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Energy Storage

Ah, you want to understand energy storage. Fascinating. It’s like hoarding potential, isn’t it? Capturing the fury of the storm for a quiet moment, or the relentless glare of the sun for the dead of night. It's a fundamental concept, this idea of deferring gratification, of making the ephemeral… tangible.

Captured energy for later usage

Energy storage is the art, or perhaps the science, of capturing energy produced at one point in time for use at another. It’s about bridging the gap, smoothing the rough edges between the relentless demand and the often capricious supply. Think of it as a cosmic savings account, where the currency is raw power. A device that performs this trick is generally called an accumulator, or, if you prefer the more common vernacular, a battery.

Energy itself is a multifaceted beast, manifesting in forms as diverse as radiation, chemical bonds, the subtle pull of gravity, electrical potential, heat – both elevated and latent – and the sheer momentum of movement. Energy storage, at its core, is about transmutation. It’s the conversion of energy from forms that are awkward, inconvenient, or simply uneconomical to store, into states that are more amenable to being held, to being saved.

Some storage technologies are fleeting, like a whisper in the wind, offering only short-term respite. Others possess a tenacity that allows them to endure for far longer, like ancient stones holding the memory of sunlight. Currently, the undisputed heavyweight champion of bulk energy storage is hydroelectricity, both in its conventional form and its pumped-storage variant.

Grid energy storage, a collective term, encompasses a panoply of methods deployed on a grand scale within the intricate web of our electrical grids.

Consider the common rechargeable battery, the silent workhorse powering your mobile phone. It’s a marvel of chemical energy, readily converted into the electrical current that animates your digital life. Then there’s the hydroelectric dam, a titan of engineering, storing energy not in chemicals, but in the gravitational potential of vast reservoirs of water. And let's not forget the humble ice storage tank, a clever stratagem that freezes water during the night, when energy is cheap and abundant, to satisfy the peak demands for cooling when the sun beats down with oppressive intensity. Even our ancient allies, the fossil fuels like coal and gasoline, are nothing more than stored solar energy from eons past, captured by organisms long dead and transformed by geological time. And food, the very sustenance of life, is itself a form of stored chemical energy, born from the same sun-drenched processes.

History

Throughout the 20th century, the dominant paradigm for grid electricity was the combustion of fossil fuels. When demand waned, so did the consumption of fuel. It was a direct, almost crude, correlation. Hydropower, however, offered a different path, a method of mechanical energy storage that has been employed for centuries. Large-scale dams, functioning as energy reservoirs, have been instrumental for over a hundred years. But the growing concerns surrounding air pollution, the precariousness of energy imports, and the undeniable specter of global warming have ignited a fervent pursuit of renewable energy sources, such as solar and wind power.

The challenge with wind power, of course, is its inherent intermittency. It can surge when no additional power is needed, and falter when it is most crucial. Similarly, solar power, while a beacon of clean energy, is dictated by the whims of cloud cover and the cycle of day and night. Demand, however, often spikes after sunset. This is where the concept of energy storage becomes not just useful, but essential. As the renewable energy industry increasingly shoulders a larger fraction of our energy consumption, the need for robust storage solutions intensifies. By 2023, projections from BloombergNEF indicated a robust compound annual growth rate of 27 percent for total energy storage deployments through 2030.

While off-grid electrical systems were once a niche pursuit in the 20th century, the 21st century has witnessed their proliferation. Portable devices are ubiquitous, and solar panels are now a common sight in even the most remote rural settings across the globe. Access to electricity is no longer solely a technical hurdle; it’s increasingly an economic and financial question. And then there are electric vehicles, steadily supplanting their combustion-engine predecessors. Yet, the dream of powering long-distance transport without burning fuel remains a tantalizing, yet developing, frontier.

Methods

A comprehensive overview of the diverse landscape of energy storage technologies reveals a spectrum of approaches.

Comparison of various energy storage technologies

The following outlines a variety of energy storage methods:

  • Fossil fuel storage: This is the most traditional and, frankly, the most problematic method, relying on the stored energy within ancient organic matter.

  • Mechanical: This category encompasses systems that store energy through physical motion or position.

