It is deeply moving to hear announcements of aggressive decarbonisation targets, and it must be deeply gratifying for politicians, such as Boris Johnson, to make them. It is also deeply disturbing to find how little the political and financial leadership understands about what it will take to hit them.
Almost all of us have been living off a legacy of inertia. I speak not, in this particular case, of our families’ idleness supported by great wealth, but the inertia of the spinning masses connected to our electricity grids. All those fossil, hydro, and nuclear generators connected together stabilise each other like spinning tops that resist toppling over.
When one (synchronous) generator slows down due to extra loads being added by nearby customers, other synchronous generators speed up to maintain the grid’s rated frequency, 50 cycles per second (Hertz, or Hz) in much of the world, or 60Hz in North America. In the past, most motors’ operating frequencies were synchronised with the grid, which further reinforced stability.
But in recent years, customers, starting with industrial users, started using more efficient variable frequency drives for motor power. This eroded demand side frequency stability. And, of course, intermittent renewables such as wind and solar took ever larger shares of the generation mix.
Intermittent renewable generators are connected “asynchronously” to the grid. The wind turbines do not spin at the continuous speeds of the legacy systems, and so they and photoelectric solar arrays are connected through power electronics that process their power to bring the output as close as possible to the standard grid frequency, while also attempting to maintain sufficient voltage, the electrical equivalent of water pressure in a pipe.
If sensors connected to a generator, transmission operator or distribution company detect the grid’s rate of change of frequency (Rocof) moving too quickly, those grid elements will quickly disconnect until the grid stabilises and technicians reconnect everything. Otherwise, unstable electric forces could twist apart a generator shaft, or, if repeated, lead to the early failure of transformers and other electrical assets.
At low levels of renewable penetration in the grid’s generation mix, it is not too challenging to keep everything balanced. As decarbonisation proceeds, it becomes more difficult, and power electronics failures, such as happened in southern England in August 2019, can cause serious blackouts.
That incident may have accelerated the plans of National Grid ESO, the UK’s electricity supply operator, to create a “market” for inertia, through which it can procure the stability previously provided for free thanks to those spinning masses. National Grid ESO announced in January that it had agreed contracts with five parties to provide 12.5 giga-volt-ampere seconds of inertia over six years for £328m.
Helpfully, the ESO explained that 12.5 GVA “is the equivalent of the inertia provided by approximately five coal fired power stations”.
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Julian Leslie, the ESO’s head of networks, elaborated that “this is not an inertia market per se, because one can only buy inertia (which is usually expressed in MW seconds) from rotating plants connected to the system”. Initially, these are either converted fossil fuel sites or existing pumped hydro generators. Where hydro is not available, the inertial energy will be provided by flywheels connected to generators, spinning away until the moment their power is required.
The initial contracts are just the beginning. Eric Lewis, a power systems engineer and adviser to the ESO’s inertia programme, says “The requirements for inertia will increase exponentially. Flywheels are good for the first few seconds [of a frequency fluctuation], but then other energy storage systems and generators have to take over.”
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The Irish electric system (which spans the Republic and Northern Ireland) has been working on post fossil fuel stability even before the UK’s National Grid, and does not have nuclear plants to provide carbon free inertia. Marios Zarifakis, the electrical asset manager of Ireland’s ESB generation, says “you could supply synthetic inertia with batteries, but there is a (brief) delay (with the power electronics) and they are really expensive.” Flywheels, and, in the future, hydrogen fired turbines, are better sources of inertia, he thinks.
Among European power people, by the way, there is a general annoyance at how Germany’s Energiewende programme presumes neighbouring countries will supply balancing energy, inertia and voltage for their renewables.
There are not many fast growing markets in the world. Power stability services are going to require a great deal of financial, as well as electrical, engineering.
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