What Is Used for Long Term Energy Storage?

Solar and wind can produce more electricity than the grid needs today, but keeping that energy available for hours, days, or seasons requires different technologies.

Long-term energy storage uses pumped-storage hydropower, flow batteries, compressed-air storage, thermal storage, hydrogen, and emerging long-duration batteries. Pumped hydro is the most established large-scale option, while flow batteries and compressed air suit extended grid storage. Hydrogen becomes particularly interesting when electricity must be stored for very long periods or seasonally.

I use the term long-duration energy storage (LDES) for this discussion. The U.S. Department of Energy uses 10 hours or longer as a key threshold in its Long-Duration Storage Shot program.

What Is Considered Long-Term Energy Storage?

Not every energy storage system is designed to perform the same job. A battery that shifts solar electricity from noon to evening has very different requirements from a system storing renewable energy for several days.

Long-term or long-duration energy storage generally refers to technologies capable of supplying energy for extended periods. DOE's current LDES program focuses on systems delivering electricity for at least 10 hours, while very-long-duration applications can extend into multiple days, weeks, or even seasonal energy shifting.

Duration Changes the Best Technology

I normally separate storage requirements approximately like this:

Storage requirement	Technologies I consider
Seconds to minutes	Supercapacitors, flywheels, batteries
1–4 hours	Lithium-ion batteries
4–10 hours	Lithium-ion, flow batteries, pumped hydro
10–24+ hours	Flow batteries, pumped hydro, compressed air, thermal storage
Multiple days	Pumped hydro, compressed air, advanced batteries, thermal storage, hydrogen
Weeks or seasonal	Hydrogen and other chemical storage, selected large mechanical or thermal systems

These categories are practical rather than strict technical boundaries. DOE evaluates several technologies for LDES, including lithium-ion, sodium batteries, zinc batteries, flow batteries, hydrogen, compressed air, pumped storage hydropower, and thermal storage.

The correct choice therefore depends on more than discharge duration.

I also evaluate:

• Required MW of power
• Required MWh or GWh of energy
• Charging frequency
• Round-trip efficiency
• Site geography
• Capital cost
• Cycle life
• Self-discharge
• Construction time
• Expected number of cycles
• Need for seasonal storage

Is Pumped-Storage Hydropower Used for Long-Term Energy Storage?

Yes. Pumped-storage hydropower is one of the most mature ways to store large quantities of electricity for extended periods.

Pumped-storage hydropower stores electricity by pumping water from a lower reservoir to a higher reservoir. When electricity is needed, water flows back downward through turbines. It is particularly useful for large grid-scale storage because enormous quantities of energy can be stored without requiring millions of individual battery cells.

How Pumped Hydro Stores Energy

The process is straightforward:

Excess electricity → pumps → elevated water → turbines → electricity

When renewable generation or electricity supply exceeds demand, pumps move water uphill.

The upper reservoir acts as the energy store.

Later, when electricity demand rises, the stored water flows downward and drives turbines.

This makes pumped hydro fundamentally different from a battery.

A battery stores energy through electrochemical reactions.

Pumped hydro stores it as gravitational potential energy.

Why Pumped Hydro Remains Important

Pumped hydro becomes attractive when I need:

• Very large energy capacity
• Long operational life
• Multiple hours of discharge
• Grid-scale power
• Daily or repeated cycling

The IEA expects annual pumped-storage hydropower additions to rise to 16.5 GW by 2030, driven partly by the increasing requirement for grid flexibility and long-term storage.

Its major disadvantage is geography.

A battery can be installed in many industrial areas.

Pumped hydro normally needs suitable elevation differences, reservoirs, water management, permitting, major civil engineering, and long project-development periods.

For that reason, I consider pumped hydro an excellent long-duration technology where geography permits it, but not a universal solution.

Are Flow Batteries Good for Long-Term Energy Storage?

Flow batteries are one of the most promising electrochemical options when storage duration needs to extend beyond conventional short-duration battery applications.

Flow batteries store energy in liquid electrolytes held in external tanks. Their power equipment and energy-storage capacity can be scaled relatively independently, making them attractive for long-duration grid applications. Increasing energy capacity can often be achieved mainly by increasing electrolyte and tank volume rather than multiplying complete battery power systems.

Flow Batteries Separate Power and Energy

This is their most important advantage.

In a conventional lithium-ion system, adding more hours of storage usually means adding more battery cells, modules, racks, and associated equipment.

In a flow battery:

• The electrochemical stack largely determines power.
• The electrolyte volume largely determines energy.

If a project needs more storage duration, larger tanks can increase energy capacity.

PNNL identifies independent scaling of power and energy, modularity, and long cycle and calendar life as important characteristics of vanadium redox flow batteries.

Where I Consider Flow Batteries

I consider flow batteries for:

• Renewable-energy shifting
• Microgrids
• Grid capacity
• Repeated daily cycling
• Long-duration stationary applications
• Projects where physical space is available

Their disadvantages can include lower energy density, large tanks, pumps, system complexity, and a less mature manufacturing ecosystem than lithium-ion.

