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Flywheel electricity storage

Flywheel energy storage makes use of the mechanical inertia contained within a rotating mass. Electricity is used in an electric motor to spin the flywheel (i.e. charging). The process is reversed when electricity is needed with the motor that accelerated the flywheel acting as a generator extracting energy from the rotating flywheel (i.e. discharging). To reduce friction losses, it is common to place flywheels inside a vacuum with the flywheel magnetically levitated.

Flywheels are widely deployed in transport applications (e.g. regenerative braking and acceleration for trains). For stationary applications they still present a niche with <100 MW deployed. Nonetheless, the technology is mature with multiple companies offering stationary systems for short-term power flexibility services. Deployment is still hindered by the relatively high cost compared to battery technologies.


Figure 1 - Schematic of a flywheel for stationary energy storage.

Qnetic flywheels

Qnetic is a novel flywheel energy storage system designed for stationary, large-scale and multiple-hour discharge applications. This is differentiated from traditional flywheel products, and is enabled by scaling-up the rotor – being the energy storage component – to 5.5 metres height and 2.5 metres diameter, and using innovative ultra-light composites as the rotor material, leading to a unit capacity of 2,000 kWh.

According to Qnetic, optimisation of the rotor structure and materials enables storage of a comparatively very large amount of energy (due to its light weight, high strength, and high speed) for a comparatively low cost (due to its minimisation of materials used). Reduced mechanical losses further improves cost. The technology is developed by the Qnetic corporation and prototypes are planned to be deployed in 2023/24. 

Qnetic cost.png

Figure 2 - Levelized cost of storage (LCOS) for four energy storage technologies in a sample energy arbitrage application in 2030. Application requirements are shown in text box on top right, including the system’s power capacity, discharge duration, annual number of full equivalent charge-discharge cycles, the power price for charging, and discount rate. Colours and legend indicate contribution of different lifetime cost components.

The figure above shows the rendering of an energy storage system with multiple Qnetic units and its lifetime cost compared to other promising energy storage technologies. Qnetic flywheels display the lowest lifetime cost of 101 US$/MWh in the given energy arbitrage application. They are designed for multiple-hour discharge applications and therefore significantly cheaper than standard flywheels, which are typically designed for power applications with much shorter discharge durations and so are ill-suited for the sample energy arbitrage use case investigated here. The cost and performance input data for Qnetic flywheels was provided by Qnetic. The data for the comparison technologies were taken from peer-reviewed literature and industry reports.

Schmidt, O, Staffel, I. Levelized cost of storage for Qnetic flywheel energy storage systems. Imperial College Consultants and Storage Lab. 2022. Please contact us for a copy of the report. 

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