[sloucher]

As I understood the article, this isn't about storing energy so much as just stabilising the frequency of the grid.

(Yes, if we used more hydro-electric then that would do the job for us, but it's not the case that we need this in order to store energy).

[LeoPanthera]

Frequency is proportional to voltage, and the voltage drops when the supply is insufficient, so this is absolutely about storing energy.

[jrockway]

That's not quite how the grid works. Consider a case where you wire two AC motors together. You spin one, and the other one spins. That is kind of like how the grid works; many loads are AC motors that spin at the line frequency, and generators try to also spin at that frequency. If the load on the motors increases, the load on the generators increases. Eventually there won't be enough energy going into the generator to spin at the nominal line frequency, and the motors attached to the mains will spin more slowly (and correspondingly use less power, and output less work).

This all becomes more complex when the loads are switch-mode power supplies and the generators are solar panels. The flywheel project adds some smoothing to the grid, as spinning generators are replaced with DC devices. But the principles are the same; without any extra devices, electricity has to be generated at the exact instant that it is consumed, and the consumers have a large physical effect on the generators.

(Frequency is not proportional to voltage in general, only in this spinning-generator connected to a spinning load case. You can obviously switch DC on and off at whatever frequency you desire, and a perfect on/off cycle consists of infinitely many frequencies at various amplitudes.)

[VBprogrammer]

I'm not sure where you are getting that frequently is proportional to voltage. Voltage in a synchronous generator is controlled by varying rotor excitation current. Frequently is controlled by varying the power to the prime mover.

But your main point is correct. This has to store energy to control frequency.