Multi-day energy storage, a solution for a clean and reliable grid in the UK and Ireland


The Challenge: Meeting the UK & Ireland’s grid decarbonization goals reliably and affordably

The United Kingdom and Ireland have set some of the most ambitious goals for electric grid decarbonization in Europe, with the United Kingdom setting a target of achieving a net-zero grid by 2035 and Ireland a target of at least 50% emissions reductions on its grid by 2030. However, both countries are projected to fall short of these goals unless more aggressive measures to curb emissions and deploy new, clean energy technologies are taken.1

As the UK and Ireland utilise more renewable energy sources, which are inherently weather-dependent and intermittent, energy storage technologies will increasingly be needed to ensure 24/7 grid reliability. The predominant type of energy storage technologies deployed in the UK and Ireland today are pumped hydro storage reservoirs, compressed-air energy storage facilities, and lithium-ion batteries, however both pumped hydro and compressed air face geographic constraints, and lithium-ion batteries are currently only able to provide up to 8 hours of energy storage. For longer stretches of low renewable power generation – or “dunkelflaute”, as they’ve been termed – new, long-duration and multi-day energy storage (MDS) technologies will be needed to ensure grid reliability, cleanly and affordably.

A key barrier that long-duration and multi-day energy storage technologies face is that existing market signals are not structured to incentivize widespread development and adoption. As acknowledged by the European Commission, as well as the UK Government and Irish System Operator, Eirgrid,2 market signals are currently oriented around meeting electricity demand day-to-day and lowering costs over the short run. Markets do not send investment signals for assets which can cost-effectively minimise volatility in highly renewable systems over the long run. Without acknowledging and incentivizing the full benefits that these assets can provide, the UK and Ireland’s electric grids risk procuring a sub-optimal mix of energy technologies in their future energy portfolios.

Recently, Form Energy conducted research to understand the full value that long-duration and multi-day energy storage technologies could bring to the UK and Ireland’s future, decarbonized electric grids. This analysis was conducted using Formware, Form Energy’s electric system simulation model, with data from Aurora Energy Research, Baringa Management Consulting and various government sources. We included a number of technologies in our analysis: different durations of lithium-ion batteries, hydrogen, Form Energy’s 100-hour iron-air batteries, and other technologies. Ultimately, it was found that including multi-day energy storage in the UK and Ireland’s 2030 portfolios has the potential to lower overall renewable resource build, resulting in energy generation cost savings of 25-40% annually. The findings from this analysis are discussed in detail below.

1 Climate Change Advisory Council, Annual Report 2023,

Climate Change Committee, 2023 Progress Report to Parliament,

2 European Commission, Staff Working Document on Energy Storage – Underpinning a decarbonised and secure EU energy system,

Results: Ireland analysis

We examined two scenarios that would allow Ireland to meet its 2030 emissions target:3 an “MDS” scenario, in which the capacity expansion model was able to select multi-day storage technologies in a least-cost resource portfolio, and a “No MDS” scenario which limited storage technologies to 8 hours or less in duration. Results of these modelled scenarios demonstrate that multi-day storage can play a key role in decarbonizing the Irish electric grid.

Ireland’s least-cost resource portfolio includes 3.6 GW of multi-day storage in 2030.

Our modelling shows that multi-day storage is part of Ireland’s least-cost resource portfolio in 2030, with the model selecting 3.6 gigawatts (GW) of MDS as part of the optimal portfolio. This is because inclusion of MDS lowers the total resource build necessary to reliably serve Ireland’s demand for electricity from renewable energy. In Figure 1, total expected installed capacity in 2030 is shown, with the “MDS” scenario allowing for 16 GW fewer total resources to be built out.

Figure 1. Total installed capacity, 2030 (Ireland)

A portfolio with MDS results in annual generation cost savings of 28% per year, amounting to over €1.4bn annually.

In the MDS scenario, reduction in total resource build results in lowered total annual generation costs4 of 28% per year (€1.41 billion) relative to the “No MDS” scenario. These annual cost savings are a result of MDS technologies being able to capture excess wind and solar energy during long periods of oversupply and discharge it during renewable lulls or other periods of grid stress. In the absence of these multi-day storage technologies, Ireland would otherwise need to overbuild both renewables and lithium-ion batteries in order to reliably serve electric demand. These projected cost savings are shown in Figure 2.

Figure 2. Optimised generation costs, annualised, 2030 (Ireland)

Annual renewable curtailment is reduced by 70% in a portfolio that includes MDS.

