Energy storage

thumb|The Llyn Stwlan dam of the [[Ffestiniog Power Station|Ffestiniog Pumped-Storage Scheme in Wales. The lower power station has four water turbines which can generate a total of 360 MW of electricity for several hours, an example of artificial energy storage and conversion.]]

Energy storage is the capture of energy produced at one time for use at a later time to reduce imbalances between energy demand and energy production. A device that stores energy is generally called an accumulator or battery. Energy comes in multiple forms including radiation, chemical, gravitational potential, electrical potential, electricity, elevated temperature, latent heat, potential and kinetic. Energy storage involves converting energy from forms that are difficult to store to more conveniently or economically storable forms.

Some technologies provide short-term energy storage, while others can endure for much longer. Bulk energy storage is currently dominated by hydroelectric dams, both conventional as well as pumped. Grid energy storage is a collection of methods used for energy storage on a large scale within an electrical power grid.

Common examples of energy storage are the rechargeable battery, which stores chemical energy readily convertible to electricity to operate a mobile phone; the hydroelectric dam, which stores energy in a reservoir as gravitational potential energy; and ice storage tanks, which store ice frozen by cheaper energy at night to meet peak daytime demand for cooling. Fossil fuels such as coal and gasoline store ancient energy derived from sunlight by organisms that later died, became buried and over time were then converted into these fuels. Food (which is made by the same process as fossil fuels) is a form of energy stored in chemical form.

History

In the 20th century grid, electrical power was largely generated by burning fossil fuel. When less power was required, less fuel was burned. Concerns with air pollution, energy imports, and global warming have spawned the growth of renewable energy such as solar and wind power. Wind power is uncontrolled and may be generating at a time when no additional power is needed. Solar power varies with cloud cover and at best is only available during daylight hours, while demand often peaks after sunset (see duck curve). Interest in storing power from these intermittent sources grows as the renewable energy industry begins to generate a larger fraction of overall energy consumption. In 2023 BloombergNEF forecast total energy storage deployments to grow at a compound annual growth rate of 27 percent through 2030.

Off grid electrical use was a niche market in the 20th century, but in the 21st century, it has expanded. Portable devices are in use all over the world. Solar panels are now common in the rural settings worldwide. Access to electricity is now a question of economics and financial viability, and not solely on technical aspects. Electric vehicles are gradually replacing combustion-engine vehicles. However, powering long-distance transportation without burning fuel remains in development.

Methods

thumb|right|400px|Comparison of various energy storage technologies

Outline

The following list includes a variety of types of energy storage:

Fossil fuel storage
Mechanical
Spring
Compressed-air energy storage (CAES)
Fireless locomotive
Flywheel energy storage
Solid mass gravitational

Hydraulic accumulator
Pumped-storage hydroelectricity (a.k.a. pumped hydroelectric storage, PHS, or pumped storage hydropower, PSH)
Thermal expansion
Electrical, electromagnetic
Capacitor
Supercapacitor
Superconducting magnetic energy storage (SMES, also superconducting storage coil)
Biological
Glycogen
Starch
Electrochemical (battery energy storage system, BESS)
Flow battery
Rechargeable battery
UltraBattery
Thermal
Brick storage heater
Cryogenic energy storage, liquid-air energy storage (LAES)
Liquid nitrogen engine
Eutectic system
Ice storage air conditioning
Molten salt storage
Phase-change material
Seasonal thermal energy storage
Solar pond
Steam accumulator
Thermal energy storage (general)
Chemical
Biofuels
Hydrated salts
Hydrogen peroxide
Power-to-gas (methane, hydrogen storage, oxyhydrogen)
Power-to-solid (metal energy carriers, sulfur)

Mechanical

thumb |Energy from sunlight or other renewable sources is converted to potential energy for storage in devices such as electric batteries. The stored potential energy is later converted to electricity that is added to the power grid, even when the original energy source is not available. In pumped hydro systems, energy from the source is used to lift water upward against the force of gravity, giving it potential energy that is later converted to electricity provided to the power grid.

