Page - 157 - in Water, Energy, and Environment - A Primer

megawatts,andenergystoragerequirementsrangingfromseveral hundredkWhtohundredsofmegawatt-hours.Theyarealsoeasy tocontrol andmaintain, and fluid flowcanbestoppedquickly in an emergency situation. 9.2.3 Flywheels Flywheels store energy by using electrical power to accelerate a cylindrical assembly called a rotor (the flywheel) to a very high speed and maintaining the energy in the system via rotational motion. The rotational energy is converted back to electricity by slowing down the flywheel. The flywheel system itself is a kinetic, or mechanical, battery, spinning at very high speeds to store energy that is instantly availablewhenneeded. At the core of most modern-day flywheels is a carbon-fiber composite rim, supported by a metal hub and shaft, with a motor/generator mounted on the shaft. Together, the rim, hub, shaft and motor/generator assembly form the rotor. When charging (i.e., absorbing energy), the flywheel’smotor acts like an electrical load and draws power from the grid to accelerate the rotor to a higher speed. When discharging, the motor is switched into generator mode, and the inertial energy of the rotor drives the generator which, in turn, creates electricity that is then injected back into the grid. Multiple flywheels may be connected together to provide various megawatt-level power capacities. To illustrate the industry’s current capabilities, one major flywheelmanufacturer offers a high-performance rotor assembly that is sealed in a vacuum chamber and spins at up to 16,000 rpm. At that rotational speed, the speed at the rim would be approximatelyMach2, about 1500mph, if itwere operated in a normal atmosphere.At that speed the rotormustbeenclosed ina high vacuum to reduce friction and energy losses. To reduce losses even further, the rotor is levitated with a combination of permanent magnets and an electromagnetic bearing. At 16,000 Energystorage 157