Oversupply and curtailment of renewables on the Irish grid is already a concern, with 1,280 GWh or 7.6% of renewables being curtailed in 2022 – enough to power over 400,000 households.5 The challenge of curtailment is only expected to grow as Ireland increasingly shifts to a renewable electricity system. Our analysis finds that a “No MDS” scenario relying heavily on renewables and lithium-ion batteries would result in annual curtailment of 42% in Ireland. In contrast, inclusion of MDS could lower annual curtailment to 15% – and weekly curtailment by up to 30%. This can be seen in Figure 3.

Figure 3. Annual renewable energy curtailment, 2030 (Ireland)

Figure 3

In Figure 4 below, we see the reliability benefits that MDS can bring during a typical winter lull in renewable generation. Specifically, system dispatch is simulated over what is projected to be a typical week in March 2030. This week contains a multi-day wind lull, and in the “No MDS” scenario, we see that both 8-hour lithium-ion batteries and existing thermal generators are dispatched to serve electric load during the lull (March 4-7). In the “MDS” scenario, in contrast, we see MDS dispatching over the entirety of this period and displacing both the shorter duration batteries as well as a portion of the thermal generation that would otherwise be needed to reliably meet demand for electricity during this grid stress period.

Figure 4. Simulated resource dispatch during a wind lull period, 2030 (Ireland)

Weekly dispatch, “No MDS” scenario

Weekly dispatch, “MDS” scenario

These results highlight the potentially pivotal role that MDS will play in integrating renewables and keeping Ireland on track for its budgeted carbon emissions. By capturing energy that would otherwise be wasted, MDS can substantially reduce the amount of renewables needed. Reducing renewable generation assets not only helps bring down the cost of the system, but also accelerates progress by circumventing other barriers like planning permission and grid connection – all in all, enabling Ireland to meet its carbon emissions goals more quickly and effectively.

3 This analysis models a 2030 emissions target of 0.2 MMT.

4 Generation costs include the capital cost of new resources as well as fixed and variable operations and maintenance costs for all resources (new and existing).

5 Eirgrid, 2023, Annual Renewable Energy Constraint and Curtailment Report 2022,
Household consumption taken from Central Statistics Office, Metered Electricity Consumption 2022,,177,every%20county%20compared%20with%202021.

Results: Great Britain Analysis

Our modelling of Great Britain (which included England, Scotland and Wales) examines three different scenarios. The “No MDS” scenario limits selectable battery storage to lithium-ion technologies with a maximum duration of 8 hours. We then include two scenarios in which MDS technologies are selectable resource options – a “No Iron-Air” scenario and an “All Tech” scenario. The “No Iron-Air” scenario is used to help isolate the value of iron-air batteries in Great Britain’s 2030 grid, while the “All Tech” scenario demonstrates the role that iron-air batteries can play alongside other resource options.

Approximately 22 GW of iron-air battery storage is part of the UK’s least-cost resource portfolio in 2030.

Multi-day storage will play a vital role in achieving deep decarbonization in the UK while avoiding the substantial resource overbuild that would occur in the absence of MDS technologies. In Figure 5, total installed capacity in 2030 is shown for the three modelled scenarios. The “All Tech” scenario includes approximately 22 GW of iron-air batteries and requires 1 GW fewer total resources to be built out vs. the “No Iron-Air” portfolio, which relies on hydrogen and an increased deployment of lithium-ion batteries. And both the “All Tech” and “With Iron-Air” portfolios require 110 GW less total capacity to be built out, reducing needed storage capacity by 70% relative to the “No MDS” scenario.

Figure 5. Total installed capacity, 2030 (UK)

MDS can help the UK reach net-zero emissions with annual generation cost savings of 30-40%.

Similar to the Irish results, inclusion of MDS in the UK’s resource portfolio in the UK results in substantial savings in annual generation costs, as MDS has the ability to displace a large volume of shorter duration storage assets. Annual generation costs for each of the modelled scenarios are shown in Figure 6. The “No-Iron Air” scenario, which relies on hydrogen for multi-day storage, saves 30% relative to the “No MDS” scenario. The “All Tech” scenario increases these cost savings even further, with the addition of iron-air saving up to 40% in annual generation costs.

Figure 6. Optimised generation costs, annualised, 2030 (UK)

MDS reduces renewable curtailment by >90% per year compared to the No MDS scenario.

