mechanical battery

Most batteries that we use on a daily basis are chemical batteries. They contain chemicals that react in some way to create a flow of electrons.
Lead-acid batteries, alkaline batteries, Nickel-metal hydride batteries and lithium-ion batteries are all chemical batteries. Chemical batteries are especially handy because they can be small -small enough to fit inside a hearing aid if need be.
But there is an entirely different type of battery that is available if you don't need it to fit inside your ear canal or your pocket. These are called mechanical batteries, because they rely on mechanical energy rather than chemical energy.
A flywheel battery is one form of mechanical battery. It uses the kinetic energy stored in a flywheel to create a flow of electrons. To "charge the battery", electricity drives an electric motor that spins up a flywheel. In the simplest possible flywheel battery, the flywheel could be a heavy steel wheel mounted on an axle that might spin at a maximum of 5,000 RPM. To use the battery, a generator converts the kinetic energy stored in the spinning flywheel back into electricity. It is easy to imagine that a big steel wheel weighing 1,000 pounds could store a lot of energy, and you would be right.
Modern technology has made flywheel batteries more sophisticated than you might imagine. The problem with steel is that it can only spin so fast before it flies apart, and if it does fly apart it can be dangerous. Many of the newer flywheels are therefore made out of carbon fiber. The strength of carbon means that these flywheels can spin much more rapidly. Modern flywheels also spin in a vacuum and use magnetic bearings so there is zero friction. A flywheel about the size of a trash can can store enough power to run a typical American household for a day.
A flywheel battery has a number of advantages compared to a chemical battery. The first advantage is the ability to absorb and store quick bursts of energy much more efficiently than a chemical battery can. The second advantage is high efficiency -very little energy is lost when charging the battery (spinning up the flywheel) or discharging the battery. The third is long life -chemical batteries tend to wear out after some number of charge/discharge cycles (500 to 1,000 cycles is typical). Flywheel systems do not. That also leads to lower maintenance costs and better reliability compared to battery systems.
The main disadvantage of flywheel batteries is their minimum size. It is unlikely that a flywheel battery would ever be found in a cell phone, for example. On the other hand, a flywheel battery would be ideally suited for things like whole-house batteries or other batteries where size is not a big issue.
You might wonder why you would need a whole-house battery. One obvious application is as an emergency power supply in case of a power failure. Or, if you are powering your house off of solar panels, a whole-house battery would let you store electricity for use at night. A whole-house battery could also allow time shifting. You could buy power from the power company at night when it is much less expensive, and then use the power during the day for air conditioning. Over time, a flywheel battery used in this way would pay for itself.
There are other types of mechanical batteries available as well. At some wind farms there have been experiments with storing compressed air in huge underground salt caverns. Salt caverns are handy because they are pressure-tight and self-sealing. When the wind is blowing, electricity drives air compressors that fill the caverns with pressurized air. When the wind stops, the pressurized air drives turbines that spin generators.
Another large-scale mechanical battery can be found near select power plants. Two lakes are arranged one above the other on a hillside. At night when the power plant has excess capacity, it pumps water to the upper lake. During the day, the water falls from the upper lake to the lower lake and drives generators like those found in a hydroelectric dam.
If flywheel batteries become cheap enough and reliable enough, it is easy to imagine finding them in homes and in electric cars. They would take the place of chemical batteries in a way that is lighter and less expensive.