Beacon Power has updated the old idea of short-term energy storage via flywheels:-
Flywheel energy storage works by accelerating a cylindrical assembly called a rotor (flywheel) to a very high speed and maintaining the energy in the system as rotational energy. The energy is converted back by slowing down the flywheel. The flywheel system itself is a kinetic, or mechanical battery, spinning at very high speeds to store energy that is instantly available when needed.
The primary use at the moment appears to be frequency regulation for electricity generation, a well-known problem with wind and solar, especially solar. As Beacon Power says:-
To ensure a functional and reliable grid, the Independent System Operators (ISOs) that operate the various regional grids must maintain their electric frequency very close to 60 hertz (Hz), or cycles per second (50 Hz in Europe and elsewhere). When the supply of electricity exactly matches the demand (or "load"), grid frequency is held at a stable level. Grid operators, therefore, seek to continuously balance electricity supply with load to maintain the proper frequency. They do this by directing about one percent of total generation capacity to increase or decrease its power output in response to frequency deviations.
Not all generators can operate reliably in such a variable way. Changing power output causes greater wear and tear on equipment, and fossil generators that perform frequency regulation incur higher operating costs due to increased fuel consumption and maintenance costs. They also suffer a significant loss in "heat rate" efficiency and produce greater quantities of CO2 and other unwanted emissions when throttling up and down to perform frequency regulation services (my emphasis).
Flywheel storage technology must add substantial frequency regulation costs to wind and solar. In the case of wind in the UK, these are costs which its proponents have so far succeeded in spreading around elsewhere.
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