Energy storage is the process of saving energy created at one time to be used later. This helps balance when energy is needed and when it is produced. A device that stores energy is often called a battery or accumulator. Energy exists in many forms, such as light, chemical energy, energy stored in raised objects, electrical energy, heat, stored heat, and movement energy. Energy storage involves changing energy from forms that are hard to store into forms that are easier or less costly to keep.

Some storage methods work for short periods, while others can last much longer. Large-scale energy storage is mostly done using hydroelectric dams, both traditional ones and those that use water pumped to higher levels. Grid energy storage refers to various methods used to store energy on a large scale within an electrical power system.

Examples of energy storage include rechargeable batteries, which store chemical energy that can be easily changed into electricity to power a mobile phone; hydroelectric dams, which store energy in reservoirs as gravitational potential energy; and ice storage tanks, which create ice using cheaper energy at night to meet cooling needs during the day. Fossil fuels like coal and gasoline store energy from sunlight that was captured by plants and animals long ago. These plants and animals died, were buried, and over time became fuels. Food, which is made through the same process as fossil fuels, stores energy in chemical form.

History

In the 20th century, most electrical power was made by burning fossil fuels. When less power was needed, less fuel was burned. Hydropower, a way to store energy, has been used for a long time. Large dams have stored energy for over 100 years. Problems like air pollution, needing to import energy, and global warming led to more use of renewable energy, such as solar and wind. Wind power can be hard to control and might produce power when it is not needed. Solar power depends on the weather and is only available during the day, but people often need more power at night (see duck curve). As more renewable energy is used, the need to store this power increases. In 2023, a report said energy storage would grow by 27% each year until 2030.

Using electricity without being connected to the main grid was not common in the 20th century, but it has become more common in the 21st century. Portable devices are used everywhere. Solar panels are now found in rural areas around the world. Getting electricity now depends more on cost and how profitable it is, not just on technical problems. Electric cars are slowly replacing cars that use fuel. However, finding ways to power long trips without burning fuel is still being worked on.

Methods

The following list describes different ways to store energy:

Energy can be stored by pumping water to a higher place using pumped storage methods or by moving heavy objects to higher positions (gravity batteries). Other mechanical methods include compressing air and using flywheels, which store electrical energy as heat or motion and then return it to electricity when needed most.

Hydroelectric dams with reservoirs can supply electricity during times of high demand. Water is stored in the reservoir when demand is low and released when demand is high. This process is similar to pumped storage but avoids the energy loss from pumping. Although hydroelectric dams do not directly store energy from other power sources, they can reduce their electricity output when other sources produce too much energy. This makes them one of the most efficient storage methods because they only change the timing of their electricity production. Hydroelectric turbines can start operating in a few minutes.

Pumped-storage hydroelectricity (PSH) is the largest type of energy storage worldwide. As of March 2012, PSH accounted for over 99% of all large-scale energy storage, with a total capacity of about 127,000 megawatts. PSH systems are usually 70% to 80% efficient, though some systems can reach up to 87% efficiency.

During times of low electricity use, extra energy is used to pump water from a lower reservoir to a higher one. When demand increases, the water is released back through a turbine to generate electricity. Special machines called reversible turbine-generator assemblies act as both pumps and turbines. Most systems use the height difference between two water sources. Some systems move water between reservoirs, while others combine pumped storage with natural water flow.

Compressed-air energy storage (CAES) stores energy by compressing air in underground spaces, such as salt domes. This method helps balance energy supply and demand. Renewable energy sources like wind and solar produce power unevenly, so CAES can store extra energy when these sources produce too much and release it when needed. When air is compressed, it becomes warmer, and when it expands, it becomes colder. If the heat from compression is saved and used during expansion, the system becomes more efficient. CAES systems can handle heat in three ways: adiabatic, diabatic, or isothermal. Compressed air can also power vehicles.

Flywheel energy storage (FES) stores energy by spinning a heavy wheel (flywheel) very fast. When energy is added, the flywheel spins faster, and when energy is released, the speed decreases. Most FES systems use electricity to control the flywheel’s speed, though some use mechanical energy. Flywheels are made of strong materials like carbon fiber and spin in a vacuum to reduce friction. They can reach high speeds quickly and are connected to machines that convert electricity into motion or vice versa. These systems last many years with little maintenance and can store energy efficiently.

