Mechanical Battery | Flywheel Energy Storage

In the late 1940s a Swiss company- Oerlikon, developed a peculiar kind of public bus, the Gyrobus. It was a electrically powered bus made to operate over short distances and low traffic regions with zero greenhouse gas emission. The thing that made the Gyrobus so unique was that instead of any electro-chemical batteries or any internal combustion engines it was powered by a large, high-speed rotating 1500 kg flywheel sealed in a low resistance hydrogen filled chamber, which spun at about 3000 rpm. Electrical energy was transferred into the system  by a induction motor and converted into rotational kinetic energy of the flywheel. The induction motor was powered by metal rods mounted at the top of the bus. At every stop the the rods would contact overhead charging for spinning up the flywheel. For driving the wheels of the bus the charging motor would reconfigure as a generator extracting electrical energy from the flywheel, this electrical energy is then used to power a motor at the back wheels to propel the bus. The motor was also used as a regenerative brake which converted the kinetic energy of the wheels back to electricity for further charging of the flywheel. But these flywheel systems showed several drawbacks like excessive wear and reliability problems. 

In 2013 Volvo came up with a flywheel storage system fitted to the rear axle of it's S60 Sedan. It used regenerative braking to charge the flywheel. The braking action spins the flywheel at upto 60,000 rpm and this would stop the main engine. The energy from the flywheel is applied through a special transmission to partially or completely power the vehicle. When coupled with a four cylinder engine it reduced the fuel consumption upto 25%. 

The flywheel energy storage or using the energy of a spinning flywheel is one of the earliest techniques of storing mechanical energy by mankind. The potter's wheel is one such earliest examples. Flywheels were also used in lathe machines, water wheels and early steam engines. In the 18th century, with onset of Industrial revolution the term Flywheel was coined. The advantageous characteristics of heavy metal flywheels to convert linear long reciprocating strokes of a steam  engine into usable  rotational energy literally powered the industrial revolution. This capability of a  flywheel later migrated from steam engines into internal combustion engine. From the first car by Carl Benz to the modern engines all internal combustion engines need flywheels. Major developments in the flywheel storage systems- FES came in 20th century when the mechanical effects of rotating flywheels were properly investigated. 

Mechanical energy storage devices utilize the energy due to the dynamic motion of objects or substances to store energy. One such technology is the Flywheel energy storage. Conventional batteries store energy in form of the chemical energy where as the flywheel energy storage takes advantage of the rotational kinetic energy of a flywheel. The physics  of such technologies in paper is quite simple, the device uses a motor to rotate a flywheel to store energy in for of the rotational energy and when needed the same flywheel in motion can be used to drive a generator shaft for electricity generation. But the engineering of such technologies must be advanced with high-tech materials for components, efficient mechanisms and complex designs. 

Flywheel Energy Storage :-

It is a mechanical device that uses an electric motor to accelerate a rotor to a very high speed so that the electrical energy input is transformed into mechanical energy(Rotational). Hence, it accumulates energy in form of rotational kinetic energy of flywheel that can be used instantaneously whenever required. For extraction of electricity the flywheel drives a electric generator, the inertia of flywheel allows the rotor to keep spinning and the kinetic energy is converted to electrical energy. The flywheel is recharged by again using an electric motor to increase it's rotational speed and kinetic energy. The rotor of the FES spins in a nearly frictionless enclosure to avoid loses. The FES is operated in a vacuum to reduce drag effects on the rotor.

Flywheels can patch the gap between short-term ride-through power and long-term energy storage with excellent cyclic and load following characteristics.

The Flywheel storage technology seeks more R&D as it has several beneficial aspects. The FES uses electricity for driving the flywheel, but devices are being developed that can directly use mechanical energy for accelerating the flywheel such as using regenerative braking along with flywheels.

The flywheel of a conventional FES system are made up of steel and rotate on traditional bearings. In advanced FES the flywheel  is made out of high strength carbon-fiber composite materials, uses magnetic bearings and contained in a vacuum which allows it to revolve at speeds from 20,000 to over 50,000 rpm. 

Physical Characteristics:-

Governing Equation:-

In comparison to chemical batteries the FES have long lifetime, low maintenance, high specific energy and high power output. FES systems can be cycled frequently without much effect on their performance. The energy efficiency can be upto 90%. These systems charges very rapidly within 15 minutes. To optimize the energy-to-mass ratio, the flywheel must rotate at it's maximum possible speed.

