# A Review of Flywheel Energy Storage System Technologies and Their Applications
## 4 FESS Applications
### Frequency Regulation  
Frequency fluctuations occur as a result of variations between the loads and supply, where one exceeds the other. When demand exceeds supply, the generating plants are slowed down by the extra load, thus decreasing the system frequency. On the other hand, the generators accelerate and the frequency increases whenever the generation exceeds the demanded loads [68; 69]. The frequency fluctuates every second, as demand varies and generators turn on and off. In order to avoid this, frequency regulation is applied, which demands the generators to hold capacity in reserve, to maintain the stability of generation and consumption. This ramping up and down of the generators not only increases the fuel cost and emissions, but also takes a minute or longer for some generating power plants to respond [68; 70].  
A frequency regulation service is a very cyclic application requiring thousands of charge-discharge cycles in a year. Any storage device in this application will be very rarely \"resting\", due to the fact that it includes constant charging and discharging at variable rates, from very slow to rapid and deep cycling [70]. Today, grid operators look for 'fast-acting regulators' including flywheels and batteries to respond to the frequency regulation issue. Because of their fast response and frequent charge-discharge capabilities, flywheels are likely to dominate over batteries in this application. Similarly, the capability of flywheels to switch from full output to full absorption in seconds, puts them on a par with the immediate energy produced by gas fired power plants. Flywheel energy storage systems can deliver twice as much frequency regulation for each megawatt of power that they produce, while cutting carbon emissions in half [68; 71]. The earliest, but shortest lifespan of a flywheel system reportedfor frequency regulation using renewables, was installed in Shimane, Japan, in 2003. This 200 kW Urenco Power Technology Flywheel was installed by Fuji electric to reduce system fluctuations due to wind generation. The diesel generators supporting the 1.8 MW wind turbines were operating at a higher efficiency when the flywheel system was integrated. The system was decommissioned in 2004, as Urenco abandoned power quality operated flywheels and removed all previously installed units [65]. The largest power rated FESS for frequency regulation is the Joint European Torus (JET) Fusion Flywheel of the European Atomic Energy Community (EURATOM) located in Oxfordshire, UK [72]. JET uses the grid for powering electromagnets, and for generating fields to confine and ohmically heat hydrogen plasma. Experiments typically last 20 s and draw 1000 MW--a large fraction of the local Didcot Power Station's capacity [73]. There are two large flywheels capable of supplying up to 400 MW for 30 s and an additional 300 MW power is pulled from the grid to combine with the FESS, in order to satisfy the peak consumption of the JET pulse. The JET flywheels are charged up from the grid for several minutes, then quickly discharged into the required loads. Hence, distribution network congestion is avoided by reducing the demand on the grid during experiments [74]. A 20 MW flywheel-based facility provides frequency regulations services to New York Independent System Operator (NYISO) in Stephentown, New York [29]. The facility is built and operated by Beacon Power and comprises 200 flywheels, each with a storage capacity of 100 kW. The tests during early trials showed that 1 MW of fast response flywheel storage could produce up to 30 MW of regulation service; two to three times better than an average Independent System Operator (ISO) generator. A second 20 MW frequency-regulation facility in the Hazle Township of Pennsylvania is commissioned by Spindle Grid Regulation, LLC. This zero emission facility is designed for 20 year-life and at least 100,000 full-depth discharge cycles. It is comprised of 200 Beacon Power's 100 kW (25 kWh) flywheels connected in parallel, which can respond in less than 2 s [75].