It's happening, the power grid is changing.

[Delta]

have a giant elevated reservoir and pump water in it when having excess electricity supply. Quite cheap and gravity is pretty fast.

That's called pumped hydro is is already in use, however the response times are far too slow for this specific application.

[Ian.M]

https://en.wikipedia.org/wiki/Dinorwig_Power_Station
75 seconds from stationary or 16 seconds from spinning synchronised 'motoring' standby.
The water in the penstocks has significant inertia, and valves take finite time to open.

The flywheel idea is more promising, but it cannot be kept at a rotational speed equivalent to 50.5Hz then cut in as required, as it will take time of up to 1/freq_diff to synchronise before it can be cut onto the grid.  That could easily be 0.7 seconds (grid dropped to 49.8Hz).  It also wouldn't be able to accept excess power from the grid during a sudden demand drop.  More promising would be to interpose a differential gear, with the flywheel and generator on either side of it and the main shaft normally locked.  Add a torque converter and reduction gear running off the flywheel to drive the main shaft in either direction and you could pick up the load or accept power as quick as you could release a brake band and slam in a clutch.   However the mechanical complexity and  maintenance requirements would be severe.

I suspect that the only commercially viable solutions will be electronic ones using storage batteries or supercaps.  Reversible high voltage DC<>AC inverters are a well understood and mature technology that's already on the grid (see https://en.wikipedia.org/wiki/HVDC_Cross-Channel), just build one with a storage bank instead of a link cable and updated control systems to handle cutting in in one cycle.

[Kleinstein]

The fly-wheel could use a slowly electronic rotating field instead of a fixed DC field. So the mechanics would provide something like 45  - 55 Hz and the electronics would compensate for the +-5 Hz only.  This works reasonably well with some windmills and the electronic DC/AC converter only needs to work on a faction (e.g. 10%) of the power of the whole setup.

For generators  and motors the efficiency gets better and specific costs usually go down when they get larger - so there is an incentive to make rotating machines relatively large, though it still small enough to allow transport. So it likely won't be a 500 tons fly-wheel, but also not many 0.5 ton ones. For a flywheel there is also given size / speed relationship. So they would run to fast (thus no easy generator / motor and more bearing problems) if too small.

One big candidate for this might be battery storage at those windmills that already use electronic DC/AC converts and larger solar power installations - though likely not much solar in England.

[Someone]

In general a fast starting Diesel backup scales up to MW sizes and can be assisted in starting with compressed air. They form the essential safety systems of power stations.

In the UK the power source of last resort, i.e. "black start" if the entire network is down, is pumped hydro.

These examples are not black start resources, they are immediate ride through and backup to provide uninterruptible power for life critical systems (such as the safety systems of power stations) and flywheel + diesel is a very mature and scalable technology. As above hydro can provide massive energy storage but comes on slower than the tender requires, so it could be a component of a bid but there still remains the need for an extremely fast dispatchable generator to ride through until the slower generators come on (hydro and open cycle gas turbines would be currently called "fast" and have similar dispatch speeds of many seconds but less than a minute).

[DenzilPenberthy]

In the UK the power source of last resort, i.e. "black start" if the entire network is down, is pumped hydro.

These examples are not black start resources,....

"The Cruachan Power Station (also known as the Cruachan Dam) is a pumped-storage hydroelectric power station in Argyll and Bute, Scotland.  It is one of only four pumped storage power stations in the UK, and is capable of providing a black start capability to the National Grid."

"Another important feature of Dinorwig [pumped hydro site] is that it has been designed to assist restarting the National Grid on the occasion of a complete power failure (a black start). It includes diesel generators and large batteries which would allow the plant to restart even in the event of a complete shutdown of the grid (Dinorwig is not unique in this respect as some fossil-fuel plants are also able to self-start)."

...

[Someone]

In the UK the power source of last resort, i.e. "black start" if the entire network is down, is pumped hydro.

These examples are not black start resources,....

"The Cruachan Power Station (also known as the Cruachan Dam) is a pumped-storage hydroelectric power station in Argyll and Bute, Scotland.  It is one of only four pumped storage power stations in the UK, and is capable of providing a black start capability to the National Grid."

"Another important feature of Dinorwig [pumped hydro site] is that it has been designed to assist restarting the National Grid on the occasion of a complete power failure (a black start). It includes diesel generators and large batteries which would allow the plant to restart even in the event of a complete shutdown of the grid (Dinorwig is not unique in this respect as some fossil-fuel plants are also able to self-start)."

...

Yes, thats a black start resource. We've been talking about fast starting Diesels generators for riding out interruptions and providing a fast energy supply, often coupled to flywheels to provide immediate (cycle by cycle) availability.

[ConKbot]

In the UK the power source of last resort, i.e. "black start" if the entire network is down, is pumped hydro.

These examples are not black start resources,....

"The Cruachan Power Station (also known as the Cruachan Dam) is a pumped-storage hydroelectric power station in Argyll and Bute, Scotland.  It is one of only four pumped storage power stations in the UK, and is capable of providing a black start capability to the National Grid."

"Another important feature of Dinorwig [pumped hydro site] is that it has been designed to assist restarting the National Grid on the occasion of a complete power failure (a black start). It includes diesel generators and large batteries which would allow the plant to restart even in the event of a complete shutdown of the grid (Dinorwig is not unique in this respect as some fossil-fuel plants are also able to self-start)."

...

Black-start isnt relevant to this fast transient capable grid improvement though. Black-start is for re-starting the grid after grid-wide outages. I.e. everything has gone black. You start the diesel backup generator at the hydro station, open up the sluices, provide excitation for the generators, provide power to a coal/NG/nuclear power station which provides base load, start getting other power stations online and re-synchronized while re-connecting load.  Hydro power (pumped or standard) is generally a good starting point for a black-start capable power station, no multi megawatt cooling pumps and air blowers, etc.  And since you're just changing a valve to adjust flow to a turbine, and don't have an equilibrium to manage, or massive amounts of water to heat or cool (nuclear/coal/nat-gas steam plants) they can be pretty responsive too, but black-start doesn't imply fast transient response, and fast transient response doesn't imply black-start

On the grid transient handling issue. Obviously for a grid-scale thing you need big things where scaling them up provides economy and doesn't cause safety issues (like storing all the energy in a massive flywheel) I wonder if flow-battery technology has improved enough to be useful for something grid-scale, without using chemicals conceived by satan himself. Or for hydro-electric power, if it would be possible to use smaller, faster-responding turbines  and a different intake design to improve transient response. I.e. a dam with 30  2 MW turbines intead of 4 15MW turbines, even if they were grouped together to keep from increasing the amount of support equipment excessively.