Advanced and Long-Duration Energy Storage: A Review of Emerging Technologies, Applications, and Techno-Economic Assessment

The power grid is a delicate balancing act. For decades, it relied on large power plants that could be turned up or down at will to match the exact moment-by-moment needs of homes and factories. Today, that system is shifting toward wind and solar energy, which are clean but unpredictable. The sun does not always shine, and the wind does not always blow, yet the lights must stay on. This creates a new problem: we need to store electricity when it is abundant and release it when it is scarce. While batteries have become excellent at holding power for a few hours—enough to cover the evening peak after the sun sets—they struggle when the weather turns bad for days at a time. A cloudy week or a calm stretch of wind can leave a grid with short-duration batteries empty and vulnerable. The question facing engineers and planners is no longer just about how much energy we can store, but how long we can keep it there.

A recent review by researchers at Gujarat Technological University explores this frontier of long-duration energy storage. They examined a wide array of technologies designed to hold power for days, weeks, or even seasons, moving far beyond the standard lithium-ion batteries that dominate the market today. The authors combined data from global agencies, industry reports, and their own computer simulations to understand which technologies work best for different lengths of time and how much they might cost. Their work reveals a clear dividing line: while familiar batteries are the most economical choice for storing energy for a few hours, they become prohibitively expensive for longer periods. For those extended durations, the most promising solutions are often mechanical or chemical systems that use abundant, cheap materials like air, water, iron, or heat, even if they are less efficient at converting energy back and forth.

The researchers began by mapping the landscape of storage, from the century-old technology of pumped hydro, which uses gravity and water reservoirs, to emerging systems like compressed air, liquid air, and flow batteries. They also looked at iron-air batteries, which store energy through a reversible rusting process, and thermal systems that keep heat in bricks or molten salt. A key insight from their analysis is that the cost structure of these technologies changes depending on how long they need to last. For short periods, the cost is driven by the power equipment needed to charge and discharge quickly. But as the duration extends to days or weeks, the cost is dominated by the price of the storage medium itself—the water, the air, or the iron. This means that a technology that is too expensive for a four-hour shift might become the cheapest option for a hundred-hour shift simply because its storage material is so cheap.

To test these ideas, the authors built a model to calculate the levelized cost of storage, which represents the average price per unit of energy delivered over the system's lifetime. They ran this model across eight different technologies, assuming a standard usage pattern where the system discharges fully once every few days. The results showed a sharp crossover point. Lithium-ion batteries, which are currently the industry standard, remained the cheapest option only up to about two or three hours. Beyond that, their cost climbed steeply because every extra hour of storage required adding more expensive battery cells. In contrast, technologies like adiabatic compressed air storage, which traps the heat generated when air is compressed, and iron-air batteries, which use a simple chemical reaction with oxygen, remained far cheaper as the duration increased. In their calculations, at a duration of twenty-four hours, the cost of using lithium-ion was roughly three times higher than using compressed air. At a duration of one hundred hours, the gap widened to nearly six times.

The study also included a simulation of a power grid facing a realistic weather challenge: a four-day period where the wind stopped blowing. In this scenario, the researchers modeled a system with solar and wind power trying to meet a steady demand. Without any storage, the system could only meet about 70 percent of the demand. Adding a standard four-hour battery improved this to 83 percent, but the battery would run out on the first night of the wind lull and never recover. To get through the entire four-day gap without failing, the simulation showed that the grid would need a storage system capable of discharging for about one hundred hours. This finding highlights that the value of storage is not linear; the first few hours provide a significant boost, but the final stretch of reliability requires a completely different kind of technology—one that can hold a massive amount of energy for a long time without breaking the bank.

The researchers paid special attention to the situation in India, where the gap between planning and reality is stark. Government plans call for hundreds of gigawatt-hours of storage by the early 2030s to support a massive expansion of renewable energy, yet the amount of battery storage actually operating in 2025 was less than one gigawatt-hour. The country has identified a huge potential for pumped hydro storage, which uses water and gravity, and has begun tendering projects for it. However, the review notes that converting these tenders into operating plants is a major challenge, and there is a pressing need to test non-lithium technologies in local conditions. The authors suggest that while pumped hydro and lithium-ion are ready for immediate large-scale use, the future of multi-day storage likely lies in a mix of emerging options like iron-air and compressed air, provided they can be proven in real-world settings.

Ultimately, the paper argues that there is no single "best" technology for all energy storage needs. Instead, the grid requires a portfolio of solutions matched to the specific duration of the problem. Short bursts of power are best handled by fast, efficient batteries, while long, slow gaps in supply require systems that prioritize low-cost storage materials over high efficiency. The transition to a renewable-powered world will depend on successfully deploying these long-duration technologies, moving from the current era of short-term storage to a future where the grid can weather days of calm or clouds without interruption. The path forward involves not just building more of what we already have, but embracing a diverse range of mechanical and chemical systems that can hold the power of the sun and wind for as long as the world needs it.