# A Review of Flywheel Energy Storage System Technologies and Their Applications
## 4 FESS Applications
### Transportation  
In transportation, flywheels are used in hybrid and electric vehicles to store energy, for use when harsh acceleration is required or to assist with uphill climbs. In hybrid vehicles, the constant power is provided by the internal combustion engines to keep the vehicle running at a constant optimum speed, reducing fuel consumption, air and noise pollution, and extending the engine life by reducingmaintenance requirements [7; 8]. At the same time, energy from regenerative braking during vehicle slowdown is stored in flywheels, which will be supplied back to provide a boost during acceleration or climbing hills [7; 8; 59]. The only competitors to flywheels in hybrid vehicle applications are chemical batteries and ultra-capacitors. However, ultra-capacitors suffer from a low energy density and higher cost. Flywheels rank better than batteries based on their longer life time, higher power density, higher efficiency, and frequent charge-discharge capability [8]. Furthermore, flywheels are developed for rail applications, both for hybrid and electric systems. They also find a place in gas turbine trains for the same purpose. The desired speed and maximum weight of the train determines the power and energy requirements. It is estimated that 30% of the braking energy could be recovered by this system, due to receptivity issues [8]. In electrical vehicles with chemical batteries as their source of propulsion, flywheels are considered to cope well with a fluctuating power consumption. This will prolong the lifetime of the battery as its charge-discharge cycles become more regular [10]. In train energy recovery systems, flywheels are installed at stations or substations to recover energy through regenerative braking, and supply it back into the system for traction purposes. Flywheels are well suited for this application due to the high rate of charge-discharge cycles needed. In addition, it allows voltage sag control for transmission and distribution lines, without increasing the line capacity of the railway. A number of flywheels for trackside energy recovery systems have been demonstrated by URENO and Calnetix [78]. In April 2014, VYCON Inc. installed a FESS for the Los Angeles Country Metropolitan Transportation Authority (LA Metro) Red line (MRL), to recover the braking energy from trains. MRL provides rail subway service connecting downtown to San Fernando Valley through six-car trains with AC or DC traction systems [79]. VYCON's flywheel, known as Metro's Wayside Energy Storage Substation (WESS), can recover 66% of the braking train energy [80]. The collected data, after six months of operation, showed 20% energy savings (approximately 541 MWh), which is enough to power 100 average homes in California [79]. A total of 190 metro systems operating in 9477 stations and approximately 11,800 km of track has been reported globally [13]. The introduction of energy storage into rail transit for braking energy recovery can potentially reduce 10% of the electricity consumption, while achieving cost savings of $90,000 per station [81]. Flywheels are also used in roller coaster launch systems to accumulate the energy during downhill movements and then rapidly accelerate the train to reach uphill positions, using electromagnetic, hydraulic, and friction wheel propulsion [82]. The Incredible Hulk roller coaster at an adventure theme park in Orlando, Florida, uses several 4500 kg flywheels to propel the system. The flywheels charge continuously at about 200 kW and then discharge at 8 MW, to launch the train [13]. Since the late 2000s, the use of flywheel hybrid storage systems in motorsports has seen major developments, beginning with Formula 1 and followed by the highest class of World Endurance Championship (WEC) [13]. Williams Hybrid Power (WHP), part of Williams Group of companies, pioneered the use of flywheel energy storage in motorsport. WHP's electric flywheel was used in Porsche Motorsport on their 2010 911 GT3 R Hybrid endurance racing car.