Energy storage is the process of saving energy produced at one time to use later. This helps balance when energy is needed and when it is made. A device that stores energy is often called a battery or accumulator. Energy exists in many forms, such as radiation, chemical energy, gravitational potential energy, electrical potential energy, electricity, heat, latent heat, and kinetic energy. Energy storage involves changing energy from forms that are hard to store into forms that are easier or cheaper to keep.

Some energy storage methods work for short periods, while others can last much longer. Large-scale energy storage is mostly done using hydroelectric dams, both traditional and pumped storage types. Grid energy storage includes various methods used to store energy on a large scale within an electrical power grid.

Examples of energy storage include rechargeable batteries, which store chemical energy that can be easily changed into electricity to power devices like mobile phones. Hydroelectric dams store energy as gravitational potential energy in reservoirs. Ice storage tanks use cheaper energy at night to freeze water into ice, which is later used to cool buildings during hot days. Fossil fuels like coal and gasoline store energy that was originally from sunlight. This energy was captured by plants and other organisms long ago, which were later buried and turned into these fuels over time. Food, which is made through a similar process to fossil fuels, stores energy in chemical form.

History

In the 20th century, most electrical power was created by burning fossil fuels. When less power was needed, less fuel was burned. Hydropower, a way to store mechanical energy, is the most widely used method of this type and has been used for hundreds of years. Large hydropower dams have stored energy for more than 100 years. Problems like air pollution, reliance on energy from other countries, and global warming have led to the growth of renewable energy sources such as solar and wind power. Wind power is unpredictable and may produce electricity when it is not needed. Solar power depends on weather conditions and is only available during the day, but energy demand often increases after sunset (see duck curve). As renewable energy becomes a larger part of total energy use, the need to store power from these unpredictable sources has increased. In 2023, BloombergNEF predicted that total energy storage use would grow by 27% each year through 2030.

Off-grid electricity use was uncommon in the 20th century but has grown in the 21st century. Portable electronic devices are now used worldwide. Solar panels are now common in rural areas around the world. Access to electricity now depends more on cost and financial feasibility than on technical challenges. Electric vehicles are slowly replacing vehicles that use fuel. However, finding ways to power long-distance transportation without burning fuel is still under development.

Methods

Energy can be stored in different ways. One method uses water pumped to a higher elevation using a process called pumped storage. Another method, called gravity batteries, stores energy by moving heavy objects to higher places. Other mechanical methods include compressing air and using flywheels, which store energy as heat or movement and return it when needed.

Hydroelectric dams with reservoirs can generate electricity during times of high demand. Water is stored in the reservoir when demand is low and released when demand is high. This works similarly to pumped storage but avoids the energy loss from pumping. While dams do not directly store energy from other sources, they can adjust their output by reducing electricity production when other sources generate too much power. Hydroelectric turbines can start operating within a few minutes.

Pumped-storage hydroelectricity (PSH) is the largest form of energy storage worldwide. As of March 2012, PSH accounted for over 99% of global bulk storage capacity, with a total capacity of about 127,000 MW. PSH systems are typically 70% to 80% efficient, with some reaching up to 87% efficiency. During low demand, excess energy is used to pump water from a lower reservoir to a higher one. When demand increases, water flows back down through a turbine to generate electricity. Most systems use the height difference between two water sources. Some facilities move water between reservoirs, while others combine pumped storage with natural water flow.

Compressed-air energy storage (CAES) stores energy by compressing air into underground reservoirs, such as salt domes. This method helps balance energy supply and demand, especially when renewable sources like wind and solar produce inconsistent power. When air is compressed, it becomes hot, and when it expands, it needs heat to function properly. If the heat from compression is saved and reused, the system becomes more efficient. CAES systems can operate in adiabatic, diabatic, or isothermal ways. Compressed air can also be used to power vehicles.

Flywheel energy storage (FES) stores energy by spinning a heavy wheel at very high speeds. When energy is added, the wheel spins faster, and when energy is released, the wheel slows down. Most FES systems use electricity to control the wheel’s speed, though some use mechanical energy directly. Flywheels are made of strong materials and use magnetic bearings to spin without friction. They can reach high speeds in minutes and are connected to electric motors or generators. FES systems last for many years with little maintenance and have high energy and power storage capabilities.