Achieving net-zero emissions requires increased renewable resource build, which naturally lends itself to increased curtailment, absent appropriate energy storage technologies. Inclusion of MDS in net-zero resource portfolios in the UK is found to decrease annual renewable curtailment by 90% or more, as shown in Figure 7. In our simulation, iron-air batteries capture more than 100 TWh of energy which would otherwise be curtailed. For context, the UK generated 138 TWh of renewable electricity in total in 2022.6 A net-zero scenario necessarily has higher volumes of renewable overbuild to ensure that load can be served every hour of the year, even during periods of “dunkelflaute.” Inclusion of MDS both reduces the volume of overbuild needed to maintain a reliable electric grid and absorbs excess renewable energy for later deployment, thereby reducing renewable curtailment.

Figure 7. Annual renewable energy curtailment, 2030 (UK)

In Figure 8, total system dispatch is shown during a typical winter week. In the “No MDS” scenario in the upper chart, substantial overbuild of resources is required to meet demand amid shifting weather patterns. Energy storage plays a role in delivering flexibility, but shorter durations are insufficient to manage annual shifts without the complement of enormous generation capacity. In contrast, the “All Tech” scenario in the lower chart shows a larger portion of excess renewable generation being stored at the beginning of the week and discharged over this multi-day period of lower-than-expected renewable production. The additional duration plays a key role in ensuring reliability in the face of fluctuating weather.

Figure 8. Simulated resource dispatch during a typical winter week, 2030 (UK)

Weekly dispatch, “No MDS” scenario

Weekly dispatch, “All Tech” scenario

A final key finding from our analysis is that there is a natural pairing between iron-air batteries and the region’s offshore wind resources, as can be seen in Figure 8 (UK) and also in the earlier Figure 4 (Ireland). These Figures illustrate the ability of MDS to charge for consecutive days during periods of high offshore wind production, and discharge over consecutive days during lull periods. This pairing between MDS and offshore wind has the potential to result in overall increased renewable energy utilisation.

6 National Grid, How much of the UK’s energy is renewable?,

Conclusion: Pathways to Accelerate Multi-Day Storage Adoption in the UK & Ireland

This analysis echoes previous studies which demonstrate that multi-day storage is a valuable component of a decarbonized electric system.7 Analysis using Formware shows that multi-day storage technologies, such as Form Energy’s 100-hour iron-air batteries, would allow the UK and Ireland to meet emissions reductions targets at lower costs relative to a resource portfolio that does not include these technologies. These lower costs are driven by a need for significantly less renewable power generation resources to be built out, given that storage can help fill in for the variations of a weather driven system.

To enable this future, it is critical to maintain the right policies, rewarding these assets for the essential role they play on the grid. Specifically, it is important that policymakers:

  1. Understand needs: These analyses are a key first step, but cannot substitute the planning undertaken by governments, system operators and other stakeholders. It is important that further studies be rapidly delivered, specifically exploring the role of long-duration and multi-day energy storage technologies in net-zero power systems – and that they utilise best-in-class methodologies when doing so.
  2. Begin procurement: Undertaking early procurement of multi-day storage technologies will bring added reliability to future power systems, deliver early learnings on best integration practices, and send clear market signals to developers and technology suppliers. As such, multi-day storage specific procurement should begin without delay.
  3. Deliver Long-term frameworks: Aside from initial procurement of multi-day storage, policymakers should assess the frameworks required to deliver long-term, market-driven procurement of long duration energy storage. Specifically, capacity markets should be aligned with the needs of decarbonised systems, and market operators should be incentivized to hedge against renewable variability through, for example, 24/7 power purchase agreements.

Actions such as these can help accelerate adoption of long-duration and multi-day energy storage technologies, enabling a more reliable, affordable, and clean electric grid for all in the UK and Ireland.

7 For example, Sepulveda et al., 2021, The Design Space for Long-Duration Energy Storage in Decarbonized Power Systems , Guerra et al., 2020, The Value of Seasonal Energy Storage Technologies for the Integration of Wind and Solar Power

Methods and Data

This analysis was conducted using Formware, a least-cost capacity optimization and production cost tool designed to capture the chronology and multi-scenario optimization needed to accurately model grids with substantial renewables. Formware simulates the least-cost build and operation of generation and storage resources. It considers growth in electricity demand due to electrification, policy goals around decarbonization, reliability targets and reserve margins, and weather-driven variability in electric demand and renewable production.

Resource Cost and Performance Specifications

Capital and operating costs for generators in Ireland and the UK were taken from Aurora Energy Research. Assumed costs for storage technologies are shown in Table 1.

Table 1. Cost and operating assumptions for modelled storage resources

Portfolio Inputs and Assumptions

Technical assumptions were taken from a combination of Aurora Energy Research, Baringa Management Consulting, and the UK’s Department of Business, Energy & Industrial Strategy.

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