Energy can be stored in water pumped to a higher elevation using pumped storage methods or by moving solid matter to higher locations (gravity batteries). Other commercial mechanical methods include compressing air and flywheels that convert electric energy into internal energy or kinetic energy and then back again when electrical demand peaks.

Hydroelectricity

Hydroelectric dams with reservoirs can be operated to provide electricity at times of peak demand. Water is stored in the reservoir during periods of low demand and released when demand is high. The net effect is similar to pumped storage, but without the pumping loss.

While a hydroelectric dam does not directly store energy from other generating units, it behaves equivalently by lowering output in periods of excess electricity from other sources. In this mode, dams are one of the most efficient forms of energy storage, because only the timing of its generation changes. Hydroelectric turbines have a start-up time on the order of a few minutes.

Pumped hydro

thumb|The [[Sir Adam Beck Hydroelectric Generating Stations|Sir Adam Beck Generating Complex at Niagara Falls, Canada, which includes a large pumped storage hydroelectricity reservoir to provide an extra 174 MW of electricity during periods of peak demand]]

Worldwide, pumped-storage hydroelectricity (PSH) is the largest-capacity form of active grid energy storage available, and, as of March 2012, the Electric Power Research Institute (EPRI) reports that PSH accounts for more than 99% of bulk storage capacity worldwide, representing around 127,000 MW. Small-scale systems have long been used in such applications as propulsion of mine locomotives. The compressed air is stored in an underground reservoir, such as a salt dome.

Compressed-air energy storage (CAES) plants can bridge the gap between production volatility and load. CAES storage addresses the energy needs of consumers by effectively providing readily available energy to meet demand. Renewable energy sources like wind and solar energy vary. So at times when they provide little power, they need to be supplemented with other forms of energy to meet energy demand. Compressed-air energy storage plants can take in the surplus energy output of renewable energy sources during times of energy over-production. This stored energy can be used at a later time when demand for electricity increases or energy resource availability decreases.

Compression of air creates heat; the air is warmer after compression. Expansion requires heat. If no extra heat is added, the air will be much colder after expansion. If the heat generated during compression can be stored and used during expansion, efficiency improves considerably.

FES systems have rotors made of high strength carbon-fiber composites, suspended by magnetic bearings and spinning at speeds from 20,000 to over 50,000 revolutions per minute (rpm) in a vacuum enclosure. Such flywheels can reach maximum speed ("charge") in a matter of minutes. The flywheel system is connected to a combination electric motor/generator.

FES systems have relatively long lifetimes (lasting decades with little or no maintenance; high specific energy (100–130 W·h/kg, or 360–500 kJ/kg) and power density.

Solid mass gravitational

Changing the altitude of solid masses can store or release energy via an elevating system driven by an electric motor/generator. Studies suggest energy can begin to be released with as little as 1 second warning, making the method a useful supplemental feed into an electricity grid to balance load surges.

Efficiencies can be as high as 85% recovery of stored energy.

Potential energy storage or gravity energy storage was under active development in 2013 in association with the California Independent System Operator. cranes, or elevators to move weights up and down

using high-altitude solar-powered balloon platforms supporting winches to raise and lower solid masses slung underneath them
using winches supported by an ocean barge to take advantage of a 4 km (13,000 ft) elevation difference between the sea surface and the seabed

thumb|District heating accumulation tower from Theiss near [[Krems an der Donau in Lower Austria with a thermal capacity of 2 GWh]]

Thermal

Thermal energy storage (TES) is the temporary storage or removal of heat.

Sensible heat thermal

Sensible heat storage take advantage of sensible heat in a material to store energy.

