinsights.org/energy/e_flywheels/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_flywheels/correlation/index.md)
- [Science and technology](https://www.materialinsights.org/energy/e_flywheels/sg-sci/index.md)
- [Economics](https://www.materialinsights.org/energy/e_flywheels/sg-econ/index.md)
- [Impact and additionality](https://www.materialinsights.org/energy/e_flywheels/sg-impact/index.md)
- [What to track](https://www.materialinsights.org/energy/e_flywheels/tracking/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_flywheels/links/index.md)