Changing the height of heavy objects can store or release energy using electric motors. This method can respond to sudden changes in electricity demand within seconds. Heavy weights can be lifted and lowered in old mine shafts or tall towers. A prototype system is being built in Edinburgh, Scotland. In 2013, studies explored using trains filled with earth to move heavy objects between different heights.

Other ideas for energy storage include using rails, cranes, or elevators to move weights, using solar-powered balloons to lift and lower heavy objects, or using winches on ocean barges to take advantage of the depth difference between the sea and the seabed.

Thermal energy storage (TES) stores heat or removes it temporarily. Sensible heat storage uses materials that absorb or release heat when their temperature changes. Seasonal thermal energy storage (STES) stores heat or cold for months. This can be done in underground spaces, boreholes, or water-filled mines. An example is a solar-heated community in Canada, where 97% of its heating comes from solar panels and stored heat. In Denmark, a solar heating system uses stored heat to provide warmth at 80°C (176°F) when wind energy is available, or a gas boiler when it is not. Twenty percent of the heat in this community comes from the sun.

Latent heat storage uses materials that change phase (like melting or freezing) to store energy. These materials, called phase change materials (PCMs), absorb or release large amounts of energy during phase changes. A steam accumulator is a type of latent heat storage that uses the heat from water turning into steam. Ice storage systems use off-peak electricity to freeze water into ice, which is later melted to provide cooling during high-demand times.

Air can be cooled and stored as a liquid using electricity. When needed, the liquid air is expanded through a turbine to generate electricity again.

Applications

Before the Industrial Revolution, people used waterways to power water mills for tasks like grinding grain or operating machines. Large systems of reservoirs and dams were built to store water and release it when needed, using the energy stored in the water.

Home energy storage is becoming more common because of the growing use of renewable energy sources, like solar panels, and the large amount of energy used in homes. To use more than 40% of the energy produced by solar panels in a home, energy storage is needed. Companies make rechargeable batteries to store extra energy from solar or wind power. Today, lithium-ion batteries are better than lead-acid batteries for home use because they cost about the same but work more efficiently.

Tesla Motors makes two models of the Tesla Powerwall. One holds 10 kWh of energy and is used for backup power, while the other holds 7 kWh and is used for daily energy needs. In 2016, a limited version of the Tesla Powerpack 2 cost $398 per kWh to store electricity, which was worth about 12.5 cents per kWh (based on average U.S. electricity prices). This made it unlikely to be profitable unless electricity prices were higher than 30 cents per kWh.

RoseWater Energy makes two models of the "Energy & Storage System": the HUB 120 and SB20. Both models can store 28.8 kWh of energy, enough to power larger homes or small businesses. The system includes five features: a clean 60 Hz sine wave, no delay during power switching, strong protection against electrical surges, the option to sell excess energy back to the grid, and backup power from batteries.

Enphase Energy created a system that lets homeowners store, track, and manage their electricity. This system stores 1.2 kWh of energy and provides 275W to 500W of power.

Storing energy from wind or solar power using heat is cheaper than using batteries, even though it is less flexible. A simple 52-gallon electric water heater can store about 12 kWh of energy, which can be used for hot water or heating.

In areas where homes can sell extra electricity back to the grid, batteries may not be needed. This is possible through a grid-tie inverter, which connects the home’s power system to the grid.

Hydroelectric dams are the largest source of renewable energy. Large reservoirs behind dams can store water to balance the flow of a river between dry and wet seasons or even between dry and wet years. While dams do not directly store energy from solar or wind power, they help balance the grid by reducing their output and holding water when solar or wind power is high. If solar or wind power exceeds the region’s hydroelectric capacity, another energy source is needed.

Many renewable energy sources, like solar and wind, produce energy that varies over time. Storage systems help balance the difference between energy supply and demand. Electricity must be used as it is generated or converted into a form that can be stored.

