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Please be warned that I might misuse technical terms in my below question. I am a German native speaker and not in any case you find translations in common dictionaries.

Part of the operating reserve of a grid is spinning reserve. As I understand it, common examples for it are the turbines of thermal power plants and dedicated flywheels. In both cases something heavy rotates and drives an electromagnetic generator. If there is a sudden spike in resistance in the grid (e.g., a factory machine is powered up) the spinning reserve can catch this spike with its inertia.

Due to Germany now using more than 50% renewables over the course of a year the German grid is in dire need of spinning reserve. We call it "Momentanreserve", which is translatable to "instant reserve".

Therefore the German grid authority (BNetzA) published specifications about how transmission system operators are supposed to handle "instant reserve". https://www.bundesnetzagentur.de/DE/Beschlusskammern/1_GZ/BK6-GZ/2023/BK6-23-010/BK6-23-010_zweite_konsultation.html?nn=660086

They mention something they call "synthetische Schwungmasse" which I would translate to "synthetic inertia having mass".

Can you explain in laymen's terms how it is possible to provide "instant reserve" without physical mass? Are the magical terms I need to look up "Grid Forming Inverters"?

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    \$\begingroup\$ People can find here and there cargo container like new buildings which are not easy to see from busy roads. In rural areas the surface of the ground reveals that some underground cables are drawn to them. There's no other info than "Keep out!" and name of an unknown foreign company. Someone says they are preparations for the soon coming putch. Others say they are battery stations rented by electric power companies to store electric power when its stock market price is low and and to output it when the price is high. If that's true they very likely can also handle fast demand peaks. \$\endgroup\$
    – Absus4
    Commented Sep 23 at 11:26

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Spinning metal is just one way to store energy. As long as you have:

  • an energy storage system, and
  • a quickly-responding means to couple it to the grid,

you got the Schwungmasse. It's considered synthetic when the energy isn't stored in angular momentum. So, a battery storage plant would be a synthetic Schwungmasse. It would be so, to an extent, even at full load, since batteries and inverters can take momentary overloads if they are designed for it.

So would be a solar plant running under-capacity, since its inverters can react very quickly to changes in demand.

In principle, renewable energy is not a problem in itself - but its high utilization is. A solar plant running at 50% capacity may not be very economical to operate, but it sure has quite a bit of equivalent Schwungmasse as it can double its output on the scale of one half-cycle.

With renewable sources running at full capacity, their equivalent Schwungmasse is zero, more-or-less. That's the source of the problem. A thermal plant running at full capacity has same Schwungmasse as an idle one, since the stored angular momentum does not depend on the load at all. It's constant.

This is the fundamental difference between plants with rotating generators, and those without them. A rotating generator and turbine provide a constant Schwungmasse at all times. The Schwungmasse of a wind or solar plant is maximum at no load, and zero at full load, approximately speaking.

A wind turbine has some Schwungmasse at all times as well. The machinery in the nacelle has to be light so there's not much margin left when running at full load. Thus the turbine controller will keep the load limited to 100% or close to it, even if with sufficient overbuilding, it could output 500% for one turbine revolution for example.

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  • \$\begingroup\$ Are large scale capacitors also a viable option to provide this kind of instant operating reserve? \$\endgroup\$
    – Merlin Nestler
    Commented Sep 23 at 8:25
  • \$\begingroup\$ Theoretically? Of course. Except nobody can or wants to pay for them, because of their high cost relative to energy stored. We're not talking, say, 2x as expensive as batteries. We're talking order of magnitude or more expensive. A dense capacitor bank will thermally run away just as batteries do if there's a fault that causes capacitor failure. The only reason thermal runaway is not a big problem for capacitor storage yet is because capacitors have such low energy density. Thermal runaway has to do with energy density, not tech (battery vs capacitor). Enough heat in a small volume and it goes \$\endgroup\$
    – Kuba hasn't forgotten Monica
    Commented Sep 23 at 8:32

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