flywheel to keep the Hz up to prevent blackouts

These flywheels are to assist with short term stability. Any sudden drop in grid frequency will result in the flywheel slowing and therefore giving up energy to the grid and thereby limiting the frequency fall.

Historically the large rotating mass of steam turbines and alternators coupled thereto provided this inertia, but these days more and more input to the grid is via static converters that provide no inertia.


Flywheels of any reasonable size wont help for other than "second to second" stability. Bulk energy storage requires either pumped storage, or increasingly utility scale batteries. IIRC we already have about 0.5 GWH of battery storage, and another 4 GWH are proposed. 

For the foreseeable future we will still need to burn gas at times of low renewable input, but it seems prudent to reduce gas reliance as far as possible.

If you had such a flywheel, would not it's inertia also cause a delay in the grid recovering its frequency back to 50.0 Hz from whatever it had dropped to? Unless it could be mechanically decoupled and sped up in a non-locked to grid frequency mode?

Clive

Yes Clive I agree. Where does the energy required to get the flywheel back to speed come from? Of course how silly, more windmills. To store 30 degrees of phase error for the grid in a flywheel, it would need to weigh something around 100,000 tonnes, and would be very dangerous indeed if anything went wrong and it got free. The steam turbines can virtually instantly get more steam as required to correct the phase error, and the inertial energy storage requirement is quite small. That is basically how the speed governer works, from the reactive power measurement.

Flywheels for energy storage can work but they are not cheap or simple.

There is a design used in large UPS systems which has a device that looks a bit like a motor or generator except that the "stator" is rotating round the rotor at twice the synchronous speed. The rotor is on the same shaft as a standard synchonous motor / generator. If the power fails the generator shaft starts to slow down and rotating mass is connected magnetically to its rotor and speeds the shaft up again. This continues until the rotating mass has dropped to the same speed as the shaft - typically 3 secs but 3 secs is long enough for a diesel generator - also on the same shaft - to start and, because the whole system has never lost sync with its neighbours there is no time lost trying to sync anything.

But 3 secs is all the storage that this arrangement provides and it consumes about 50kW in overcoming air resistance to keep it running. They come in 2MW modules and a typical computer data centre might have 30 of them.

This is all assuming that the flywheel is connected to a synchronous generator but the reality is that most flywheel energy storage uses the stored energy to generate DC which then uses inverters to get the desired frequency. Flywheel storage sits alongside Batteries and Supercapacitors as the storage of choice for small to medium sized installations. The US Navy have tested flywheel storage as a source for energy weapons and aircraft carrier catapults, though I don't know how successful the experiments were (and am unlikely to find out....)

The "giant flywheel project" tag is misleading as it is inconceivable that a single flywheel would be used. Even ignoring the mass, stresses, etc. that would create so many problems there is the issue that the whole system needs to be shut down if any maintenance is required. So much simpler to have a string of flywheels and take one out for maintenance as and when required.

There are actually at least 3 sorts of systems, the induction machine

(IM),  permanent magnet machine (PM), and variable reluctant machine (VRM).

Permanent magnet machines are physically smaller, but lose energy all the time to magnetic hysteresis,  where the others can be spun up and left with the magnetic field reduced  until needed.

Induction machines, especially squirrel cage  are simpler, but of course are not quite synchronous - the analogy of motor slip applies in reverse while generating, and the low frequency difference frequency flows in the core. More complex designs can force a specific frequency in the core ,so that the output is synchronous, and some earlier wiind turbines did this too.

The variable recluctance motors are more interesting, being more of notched cog between coils, so that the inductance of the coils is modulated while current is flowing giving a varying voltage. This can generate or motor depending on the phase of the applied voltage to the coils to force the phase of the current or vice versa,  these are the best sort of genset where speeds are very high - like an un-geared gas turbine or small diameter wheel.

Nearly all serious (megawatt-minute) systems are indeed AC/DC/AC and have large banks of thyristors or IGBTs depending on the era it was designed in.


A Diesel Rotary UPS wikki article DRUPS is something else as the prime mover is the diesel engine, but also worth understanding to get a scale of what is being asked for on big systems