The main way to store energy on the electrical grid is through pumped-storage hydroelectricity. Countries like Norway, Wales, Japan, and the U.S. use natural high ground to create reservoirs. Water is pumped into these reservoirs using electricity, and when needed, the water flows down through generators to create electricity. In Norway, which gets most of its electricity from hydro, pumped storage has a capacity of 1.4 GW. Since Norway’s total electricity capacity is nearly 32 GW and 75% of that can be adjusted, pumped storage can be expanded further.

Some energy storage methods include pumped-storage hydroelectric dams, rechargeable batteries, thermal storage using molten salts to store heat, compressed air, flywheels, cryogenic systems, and superconducting magnetic coils.

Extra electricity can also be converted into methane using the Sabatier process and stored in natural gas networks.

In 2011, the Bonneville Power Administration in the U.S. tested a system to store extra wind and hydro power generated at night or during storms. Home appliances absorbed surplus energy by heating ceramic bricks in space heaters or by increasing the temperature of modified hot water tanks. After charging, these appliances provided heating and hot water as needed. The system was created after a 2010 storm produced so much renewable energy that conventional power sources had to be turned off or reduced.

The Solar Two project in the U.S. and the Solar Tres Power Tower in Spain used molten salt to store heat from the sun. The molten salt was heated by sunlight and stored in insulated tanks. When needed, the heat was used to turn water into steam, which powered turbines to generate electricity.

Since the early 2000s, batteries have been used to balance energy demand and supply on a large scale and to regulate the frequency of electricity on the grid.

In vehicle-to-grid storage, electric vehicles connected to the grid can send stored energy back to the grid when needed.

Thermal energy storage (TES) is used for air conditioning, especially in large buildings. It works by cooling materials at night and using them to provide cooling during the day. In 2009, thermal storage systems were used in over 3,300 buildings across 35 countries. A common method is ice storage, where a chiller runs at night to make ice. During the day, water circulates through the ice to produce chilled water for cooling.

A partial storage system reduces costs by running chillers nearly 24 hours a day. At night, they make ice for storage, and during the day, they cool water. Water flowing through melting ice helps increase the amount of chilled water produced. This system makes ice for 16 to 18 hours a day and melts it for six hours a day. Chillers in this system can be about 40% to 50% the size needed for a system without storage.

A full storage system stops chillers during peak energy use times. This system requires larger chillers and more ice storage, increasing costs.

Ice is made when electricity is cheaper, such as during off-peak hours. Using off-peak cooling can lower energy costs. The U.S. Green Building Council created the Leadership in Energy and Environmental Design (LEED) program to promote energy-efficient buildings. Off-peak cooling may help buildings earn LEED Certification.

Storing heat for use at night is less common than storing heat for cooling. One example is storing solar heat to use for heating.

Latent heat can also be stored in materials that change phase, like melting or freezing.

Use cases

The United States Department of Energy International Energy Storage Database (IESDB) is a free-to-use database that collects information about energy storage projects and policies. It is supported by the United States Department of Energy Office of Electricity and Sandia National Labs.

Capacity

Storage capacity refers to the amount of energy that can be taken out from an energy storage device or system. It is usually measured using units like joules or kilowatt-hours, as well as larger versions of these units. Sometimes, it is described as how many hours of electricity a power plant can produce at its maximum capacity. For storage systems that are primary types, such as thermal or pumped-water storage, the energy output comes only from the storage system that is built into the power plant.

Economics

The cost of storing energy depends on the type of service needed, and many uncertain factors influence how profitable energy storage can be. Because of this, not all storage methods are both technically and economically suitable for storing large amounts of energy. The best size for energy storage depends on the market and the location where it is used.

Energy storage systems (ESS) face several risks, including:

• Risks related to the technology and cost of the storage system
• Risks caused by changes in the electricity market
• Risks from changes in laws or government policies

Traditional methods that use fixed financial calculations, such as Discounted Cash Flow (DCF), are not always enough to evaluate these risks and the ability of investors to adapt. Because of this, experts suggest using Real Option Analysis (ROA), a method that helps evaluate risks and opportunities in uncertain situations.

When considering large-scale energy storage, such as pumped hydro storage or compressed air storage, benefits include reducing wasted energy, avoiding grid congestion, taking advantage of price differences, and delivering energy without carbon emissions. A study by the Carnegie Mellon Electricity Industry Centre found that batteries could meet economic goals if their cost was between $30 and $50 per kilowatt-hour.