They are therefore much better suited to stationary projects than mobile applications.

Is Compressed Air Used for Long-Term Energy Storage?

Compressed-air energy storage, or CAES, is another mechanical method for storing large amounts of electricity.

Compressed-air energy storage uses electricity to compress air and store it under pressure. When power is required, the air is released through an expansion system to help generate electricity. DOE includes CAES among its principal long-duration storage technology families alongside pumped hydro, batteries, thermal systems, and hydrogen.

How CAES Works

The basic energy path is:

Electricity → air compression → stored compressed air → expansion → electricity

Large-scale systems may use underground formations or engineered storage vessels.

CAES can make sense when I need substantial energy capacity and the site has suitable geological or engineering conditions.

Like pumped hydro, its energy-storage medium is not a conventional battery cell.

That can make very large capacity attractive.

However, the site requirements can limit deployment.

I therefore see CAES as a long-duration grid technology rather than a solution for homes or most small businesses.

Can Thermal Energy Be Used for Long-Term Storage?

Yes. Electricity does not always need to remain electrical while it is being stored.

Thermal energy storage converts energy into heat or cold and preserves it for later use. Materials such as molten salt, water, rocks, or other thermal media can store large amounts of energy. The stored heat can later serve heating applications directly or be converted back into electricity when the system is designed for power generation.

Molten Salt Is an Important Example

Molten-salt storage is frequently associated with concentrated solar power.

Solar energy heats the storage medium.

The heat remains inside insulated tanks.

Later, it can create steam and produce electricity.

DOE includes molten salt and other thermal-storage technologies among the major LDES technology families being evaluated for future cost reductions.

Thermal Storage Can Avoid Unnecessary Conversion

Thermal storage becomes especially attractive when the final requirement is heat.

Suppose an industrial facility needs high-temperature process heat tomorrow.

One option is:

Electricity → battery → electricity → heater → heat

Another is:

Electricity → heat → thermal storage → heat

The second route may avoid unnecessary electrical conversion stages.

I therefore evaluate the final energy requirement before assuming that a battery is the best storage solution.

Is Hydrogen Used for Very Long-Term Energy Storage?

Hydrogen becomes particularly interesting as required storage duration becomes very long.

Hydrogen can store renewable electricity by using electrolysis to convert electricity into hydrogen. The hydrogen can later be used in a fuel cell, turbine, industrial process, or other application. DOE specifically includes bidirectional hydrogen storage among long-duration technologies and considers both above-ground tanks and underground storage approaches.

Hydrogen Separates Energy Capacity From Power Equipment

The energy path can look like this:

Electricity → electrolyzer → hydrogen → storage → fuel cell/turbine → electricity

The electrolyzer determines how rapidly hydrogen can be produced.

The storage vessel or underground cavern determines how much hydrogen can be held.

The fuel cell or generator determines the power output when electricity is needed again.

This separation can become useful when enormous amounts of energy must be stored for long periods.

Hydrogen Is Particularly Relevant to Multi-Day and Seasonal Storage

For a four-hour application, a lithium-ion battery normally offers a much simpler energy pathway.

As storage duration becomes extremely long, however, the economics can change because battery-based systems need progressively more electrochemical capacity.

NREL research comparing long-duration options found that hydrogen with geologic storage could become particularly competitive at a modeled 120-hour duration under the study's assumptions.

I would not interpret that finding to mean hydrogen is automatically the cheapest five-day system everywhere.

Site conditions, electrolyzer cost, power-generation equipment, efficiency, hydrogen infrastructure, and storage geology all matter.

Hydrogen's main tradeoff is lower electricity-to-electricity efficiency compared with many batteries.

Its strength is the ability to store very large quantities of chemical energy for extended periods.

Are Lithium-Ion Batteries Used for Long-Term Energy Storage?

Lithium-ion batteries dominate much of today's battery-storage market, but their strongest economic role has traditionally been shorter-duration applications.

Lithium-ion batteries can technically be designed for 10-hour or longer storage, and DOE includes them in its LDES technology research. However, continually increasing duration requires more battery cells, so alternative technologies that separate power from stored-energy capacity can become increasingly attractive for very-long-duration applications.

Lithium-Ion Is Excellent for Daily Energy Shifting

I still prefer lithium-ion, especially LFP, for many applications such as:

• Solar evening shifting
• Commercial peak shaving
• Residential backup
• Frequency regulation
• Four-hour grid storage

Lithium-ion provides:

• Fast response
• High efficiency
• Modular installation
• Mature manufacturing
• Small physical footprint

The challenge appears as duration increases.

A 100 MW/4-hour system requires:

100 MW × 4 hours = 400 MWh

A 100 MW/20-hour battery requires:

100 MW × 20 hours = 2,000 MWh

The power requirement remains 100 MW, but the second project needs five times as much energy capacity.

