Energy Storage Technologies

Summary

Energy storage technologies provide primary power sources for portable devices and vehicles and are employed in electrical grids to act as backups in order to ensure a stable, steady energy supply. Energy storage is particularly needed for grids that rely on renewable energy sources, such as solar and wind power, so that during periods without sunlight or wind when generators are not operating, electricity can still be sent to consumers. Storage technologies fall into three broad categories: mechanical energy (kinetic or potential) and thermal energy systems; electrochemical systems; and electrical storage systems.

Definition and Basic Principles

Energy storage is the artificial containment of energy for controlled release. It is different in kind from the various forms of natural energy storage, such as the storage of radiant energy from the sun in wood, oil, or coal, which have historically supplied the energy for the generation of the electrical power that supports modern technological civilization. Energy storage technologies provide the physical means for energy containment and are based on a variety of mechanical, electrochemical, or electromagnetic principles. As such, they are rated for usefulness and efficiency based upon how much energy they can contain (energy density) and how they release that energy (power density).

Energy storage devices can supply either primary power or secondary power. Devices such as batteries, capacitors, and fuel cells, for instance, may provide primary power, usually for portable electronics or vehicles—anything that must be used apart from the steady supply of an electrical power grid. Portable radios, smoke detectors, watches, electric cars, and emergency lighting are examples. As secondary, or supplemental, power sources, energy storage technologies are charged by a power grid and then return the energy back to the grid as needed to manage peak electrical loads, improve power quality, ensure frequency regulation, or make up for failing production, as when a turbine must be taken off-line or when absence of sunlight or wind idles a solar- or wind-powered generator.

Background and History

Archaeologists found evidence of what might have been a battery dating from the third century BCE in the form of a clay pot containing iron and copper rods; the pots could have produced electrical discharges if filled with vinegar, although scholars disagree about this interpretation. It is even less well known when another major energy storage device, the reservoir or millpond, first saw use. It consisted of a dam or diverted stream that filled a pond that supplied water on demand to turn a waterwheel.

The first modern artificial energy storage systems were the Leyden jar and the electrochemical battery. German physicist Ewald Georg von Kleist invented the Leyden jar in 1744. A capacitor, it stored static electricity in a glass jar coated inside and out with metal foil and had a conducting rod stuck through an insulating stopper. In an 1800 letter to England's Royal Society, Italian physicist Alessandro Volta described the first electrochemical battery, an arrangement of stacked disks of tin or zinc paired with disks of copper, brass, or silver, each pair separated by a disk of paper or leather moistened in an electrolyte solution. Whereas the Leyden jar delivered electricity only in a burst, the battery gave a steady current, which proved invaluable to experiments in physics and chemistry.

Other devices for storing energy followed steadily: the fuel cell in 1839, compressed air energy storage in 1870, the rechargeable flow battery in 1884, thermal energy storage in 1886, pumped hydro storage in 1908, flywheel energy storage in 1950, supercapacitors in 1966, and superconducting magnetic energy storage in 1986.

How It Works

Mechanical Systems. Pumped water (pumped hydro) is the most common system worldwide for storing energy to serve electrical grids. During low demand on the grid, usually at night, water is pumped from a lower to an upper reservoir. When electrical demand increases and load leveling or supplemental electricity is needed, water in the upper reservoir is released to flow downhill and turn generators. It is also possible to use underground cavities or the open sea for storage. Compressed air energy storage (CAES) also takes advantage of off-peak electricity. Air is compressed, stored underground, and used when needed to turn a gas turbine. The main difficulty with pumped hydro and CAES is finding suitable terrain for reservoirs or underground cavities to store air.

Flywheels store kinetic energy in a rotating mass in the form of a disk or rotor. The flywheel is accelerated during off-peak load by an electrical motor. Later this stored energy can be used to turn an electrical generator and return energy to a grid. Flywheels are categorized as low speed if the rotor spins at fewer than 6,000 revolutions per minute (rpm). These usually have steel rotors and turn on metal ball bearings. High-speed flywheels turn at up to 50,000 rpm and have rotors of composite materials that turn on magnetic bearings. The drawback of flywheels is primarily the degree of standby energy loss. Springs, long used to store mechanical energy in clocks, for instance, are also proposed to store energy for electrical generation.

Thermal energy storage (TES) involves heat stored in saltwater ponds, molten salt, bricks, water reservoirs, pressurized water, or steam by converting electrical energy or by collecting heat directly, as through geothermal or solar heating. Conversely, during low demand in a cooling system, electricity can run refrigerators to make ice, which during high demand can be returned to the system to enhance cooling. Waste electricity during off-peak demand can also be used to produce liquid oxygen and nitrogen that can later be boiled to operate a turbine. In all cases, mechanical and thermal systems are, in theory, continually renewable as long as machinery and materials remain sound.

