Recent technical advances have enabled flywheel energy storage systems (FESS) to become more compact and able to support higher-power applications. Due to their proven reliability, low cost of ownership, and favorable green environmental aspects, engineers and managers of data centers, hospitals, industrial systems, electric rail, and microgrid applications are reaping the benefits of clean energy storage that flywheels offer.
Flywheel systems hold kinetic energy in the form of a rotating mass and convert this energy to electric power within the flywheel system. A high-speed motor generator, active magnetic bearings used to levitate and sustain the rotor during operation (see figure), and a control system provide information on system performance. These technologies enable flywheels to charge and discharge at high rates for countless cycles, making conventional technologies like batteries obsolete in many circumstances.
Delivering up to 450 kW of predictable DC power from a single flywheel unit, the flywheel’s motor generator rotates at speeds of up to 36,750 RPM. During discharge, the rotor speed decreases to a lower minimum speed, typically 10,000 to 12,000 RPM.
The flywheel can charge and discharge at high rates for countless cycles without degradation throughout its 20-year life. The amount of energy available and its duration is proportional to its mass and the square of its revolution speed. For flywheels, doubling mass doubles energy capacity, but doubling rotational speed quadruples energy capacity:
In this equation, κ depends on the shape of the rotating mass, M is the mass of the flywheel, and ω is the angular velocity.
For data centers, flywheel systems can be paired with three-phase uninterruptible power systems (UPSs) that include batteries. When there is a power glitch, the flywheel absorbs all of the short-duration discharges to prolong battery life; however, more data center managers are eliminating the batteries altogether — due to the high cost of battery replacement, cooling costs, size, and carbon footprint goals — and using UPS with flywheel systems alone. When the 30 × 30” flywheel system is used with the UPS and no batteries, the flywheel will provide instant power to the connected load exactly as it would with a battery string; however, if the power loss continues for more than 15 seconds, the load will quickly transfer to the facility’s engine-generator.
For energy-intensive applications such as electric rail, industrial equipment, and microgrids, flywheels recycle energy by taking the energy component of an application that would otherwise be wasted and converting it into usable electricity or thermal energy. For these applications, the flywheel can cycle hundreds of kilowatts of power, discharging and recharging every two minutes over one million times, without degradation of the energy storage capacity.
This article was written by Frank DeLattre, President of VYCON, Inc. For more information, visit here.