# A Review of Flywheel Energy Storage System Technologies and Their Applications
## 4 FESS Applications
### Ups  
A short term (seconds to minutes) energy storage device with control electronics is referred to as uninterruptible power supply (UPS). A UPS is one of the existing markets and the most successful application for high power flywheels to supply power for occasions which usually don't last longer than 15 s. More than 80% of the power outages last for less than a second [8] and 97% of them last for less than 3 s [10]; however, this causes voltage and frequency problems, as well as power interruptions. In these applications, the UPS, as a backup storage, bridges the gap between the loss of the grid and the start of backup sources during an interruption. The most developed and widely used storage medium in UPS applications is batteries. FESS can be used as a substitute for batteries or in combination with batteries in UPS systems [12]. In cases with only flywheels as a backup storage, sufficient power is provided by the flywheel to run the system, until the power source is restored or a standby power source comes online. Depending on the level of power required, 10-15 s of back up support is enough to meet the demand loads, without transferring to the generator set power [8; 10]. This is the case when power outages last less than 15 s. Meanwhile, in diesel-rotary UPS with diesel generators for long-term outages, the diesel engines commonly start and accept 100% load within 3-4 s and flywheels are best suited to bridge the power, until the generators are fully operated and synchronized. As a result, either scenario can be accomplished, with flywheels acting as energy storage systems for UPS applications. In addition, flywheels are used in combination with batteries in UPS systems requiring longer durations. A flywheel can deal with shorter interruptions, while batteries can be saved for longer outages. This will save the battery from frequent charge-discharge, which will further increase its lifetime [12]. Usually, flywheels and batteries are combined for applications requiring a mix and match between energy density and cost, which cannot be otherwise achieved with one of these storage systems [71]. Many manufacturers around the world have developed flywheel systems for UPS. To name a few, one of the earliest flywheels for on-site power applications was built in Munchen, Germany, in 1973. It was rated for 155 MW power and 0.93 power factor, for a pulsed duration of 9.7 s. In the following years, this flywheel was complemented with two more flywheels to utilise a flywheel generator system for high energy fusion experiments. The system was fully commissioned in 1987 and the total rated power and pulsed duration capability of the system was increased to 387 MW and 12 s, respectively [65]. A hybrid microgrid supplying heat and electricity to an industrial/airport facility in an island in Alaska was commissioned in 1999. Originally consisting of a 225 kW wind turbine, and two 150 kW diesel generators, the system was upgraded with a 160 kW FESS by Beacon technology in 2014. In addition to efficiency improvements, it has also provided fuel savings of up to 30% [65]. Piller GmbH has installed a FESS in a combined heat and power station to supply a semiconductor facility in Dresden, Germany. The integrated 5 MW flywheel subsystem can supplement the 30 MW plant with 5 s storage [7]. A battery-free UPS system has been announced by VYCON to protect a light-out data centre located in Texas, US. The system will involve multiple 750 kVA double conversion UPS modules, paired with an 8 MW (300 kW power rating per unit) FESS. Deploying approximately 1 MW of clean energy, a similar flywheel system has also been planned to protect EasyStreet Online Service's data centre in California, US [77]. Similarly, the control centre of Austin Energy is protected by a 4.8 MW flywheel UPS by VYCON. Austin Energy, one of the largest electric utilities in US, supplies approximately 400,000 customers and of a population of about one million.