Abstract
A review of flywheel energy storage technology was made, with a special focus on the progress in automotive applications. We found that there are at least 26 university research groups and 27 companies contributing to flywheel technology development. Flywheels are seen to excel in high-power applications, placing them closer in functionality to supercapacitors than to batteries. Examples of flywheels optimized for vehicular applications were found with a specific power of 5.5 kW/kg and a specific energy of 3.5 Wh/kg. Another flywheel system had 3.15 kW/kg and 6.4 Wh/kg, which can be compared to a state-of-the-art supercapacitor vehicular system with 1.7 kW/kg and 2.3 Wh/kg, respectively. Flywheel energy storage is reaching maturity, with 500 flywheel power buffer systems being deployed for London buses (resulting in fuel savings of over 20%), 400 flywheels in operation for grid frequency regulation and many hundreds more installed for uninterruptible power supply (UPS) applications. The industry estimates the mass-production cost of a specific consumer-car flywheel system to be 2000 USD. For regular cars, this system has been shown to save 35% fuel in the U.S. Federal Test Procedure (FTP) drive cycle.
Highlights
The flywheel is an old means of storing energy and smoothing out power variations
The focus in this review is on applications where flywheels are used as a significant intermediate energy storage in automotive applications
??, future potential flywheel energy storage is assessed by comparing contemporarycontemporary materials with a few ones
Summary
The flywheel is an old means of storing energy and smoothing out power variations. The potter’s wheel and the spinning wheel are examples of historical uses of flywheels. For example a flywheel power buffer in a hybrid bus or a Formula 1 flywheel, will release the stored energy relatively soon after charging. This requires an optimization focus on the round trip efficiency (sometimes called AC-AC efficiency), computed as the fraction of input and available output energy: ηac−ac =. A heavy steel rotor is optimized for high inertia; more energy is attained in a linear fashion by adding extra weight. Flywheels optimized for high-speed operation are usually of higher energy density, the high speed can impede power transfer capabilities
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