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Physical storage

  • Energy from intermittent electricity sources can be efficiently stored by physical methods, such as mechanical, thermal, or gravitational potential technologies

  • 93% of utility-scale energy storage in 2021 was pumped-storage hydroelectricity, where water is pumped to higher elevations and released to flow downhill through a turbine when needed

  • Other forms of gravity-based storage include lifting weights to the top of tall structures or moving railcars up a hill

  • A variety of other storage technologies exist, such as compressed air storage, mechanical energy storage, and thermal storage

How does pumped-storage hydroelectricity work?

Hydro-storage can store large amounts of energy by using gravity. In times of high electricity supply, water is pumped from a lower reservoir to a higher reservoir. Then, at times of high demand, the water is allowed to flow back down from the high reservoir by gravity, spinning a turbine in the process to regenerate electricity. 


The most common plant model uses reversible pump-turbine/motor-generator assemblies that can act as both pumps and turbines. Pumped storage stations are actually net consumers of electricity, due to hydraulic and electrical losses incurred in the cycle of pumping from lower to upper reservoirs. However, these plants are typically highly efficient (round-trip efficiencies reaching greater than 80%).


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Source. Alternative Energy Tutorials


As we move towards an increasingly electrified energy system and away from fossil fuels, storage will be essential in addressing the challenge of intermittent electricity sources such as solar and wind. Storage allows for a flexible and efficient grid, since electricity produced at peak production times (for example the middle of a sunny day for solar) can be stored and used at peak demand times (such as evenings).


Although the physics are simple, one of the most efficient ways of storing electricity is to convert it to mechanical or gravitational potential energy. This can be in the form of lifting weights to a higher elevation, using flywheels, compressing air, and more.

By far the most common and well established way of storing electricity is by hydro-storage, or pumped-storage hydroelectricity, where water is pumped to higher elevations, stored in a reservoir, and then released to flow through a turbine and generate electricity when needed. As of 2021, hydro-storage accounted for 93% of all utility-scale energy storage capacity in the United States.

Hydro-storage considerations

Building a pumped storage hydroelectricity plant takes a large upfront investment, but is very efficient and long-lasting. The largest challenge with building hydro-storage facilities is finding a site that is suitable. There needs to be a water source, as well as an elevation difference in order to establish the high and low reservoirs. Most hydro-storage plants are coupled with hydroelectric power plants situated on dynamic bodies of water like rivers. However, there are more location options for hydrostorage facilities only, since they can be situated on static bodies of water where an elevation difference exists, such as mountain lakes or human-made reservoirs.


The environmental impact of establishing these large facilities also needs to be considered, and some proposed projects look at repurposing old mines or quarries rather than disturbing natural areas.


Figure reprinted from the Energy Information Administration. Note that Kansas, the flattest state in the country, has no pumped storage facilities.

Other energy storage technologies

Although nowhere near as widespread as pumped water storage, other forms of gravity storage do exist and are in development. These are referred to by the general term gravity batteries.


One technology uses a tall structure and a large weight, either a tall tower or a deep underground shaft. Energy is used to move the weight to a higher elevation, then released and used to generate electricity on its way back down.


Another gravitational method is Advanced Rail Energy Storage (ARES), where heavy train cars are driven up a hill, then released back down the hill with regenerative braking technology to recoup electricity.


A different mechanical storage technology is Compressed Air Energy Storage (CAES), where an air compressor pressurizes air and stores it underground. When there is electricity demand, the air is released back to the surface, heated (which expands the gas, giving it kinetic energy), and is then used to turn a turbine.

Flywheels are a form of mechanical energy storage where energy is stored as rotational energy. A weight rotates around at various speeds, and its momentum stores the energy for a short time. 


Thermal energy storage is another option. A material is heated and the energy is stored as thermal energy until that heat is then used to generate steam and turn a turbine to generate electricity. There are many different types of thermal energy storage, and their efficiency depends on the specific heat of the storage material and the properties of its container. Molten salt thermal storage is often combined with concentrating solar power plants, for example.

Electricity storage is one of the largest energy challenges we are currently facing. The efficiency and cost-effectiveness of the well-established large-scale pumped water storage technology will likely play a large role, in combination with smaller, more dispersed battery or other mechanical energy storage sites.

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Questions for deeper thinking

  • What are some of the geographical considerations for these types of alternative physical storage approaches? How might the nearby population density (urban vs. rural) make one type of storage more appealing than another?

  • What are some possible environmental impacts associated with these physical storage methods?

Sources and further reading

Page last updated: September 7, 2022​

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