Flywheel is a spinning wheel that stores kinetic energy. Basically, flywheel energy system includes a cylinder with a shaft connected to an electrical generator. Electricity is stored by increasing the flywheel’s rotational speed; electric energy is converted to kinetic/mechanical energy by the generator and stored as momentum on the flywheel. To use the energy stored on the flywheel, we draw electrical energy back out by reversing the process and slowing the flywheel’s rotational speed the stored energy in the generator will be converted to electrical energy. Since the flywheel in a vacuum, we don’t lose much energy over time. It's a simple equation of mass x velocity^2. The quicker the flywheel goes; the more energy it contains [22] [23]. 

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Flywheels vs. Batteries

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Even though flywheels are more expensive than batteries in terms of first cost, they have better and longer lifetime, can be charged and discharged at high rates for many cycles without losing on their efficiency. Their maintenance is simple, and no chemicals are used to operate a flywheel, and all their materials are completely recyclable. Flywheels can be classified as low speed or high speed. Low speed flywheels are usually made from steel, and high-speed flywheels are made from carbon or carbon and fiberglass.

On the other hand, batteries have a lower first cost, however, they have high-cost maintenance and their lifetime is shorter than flywheels' because repeated charging and discharging will damage the batteries [22][23]

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Flywheels Cost Overview

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Flywheel on its own rotates at thousands revolutions per minute (rpm). Flywheels’ standby losses range from 0.1% to 1.0% of rated power. Lifetime for flywheels is normally about 20 years, and could be kept in normal outdoor ambient temperature conditions. Also, their reliability is equal to two battery strings operating in parallel. Manufacturers typically provide 2-year and 5-year warranties as part of their standard packages.

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Purchase cost ranges from $100/kW to $300/kW.

Installation ranges from $20/kW to $40/kW.

Routine maintenance is simple and minimal for few $/kW/year.

Some vendors offer Service contracts for $5/kW/year.

Bearing replacement cost ranges from $5/kW to $15/kW.

Replacing the vacuum pump costs $5/kW.

Standby power consumption will run about $5/kW/year.

[8]

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