Seasonal thermal energy storage (STES) allows heat or cold to be used months after it was collected from waste energy or natural sources. The material can be stored in contained aquifers, clusters of boreholes in geological substrates such as sand or crystalline bedrock, in lined pits filled with gravel and water, or water-filled mines. In Braedstrup, Denmark, the community's solar district heating system also uses STES, at a temperature of . A heat pump, which runs only while surplus wind power is available. It is used to raise the temperature to for distribution. When wind energy is not available, a gas-fired boiler is used. Twenty percent of Braedstrup's heat is solar.

A steam accumulator is a type of LHTES where the phase change is between liquid and gas and uses the latent heat of vaporization of water. Ice storage air conditioning systems use off-peak electricity to store cold by freezing water into ice. The stored cold in ice releases during melting process and can be used for cooling at peak hours.

Cryogenic thermal energy storage

Air can be liquefied by cooling using electricity and stored as a cryogen with existing technologies. The liquid air can then be expanded through a turbine and the energy recovered as electricity. The system was demonstrated at a pilot plant in the UK in 2012.]]

A rechargeable battery comprises one or more electrochemical cells. It is known as a 'secondary cell' because its electrochemical reactions are electrically reversible. Rechargeable batteries come in many shapes and sizes, ranging from button cells to megawatt grid systems.

Rechargeable batteries have lower total cost of use and environmental impact than non-rechargeable (disposable) batteries. Some rechargeable battery types are available in the same form factors as disposables. Rechargeable batteries have higher initial cost but can be recharged very cheaply and used many times.

Common rechargeable battery chemistries include:

Lead–acid battery: Lead acid batteries hold the largest market share of electric storage products. A single cell produces about 2V when charged. In the charged state the metallic lead negative electrode and the lead sulfate positive electrode are immersed in a dilute sulfuric acid (H<sub>2</sub>SO<sub>4</sub>) electrolyte. In the discharge process electrons are pushed out of the cell as lead sulfate is formed at the negative electrode while the electrolyte is reduced to water.
Lead–acid battery technology has been developed extensively. Upkeep requires minimal labor and its cost is low. The battery's available energy capacity is subject to a quick discharge resulting in a low life span and low energy density.
Nickel–cadmium battery (NiCd): Uses nickel oxide hydroxide and metallic cadmium as electrodes. Cadmium is a toxic element, and was banned for most uses by the European Union in 2004. Nickel–cadmium batteries have been almost completely replaced by nickel–metal hydride (NiMH) batteries.
Nickel–metal hydride battery (NiMH): First commercial types were available in 1989.

Flow battery

A flow battery works by passing a solution over a membrane where ions are exchanged to charge or discharge the cell. Cell voltage is chemically determined by the Nernst equation and ranges, in practical applications, from 1.0 V to 2.2 V. Storage capacity depends on the volume of solution. A flow battery is technically akin both to a fuel cell and an electrochemical accumulator cell. Commercial applications are for long half-cycle storage such as backup grid power.

Supercapacitor

thumb|One of a fleet of [[capa vehicle|electric capabuses powered by supercapacitors, at a quick-charge station-bus stop, in service during Expo 2010 Shanghai China. Charging rails can be seen suspended over the bus.]]

Supercapacitors, also called electric double-layer capacitors (EDLC) or ultracapacitors, are a family of electrochemical capacitors Fraunhofer has patented their invention in the United States and EU. Fraunhofer claims that Powerpaste is able to store hydrogen energy at 10 times the energy density of a lithium battery of a similar dimension and is safe and convenient for automotive situations.

Biofuels

Various biofuels such as biodiesel, vegetable oil, alcohol fuels, or biomass can replace fossil fuels. Various chemical processes can convert the carbon and hydrogen in coal, natural gas, plant and animal biomass and organic wastes into short hydrocarbons suitable as replacements for existing hydrocarbon fuels. Examples are Fischer–Tropsch diesel, methanol, dimethyl ether and syngas. This diesel source was used extensively in World War II in Germany, which faced limited access to crude oil supplies. South Africa produces most of the country's diesel from coal for similar reasons. such as iron, aluminium, and non-metallic materials such as sulfur. Energy in the form of electricity or solar heat is stored chemically and can be released on-demand. Historically, solid energy carriers have been long used in fireworks and rockets.

