**Figure 4.**

*Pumped hydro storage [45].*

cylinder or disc at speeds up to 60,000 rpm. Flywheels are considered "dynamic" energy storage systems because they must be accelerated by an external force before they can store energy. A flywheel is a rotating wheel or disk, mostly made from steel or carbon fiber, that turns around an axis and is usually used in short-term energy storage devices for motion applications such as powertrain engines and road cars. In these applications, the flywheel soothes the power load during deceleration by dynamic braking operation and provides a lift during acceleration [47]. A flywheel generates power as a rotational kinetic energy and later converts the power into electrical energy. The flywheel is an electric motor, a holding system for energy, and a generator at the same time. As excess power is available, the power and electric motor spins the flywheel roughly at a high speed. For the energy applied to the flywheel, a small amount is used to hold the flywheel rotated; the remainder is storage energy [18]. Flywheels are suitable for storing surplus electricity from intermittent solar and wind power on the electrical grid; on the other hand, flywheels cannot conserve an immense amount of power due to the fact that they have a high loss rate, so the generated electrical energy can be consumed very rapidly and has a high loss rate.

Because of these losses, the cost per kWh of energy stored is high, and powerspecific costs are relatively low [48].

Simply put, the advantages and disadvantages of flywheels can be summarized as systems notable for their longevity (up to decades), easy maintenance, and fast response time. But they can only operate for a short period of time (**Figure 5**).

**Figure 5.** *Flywheel energy storage [49].*
