Energy storage, as defined by the EU, is a means of "deferring the final use of electricity to a time later than when it was generated". Considering that renewable energy generation technologies pose grid stability issues due to their intermittent nature - making it difficult to match supply to demand - energy storage technologies allow for effective management of variations in energy production and use, on time scales that match supply and demand, which is essential for achieving decarbonization goals.
Energy storage technologies have evolved rapidly over the last decade. Although they are at different levels of development, they are generally classified according to the processes involved. In general, there are five groups of energy storage: mechanical, electrochemical, electrical, chemical and thermal. These five groups are further subdivided into approximately fifty individual technologies. Some represent the state of the art in energy storage.
Most studies conclude that the most important storage technology to replace fossil fuels is lithium-ion batteries, thanks to their installation flexibility, low cost and safety. In addition, they have a high energy density and a lifetime of approximately fifteen years. However, concerns about material shortages have led to lithium-based batteries being replaced by increasingly safe and cost-effective alternatives, such as zinc or sodium-ion batteries. These batteries are excellent candidates as storage systems for power grid applications because their components are more abundant and cheaper to extract. Zinc batteries, in particular, offer an alternative with higher stationary storage capacity, lower flammability, higher specific power and low cost.
Flow batteries also stand out for their long-life storage capacity. These batteries store energy in external tanks containing electrolytes, allowing greater scalability and a longer life cycle. Their main challenge is to improve their efficiency and reduce manufacturing costs to make them more competitive.
Thermal energy storage (TES) is an innovative technology that converts electricity into heat, storing it for several hours, or even days, and then converting it back into electricity or using it directly as heat. One of the main advantages of TES is that it is usually built with abundant materials, low cost and maintenance, and long life. The technology is also highly scalable, which means it can be adapted to a variety of applications, from large power plants to small residential buildings.
Another leading technology is kinetic and gravitational (mechanical storage) systems. On the one hand, flywheels store energy kinetically through high-speed rotating cylinders. Despite their effectiveness in rapid energy discharge scenarios, they have a high cost and little competitive feasibility. On the other hand, gravitational storage systems harness the force of gravity and regenerative braking to send renewable energy to the grid. Gravitational batteries use the potential energy of massive weights that are lifted and then dropped to generate power, which can be stored for months or even years. Although gravitational storage systems are a promising technology, there are still many drawbacks to their adoption, such as their high cost or scalability in different environments.
Green hydrogen generated by electrolysis through renewable energy sources (such as solar or wind) also offers an innovative solution for long-term energy storage. Once produced, hydrogen can be stored and used to generate electricity again when needed. One of the advantages of hydrogen is its high storage capacity, as it can last for months without losing energy through discharge. This makes it a more attractive option compared to other technologies, such as lithium-ion batteries that can only store energy for a short period of time. However, the initial costs are still very high, and the logistics and infrastructure for its production are not yet sufficiently developed.
While the benefits of energy storage are clear, all of these technologies share a number of technical, regulatory and economic barriers that hinder their widespread adoption.
One of the most notable challenges in the storage landscape is related to the supply chain. For example, increased demand for critical materials for battery technology, such as semiconductors or lithium, has led to shortages and price increases, creating bottlenecks and increased volatility.
In addition, following the principles of circular economy, a key aspect to be developed in relation to industrial processes and business models is the reuse and recycling of materials and components of energy storage technology, considering, in this sense, its entire life cycle.
On the other hand, lengthy and complex permitting procedures or lack of long-term revenue visibility may hinder access to financing for projects deploying new storage technologies. Since perceived risk is a major barrier to investment in such projects, the availability of financing remains a relevant concern, especially for commercial projects and pioneering technologies without state aid.
Public funding can therefore contribute to the development of energy storage technologies, addressing these barriers and making it more attractive to invest in such projects. In fact, the EU identified energy storage as one of the reform and investment priorities to be considered by Member States when drafting their Recovery and Resilience Plans and is therefore eligible for funding under the Recovery and Resilience Mechanism. This is the case for measures approved under EU state aid rules to support innovative electricity storage projects in Spain (Case SA.103068 https://competition-cases.ec.europa.eu/cases/SA.103068 ).
Ultimately, the stability and reliability of the energy system does not depend on a single technology, but requires all of them to effectively address this goal. The diversity of energy storage technologies makes them suitable for different contexts, as each has its own unique characteristics in terms of capacity, efficiency, lifetime or cost-effectiveness. Ultimately, the importance of the regulatory and institutional framework cannot be overemphasized. Clear and supportive policies are essential to incentivize the deployment of energy storage technologies and drive innovation in this sector. By creating an enabling environment in all respects, the potential of energy storage can be exploited and a stable, reliable and sustainable energy system can be achieved sooner rather than later.