The specific energy of a FES mainly depends upon:- 

1. Geometry of the rotor.
2. Material properties of the rotor

Composite materials are often used since they have high strength which can withstand high hoop stresses due to rotation and low densities  because high density materials are subjected to more centrifugal forces. Examples include the carbon-fiber composite and aerospace-grade high-performance steel. Usage of mechanical bearings in FES can cause frictional energy loses, about 20% to 50% in two hours. Much frictional loses are due to changes in orientation of the flywheel, which is a result of rotation of earth. The change in orientation is resisted by the gyroscopic forces  exerted by the flywheel. This results in increased frictional forces in the mechanical bearings. Hence, modern flywheels use magnetic levitation (magnetic bearing) in a high vacuum chamber, which can maintain 97% mechanical efficiency.

Advancement in FES system:-

Advanced FES systems achieve high energy density, mechanical efficiency and low loses by the following ways:-

1. Rotating mass made with composite materials with high strength to mass ratio.
2. A vacuum environment is maintained for the flywheel to minimize energy losses.
3. Air or magnetic bearing are used for higher rotational speed and reduce frictional losses at the bearings.

Advanced FES operate at a rotational frequency in excess of 100,000 RPM with tip speeds in excess of 1000 m/s. FESS are best used for high power, low energy applications that need many cycles.

Applications:-

Uninterruptible power supply (UPS):-

From the point of view of clean energy storage system flywheel storage is comparable with batteries and has fast discharge. FES systems are appropriate for uninterruptible power supply for Data centers, hospitals and other critical infrastructures. Maintenance cost of flywheels are about half the of cost traditional UPS. Introduction of magnetic bearings eliminate the mechanical bearing maintenance. Costs of a fully installed flywheel UPS (including power conditioning) are (in 2009) about $330 per kilowatt (for 15 seconds full-load capacity) which is significantly less than battery systems.

NASA G2 flywheel :-

The G2- a modular flywheel energy storage device funded by NASA's Glenn Research Center with an aim of using flywheels to aerospace energy storage and integrated power and altitude control. G2 system used a multilayer carbon fiber rim with a titanium hub designed to spin at 60,000 rpm, mounted on magnetic bearings. Weight was limited to 250 pounds. Storage was 525 W-hr (1.89 MJ) and could be charged or discharged at 1 kW.

Aircraft Launching :-

The Gerald R. Ford-class aircraft carrier will be using a flywheel energy storage system for quick discharge of power into the electromagnetic launch system. The shipboard power system on it's own cannot supply such transients needed to launch aircrafts. Each rotor will store about 121 MJ (34 kWh) at 6400 rpm. They can store 122 MJ (34 kWh) in 45 secs and release it in just 2–3 seconds.

Tokamak Fusion Experiments:-

Tokamak fusion experimental  reactors requires very high currents for short interval of time primarily to power large electromagnets which confine the hot plasma in the reactors.

• The Joint European Torus has two 775 tonne flywheels that spin up to 225 rpm. And each stores upto 3.75 GJ and can deliver at up to 400MW.
• The Helically Symmetric Experiment at the University of Wisconsin-Madison has 18 one-ton flywheels, which are spun to 10,000 rpm using repurposed electric train motors.

Kinetic Energy Recovery system :-

Fig:- Flybrid Systems Kinetic Energy Recovery System for a Formula 1 car.

It is a automotive component used for recovering kinetic energy of a moving vehicle under braking condition. This recovered energy is stored using a FESS for later use under acceleration. In motor sports applications this recovered energy is used to improve acceleration. The same technology can be used in road cars to reduce fuel consumption and increase efficiency. 

Efficient storage systems in present scenario of energy supply system is one of the most important support systems for both Renewables and Non-renewables like nuclear energy. At present gas turbines or diesel engines are used for backup power supply or to balance power demand fluctuations, which in turn fails to achieve goals of decarbonizing the energy grid. Flywheel energy storage on the other hand has zero carbon footprint and finds numerous applications for high and transient power supplies. Apart from this FESS has several advantages over chemical batteries like low maintenance (time and cost), longer lifetime, better performance under fluctuating power supplies and can be introduced into automotive systems as regenerative storage. Energy sources like nuclear also require storage systems for backups during accident situations which needs to be supplied quickly into the coolant pumping systems to prevent the reactor from heating up. FESS are used in power grids to get a balance between supply and consumption rates. Several such installations offering at most 20 MW storage capacity with 15 minutes of discharge time has been installed in U.S and Canada. 

Although flywheels are some of the earliest mechanical devices that was used to store and transfer energy, recent development of modular, reliable, cost effective and green FESS is going to be vital in the near future. With the advent of manufacturing and material science the FESS shows better prospects and can also pose good competition to chemical batteries in the market. With minimum environmental impacts and longevity, the FESS one of the mankind's oldest ways of storing energy can be the key to our sustainable future.