Electrochemical Systems. Large or small, batteries combine cells in a sequence. A cell comprises an electron source, the anode, and an electron acceptor, the cathode, that, immersed in an electrolyte, directly converts chemical energy to electricity. Batteries come in two major types: primary, or disposable (alkaline, lithium cells, and zinc-carbon batteries), and secondary, or rechargeable (lead-acid, nickel-cadmium, nickel-iron, nickel-metal hydride, and lithium-ion batteries). The number of charge-discharge cycles that secondary batteries can go through before wearing out, their charging and storage times, and their energy density and power density vary and determine their range of applications.

Flow cells (also called "redox flow cells" or "flow batteries") are similar to batteries, except that the electrodes are catalysts for the chemical reaction, which occurs as a microporous membrane allows ions to pass from one electrolyte solution to another. Among flow cells are types that use zinc and bromine, vanadium in two types with different states, or polysulfide and bromine as the pairs of electrolytes. The advantages of flow cells are that they are capable of a large number of cycles, and the electrolytes can be replenished.

Hydrogen fuel cells store energy by employing an electrolyzer to produce hydrogen. It is stored until a fuel cell splits the hydrogen into ions and electrons. The electrons flow through a wire, producing an electric current, while the ions, after passing through an electrolyte between the anode and cathode, are reattached to the electrons reentering the cell from the wire and in the presence of oxygen-form wastewater.

Biological storage is yet another means of storing energy chemically through biological conversion, as is done with adenosine triphosphate (ATP) in the mitochondria of cells. Although interest in synthetic biology is likely to increase, it is a nascent technology.

Electricity Storage Systems. Superconducting magnetic energy storage (SMES) entails storing energy in a magnetic field produced by passing direct current through a coil. This technique does not work with normal electrical wire because resistance and heat loss dissipates energy from the coil. Superconducting wire, having almost no resistance, is required. The superconductors, however, must be kept at very low temperatures to function. Niobium-based wires, for instance, require a temperature of just more than nine kelvins. Ceramic materials that superconduct at much higher temperatures may be used, and room-temperature superconductors are theoretically possible, obviating the need for complex cooling systems for the coil.

Supercapacitors are also known as double-layer capacitors, electrochemical capacitors, ultracapacitors, and pseudocapacitors. They store energy by separating electrostatic charges in electrode plates on either side of a liquid or organic electrolyte. One plate attracts positive ions from the electrolyte, the other negative ions, thereby storing electrical energy in two layers. Supercapacitors are reversible and capable of 10,000 charge-discharge cycles.

Applications and Products

Batteries and Cells. Batteries are ubiquitous in technological society, needed to power virtually all devices that are portable and many vehicles, but also used in utility-scale energy storage. Disposable batteries commonly power household devices, such as the D-cell 1.5-volt batteries in flashlights and toys. Still, rechargeable batteries are the power source in technologies such as laptop computers and cell phones. Lithium-ion batteries quickly dominated the portable devices market because they have high-energy-density (250 to 693 watt-hours per liter, or Wh/L), high efficiency, and long recharging life (about 3,000 cycles). Arrays of batteries are also used for large-scale energy storage. According to the Energy Storage Association, zinc-bromine battery systems, sometimes mounted on trailers for transportation, have capacities of one megawatt (MW) for three-megawatt hours (MWh); units can be linked for further capacity. Sodium-sulfur batteries have been widely used in Japan, storing up to 34 MW per 245 MWh for frequency regulation and to receive energy from wind generators. The lead-acid battery, the most developed battery technology, provided the storage core for vehicles for generations. Nickel-metal hydride and lithium-ion batteries are competing systems, especially for hybrid and electric vehicles, because they are more efficient and powerful. Home energy storage from photovoltaic cell arrays includes lead-acid, lithium, and metal-hydride batteries connected in banks. Nickel-hydrogen, nickel-cadmium, and lithium-ion batteries power space exploration and have served on satellites, the Hubble Space Telescope, interplanetary probes, and Opportunity and Spirit, the two rovers that landed on Mars in 2004.

Flow cells are ideal for storage systems in remote locations. Vanadium redox systems, for instance, deliver up to 500 kW for up to ten hours and can be linked together to provide more energy. Zinc-bromine systems have been produced for 50-kWh and 500-kWh systems to reinforce weak distribution networks or prevent power fluctuations.

Hydrogen fuel cells can potentially do almost anything a battery can do: provide backup power, perform power leveling, run handheld devices, and supply primary or auxiliary power to cars, trucks, buses, and boats. In many cases, they are more efficient than petrochemical fuels. A hydrogen fuel cell in a vehicle that uses an electric motor, for example, can be 40 to 60 percent efficient, compared with the 35 percent peak efficiency of the internal combustion engine. Because they do not emit carbon monoxide, which can be deadly to humans, fuel cells can also be used in enclosed spaces, such as refrigerated warehouses.