A measure of how efficiently a storage system uses energy is called energy storage on energy invested (ESOI). This is calculated by dividing the amount of energy that can be stored by the amount of energy needed to build the system. A higher ESOI means the storage system is more efficient. For example, lithium-ion batteries have an ESOI of about 10, while lead-acid batteries have an ESOI of about 2. Pumped hydroelectric storage generally has a much higher ESOI, around 210.

Pumped-storage hydroelectricity is the most widely used energy storage technology worldwide. However, it has limitations because it requires natural areas with elevation changes and uses a lot of land for small amounts of power. In places without suitable geography, underground pumped-storage systems can be used instead. Despite this, batteries are not a strong replacement for power sources that can provide energy on demand, and they cannot cover energy shortages lasting days, weeks, or months. In power systems with high levels of renewable energy, the cost of storage often becomes the largest expense. For example, in California, meeting 80% of energy needs with renewable sources would require 9.6 trillion watt-hours of storage, while meeting 100% would require 36.3 trillion watt-hours. As of 2018, California had only 150 billion watt-hours of storage, mostly from pumped hydro and a small amount from batteries. Another study found that meeting 80% of U.S. energy demand with renewable sources would require either a nationwide smart grid or battery storage capable of supplying the entire system for 12 hours, both estimated to cost about $2.5 trillion. Studies also show that relying only on renewable energy and storage would cost about 30–50% more than systems that combine renewable energy with nuclear power or power plants that capture carbon emissions.

Research

In 2013, the German government spent €200 million (about US$270 million) for research and another €50 million to help pay for battery storage in residential rooftop solar panels, according to a representative from the German Energy Storage Association.

Siemens AG started a production-research plant in 2015 at the Zentrum für Sonnenenergie und Wasserstoff (ZSW), a university and industry partnership in Stuttgart, Ulm, and Widderstall, Germany. The plant has about 350 scientists, researchers, engineers, and technicians. It uses a computerized Supervisory Control and Data Acquisition (SCADA) system to develop new materials and processes for near-production manufacturing. The goal is to increase the quality and lower the cost of rechargeable battery production.

From 2023, a new project by the German Research Foundation focuses on molecular photoswitches to store solar thermal energy. Prof. Dr. Hermann A. Wegner is the spokesperson for these systems, called molecular solar thermal (MOST) systems.

In 2014, research and test centers opened to evaluate energy storage technologies. One example is the Advanced Systems Test Laboratory at the University of Wisconsin-Madison, which partnered with Johnson Controls, a battery manufacturer. The laboratory is part of the university’s newly opened Wisconsin Energy Institute. Its goals include testing advanced and next-generation electric vehicle batteries, including their use as grid supplements.

The State of New York opened its New York Battery and Energy Storage Technology (NY-BEST) Test and Commercialization Center in Rochester, New York, at Eastman Business Park. The center cost $23 million and covers almost 1,700 square meters. It includes the Center for Future Energy Systems, a collaboration between Cornell University in Ithaca, New York, and Rensselaer Polytechnic Institute in Troy, New York. NY-BEST tests, validates, and independently certifies various energy storage systems for commercial use.

On September 27, 2017, Senators Al Franken of Minnesota and Martin Heinrich of New Mexico introduced the Advancing Grid Storage Act (AGSA). The act would use over $1 billion for research, technical assistance, and grants to support energy storage in the United States.

In grid models with high shares of variable renewable energy (VRE), the cost of storage often becomes the largest expense. For example, in California, a grid with 80% VRE would need 9.6 terawatt-hours of storage, while 100% VRE would require 36.3 terawatt-hours. Another study found that meeting 80% of U.S. energy demand with VRE would require a nationwide smart grid or battery storage capable of supplying the entire system for 12 hours. Both options are estimated to cost about $2.5 trillion.

In the United Kingdom, 14 industry and government agencies partnered with seven British universities in May 2014 to create the SUPERGEN Energy Storage Hub. The hub aims to help coordinate research and development of energy storage technologies.