That is why LDES development increasingly examines storage technologies where additional hours can be added more economically.

What New Battery Technologies Can Store Energy for Days?

Long-duration battery development is not limited to conventional lithium-ion or vanadium flow batteries.

DOE is evaluating several electrochemical technologies for long-duration storage, including flow, lithium-ion, sodium, zinc, lead-acid, and other battery approaches. These technologies are being developed because no single chemistry currently provides the ideal combination of low cost, long duration, high efficiency, long life, safety, and geographic flexibility.

Sodium and Zinc Are Important Alternatives

Sodium-based batteries may reduce dependence on lithium and use more widely available raw materials.

Zinc-based batteries offer another path for stationary applications.

DOE's Storage Innovations 2030 assessments include both sodium and zinc batteries among technologies being investigated for long-duration storage cost reductions.

I view these technologies as part of a broader diversification of stationary storage.

The objective is not necessarily to replace lithium-ion everywhere.

Instead, different battery chemistries can specialize.

Lithium-ion can dominate high-efficiency shorter-duration applications while other chemistries target longer discharge periods, lower material costs, or different safety requirements.

Which Long-Term Energy Storage Technology Is Best?

There is no single best technology across every duration and location.

Pumped-storage hydropower is my preferred established option for very large long-duration grid storage where geography is suitable. Flow batteries are attractive for repeated long-duration cycling, compressed air for suitable large-scale sites, thermal storage for heat-related applications, and hydrogen for extremely long or potentially seasonal storage requirements.

My Technology Comparison

Technology	Strongest use	Main advantage	Main limitation
Pumped hydro	Large grid storage	Mature, very large scale	Geographic requirements
Flow battery	Daily long-duration cycling	Power and energy scale separately	Tanks and lower energy density
Compressed air	Large long-duration projects	Large energy capacity	Site/geology requirements
Thermal storage	Heat and power applications	Low-cost storage media possible	Conversion back to electricity
Hydrogen	Multi-day/seasonal storage	Extremely large energy potential	Lower round-trip efficiency
Lithium-ion	Shorter daily storage	Efficiency and maturity	More cells needed as duration grows
Sodium/zinc batteries	Emerging LDES	Alternative materials	Less mature deployment

DOE's current long-duration strategy is similarly technology-neutral, assessing electrochemical, chemical, mechanical, and thermal approaches rather than identifying one universal winner.

Geography Often Determines the Answer

A mountainous region with existing reservoirs may strongly favor pumped hydro.

A flat industrial site cannot simply create the same geography.

A location with suitable underground caverns may make compressed air or hydrogen more attractive.

An industrial plant with a large heat requirement may benefit from thermal storage.

A constrained urban site may favor batteries.

I therefore choose the technology after understanding the project rather than choosing a technology and forcing the project to fit it.

My Insights: What Is Used for Long Term Energy Storage

The best answer is a portfolio of technologies rather than one battery chemistry.

Long-term energy storage uses pumped-storage hydropower, flow batteries, compressed-air storage, thermal systems, hydrogen, and emerging long-duration batteries. I consider pumped hydro the most established large-scale choice where geography allows it, while hydrogen becomes increasingly interesting for very-long-duration or seasonal requirements and flow batteries offer strong potential for repeated extended cycling.

I Match Technology to Duration

My first decision is how long energy must remain available.

For several hours, LFP battery storage may be completely appropriate.

For 10–24 hours, I compare lithium-ion with flow batteries, pumped hydro, thermal storage, and other LDES technologies.

For multiple days, technologies with inexpensive energy-storage capacity become more important.

For seasonal storage, I pay much more attention to hydrogen and other chemical energy carriers.

The cheapest technology at four hours does not automatically remain the cheapest at 100 hours.

I Separate Power Cost From Energy Cost

This is one of the most important concepts in long-term storage.

A grid may need:

100 MW of output

but

10,000 MWh of stored energy.

That represents 100 hours of storage.

For this type of application, a technology that can cheaply add energy capacity without equally increasing expensive power equipment can have a major advantage.

Flow batteries, pumped hydro, compressed air, thermal storage, and hydrogen all address this challenge in different ways.

Long-Term Storage Will Probably Remain Diverse

DOE's strategy evaluates technologies across four families—electrochemical, chemical, mechanical, and thermal—because each solves a different part of the storage problem.

I expect the future grid to use combinations rather than one universal technology.

A power system could use:

• Lithium-ion for rapid four-hour shifting
• Flow batteries for longer daily cycles
• Pumped hydro for large regional storage
• Thermal storage for industrial heat
• Hydrogen for rare multi-day or seasonal shortages

That layered approach lets each technology operate where its characteristics create the most value.

Conclusion

Long-term energy storage uses pumped hydro, flow batteries, compressed air, thermal storage, hydrogen, and advanced batteries. The best option depends mainly on duration, scale, geography, efficiency, and cost.