Why Lots Of Rabbits Have To Be Harnessed To Keep Grid Clocks Aligned – CleanTechnica

Sign up for daily news updates from CleanTechnica on email. Or follow us on Google News!

---

The UK now has an active market for grid inertia, marking a significant step toward ensuring stability in an electricity grid increasingly reliant on intermittent renewable energy sources. The first successful bidder to provide this inertia service was a notably straightforward yet effective technology: an electric motor spinning a massive heavy metal flywheel paired with a condenser, an electrical component used to store energy electrostatically in an electric field, also known as a capacitor. According to ABB, the global technology giant that provided this solution, established industrial technologies continue to play critical roles in modern grid management.

This brief explainer article on inertia, its value propositions for the biggest machine in the world — our electrical grid — and how it is changing was based on a request for clarification and expansion on the transcript of the recent discussion I had with Mark O’Malley, Leverhulme Professor of Power Systems at the Imperial College of London and founder of the Global Power System Transformation organization. The entire conversation was about inertia, but that was the starting point for the conversation, not a basic explainer with clear examples. I’ve been dealing with the subject on and off for a decade, and have a fraction of the knowledge that O’Malley and his global collaborators in industry, academia and government do on the subject, but still have so much context that I could have an interesting conversation with O’Malley (at least, on my side as I won’t presume to speak for him). My bad for not more clearly establishing the basics.

Initially, it may seem surprising that mechanical inertia, involving such straightforward rotating equipment, remains competitive against sophisticated power electronics. However, inertia is fundamentally about storing and instantly releasing kinetic energy to stabilize frequency and voltage fluctuations, something mechanical systems inherently excel at.

Synchronous generation refers to power plants — typically fossil fuel, nuclear, or hydroelectric facilities — whose spinning turbines operate in precise alignment (or “in sync”) with the grid’s frequency, measured in hertz (Hz). For instance, the UK’s electricity grid runs at exactly 50 Hz, meaning synchronous generators must rotate at speeds exactly matching this frequency — 3,000 revolutions per minute, an exact multiple of 50, for many UK generators. When grid frequency rises or falls even slightly from 50 Hz, synchronous generators naturally resist this change by either absorbing or releasing kinetic energy stored in their rotating mass. This stabilizing effect, known as grid inertia, is essential for maintaining stable frequency and voltage, ensuring the lights stay on and equipment isn’t damaged.

In contrast, inverter-based renewable generation, like solar and wind, doesn’t naturally synchronize with the grid’s frequency. These systems initially produce direct current (DC), which power electronics convert into alternating current (AC) at the grid’s frequency. Without additional controls, these inverters do not inherently provide inertia.

The closure of fossil-fuel power plants, essential for achieving climate targets, has significantly reduced traditional grid inertia, creating the need for new solutions. As renewables such as wind and solar increasingly dominate, the inertia traditionally provided by large synchronous generators is diminishing, requiring alternative mechanisms to prevent instability.

ABB, which won the UK’s initial inertia market auction, delivered a system comprising a large electric motor spinning a heavy mass with an integrated condenser. ABB highlights that this setup allows the kinetic energy stored in the spinning mass to respond instantaneously to grid disturbances, providing the short-term inertia needed to maintain frequency stability.

At the time, almost two years ago now, I was surprised that a mechanical system won the first auction, as I had been aware of the ability of power electronics to provide this ancillary service and one of the principles in my decarbonization decision-making framework is “Solid-state outperforms the mechanical.” It’s not a hard and fast rule, clearly, but it’s more true than not.

The discussion with O’Malley was all about solid-state inverters — the wildly multiplying rabbits of many different sub-species in the metaphor for the discussion — performing the services of the big chunks of spinning metal, but I’ve been interested to find that this may be a place where the principle has exceptions, just as with pumped hydro.

Similar mechanical inertia concepts have been explored previously, such as compressed air passing through turbine systems, a method demonstrated at smaller scales elsewhere. For instance, turbines used by the UK energy sector have adopted compressed air to rotate turbines during low-load conditions, maintaining inertia without fuel combustion. This approach has proven effective enough to be considered a viable ancillary service by grid operators, underlining mechanical inertia’s enduring value in a modern electricity system.