Aluminum

Aluminum has been proposed as an energy store by a number of researchers. Its electrochemical equivalent (8.04 Ah/cm3) is nearly four times greater than that of lithium (2.06 Ah/cm3).

Electrical methods

Capacitor

thumb|175px|right|This mylar-film, oil-filled capacitor has very low inductance and low resistance, to provide the high-power (70 megawatts) and the very high speed (1.2 microsecond) discharges needed to operate a [[dye laser.]]

A capacitor (originally known as a 'condenser') is a passive two-terminal electrical component used to store energy electrostatically. Practical capacitors vary widely, but all contain at least two electrical conductors (plates) separated by a dielectric (i.e., insulator). A capacitor can store electric energy when disconnected from its charging circuit, so it can be used like a temporary battery, or like other types of rechargeable energy storage system.

Capacitance is greater given a narrower separation between conductors and when the conductors have a larger surface area. In practice, the dielectric between the plates emits a small amount of leakage current and has an electric field strength limit, known as the breakdown voltage. However, the effect of recovery of a dielectric after a high-voltage breakdown holds promise for a new generation of self-healing capacitors. The conductors and leads introduce undesired inductance and resistance.

Research is assessing the quantum effects of nanoscale capacitors for digital quantum batteries.

Homes

Home energy storage is expected to become increasingly common given the growing importance of distributed generation of renewable energies (especially photovoltaics) and the important share of energy consumption in buildings.

Tesla Motors produces two models of the Tesla Powerwall. One is a 10 kWh weekly cycle version for backup applications and the other is a 7 kWh version for daily cycle applications. In 2016, a limited version of the Tesla Powerpack 2 cost $398(US)/kWh to store electricity worth 12.5 cents/kWh (US average grid price) making a positive return on investment doubtful unless electricity prices are higher than 30 cents/kWh.

RoseWater Energy produces two models of the "Energy & Storage System", the HUB 120 and SB20. Both versions provide 28.8 kWh of output, enabling it to run larger houses or light commercial premises, and protecting custom installations. The system provides five key elements into one system, including providing a clean 60 Hz Sine wave, zero transfer time, industrial-grade surge protection, renewable energy grid sell-back (optional), and battery backup.

Enphase Energy announced an integrated system that allows home users to store, monitor and manage electricity. The system stores 1.2 kWh of energy and 275W/500W power output.

Storing wind or solar energy using thermal energy storage though less flexible, is considerably cheaper than batteries. A simple 52-gallon electric water heater can store roughly 12 kWh of energy for supplementing hot water or space heating.

For purely financial purposes in areas where net metering is available, home generated electricity may be sold to the grid through a grid-tie inverter without the use of batteries for storage.

Grid electricity and power stations

Renewable energy

thumbnail| Construction of the Salt Tanks which provide efficient [[thermal energy storage so that electricity can be generated after the sun goes down, and output can be scheduled to meet demand. The 280 MW Solana Generating Station is designed to provide six hours of storage. This allows the plant to generate about 38% of its rated capacity over the course of a year.]]

thumb|right| The 150 MW [[Andasol solar power station in Spain is a parabolic trough solar thermal power plant that stores energy in tanks of molten salt so that it can continue generating electricity when the sun is not shining.

Some forms of storage that produce electricity include pumped-storage hydroelectric dams, rechargeable batteries, thermal storage including molten salts which can efficiently store and release very large quantities of heat energy,

Thermal storage for heating is less common than for cooling. An example of thermal storage is storing solar heat to be used for heating at night.

