Long-duration energy storage gains importance in renewable-rich grids
- Editorial Team SDG7
- Aug 29
- 5 min read
Published on 29 August 2026 at 03:14 GMT
By Editorial Team SDG7
Electricity systems built around wind and solar power face a timing problem as well as a generation problem. Solar output falls every evening, wind conditions can weaken across a region, and periods of high demand do not always coincide with abundant renewable production. Long-duration energy storage offers a way to move surplus electricity into those longer gaps, complementing transmission, flexible demand and other sources of firm capacity.
There is no universal boundary. The US Department of Energy uses more than 10 hours for its Long Duration Storage Shot, while an NREL Storage Futures Study notes that 10 to 100 hours is often used. Storage for a two-hour evening peak faces different demands from a plant expected to discharge through a wind lull lasting several days.
The need changes as renewable penetration rises
At modest shares of solar power, batteries can charge around midday and discharge into a relatively narrow evening peak. As more storage flattens that peak, the remaining period of net demand becomes wider. The NREL study found that this progression increases the duration required for storage to retain the same capacity value. Wind-heavy systems add another pattern: output can remain low for longer than a daily solar cycle and can coincide with cold or hot periods of elevated demand.
No storage plant creates energy. It shifts it, losing part of the input in the process. A reliable system also depends on sufficient generation, transmission, demand flexibility and resources able to cover rare but severe shortfalls. Multi-hour and multi-day storage can reduce renewable curtailment during surpluses and return electricity when it has greater system value.
The International Renewable Energy Agency reported that conventional pumped storage remained the dominant electricity-storage technology, with about 150 gigawatts of global power capacity in 2024. It also stressed that flexibility requirements extend from seconds to seasons. This range helps explain why the emerging market is a portfolio rather than a single contest between battery chemistries.
Efficiency is only one part of the comparison
Round-trip efficiency measures how much electricity returns after charging and discharging. High efficiency lowers the amount of generation required to refill a store, which is valuable for frequently cycled assets. Yet a lower-efficiency technology can remain competitive for infrequent, long events when its energy-holding component is inexpensive and its standby losses are limited.
Cost comparisons require similar care. Power capacity determines how quickly a system can charge or discharge; energy capacity determines how long it can sustain that output. Technologies that separate them can add duration comparatively cheaply, even when power-conversion equipment remains costly. Project life, cycling frequency, financing, construction risk and charging prices all influence levelised cost.
The US Department of Energy's commercial-liftoff analysis illustrates the uncertainty by separating power-related costs from energy-capacity costs and distinguishing inter-day from multi-day systems. Its figures are indicative category benchmarks rather than prices for every project.
Six routes to longer storage
Flow batteries circulate liquid electrolytes through an electrochemical stack. Their tanks determine energy capacity while stacks determine power, making longer duration possible without multiplying every system component. They can cycle deeply, and many aqueous designs avoid flammable electrolytes, although pumps, membranes and electrolyte materials add cost and complexity. Vanadium systems have commercial installations, while other chemistries remain at earlier stages of scale-up. Their siting is less geographically constrained than large civil works, but deployment remains far below lithium-ion volumes.
Thermal energy storage converts electricity into heat held in materials such as molten salt, bricks, rocks or other media. Systems supplying industrial heat can avoid converting the energy back to electricity, reducing conversion losses in factory and district-energy applications. Electricity-to-electricity projects require a heat engine, which lowers round-trip efficiency. The storage media can be abundant and inexpensive, but commercial readiness varies widely between established heat-storage applications and newer power-return designs.
Compressed-air energy storage uses electricity to compress air, generally storing it in underground caverns before expansion through machinery generates power. Its energy reservoir can be large and relatively inexpensive, and plant lifetimes can extend over decades. Conventional projects have a limited operating history and suitable geology is a major constraint. Designs that manage compression heat or use purpose-built vessels seek broader siting and improved performance, but they add equipment and cost.
Pumped hydropower storage moves water to an upper reservoir and releases it through turbines. It is the most commercially mature option in this group, offers large power ratings and can store energy for many hours or days. Its constraints are equally physical: elevation difference, water management, grid connection, environmental assessment, long permitting and construction periods, and high upfront capital. Closed-loop projects that are separated from continuously flowing rivers can reduce some impacts, but they do not remove land and infrastructure requirements.
Gravity storage systems lift solid masses or use mine shafts, then recover electricity as the mass descends. Familiar mechanical equipment avoids electrochemical degradation, but stored energy depends on mass and height, creating substantial material and site requirements. Demonstrations exist, but grid-scale operating evidence is thinner than for pumped hydropower and cost estimates remain design-sensitive.
Iron-air batteries store energy through reversible oxidation, often described as rusting and de-rusting iron. Iron is abundant, and developers target low energy-capacity costs over multi-day discharge while accepting lower round-trip efficiency than lithium-ion or many flow batteries. Commercial readiness is still being established. A US Department of Energy demonstration programme with Xcel Energy and Form Energy has planned two 10-megawatt, 100-hour systems at retiring coal plants, with early phases covering design and permitting before construction.
Commercial readiness is a system question
Pumped hydropower has the deepest operating record, while flow batteries and thermal storage include commercial products alongside less mature designs. Compressed air has a small number of established large plants and several newer configurations. Gravity and iron-air technologies are moving through demonstrations and early commercial projects. Readiness also depends on bankable warranties, supply chains, permitting knowledge, market rules and revenue certainty, not only whether equipment works.
That market design creates a central difficulty. Multi-day stores may operate rarely, yet provide value by reducing exposure to extreme scarcity. Energy-arbitrage revenue alone may not pay for that insurance function. Capacity mechanisms, reliability procurement and long-term contracts are among the approaches used or considered by electricity markets, but their design and cost allocation remain contested.
The public-interest link is clearest through SDG 7 (affordable and clean energy). Storage can support the integration of renewable electricity, but affordability depends on choosing duration and technology for a system's actual weather, network and demand patterns. The technologies' differing characteristics leave no universally applicable option: geography favours some mechanical systems, frequent cycling rewards efficiency, low-cost storage media favour long duration, and commercial maturity can reduce financing and execution risk.
Further information:
• International Renewable Energy Agency, Renewable power generation costs in 2024, used for global storage capacity, technology categories and comparative installed-cost context.
• US Department of Energy, Long-duration energy storage overview, used for duration categories and commercialisation barriers.
• National Renewable Energy Laboratory, Storage Futures Study: Key learnings for the coming decades, used for the definition of long duration and the widening net-load peak as storage deployment grows.
• US Department of Energy, 2022 Grid Energy Storage Technology Cost and Performance Assessment, used for the comparison framework and coverage of flow, thermal, compressed-air, pumped-hydro and gravity storage.
• US Department of Energy Office of Clean Energy Demonstrations, Multiday Iron Air Demonstration, used for the planned 10-megawatt, 100-hour iron-air projects and their development status.
