The global energy transition is accelerating towards a model increasingly based on renewable sources. Solar and wind power are becoming the core of electricity generation in many countries, thanks to their economic competitiveness and their capacity to reduce CO₂ emissions.

However, the growing deployment of Variable Renewable Energy (VRE) poses a new challenge: ensuring grid stability even when the sun and wind are unavailable.

In this context, long-duration energy storage systems (LDES), are emerging as one of the most strategic technologies for the future of the energy system.

Indeed, the latest sector analyses highlight how highly decarbonised electricity grids require new forms of flexibility, capable of managing energy imbalances that are no longer limited to a few hours but can extend for days or even on a seasonal basis.

What are LDES systems?

The term LDES, an acronym for Long-Duration Energy Storage, identifies storage technologies capable of storing energy and releasing it for prolonged periods, generally exceeding 10 hours.

While lithium-ion batteries currently dominate the short-duration storage market, typically ranging between one and four hours, LDES systems are designed to address much more complex needs linked to the growing penetration of variable renewable energy.

According to the scientific report, Long-Duration Energy Storage - A Literature Review on the Link between Variable Renewable Energy Penetration and Market Creation published in 2024 in the journal Energies, the need for long-duration storage arises precisely from the growing difficulty of balancing an electricity grid powered predominantly by intermittent sources such as solar and wind.

The objective of LDES is therefore to guarantee continuity and reliability to the electricity system, making energy available even at times when renewable production drops drastically or stops completely.

Why traditional batteries are no longer enough

In recent years, lithium-ion batteries have played a fundamental role in integrating renewable sources and managing the electricity grid.

These systems have proven particularly effective in frequency regulation, rapid grid balancing, and managing short-duration peak demand. Furthermore, they have contributed to more efficient energy use through daily energy arbitrage operations, storing electricity during periods of low demand to release it when demand and prices are higher.

With the increase in the share of variable renewables, however, the needs of the system are changing profoundly. Solar production is concentrated in the central hours of the day, whilst demand peaks often occur during evening slots. Wind power, on the other hand, presents less predictable fluctuations, which can last for several consecutive days.

This temporal misalignment between production and consumption generates imbalances that traditional storage technologies can no longer compensate for efficiently.

A grid based exclusively on short-duration storage systems would risk producing large quantities of unused renewable energy during times of overproduction, a phenomenon known as curtailment, but also energy deficits during long periods characterised by low availability of sun and wind.

When LDES becomes indispensable

One of the most debated aspects concerns the so-called tipping point, which is the level of renewable energy penetration beyond which LDES systems become economically and technically necessary to guarantee grid stability.

The scientific report published in Energies shows that there is no universal threshold valid for all electricity systems. The studies analysed identify values ranging between 20% and 100% of VRE penetration. Despite this high variability, the value most frequently cited as a turning point is around 80%.

Beyond this threshold, the costs required to maintain grid stability through traditional tools or short-duration storage systems tend to rise rapidly, making LDES technologies an essential component of the energy transition.

The study also highlights how the true value of LDES does not depend solely on energy arbitrage, but above all on the capacity to replace fossil fuel "peaker" plants - namely the gas or coal plants used to cover peak demand or compensate for sudden drops in renewable production.

The main LDES technologies

Unlike short-duration storage, which is currently dominated almost exclusively by lithium-ion batteries, the LDES systems sector is still in a phase of strong technological evolution and encompasses very diverse solutions, including:

• pumped hydro storage (PHS);
• compressed air energy storage (CAES);
• flow batteries;
• thermal storage;
• hydrogen-based systems;
• gravitational technologies.

Despite the technological differences, these solutions share some fundamental characteristics, starting with the use of relatively abundant and less expensive materials compared to those employed in traditional batteries.

What makes them particularly suitable for long-duration storage is also the possibility of increasing storage capacity without a proportional growth in costs, an essential aspect when dealing with time horizons that can extend for days or weeks.

Added to this is the capacity to play a structural role within future electricity grids, contributing not only to short-term system stabilisation but also to the management of prolonged imbalances between renewable production and energy demand.

