Energy storage closes the gap between the generation and demand of energy, enabling excess energy to be stored and used when demand surpasses generation, such as when energy generation from renewable sources is low. This storage reduces curtailment of renewable energy generation and, in turn, reliance on dispatchable fossil-based energy sources such as natural gas. These technologies can also provide power system services across timescales, increasing grid stability reliability. A series of articles in this issue explore well-established and emerging grid-scale energy storage technologies.

Grid-scale energy storage is not new — pumped storage hydropower (PSH) has been used for more than a century to provide large-scale and long-duration energy storage. PSH operates by using energy to pump water from a lower reservoir to an upper reservoir, where it is stored and later released to generate electricity when needed. As renewable energy sources are increasingly used on the grid, the importance of PSH in pumping water during times of high generation to store energy also increases. This importance is giving urgency and direction to PSH research, as Yang et al. highlight in a Review on the role of PSH in power systems. Technology to allow PSH to operate more flexibly, new system configurations to reduce impacts and increase geographical flexibility, and more advanced monitoring and management systems are among the areas being actively researched.

Battery energy storage systems also have been used historically for grid-scale energy storage, but, owing to advances in battery technology and the increasing importance of energy storage, they are now being rapidly deployed. As Chen and colleagues outline, there are a plethora of battery energy storage technologies under development, including high-energy, aqueous, redox flow, high-temperature and gas batteries. Each battery system has strengths and drawbacks in terms of cost, safety, lifetime, response time, round-trip efficiency, temperature adaptability and end-of-life management.

The authors highlight that lithium-ion battery-based systems, which dominate the market, are generally useful across applications owing to their high energy density, cost-effectiveness and high round-trip efficiency. However, there are drawbacks to these batteries (such as safety concerns) and they emphasize that further research and development of these batteries and others for tailored applications is needed.

Encouragingly, this work is underway. For example, Wang and colleagues describe advances in organic battery research and argue that they might be a future option for high-energy and large-scale applications. Other battery types are also on the path to commercialization. In this issue, a Down to Business article describes the approach one company has taken to develop sodium–metal chloride batteries for grid-scale energy storage, and the challenges they have faced.

An important aspect of energy storage systems, as highlighted in this issue, is safety. Battery energy storage systems dissipate heat during operation. If this heat is not properly managed, it can lead to thermal runaway, which in turn might result in fire and explosion incidents. High-energy batteries such as lithium-ion batteries are particularly at risk of thermal runaway, and several fire incidents have been reported in the past decade. Effective thermal management is needed to strengthen the safety and temperature adaptability of batteries. To help address the thermal issues of batteries, Rao et al. discuss in this issue how to prevent and/or mitigate excessive heat generation and expand the operating temperature range of batteries.

“There is no one-size-fits-all solution in energy storage”

While the articles in this issue focus on widely used energy storage technologies, energy storage systems must fit specific — and sometimes case-by-case — requirements. There is no one-size-fits-all solution in energy storage, and, as the articles here suggest, a mix of energy storage technologies is needed to cover the range of timescales and use cases our grids need. Excitingly, many systems are being tested and developed, such as gravity energy storage as discussed in a Q&A article in this issue, capacitors, thermal energy storage and others. We will continue to cover these technologies and their implementation in the future and cheer on their development.