What Is Long Term Energy Stored In?

Long term energy storage refers to the storage of energy over extended periods of time, from days to months. It is an important component of renewable energy systems like solar and wind, which produce intermittent power depending on environmental conditions. Long term storage enables the storage of excess renewable energy when supply exceeds demand, and discharge when demand exceeds supply. This allows renewable energy to serve as a stable and reliable energy source.

There are several methods of long term energy storage, with some key ones being chemical storage, thermal storage, electrochemical storage, and hydro or compressed air storage. An ideal long term storage system should store large amounts of energy, incur minimal losses during storage, while also being cost effective. Research is ongoing on improving existing storage methods and developing new approaches to enable widespread renewable energy adoption.

Potential Energy

Potential energy is energy that is stored and held in readiness. There are several ways potential energy can be stored for the long term:

Gravity Energy Storage

Gravity energy storage takes advantage of height differentials to store energy. Some examples include:

  • Pumped hydro storage – Water is pumped uphill into a reservoir during times of low electricity demand. The water is then released through turbines to generate electricity when demand is high.
  • Rail energy storage – Train cars are hoisted up a slope using excess electricity. They are then lowered to turn generators and produce electricity on demand.
  • Weights – Massive concrete or steel weights are lifted and then dropped on demand to turn generators.

Compressed Air Energy Storage

Air is compressed and stored under pressure in underground caverns or tanks. The high pressure air is then heated and expanded through a turbine to generate electricity when needed.

Flywheel Energy Storage

A flywheel is accelerated to a very high speed and maintains its inertia. When electricity is needed, the flywheel’s rotational energy is converted into electricity by slowing down the flywheel.

Chemical Energy

Hydrogen can be stored as a chemical for long-term energy needs by converting surplus electricity into hydrogen through electrolysis. The hydrogen can then be used directly as a fuel for transportation or generating electricity again when needed. Hydrogen is attractive because it has very high energy density and can serve as a clean fuel.

Synthetic natural gas (SNG) can also be produced by reacting hydrogen with carbon dioxide. The SNG can then be stored and transported through existing natural gas infrastructure. This provides a way to store intermittent renewable energy in the form of SNG.

Ammonia, made from nitrogen and hydrogen, is another chemical that can store hydrogen. Ammonia has some advantages over hydrogen for storage and transportation. It has a higher energy density than compressed hydrogen gas and is easier to liquefy.

Biofuels like ethanol and biodiesel can provide a way to store energy chemically from biomass feedstocks. Biofuels produced from agricultural crops, waste materials, or algae can potentially provide carbon-neutral liquid fuels.

Thermal Energy

Thermal energy storage involves storing energy in a thermal reservoir for later use. There are several ways this can be accomplished:

Molten Salt Storage

Molten salt storage uses molten salt to store thermal energy. The salt is heated to high temperatures where it melts and can retain heat. The molten salt is then stored in a thermally insulated container. When energy is needed, heat can be extracted from the molten salt to generate steam and electricity.

Ice Storage

Ice storage takes advantage of the latent heat absorbed or released when water freezes or melts. Water can be frozen into ice at night when electricity demand is low. The ice is then used for cooling during the day.

Steam Accumulator

A steam accumulator stores steam under pressure in an insulated tank. The steam can be generated when electricity prices are low and then released to operate a turbine when needed.

Electrical Energy

Electrical energy storage takes advantage of electrical or magnetic fields to store energy for later use. There are several ways to store energy in these fields:


Capacitors store energy in the form of an electrical field between two metal plates that are close together but not touching. When current is applied, opposite charges build up on each plate to create the field. Capacitors can discharge their stored energy very quickly, making them useful for applications that require short bursts of electricity.

Superconducting magnetic energy storage (SMES)

SMES systems store energy in the magnetic field created by the flow of direct current in a superconducting coil. They can charge and discharge extremely quickly and efficiently. The main limitation is the need for cryogenic cooling to maintain superconductivity.


