Mechanical batteries
R. David Whitby | TLT Worldwide September 2021
Flywheels offer new possibilities for high-efficiency, low-maintenance energy storage.
In practice, a flywheel can be considered a mechanical battery.
One of the problems of generating electricity from renewable solar or wind sources is that the sun doesn’t always shine, and the wind only blows sometimes. As a consequence, using the electricity efficiently requires storing it somehow for when it needs to be used. In recent years, the two most common methods of storage are big lithium-ion batteries or reverse hydropower, in which water is pumped back uphill, to be drawn down later. A new possible method of storage is to use the electricity to split water into hydrogen and oxygen and to use the hydrogen later.
Fortunately, there may be another way to store renewable electricity, using flywheels. A flywheel has a dual-function electric motor to store and generate energy. It uses electricity to speed up the flywheel so that kinetic energy is stored in the spinning wheel. Then, when required, the kinetic energy in the spinning flywheel spins the generator’s rotor, producing electricity. This is how regenerative braking works in an electric vehicle. In practice, a flywheel can be considered a mechanical battery.
The U.S. Navy required a system to provide energy to power new directed energy weapons. Generators provide sustained power but not sufficiently quickly for the needed short bursts of high power. The U.S. Navy currently uses banks of lithium-ion batteries and, although they can provide energy rapidly, they pose risks to warships because they contain hazardous materials, are prone to thermal runaway (catching fire) and tend not to work well at high and low temperatures.
To try to solve this problem, Vishwa Robotics in Massachusetts and the Massachusetts Institute of Technology designed a mechanical battery that uses an array of flywheels inside a box. In general, flywheels can’t compete with chemical batteries in terms of energy storage. However, the new mechanical battery is a collection of smaller units rather than a single large flywheel, so that each small flywheel can be spun much faster, thereby storing much more kinetic energy. Also, specially created bearings make the unit more efficient and cost effective, while being able to store more energy than a lithium-ion battery of the same weight and release it faster with no thermal risk.
Flywheel energy storage works by accelerating a rotor (flywheel) to a very high speed and maintaining the energy in the system as kinetic energy. Using this energy to drive a generator reduces the flywheel’s rotational speed as a consequence of the principle of conservation of energy.
The new design has software that manages the flywheel array, monitoring and drawing power from different wheels to match demand. Vishwa Robotics believes that the mechanical battery can be used in a wide range of applications, including domestic and industrial energy storage, and can be scaled to any size. Also, the materials used in the mechanical battery are more widely available than those required for chemical batteries. Unlike chemical batteries, which become less efficient after a few hundred charge/discharge cycles, the mechanical battery shows no effect after tens of thousands of cycles.
However, experience has shown that flywheels can disintegrate, so the design, mechanics and materials need to be selected carefully. Advanced flywheel energy storage systems have rotors made of high-strength carbon-fiber composites, suspended by magnetic bearings and spinning at speeds from 20,000 to over 50,000 rpm in a vacuum enclosure. These flywheels can come up to speed in a matter of minutes, reaching their energy capacity much more quickly than some other forms of storage.
Compared with other ways to store electricity, flywheel energy storage systems have long lifetimes with little or no maintenance, with full-cycle lifetimes ranging from 105 to 107 cycles of use, high specific energy (between 360 to 500 kJ/kg), large maximum power output and an energy efficiency, also known as round-trip efficiency, as high as 90%.
Unfortunately, flywheels cannot be used for longer term energy storage if they are not regularly topped-up, as energy losses will dissipate the stored energy, albeit slowly. This limitation can be overcome if one day’s production of renewable electricity can be used within a couple of days and the system is being continually topped-up.
The U.S. Navy awarded a two-year development contract for the mechanical battery in April 2021, which will include testing performance and safety under various conditions. The device will be evaluated for supplying power not just for weapons but for sensors and propulsion, for example, in uncrewed submarines, and for backup power.
David Whitby is chief executive of Pathmaster Marketing Ltd. in Surrey, England. You can reach him at pathmaster.marketing@yahoo.co.uk.