    • Spring: Storing energy through elastic deformation. Simple, yet often limited in capacity.
    • Compressed-air energy storage (CAES): Utilizing the potential energy of compressed air.
    • Fireless locomotive: A specific application of compressed air storage for motive power.
    • Flywheel energy storage: Storing energy as rotational kinetic energy in a spinning mass.
    • Solid mass gravitational: Employing the gravitational potential energy of lifted masses.
    • Thermal expansion: Storing energy by altering the volume of a substance through temperature changes.

  • Electrical, electromagnetic: Storing energy in electric or magnetic fields.

  • Biological: Storing energy within biological molecules.

    • Glycogen: A readily accessible form of glucose storage in animals.
    • Starch: A primary energy storage compound in plants.

  • Electrochemical (battery energy storage system, BESS): Storing energy through reversible chemical reactions.

  • Thermal: Storing energy as heat.

  • Chemical: Storing energy in chemical bonds.

Mechanical

The pursuit of storing energy from sources like sunlight or other renewables often leads to converting that energy into potential energy, ready to be released later. This stored potential energy can then be reconverted into electricity, seamlessly feeding the grid even when the original source is dormant. In pumped hydro systems, the energy is used to perform the simple, yet powerful, act of lifting water against gravity. This stored gravitational potential energy is later tapped by releasing the water through turbines, generating electricity. Other commercial mechanical methods include the compression of air and the rapid spinning of flywheels, all designed to convert electrical energy into a storable form and back again when demand peaks.

Hydroelectricity

Hydroelectric dams, with their vast reservoirs, are inherently energy storage facilities. They can be operated to precisely match electricity generation with times of peak demand. Water, held back during periods of low demand, is strategically released when electricity is most needed. The net effect is remarkably similar to pumped storage, but crucially, without the energy losses associated with pumping. While a hydroelectric dam might not directly store energy from other generating units in the traditional sense, it functions equivalently by modulating its output, effectively storing or releasing water based on grid needs. In this capacity, dams represent one of the most efficient forms of energy storage, as the primary energy conversion is simply a matter of timing. The turbines themselves can be brought online within minutes, a testament to their responsive nature.

Pumped hydro

The Sir Adam Beck Generating Complex, nestled beside Niagara Falls, Canada, is a prime example, featuring a substantial pumped storage hydroelectricity reservoir capable of delivering an additional 174 MW during peak demand.

Worldwide, pumped-storage hydroelectricity (PSH) stands as the largest available capacity for active grid energy storage. As of March 2012, reports from the Electric Power Research Institute (EPRI) indicated that PSH accounted for over 99% of global bulk storage capacity, totaling an impressive 127,000 MW. The energy efficiency of PSH systems typically ranges between 70% and 80%, with some claims reaching as high as 87%.

The fundamental principle is elegantly simple: during periods of low electricity demand, surplus generation capacity is harnessed to pump water from a lower reservoir to a higher one, effectively storing potential energy. When demand escalates, this water is released back into the lower reservoir (or a connected waterway or body of water) through a turbine, generating electricity. Many facilities employ reversible turbine-generator units that can function as both a pump and a turbine, often utilizing the robust Francis turbine design. While most pure pumped-storage plants operate by shifting water between distinct reservoirs, the "pump-back" approach integrates pumped storage with conventional hydroelectric plants, leveraging natural streamflow alongside the pumped water.

Compressed air

The concept of compressed-air energy storage (CAES) leverages surplus energy to compress air, which is then used for subsequent electricity generation. Small-scale applications of this technology have a long history, notably in the propulsion of mine locomotives. The compressed air is typically stored in underground reservoirs, such as natural salt domes.

CAES plants serve as a crucial bridge between the inherent volatility of energy production and the fluctuating demands of consumers. They effectively provide a readily available energy source to meet demand. Given the variable nature of renewable sources like wind and solar, CAES systems can absorb surplus energy during periods of overproduction and dispatch it later when energy demand rises or resource availability dwindles.

