Recently, a major interconnector trip sent the UK’s grid frequency plummeting. At around 8:47am on a morning in early October, the NSL interconnector linking the UK and Norway, suddenly and with no warning, halted.
Norway’s power exports into the UK suddenly dropped from 1.4GW to zero, with immediate and potentially disastrous impact on the UK’s electricity grid. Frequency on the network crashed as low as 49.59Hz in less than two seconds, a number significantly lower than the National Energy System Operator (NESO) operational limits of 49.8 – 50.2Hz. Without swift intervention, much of the nation’s grid could have failed.
Here, battery energy storage systems (BESS) answered the call. Across NESO’s network, 1.5GW of BESS assets came online to inject power into the system, bringing frequency to strong levels within two minutes. The whole story is a real testament to the crucial importance of BESS in our modern grid system, an event which Roger Hollies, CTO at Arenko Group, described as “exciting”. Hollies said: “It’s exciting to see batteries casually keeping the lights on whilst delivering diversity of activity to maximise revenue. I count nine markets and services being participated in by these 12 batteries during this 50 minute window alone.”
Despite this, battery storage developers have expressed a feeling of underutilisation by the NESO and its predecessor, ESO. An open letter penned by a coalition of BESS developers, including top names like Zenobē, Eelpower, Harmony Energy and Field, noted that energy system operators frequently favour turning off renewable energy sources, such as wind turbines, to cope with energy oversupply rather than make use of BESS assets. The coalition argued that this holds back investment and drives up consumer bills; it could be argued that such missed investment opportunities could slow BESS development and thus limit its usefulness as an asset during interconnector trips.
The advantages of BESS in emergency scenarios
BESS assets are uniquely positioned to support emergency grid and frequency balancing due to the speed at which they can respond when needed – milliseconds to seconds. The reason for this speedy response is inherent to the nature of BESS compared to other energy sources; as Hollies explains: “A BESS doesn’t have to be “on” to provide a support service. All synchronized systems, e.g. gas-powered turbines, have to be operating at a minimum power – that is, burning fuel – to be able to respond quickly to an emergency frequency event. BESS do not; they can respond very quickly from even a deep standby state.”
The modular nature of BESS assets also makes them useful in emergency scenarios; even if a BESS is partially down, the parts of the asset that are online can still provide a response.
Aside from interconnector trips, BESS assets are useful in other emergencies. Several BESS asset operators have praised the potential value of batteries in “black-starting” support services if a blackout were to occur, including a new player in the market Root-Power, whose managing director Neil Brooks noted: “BESS’ flexibility, quick deployment and compatibility with other renewable energy sources make it particularly well suited for disaster response and recovery, as well as improving Britain’s energy resilience. Batteries are also useful for “black start” support in the event of a blackout, helping to jump-start other forms of generation that typically require power themselves from the grid to start.”
What the sector needs to thrive
The storage sector has criticised the NESO extensively, with the same complaints arising repeatedly.
Grid connection timescales remain a significant concern, with Brooks noting that BESS projects frequently receive grid connection dates as far away as 2030 or even 2035. While the NESO, as well as distribution network operators (DNOs), have stated plans to implement changes to speed up the connections queue, including the removal of so-called “zombie” projects, the reality for developers is far different.
Brooks added: “To date, we have not seen the numerous NESO strategies discussed to bring forward connection dates have any real positive effects on project timescales, and there is still a large disconnect with industry-wide reforms and the reality of what is being implemented at a DNO level. Much work needs to be done to ensure that DNOs implement these industry-wide reforms to projects at the distribution level and that DNOs undertake their own reassessment of the connections queue.”
Oli Petterson, Balance Power’s connection manager, believes that the way in which network operators model the impact of BESS projects isn’t reflective of real network usage. He said: “Modelling of assets prior to going online needs more attention here. Due to the way BESS schemes are modelled by network operators, the need for reinforcements is constantly increasing when, in actual fact, the existing import availability on the network is not being fully utilised. This has pushed a lot of schemes back five to ten years at a time when BESS is critical for market and network stability.”
Meanwhile, Hollies believes that the answer lies in reforms to the capacity market (CM), noting: “Currently all BESS with CM contracts are notified four hours in advance of the need to deliver close to full output for a stress event, for an undefined length of time. This could have the unintended consequence of causing another stress event, with all BESS electing to charge in advance to be able to deliver on their obligations (where over-delivery is rewarded), but also it represents an inefficient use of the bulk of what are 1-2hr assets.”
He added: “An added coordination mechanism between BESS is needed that can fairly ‘sequence’ the discharge of these shorter-duration assets during times of system stress to fully capture their benefits.”
The way forward
The recent near-crisis on the UK’s grid highlights an undeniable truth: BESS are no longer just supplementary players; they are critical to keeping our lights on.
We need reforms that match the speed and responsiveness of BESS itself. That means a streamlined grid connection process, realistic modelling that reflects how BESS assets actually interact with the grid, and an overhaul of capacity market rules.
If we want a resilient, forward-looking energy system, NESO and regulators must stop treating BESS as a backup plan and start making it a pillar of the UK’s energy strategy. With the right policies, BESS can provide the robust, flexible support our grid urgently needs, ensuring the lights stay on.
As Roger Hollies puts it, the growth we have seen thus far in the BESS market “has nothing to do with green credentials: BESS are just better and cheaper than the alternatives.”