Supercapacitors, also known as ultracapacitors, are similar to regular capacitors but can store much more energy per unit volume or mass. They use porous material electrodes to achieve a very large surface area in a small space. Supercapacitors combine the properties of capacitors and batteries, allowing fast charging/discharging like capacitors, while storing higher energy densities like batteries.


Electrochemical storage methods utilize reversible chemical reactions to store and release energy. Two promising electrochemical storage technologies for long-term energy storage are flow batteries and metal-air batteries.

Flow batteries store energy in liquid electrolyte solutions in external tanks. During charging and discharging, the electrolytes flow through an electrochemical cell that converts chemical energy to electricity (and vice versa). Flow batteries can offer almost unlimited energy capacity simply by using larger electrolyte storage tanks. Examples include vanadium redox and zinc-bromine flow batteries.

Metal-air batteries use a metal anode and oxygen from the air as the cathode. Oxygen is reduced during discharge and evolves during charging. The oxygen electrode is an open structure allowing free flow of air. Metal-air batteries typically have high energy densities and can be made from abundant materials like zinc, aluminum, sodium, and lithium. Challenges include electrolyte stability and cyclability.


While long-term energy storage offers many benefits, there are also some key challenges that need to be addressed:

High costs – Building large-scale storage facilities requires major upfront investments. The costs per kilowatt-hour of storage capacity are still generally higher than conventional generation. More widespread deployment and technology improvements can help drive down costs over time.

Geographic constraints – Suitable geology is needed for certain technologies like pumped hydro and compressed air energy storage. This limits where these facilities can be built. New innovations may help expand geographic flexibility.

Intermittency of renewables – The variable output of wind and solar makes it more complex to operate storage facilities. Advanced forecasting and control systems can help mitigate these issues.

Grid integration – It takes careful planning and engineering to integrate large-scale storage facilities with transmission and distribution grids. The system impact and economics need to be evaluated.

Case Studies

There are several examples across the world of successful long term energy storage projects using different technologies.

One innovative approach is the hybrid solar photovoltaic and compressed air energy storage system in China. This system uses excess solar energy during the day to compress air, which is then stored in underground caverns. At night, the compressed air is released to drive a turbine and generate electricity.

The Hydrostor Advanced Compressed Air Energy Storage facility in Canada takes a different approach by using compressed air to push water up into a tank at higher elevation. When electricity is needed, water flows back down through a turbine. This allows large-scale energy storage with minimal environmental impact.

Sodium-sulfur batteries are also gaining popularity for grid-scale storage. In Japan, a 34 MW sodium-sulfur battery system provides reliable backup power and helps stabilize renewable energy generation from local wind farms.

Researchers are also developing new long-duration storage methods like flow batteries, thermal storage, and technologies inspired by photosynthesis. Field demonstrations of these innovations show promise for affordable long-term energy storage in the future.

Future Outlook

The future for long-term energy storage looks promising as storage technologies continue to improve and costs decline. However, policy support will be needed to accelerate deployment.

Researchers are working to improve energy storage across all technologies – from advancing battery chemistries to developing new mechanical, thermal and other storage methods. These advances could increase storage capacity, lifespan, efficiency and power output. Declining costs, driven by economies of scale and technology improvements, will also make storage more affordable.

Policymakers have a crucial role to play in realizing the potential of long-duration storage. Establishing clear incentives, grid integration frameworks and regulatory policies can spur investment and innovation. Targeted funding for R&D and pilot projects is also important. Ultimately, a supportive policy environment will be essential to unlock the flexibility and reliability benefits that long-term energy storage can provide.


In summary, energy can be stored long-term in various forms such as potential energy, chemical energy, thermal energy, and electrical energy. Electrochemical energy storage in batteries is one of the most promising technologies. However, there are still challenges to overcome such as developing batteries that are low-cost, lightweight and have high energy density. Case studies have demonstrated successful real-world applications of long-duration energy storage. Further research and development is critical to enable wider adoption of renewable energy sources. The ability to store energy long-term will provide clean power on demand and support the transition to a sustainable energy future.

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