Nov. 17, 2019, 9:29 a.m.(UTC)
The Mechanical Battery
Though more commonly known for its electro-chemical variant, a battery or accumulator is any device that stores energy. Batteries fundamentally allow us to decouple energy supply from demand. But a far lesser known, mechanical based rechargeable battery based on flywheel energy storage or FESS is showing a resurgence of interest.
The Gyrobus was as an electric bus designed to operate quietly, along short distance, low traffic routes where installing new traditional overhead trolley power wires was not feasible. It was powered by a large 1500 kg flywheel, sealed in a low resistance hydrogen filled chamber, that spins at up to 3,000 RPM.
The concept of flywheel energy storage, was one of man’s first forms of storing mechanical energy. The potter’s wheel, one of the earliest examples, used the flywheel effect to maintain its energy under its own inertia.
Using flywheels to convert reciprocating motion to a rotational force would migrate from steam engines into the next evolution of the engine, the internal combustion engine.
In the early 20th century, when rotor shapes and rotational stresses were thoroughly analysed, and the flywheel was now being considered as a potential energy storage systems.
Known as FESS or flywheel energy storage systems, much like the system used on the gyro bus, these typically use electricity as the working energy.
The flywheel speeds up as it stores energy and slows down when it’s discharging, to delivering the accumulated energy. The rotating flywheel is coupled to an electrical motor-generator unit that performs the interchange of electrical energy to mechanical energy, and vice versa.
Beyond the motor-generator limits, the maximum speed limit at which the flywheel rotor can operate is also determined by the tensile strength of the material it’s made from.
Because the shape of a flywheel rotor affects its moment of inertia, and inherently it energy storage capacity, how efficiently the mass of the material used is utilized is determined by the shape factor of its geometry.
Flywheel energy storage system designs generally fall under one of two strategies, low speed flywheel systems that operate under 10,000 rpm and high speed variants that can approach 100,000 rpm.
Because flywheel energy storage systems usually enclose the flywheel within a vacuum to reduce friction, the primary point of energy loss happens at the bearings that support the flywheel.
Permanent magnet bearings are passive, stiff, low cost, and suffer from low losses, due to lack of a flowing current but this comes at the cost of having limited stability.
Active magnetic bearings produce their magnetic field from current carrying coils that control the rotor position. It positions the rotor through a feedback system by applying variable forces which are determined based on the deviation of the rotor position, caused by external forces.
Superconducting magnetic bearings provide the best solution for high speed flywheel energy storage systems offering compact, friction less, long lasting and stable operation.
One of the more attractive characteristics of flywheel energy storage systems are their reliability. They can achieve high cycle life spans, easily achieving hundreds of thousands of charge/discharge cycles without degrading.
Within the limits of current technology, flywheel energy storage systems are more suitable where high bursts of power are needed for a short duration.
Flywheel energy storage systems are also used at the power grid level, providing an energy storage buffer for balancing sudden changes between supply and power consumption.
At smaller scales, flywheel energy storage systems have been used where short bursts of power are needed without taxing power supply systems.
The electromagnetic aircraft launch system on the Gerald R Ford class aircraft carriers also uses this principle.
The now commercial Flybrid system uses a continuously variable transmission to recover energy from the drive train during braking, into a flywheel.
NASA has also experimented with lightweight flywheel energy storage systems for spacecraft with its G2 FESS module design.
With no need for exotic minerals, minimal environmental impact, and unprecedented reliability and longevity, one of man’s oldest energy storage systems may prove to be the key to our energy storage future.
“Stock footage provided by Videvo, downloaded from https://www.videvo.net”
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