Long-duration energy storage (LDES) technologies are critical for addressing the intermittency of renewable energy sources like wind and solar by storing energy for extended periods—ranging from several hours to days, weeks, or even seasons—and releasing it when needed. Unlike conventional short-duration storage technologies, such as lithium-ion batteries that typically provide power for 1–4 hours, LDES is designed to provide energy over much longer timescales, ensuring grid reliability during periods of low renewable generation or high demand. Several key technologies are being developed and deployed to meet this need.
1. Pumped Hydro Energy Storage (PHES)
Pumped hydro storage is the most established form of LDES, accounting for the majority of global energy storage capacity. It involves pumping water from a lower reservoir to a higher one when surplus electricity is available and releasing it to drive turbines when energy is needed. Pumped hydro can provide energy for hours or days and is valued for its large-scale capacity and long discharge times. However, its deployment is geographically limited to areas with suitable topography and water availability.
2. Compressed Air Energy Storage (CAES)
CAES systems store energy by compressing air into underground caverns or tanks when electricity is abundant. The compressed air is later released and heated to drive turbines when power is required. Traditional CAES uses natural gas to reheat the air, but newer designs such as adiabatic CAES aim to capture and reuse the heat from compression, making the process more efficient and environmentally friendly. CAES can provide several hours to days of storage, with large-scale potential, but like PHES, it is limited by geographical factors.
3. Thermal Energy Storage (TES)
Thermal energy storage involves storing excess electricity as heat in materials such as molten salts, rocks, or phase-change materials, which can then be converted back into electricity when needed. Concentrated solar power (CSP) plants often use molten salt thermal storage, allowing energy collected during the day to be dispatched at night. Another variation, known as “grid-scale TES,” stores energy in insulated containers of heated materials that can later drive turbines. TES systems can offer long-duration storage, are relatively cost-effective, and can be integrated with industrial heat processes, but they typically have lower round-trip efficiency compared to batteries.
4. Hydrogen Energy Storage
Hydrogen energy storage involves using surplus renewable electricity to split water into hydrogen and oxygen through electrolysis. The hydrogen can then be stored and used later in fuel cells, gas turbines, or industrial applications to produce electricity or heat. This technology is highly scalable and can provide seasonal storage—storing energy for weeks or months at a time. Hydrogen can also be transported via pipelines and stored in large quantities, making it suitable for both energy storage and decarbonising other sectors like transportation and industry. However, it is currently expensive, and efficiency losses occur during conversion from electricity to hydrogen and back to electricity.
5. Flow Batteries
Flow batteries store energy in liquid electrolytes contained in external tanks, with the size of the tanks determining the energy storage capacity. Common types include vanadium redox flow batteries and iron flow batteries. Flow batteries offer flexibility in energy capacity, long cycle life, and the ability to provide several hours of storage. They are particularly suited for grid applications where long-duration, daily cycling is needed. However, their upfront costs are still higher than conventional lithium-ion batteries, and they are generally less energy-dense, making them more suited for stationary, large-scale applications.
6. Gravitational Energy Storage
Gravitational energy storage systems store energy by raising heavy objects—such as blocks, water, or rocks—when electricity is plentiful and lowering them to generate electricity when power is needed. One example is Energy Vault’s system, which uses cranes to lift and lower massive concrete blocks. These systems offer long-duration storage with the potential for many charge-discharge cycles, minimal energy degradation, and relatively low operational costs. While this technology is still in early stages of development and deployment, it is attracting attention due to its scalability and potential for providing multi-hour storage.
7. Liquid Air Energy Storage (LAES)
Liquid air energy storage (LAES) uses excess electricity to cool and compress air into a liquid state at cryogenic temperatures. When energy is needed, the liquid air is re-gasified, expanding to drive turbines and generate electricity. LAES is capable of storing energy for days to weeks and can be scaled up for large grid applications. It offers long-duration storage with high energy density and can be sited flexibly compared to CAES or pumped hydro. LAES systems are still in the development phase, but they offer significant potential for large-scale energy storage.
8. Advanced Battery Chemistries (e.g., Sodium-ion, Zinc-air, and Solid-state Batteries)
New battery chemistries are being developed to provide long-duration energy storage with improved safety, cost, and energy density compared to conventional lithium-ion technology. Sodium-ion batteries, for example, use abundant materials like sodium and have the potential to offer lower-cost, longer-duration storage. Zinc-air and aluminum-air batteries use metal oxidation to store energy, offering high energy densities and the ability to store energy for several days. Solid-state batteries, which use solid electrolytes instead of liquid ones, promise improved energy density, safety, and longevity, although they are still in the research and development phase.
9. Mechanical Energy Storage
Mechanical energy storage technologies, such as flywheels, store energy in the form of rotational kinetic energy. Flywheels can provide short bursts of power for grid balancing and frequency regulation but are being developed for longer-duration applications by using high-speed, low-friction designs. They offer fast response times, high cycle life, and are well-suited for applications requiring frequent cycling.
10. Chemical Energy Storage
Chemical energy storage technologies convert electricity into chemicals like ammonia, methane, or synthetic fuels. These solutions can store energy for long durations and be used in multiple sectors, including transportation, heating, and electricity generation. Ammonia, for example, can be stored for long periods and used in fuel cells or turbines to produce electricity. Power-to-X solutions offer flexibility and scalability, although they are less efficient than direct electrical storage technologies.
Challenges and Future Outlook
The main challenges for LDES include high upfront costs, lower round-trip efficiencies (particularly for hydrogen and thermal storage), and the need for further technological development to improve scalability and performance. Additionally, long-duration storage must be integrated into markets and policies that currently favour short-duration solutions like lithium-ion batteries.
Despite these challenges, LDES technologies are essential for achieving deep decarbonisation, ensuring grid resilience, and enabling the full integration of renewable energy. As costs decline and policies shift towards supporting clean energy, the role of LDES will likely expand, with some estimates suggesting that hundreds of gigawatts of long-duration storage will be needed by 2050 to fully decarbonise power systems.