Energy can also be stored by moving heavy objects up and down using electric motors or generators. This method can respond quickly to sudden changes in electricity demand. Studies show this method can release energy within seconds. This can be done using old mine shafts or specially built towers where heavy weights are lifted and lowered. A prototype system is being built in Edinburgh, Scotland, as of 2020.

In 2013, researchers explored using earth-filled rail cars moved by electric trains to store energy by changing their elevation. Other ideas include using cranes, elevators, or solar-powered balloons to lift and lower weights. Another proposal involves using winches on ocean barges to take advantage of the 4 km (13,000 ft) elevation difference between the sea surface and the seabed.

Thermal energy storage (TES) involves storing or removing heat. Sensible heat storage uses materials that absorb or release heat as their temperature changes. Seasonal thermal energy storage (STES) stores heat or cold for months, using materials like underground aquifers, boreholes in rock, or water-filled mines. An example is the Drake Landing Solar Community in Canada, which uses solar energy stored in underground boreholes to provide heat year-round. In Braedstrup, Denmark, solar energy is stored in underground systems and used for heating, with a gas boiler as a backup.

Latent heat thermal energy storage (LHTES) uses materials that absorb or release energy when they change state, such as melting or freezing. These materials, called phase change materials (PCMs), store more energy than materials that only change temperature. A steam accumulator is a type of LHTES 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 periods.

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

Applications

Before the Industrial Revolution, people used waterways to power water mills for grinding grain or operating machinery. 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 increasing 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. Many companies make rechargeable battery systems to store extra energy from solar or wind power. Today, lithium-ion batteries are more commonly used than lead-acid batteries because they cost about the same but work better.

Tesla Motors makes two versions 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 daily. In 2016, a version of the Tesla Powerpack 2 cost $398 per kWh to store electricity. At that time, the average cost of electricity from the grid was 12.5 cents per kWh, making it unlikely to save money 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 provide 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 when switching power, strong protection against electrical surges, the ability to sell extra energy back to the grid (optional), and backup power from batteries.

Enphase Energy created a system that lets homes store, monitor, and manage electricity. This system stores 1.2 kWh of energy and can produce 275W to 500W of power.

Storing energy from wind or solar using heat is cheaper than batteries but less flexible. A standard 52-gallon electric water heater can store about 12 kWh of energy, enough to help with 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 home-generated electricity directly 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. If solar or wind power produces more electricity than needed, hydroelectric dams can reduce their output and save water for later use. If wind or solar power exceeds the capacity of hydroelectric dams, another energy source is needed.

Solar and wind power produce electricity that varies over time. Energy storage systems help balance the difference between how much electricity is produced and how much is needed. Electricity must be used immediately or converted into a form that can be stored.

The main way to store electricity on a large scale is through pumped-storage hydroelectricity. Countries like Norway, Wales, Japan, and the United States 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. Norway, which relies heavily on hydroelectric power, has a pumped-storage capacity of 1.4 GW. Since most of its total electricity capacity is adjustable, it could expand storage further.

Some energy storage methods include pumped-storage hydroelectric dams, rechargeable batteries, thermal storage using molten salts, compressed air storage, 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 tested a system to store extra energy from wind and hydro power. During times of high production, energy was used to heat ceramic bricks in space heaters and increase the temperature of modified hot water tanks. This stored energy was later used for heating and hot water. The system was created after a storm in 2010 produced so much renewable energy that traditional power sources had to be turned off or reduced.

The Solar Two project in the United States and the Solar Tres Power Tower in Spain used molten salt to store heat from the sun. The salt was heated by sunlight and stored in insulated tanks. Water was turned into steam using the stored heat, which powered turbines to generate electricity.

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

Electric vehicles can connect to the grid and provide stored energy from their batteries when needed, a process called vehicle-to-grid storage.

Thermal energy storage (TES) is used for air conditioning, especially in large buildings. Cooling systems use less energy at night to chill materials, which are then used to cool buildings during the day. In 2009, thermal storage was used in over 3,300 buildings across 35 countries.

Ice storage is a common method for cooling. Chillers operate at night to make ice, which is used during the day to cool water. This reduces energy costs during peak hours. The U.S. Green Building Council encourages energy-efficient building designs through its LEED program. Off-peak cooling can help buildings earn LEED Certification.

Storing heat for heating is less common than cooling. An example is using solar heat to provide warmth at night.

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 database that anyone can use for free. It collects information about energy storage projects and policies. The database is supported by the United States Department of Energy Office of Electricity and Sandia National Labs.