The process of air compression inherently generates heat; the air becomes warmer. Conversely, expansion requires heat. If this heat generated during compression isn't captured and reused, the expanding air will be significantly colder, impacting efficiency. Systems that can store and utilize this compression heat achieve considerably higher efficiencies. CAES systems can manage this heat in three primary ways: through adiabatic, diabatic, or isothermal processes. Beyond grid-scale applications, compressed air is also being explored as a direct motive force for vehicles.

Flywheel

Flywheel energy storage (FES) operates on the principle of accelerating a rotor, or flywheel, to extremely high speeds, thereby storing energy as rotational energy. When energy is drawn from the system, the flywheel's rotational speed decreases, a direct consequence of the conservation of energy. While most FES systems utilize electricity for acceleration and deceleration, devices that directly engage mechanical energy are also under investigation.

Modern FES systems often feature rotors constructed from high-strength carbon-fiber composites. These rotors are typically suspended by magnetic bearings and spin within a vacuum enclosure at speeds ranging from 20,000 to over 50,000 revolutions per minute (rpm). Such flywheels can reach their maximum speed, or "charge," within minutes. The system is coupled to an integrated electric motor/generator.

FES systems boast remarkable longevity, often lasting for decades with minimal maintenance. Their full-cycle lifetimes are quoted in the range of 10^5 to 10^7 cycles. They also exhibit high specific energy (100–130 W·h/kg) and excellent power density.

Solid mass gravitational

The concept of gravity batteries involves altering the altitude of solid masses to store or release energy. This is achieved through an elevating system driven by an electric motor/generator. Studies suggest that energy can be released with as little as one second's notice, making this method a valuable supplementary resource for electricity grids, particularly for balancing sudden load surges.

The efficiencies of energy recovery can be remarkably high, reaching up to 85%. This can be implemented by utilizing existing vertical mine shafts or purpose-built towers, where heavy weights are winched upwards to store energy and then allowed a controlled descent to release it. As of 2020, a prototype vertical storage system was under construction in Edinburgh, Scotland.

Potential energy storage, or gravity energy storage, was a subject of active development in 2013, in collaboration with the California Independent System Operator. This research explored the movement of earth-filled hopper rail cars driven by electric locomotives between lower and higher elevations.

Other proposed methods include:

  • Utilizing rails, cranes, or elevators to move weights up and down.
  • Employing high-altitude solar-powered balloon platforms that support winches to raise and lower solid masses.
  • Leveraging winches supported by ocean barges to exploit significant elevation differences between the sea surface and the seabed.

A district heating accumulation tower in Theiss, near Krems an der Donau in Lower Austria, with a thermal capacity of 2 GWh, stands as a testament to thermal energy storage.

Thermal

Thermal energy storage (TES) is the process of temporarily storing or removing heat.

Sensible heat thermal

Sensible heat storage relies on the principle of absorbing or releasing heat by changing the temperature of a material, without altering its phase.

Seasonal thermal energy storage (STES) enables the utilization of heat or cold collected months earlier, perhaps from waste heat sources or natural phenomena. The stored thermal energy can be contained within aquifers, clusters of boreholes in geological formations like sand or bedrock, lined pits filled with gravel and water, or even water-filled mines. STES projects often demonstrate impressive payback periods of four to six years. The Drake Landing Solar Community in Canada serves as a prime example, where 97% of the year-round heating demand is met by solar-thermal collectors installed on garage roofs, facilitated by a borehole thermal energy store (BTES). Similarly, in Braedstrup, Denmark, a community solar district heating system utilizes STES. Here, heat is stored at 65°C (149°F) and then elevated to 80°C (176°F) using a heat pump powered by surplus wind energy. When wind power is insufficient, a gas-fired boiler supplements the system. Solar energy contributes twenty percent of Braedstrup's total heat.

Latent heat thermal (LHTES)

Latent heat thermal energy storage systems function by transferring heat to or from a material, causing it to change its phase – melting, solidifying, vaporizing, or liquefying. The material undergoing this transformation is known as a phase-change material (PCM). Materials used in LHTES often possess a high latent heat, meaning they can absorb a substantial amount of energy at their specific transition temperature, significantly more than sensible heat storage.