I became aware of this when looking at the Dinorwig storage facility during a client engagement with a European green investment fund in mid-2024. The facility, also known as Electric Mountain, is a pumped hydroelectric facility located near Llanberis in North Wales, UK. Completed in 1984, Dinorwig was specifically built to rapidly respond to fluctuations in electricity demand, providing grid stabilization and peak load management. With a capacity of about 1,728 MW, it’s one of Europe’s largest and fastest-responding pumped storage facilities, capable of reaching full output in just under 16 seconds. Dinorwig plays an essential role in balancing the UK’s electricity grid, particularly as renewable energy sources increasingly dominate generation.

When the turbines aren’t being used to push water uphill or generate electricity as it flows downhill, they are kept spinning by pumping compressed air through them, keeping the inertia on the grid for those value propositions, but also allowing the system to respond to demands for power more quickly. Traditional pumped hydro plants equipped with synchronous turbines usually require between 30 seconds to a few minutes to spool up from stationary to full power.

Of course, as I discovered, Dinorwig’s response time includes a control room with human staff who answer the phone, so it’s unclear how fast the response really is. This isn’t the lights out pumped hydro facility China Light and Power (CLP), Hong Kong’s electrical utility operate in Guangdong on the mainland, which is controlled remotely. When I was talking with CLP three or four years ago related to a battery energy storage opportunity for them, they told me that they had transitioned to fully lights out in 2010. It’s unclear to me what the 200 full time equivalent staff at Dinorwig do, and it’s clearly a place where automation and digital connection are required for it to remain a useful and cost competitive part of the UK energy landscape.

While mechanical inertia remains viable, advancements in power electronics and high-voltage direct current (HVDC) technologies provide increasingly compelling grid-forming alternatives. Variable source commutation (VSC) stations associated with HVDC connections represent particularly promising technologies. I spent time learning more about this a year ago with a deep global expert on the subject, Cornelis Plet, head of growth and technology related to HVDC at DNV.

HVDC lines, which carry gigawatts of energy, use VSC technology capable of shaping precise synchronous waveforms, effectively functioning as powerful grid-forming units that can provide synthetic inertia. Such HVDC systems can instantly adjust to grid frequency and voltage changes, offering superior control and precision compared to conventional inertia sources. According to National Grid ESO, HVDC connections terminating in VSC stations have been identified as pivotal grid-forming assets in long-distance renewable energy transmission, particularly offshore wind farms connecting to the UK grid, something O’Malley and I spoke about as well.

Similarly, power electronics are increasingly positioned as critical components for managing inertia. Modern converters and inverters associated with renewable generation and battery storage systems can rapidly inject or absorb power, stabilizing grid frequency more flexibly than traditional mechanical systems. According to WindEurope, wind turbines can be operated with slightly feathered blades to maintain reserve capacity specifically for ancillary services, providing grid-forming capabilities when necessary.

Battery storage technology further amplifies the capabilities of power electronics for inertia provision. By pairing lithium-ion battery systems with advanced inverter technology, operators can achieve rapid frequency response and voltage regulation, effectively substituting traditional inertia. Per the Australian Energy Market Operator, Australia’s Hornsdale Power Reserve demonstrated the capability of large-scale battery storage in managing rapid frequency response, significantly reducing the need for fossil-fueled spinning reserves.

The work that O’Malley and the Global Power System Transformation (GPST) Consortium is doing is dedicated to integrating these modern inertia solutions. Efforts specifically focus on integrating renewable resources, grid-scale batteries, and HVDC systems into coordinated grid-forming frameworks, defining technical standards, and operational protocols to manage inertia in decarbonized power systems. That’s harnessing the rabbits to keep the clock ticking at the right hertz, to bring the metaphor to a close.

The emergence of the UK’s inertia market underscores a critical transition in grid management, moving from dependence on fossil fuels and mechanical generators to diversified sources of inertia, including sophisticated power electronics, mechanical rotating mass, and coordinated renewable generation. The interplay between traditional and modern inertia technologies is essential in managing the grid’s stability and reliability in an era dominated by intermittent renewables. Hopefully this brief explainer provides more of a basis for following O’Malley’s insights on the podcast.