Capacity

Storage capacity refers to how much energy can be taken out from an energy storage device or system. It is usually measured in joules or kilowatt-hours, and sometimes in larger units like megajoules or megawatt-hours. It can also be shown as how many hours a power plant can produce electricity at its maximum capacity. When the storage is a main type, such as thermal or pumped-water storage, the energy output only comes 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 equally good for storing large amounts of energy, and the best size for energy storage depends on the market and location.

Energy storage systems (ESS) face several risks, such as:

• Risks related to the technology and its cost
• Risks from changes in the electricity market
• Risks from changes in laws and policies

Traditional methods, like the Discounted Cash Flow (DCF) approach, are not always enough to evaluate these risks and how flexible investors can be in handling them. Because of this, experts suggest using a method called Real Option Analysis (ROA), which helps assess risks and uncertainties in uncertain situations.

When evaluating large-scale energy storage, such as pumped hydro storage or compressed air storage, benefits include avoiding wasted energy, reducing 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 way to measure how efficient a storage system is called energy storage on energy invested (ESOI). This is calculated by dividing the amount of energy a technology can store by the energy needed to build it. Higher ESOI values mean better energy efficiency. For lithium-ion batteries, ESOI is about 10, and for lead-acid batteries, it is about 2. Other methods, like pumped hydroelectric storage, usually have much higher ESOI values, such as 210.

Pumped-storage hydroelectricity is the most widely used energy storage method worldwide. However, it is limited by the need for natural geography with elevation changes and requires large land areas for small power outputs. In areas without suitable geography, underground pumped hydro storage could be used. Batteries remain a less effective option for long-term energy needs because of high costs and limited lifespan. They cannot replace energy sources that can provide power for days, weeks, or months. In power systems with high shares of variable renewable energy (VRE), the cost of storage can become the largest expense. For example, in California, meeting 80% of energy needs with VRE would require 9.6 terawatt-hours of storage, and 100% would need 36.3 terawatt-hours. As of 2018, California had only 150 gigawatt-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 VRE would require either a nationwide smart grid or battery storage capable of supplying the entire system for 12 hours, both costing about $2.5 trillion. Similar studies show that relying only on VRE and storage would cost about 30–50% more than systems that combine VRE with nuclear energy or energy sources that capture and store carbon.

Research

In 2013, the German government gave €200M (about US$270M) to support research. It also gave €50M to help pay for battery storage in homes with rooftop solar panels, according to a representative from the German Energy Storage Association.

In 2015, Siemens AG started building a production-research plant at the Zentrum für Sonnenenergie und Wasserstoff (ZSW), a research center in Baden-Württemberg, Germany. This center is a partnership between universities and companies in Stuttgart, Ulm, and Widderstall. It has about 350 scientists, researchers, engineers, and technicians. The plant uses a computerized system called SCADA to develop new materials and methods for making rechargeable batteries more efficiently and affordably.

In 2023, the German Research Foundation began a new project to study molecular photoswitches for storing solar thermal energy. Prof. Dr. Hermann A. Wegner is the spokesperson for these systems, called molecular solar thermal (MOST) systems.

In 2014, research and testing centers opened to study energy storage technologies. One example is the Advanced Systems Test Laboratory at the University of Wisconsin-Madison, which partnered with Johnson Controls, a battery company. This laboratory is part of the university’s Wisconsin Energy Institute. Its goal is to test new battery technologies, including those used in electric vehicles and for storing energy on the power grid.

In 2014, the State of New York opened the New York Battery and Energy Storage Technology (NY-BEST) Test and Commercialization Center in Rochester, New York. The center cost $23 million and covers nearly 1,700 square meters. It includes the Center for Future Energy Systems, a partnership between Cornell University and Rensselaer Polytechnic Institute. NY-BEST tests and certifies different types of energy storage for use in businesses and industries.

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

In power grids with high shares of variable renewable energy (VRE), the cost of storage often becomes the largest expense. For example, in California, meeting 80% of energy needs with VRE would require 9.6 TWh of storage, while 100% would need 36.3 TWh. Another study suggests that providing 80% of U.S. energy demand with VRE 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.

In 2014, 14 industry and government groups in the United Kingdom partnered with seven British universities to create the SUPERGEN Energy Storage Hub. This group helps organize and plan research and development for energy storage technologies.