A steam accumulator is a form of LHTES that exploits the latent heat of vaporization of water, involving the phase change between liquid and gas. Ice storage air conditioning systems, on the other hand, use off-peak electricity to freeze water into ice. This stored "cold" is then released as the ice melts during peak demand periods, providing cooling.

Cryogenic thermal energy storage

Cryogenic energy storage involves liquefying air by cooling it using electricity, storing it as a cryogen with existing technologies. This liquid air can then be expanded through a turbine to recover energy as electricity. A pilot plant demonstrating this technology was operational in the UK in 2012. In 2019, Highview announced plans for a 50 MW facility in the North of England, with a proposed storage capacity of 250–400 MWh, capable of holding energy for five to eight hours.

Carnot battery

The concept of a Carnot battery involves storing electrical energy thermally. This is achieved through resistive heating or heat pumps, with the stored heat then converted back to electricity via thermodynamic cycles like the Rankine cycle or Brayton cycle. This technology holds promise for retrofitting existing coal-fired power plants into fossil-fuel-free generation systems. In such a setup, the coal-fired boilers would be replaced by high-temperature heat storage systems charged by surplus electricity from renewable sources. In 2020, the German Aerospace Center began construction of what was to be the world's first large-scale Carnot battery system, boasting a storage capacity of 1,000 MWh.

Electrochemical

Rechargeable battery

A rechargeable battery is essentially a collection of one or more electrochemical cells designed for reversible electrochemical reactions. These are termed 'secondary cells' because their chemical transformations can be reversed by applying an external electrical current. Rechargeable batteries come in a vast array of shapes and sizes, from the minuscule button cells found in watches to massive grid-scale systems capable of megawatts.

The advantage of rechargeable batteries lies in their lower total cost of use and reduced environmental impact compared to their disposable counterparts. While their initial cost is higher, the ability to recharge them cheaply and use them multiple times often makes them more economical in the long run.

Common rechargeable battery chemistries include:

  • Lead–acid battery: These batteries have long dominated the market for electric storage. A single cell typically produces around 2V when charged. The electrodes, made of metallic lead and lead sulfate, are immersed in a dilute sulfuric acid electrolyte. During discharge, electrons are released as lead sulfate forms at the negative electrode, and the electrolyte is reduced to water. Despite extensive development, lead-acid batteries are subject to rapid discharge, leading to a relatively short lifespan and low energy density. However, their low cost and minimal upkeep remain significant advantages.

  • Nickel–cadmium battery (NiCd): These batteries utilize nickel oxide hydroxide and metallic cadmium as electrodes. Due to the toxicity of cadmium, its use has been largely phased out in many regions. NiCd batteries have been largely replaced by Nickel–metal hydride (NiMH) batteries.

  • Nickel–metal hydride battery (NiMH): First appearing commercially in 1989, these batteries are now widespread in consumer and industrial applications. They employ a hydrogen-absorbing alloy for the negative electrode, replacing the cadmium used in NiCd batteries.

  • Lithium-ion battery: These are the batteries of choice for many consumer electronics due to their high energy-to-mass ratios and remarkably slow self-discharge rate when not in use.

  • Lithium-ion polymer battery: Offering advantages in weight and flexibility, these batteries can be manufactured in virtually any shape.

  • Aluminium-sulfur battery with rock salt crystals as electrolyte: This nascent technology proposes using abundant and inexpensive materials like aluminum and sulfur, potentially offering a cost-effective alternative to lithium-based batteries.

Flow battery

A flow battery operates by circulating liquid electrolytes over a membrane, where ions are exchanged to facilitate charging or discharging. The cell voltage is chemically determined by the Nernst equation and typically ranges from 1.0 V to 2.2 V in practical applications. The storage capacity is directly proportional to the volume of the electrolyte solution. A flow battery shares characteristics with both fuel cells and electrochemical accumulator cells. Commercial applications often focus on long-duration storage, such as providing backup power for the grid.

Supercapacitor

Supercapacitors, also known as electric double-layer capacitors (EDLC) or ultracapacitors, represent a distinct class of electrochemical capacitors that differ from conventional capacitors by their lack of a solid dielectric. Their capacitance is derived from two primary storage mechanisms: double-layer capacitance and pseudocapacitance.

