The case for synchronous energy storage

11 November 2020

Blackouts experienced in the UK last year prompted a flurry of headlines expressing concern that ‘near-misses’ were on the rise. The national grid and its ability to adapt to the changes that renewable energy is introducing were brought into question.

The UK has enjoyed safe access to electricity with very little disruption for decades. It has rarely been impacted by extreme weather events that cause problems, or a significant lack of investment in infrastructure. However, National Grid PLC clearly understands that to avoid any further incidents it needs to shore up its stability and one of the technologies that it is investigating is energy storage.

One of the reasons that outages occur is a systemic failure to maintain grid frequency and this can be as a result of a deficiency in system inertia. Frequency and inertia are crucial to the successful operation of the grid and in meeting our objective of making renewables a baseload power source.

System inertia resists changes in grid frequency caused by shifts in demand and generation. This has been achieved typically by the rotating masses of turbines and rotors electromagnetically coupled to the system (synchronous) – a useful byproduct of conventional generation. Large rotating masses are hard to stop once spinning and therefore have high inertia. Electromagnetically coupled equipment also provides short circuit power.

One of the main challenges for modern grids is that they are more reliant on renewables, and many of these, particularly wind and solar, are considered to have virtually no inertia. With the exception of pumped hydro, renewables do not include spinning reserve that is electromagnetically coupled to the system. Even as hundreds of solar arrays and wind farms connect to the grid and deliver much needed green energy, they contribute to a growing vacuum of inertia, and this presents massive challenges when it comes to maintaining consistent frequency. There is also diminishing short circuit power.

The answer is to find a way to create synchronous inertia within the grid from renewables. The National Grid in the UK is researching ways of doing this, and has invested £180 million towards developing a stable system that will allow wind, solar and lithium-ion (inverter based systems) to function in a way that allows frequency to be consistent.

One promising technology is the CRYOBatteryTM, developed by Highview Power, a long duration synchronous cryogenic (liquid air) energy storage system that includes rotating masses electromagnetically coupled to the grid that provide inertia (as well as contributing to short circuit power, and also having the scale and characteristics required for “black start”).

The instantly available inertia of rotating masses provided by a system such as CRYOBattery should not be confused with “synthetic inertia” – provided, for example, by power electronics/battery – which is necessarily delayed by measurement (approx. 80 milliseconds), by which time frequency could have deviated sufficiently to trigger protection (on RoCoF (rate of change of frequency)).

The CRYOBattery system can maintain grid synchronisation in both charging or discharging phase, as the system uses both a turbine and synchronous generator (discharging) and a compressor and synchronous motor (charging). A “Stability Island” option, with the addition of clutch and flywheel, can increase capabilities.

With a synchronous system such as CRYOBattery, the grid can enjoy a more stable frequency, the threat of blackouts can be reduced and energy storage can be maximised in support of a 100% renewable energy future.

Author information: Gary Preece, power systems development director, Highview Power

Pillars of the AC transmission system
Declining levels of inertia (UK), for various future scenarios
How liquid air energy storage works
The Stability Island option for the CRYOBattery

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