Applications of LDES systems

Long-duration storage systems are destined to take on an increasingly strategic role in the evolution of electricity grids, accompanying the transition towards energy models characterised by a growing presence of variable renewable sources.

With the increasing complexity in managing the balance between energy production and demand, these technologies will be called upon to perform different and complementary functions, contributing to making the electricity system more flexible, reliable, and resilient.

The possible applications of LDES develop across multiple levels, responding to different operational needs that will become increasingly central in the journey toward decarbonising energy grids.

1. Balancing production and consumption throughout the day

One of the primary applications concerns the transfer of energy produced during hours of greater renewable availability to evening and night slots. This allows for better exploitation of the energy generated by solar and wind plants, reducing waste and production constraints.

Analyses of electricity systems with high solar penetration indicate that, to guarantee daily grid stability, storage systems with durations between 10 and 20 hours will be required.

2. Greater resilience during critical moments

Electricity grids are increasingly exposed to extreme weather conditions, such as heatwaves, long periods of low wind, or exceptional meteorological events. In these scenarios, LDES systems can accumulate energy during periods of surplus and make it available in moments of emergency, increasing resilience and energy security.

3. Managing prolonged energy imbalances

Beyond the daily management of the grid, LDES systems will also have a decisive role during prolonged periods of imbalance between renewable production and energy demand.

In grids characterised by a strong presence of solar and wind, phases can indeed occur where the availability of renewable energy is reduced for several consecutive days. In these scenarios, the possibility of storing energy over longer time horizons becomes essential to guarantee operational continuity and progressively reduce dependence on fossil fuel plants used as a reserve during the most critical times.

Variables influencing the development of LDES

The development of long-duration storage systems depends on multiple factors that influence both their economic sustainability and their operational effectiveness within the electricity grid. The most relevant elements include:

• the cost of the technologies;
• their energy efficiency;
• the balance between solar and wind production;
• climatic and geographical conditions;
• decarbonisation targets;
• the availability of adequate transmission infrastructure.

The ability to manage curtailment - meaning the limitation of excess renewable production - also represents a decisive factor.

Furthermore, storage requirements can change significantly based on the composition of the energy mix. A system heavily dependent on solar power tends to require primarily daily forms of storage, whereas grids with a higher presence of wind energy are more exposed to production fluctuations that can last for several consecutive days, thereby increasing the need for multi-day or seasonal storage.

In scenarios oriented towards complete decarbonisation and the achievement of Net Zero targets, where recourse to backup fossil fuel plants is progressively eliminated, LDES systems become an essential component for guaranteeing continuity and stability to the electricity grid.

Outstanding challenges

Despite the growing interest in long-duration storage systems, the LDES sector still faces numerous technological, economic, and regulatory challenges. Many of the technologies currently considered promising are still in a development or industrial demonstration phase, and require further investment to foster research, industrialisation, and production scalability.

The models used so far to analyse the role of LDES also present some significant limitations. In many cases, simulations are based on time horizons that are too short - often limited to a single year - which do not allow for a full assessment of the value of seasonal storage. Added to this is a limited consideration of the costs related to technology degradation and the infrastructural constraints of electricity grids.

For this reason, there is an emerging and increasing need to develop more comprehensive and standardised models, capable of realistically representing the contribution that LDES systems can offer to future decarbonised energy grids.

A key technology for the grid of the future

With the rising share of renewable energy and the evolution of electricity grids towards increasingly decarbonised models, LDES systems are candidates to become one of the key infrastructures of the energy transition.

If lithium-ion batteries have accompanied the expansion of solar and wind power in recent years, the next evolutionary step will be represented by the capacity to store energy for much longer periods, guaranteeing continuity, flexibility, and security to the grid.

In this scenario, the issue is no longer just about how much energy to produce, but above all, how to store it and make it available over time. Precisely for this reason, long-duration storage systems no longer represent a simple technological option, but an essential component of the electricity grids of the future.