Latent heat can also be stored in technical phase change materials (PCMs). These can be encapsulated in wall and ceiling panels, to moderate room temperatures.

Transport

Liquid hydrocarbon fuels are the most commonly used forms of energy storage for use in transportation, followed by a growing use of Battery Electric Vehicles and Hybrid Electric Vehicles. Other energy carriers such as hydrogen can be used to avoid producing greenhouse gases.

Public transport systems like trams and trolleybuses require electricity, but due to their variability in movement, a steady supply of electricity via renewable energy is challenging. Photovoltaic systems installed on the roofs of buildings can be used to power public transportation systems during periods in which there is increased demand for electricity and access to other forms of energy are not readily available. Upcoming transitions in the transportation system also include e.g. ferries and airplanes, where electric power supply is investigated as an interesting alternative.

Electronics

Capacitors are widely used in electronic circuits for blocking direct current while allowing alternating current to pass. In analog filter networks, they smooth the output of power supplies. In resonant circuits they tune radios to particular frequencies. In electric power transmission systems they stabilize voltage and power flow.

Economics

The economics of energy storage strictly depends on the reserve service requested, and several uncertainty factors affect the profitability of energy storage. Therefore, not every storage method is technically and economically suitable for the storage of several MWh, and the optimal size of the energy storage is market and location dependent.

Moreover, ESS are affected by several risks, e.g.:

Techno-economic risks, which are related to the specific technology
Market risks, which are the factors that affect the electricity supply system
Regulation and policy risks

Therefore, traditional techniques based on deterministic Discounted Cash Flow (DCF) for the investment appraisal are not fully adequate to evaluate these risks and uncertainties and the investor's flexibility to deal with them. Hence, the literature recommends to assess the value of risks and uncertainties through the Real Option Analysis (ROA), which is a valuable method in uncertain contexts.

Pumped-storage hydroelectricity is by far the largest storage technology used globally. However, the usage of conventional pumped-hydro storage is limited because it requires terrain with elevation differences and also has a very high land use for relatively small power. In locations without suitable natural geography, underground pumped-hydro storage could also be used. High costs and limited life still make batteries a "weak substitute" for dispatchable power sources, and are unable to cover for variable renewable power gaps lasting for days, weeks or months. In grid models with high VRE share, the excessive cost of storage tends to dominate the costs of the whole grid — for example, in California alone 80% share of VRE would require 9.6 TWh of storage but 100% would require 36.3 TWh. As of 2018 the state only had 150 GWh of storage, primarily in pumped storage and a small fraction in batteries. According to another study, supplying 80% of US demand from VRE would require a smart grid covering the whole country or battery storage capable to supply the whole system for 12 hours, both at cost estimated at $2.5 trillion. Similarly, several studies have found that relying only on VRE and energy storage would cost about 30–50% more than a comparable system that combines VRE with nuclear plants or plants with carbon capture and storage instead of energy storage.

Research

Germany

In 2013, the German government allocated €200M (approximately US$270M) for research, and another €50M to subsidize battery storage in residential rooftop solar panels, according to a representative of the German Energy Storage Association.

In grid models with high VRE share, the excessive cost of storage tends to dominate the costs of the whole grid – for example, in California alone 80% share of VRE would require 9.6 TWh of storage but 100% would require 36.3 TWh. According to another study, supplying 80% of US demand from VRE would require a smart grid covering the whole country or battery storage capable to supply the whole system for 12 hours, both at cost estimated at $2.5 trillion.

External links

U.S. Dept of Energy – Energy Storage Systems Government research center on energy storage technology.
U.S. Dept of Energy – International Energy Storage Database The DOE International Energy Storage Database provides free, up-to-date information on grid-connected energy storage projects and relevant state and federal policies.
IEEE Special Issue on Massive Energy Storage
IEA-ECES – International Energy Agency – Energy Conservation through Energy Conservation programme.
Energy Information Administration Glossary
Energy Storage Project Regeneration