Mechanical and Thermal Systems. Although flywheels have been tested as power storage in cars and have aerospace applications, mechanical and thermal systems are primarily backups for large-scale power generation or cooling systems. Pumped hydro serves electrical grids worldwide: about 165 gigawatts (GW) of storage as of 2020, according to researchers Julian Hunt et al. Systems have efficiencies of 70 to 85 percent. Some flywheels can convert 90 percent of their kinetic energy to electricity that will last for several hours. Telecommunications systems have employed flywheels delivering 2 kW for 6 kWh. Megawatts can be stored in linked arrays of flywheels. CAES is used to store energy for electrical grids. The first large-scale commercial system, capable of 290 MW, came online in Huntorf, Germany, in 1978. Later systems varied in output from 110 to 300 MW.

TES systems have somewhat more variety of use. Passive solar power systems used in houses and commercial buildings soak up energy from the sun that can be used to heat interior water or air. Likewise, systems that use off-peak energy to make ice that later contributes to refrigeration operate in large buildings to save electricity costs. For instance, the Dallas Veterans Affairs Medical Center incorporated a chilled water TES that reduced electricity consumption by nearly 3,000 kW and saved more than $200,000 in its first year of operation.

Superconducting Magnetic Energy Storage (SMES). SMES is used by some electrical utilities to improve the system reliability and transfer capacity so that grid operators can compensate for damage from storms, voltage variation, and increasing consumer demand. The first distributed SMES system began operation in Wisconsin in 2000, employing SMES mounted on forty-eight-foot trailers that can be deployed to support the grid in rural areas. The largest users from the 1990s on, however, were industries making plastics, paper, aluminum, and chemicals.

Supercapacitors. Supercapacitors are widely used in consumer electronics of all kinds for power backup and flash charging. They also figure in braking systems, such as energy buffers for elevators and motor ignition, especially for the big engines in buses, trucks, tanks, and submarines. However, they also have applications in larger systems to compensate for voltage drops and can be used in hybrid systems with rechargeable batteries, such as electric and hybrid vehicles.

Careers and Course Work

The substantial increase in research and development since the 1990s and proliferating venture capital investments in energy storage technologies means that they will continue to offer career opportunities for scientists, engineers, and businesspeople worldwide through the twenty-first century.

College students interested in pursuing a career in energy storage can begin by taking science, mathematics, and engineering courses. At some point, selecting a major will prepare one for a particular type of energy storage technology. For example, physics for those interested in SMES and flywheels; electrical engineering, chemistry, or automotive engineering for those interested in batteries, capacitors, and fuel cells and their applications; and mechanical and civil engineering for CAES and pumped hydro. Refining the specialty through graduate school coursework and research to obtain a master's degree or doctorate would position a person to work at the forefront of a crucial technology. Aspiring energy storage engineers can also benefit from pursuing related internships. Alternatively, someone with a science background would be prepared for a marketing, management, or sales position within an energy storage company or utility after obtaining a master of business administration degree (MBA).

For instance, a physicist who wrote a doctoral dissertation on high-temperature superconducting materials in energy storage systems might find employment at a company such as American Superconductor, a utility, a university or government research laboratory, or a university faculty. An undergraduate degree in mechanical engineering could lead a person, after obtaining an MBA, to join junior management for a corporation, work for a utility or construction company, or start an energy storage company. Electrical engineers who specialize in high-power electronics are often in demand. Because research into systems based on biological energy storage for utilities, consumer devices, or industry is not far advanced, experts in biochemistry have the opportunity to be pioneers.

Social Context and Future Prospects

The energy storage industry continues experiencing global expansion as countries seek to reduce carbon emissions and mitigate climate change. Energy storage helps solve pressing problems related to costs and power availability. Studies repeatedly argue that electrical grids cannot be expanded sufficiently or fast enough to meet projected consumer demand during the twenty-first century without incorporating energy storage systems. Historically, the initial construction or purchasing and installation have been costly, though upfront costs have fallen throughout the early twenty-first century. Energy storage systems can save money in the long run by obviating investment in expensive new generators, load management systems, and transmission lines. The savings can help keep the price of electricity low, especially for manufacturers, and reduce the power industry's contribution to inflation. Energy storage will also prove crucial to ensure a steady supply of solar-, wind-, or wave-powered energy generation for grid-level distribution. This is true on a smaller scale for houses or buildings that rely on autonomous renewable energy systems—most likely solar or wind—for their electricity and heating. As consumers demand and governments require better batteries for everything from portable devices to vehicles, research in batteries and fuel cells will likely concentrate on reducing recharging time and increasing cycling life, storage time, power density, and energy density.

Most energy storage systems do not emit greenhouse gasses or disrupt the ecosystem where they are used. However, the construction of CAES or pumped hydro is disruptive to the environment, and the manufacture of the other systems may require mining comparatively rare elements and frequently entails toxic waste products. Additionally, more storage could mean greater energy use and, thus, more carbon emissions unless renewable energy generation is further incentivized. On balance, however, energy storage systems for renewable energy generators will have less environmental impact than electricity produced from coal-, nuclear-, oil-, or gas-powered or hydroelectric generators.

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