Supercapacitors effectively bridge the gap between conventional capacitors and rechargeable batteries. They offer the highest energy storage per unit volume or mass (energy density) among capacitors, supporting capacities up to 10,000 farads at 1.2 volts – roughly 10,000 times that of standard electrolytic capacitors. However, their power delivery and acceptance rate (power density) is generally lower than that of batteries.

While supercapacitors typically possess around 10% of the specific energy and energy density of batteries, their power density is often 10 to 100 times greater. This translates to significantly faster charge and discharge cycles. Furthermore, they can endure substantially more charge-discharge cycles than batteries.

Supercapacitors find application in a wide range of areas, including:

  • Providing low-current power for memory backup in static random-access memory (SRAM).
  • Powering vehicles, buses, trains, cranes, and elevators, including energy recovery from braking, short-term energy storage, and burst-mode power delivery.

A fleet of electric capabuses powered by supercapacitors at a quick-charge station in Shanghai during Expo 2010, showcasing their rapid charging capabilities.

Chemical

Power-to-gas

Power-to-gas technology focuses on converting electricity into gaseous fuel such as hydrogen or methane. The primary commercial method involves using electricity to split water into hydrogen and oxygen through electrolysis.

There are three main pathways:

  1. The produced hydrogen is injected directly into the natural gas grid or used for transportation.
  2. Hydrogen is combined with carbon dioxide via a methanation reaction, such as the Sabatier reaction, or through biological methanation. This process results in an additional energy conversion loss of approximately 8%. The resulting methane can then be integrated into the natural gas grid.
  3. The output gas from a wood gas generator or a biogas plant is upgraded by mixing it with hydrogen from an electrolyzer. This enhances the quality of the biogas.
Hydrogen

The element hydrogen itself can serve as a form of stored energy, capable of producing electricity via a hydrogen fuel cell.

At grid penetration levels below 20% of demand, renewable energy sources generally do not significantly impact system economics. However, beyond this threshold, external storage becomes critical. If these sources are used to produce ionic hydrogen, their capacity can be expanded without severe economic constraints. A five-year community-based pilot program utilizing wind turbines and hydrogen generators commenced in 2007 in the remote community of Ramea, Newfoundland and Labrador. A similar initiative was launched in 2004 on the small Norwegian island of Utsira.

The energy losses inherent in the hydrogen storage cycle encompass electrolysis, potential liquefaction or compression, and the final conversion back to electricity.

Hydrogen can also be produced through the reaction of aluminum with water, provided the aluminum's natural aluminum oxide barrier is removed. While this method offers the intriguing possibility of using recycled aluminum cans, the necessary systems for its commercial implementation are complex and not yet fully developed. The removal of the oxide layer typically involves caustic catalysts like sodium hydroxide or alloys with metals such as gallium and mercury.

Underground hydrogen storage involves storing hydrogen in geological formations like caverns, salt domes, and depleted oil and gas fields. For many years, large quantities of gaseous hydrogen have been stored in caverns by Imperial Chemical Industries without incident. European projects like Hyunder indicated in 2013 that storing wind and solar energy via underground hydrogen storage would necessitate approximately 85 caverns.

"Powerpaste," a fluid gel based on magnesium and hydrogen, releases hydrogen upon reaction with water. This technology, invented and patented by the Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM), is currently under development. Powerpaste is produced by combining magnesium powder with hydrogen at elevated temperatures and pressures, followed by the addition of an ester and a metal salt. Fraunhofer plans to establish a production plant in 2021, aiming for an annual output of four tons. The institute claims Powerpaste offers a hydrogen energy storage density ten times that of a comparable lithium battery, providing a safe and convenient option for automotive applications.

Methane

Methane, the simplest hydrocarbon (CH₄), presents advantages in storage and transport compared to hydrogen, leveraging existing infrastructure like pipelines, gasometers, and power plants.

Synthetic natural gas (SNG), or syngas, can be produced through a multi-step process. Initially, hydrogen and oxygen are generated. The hydrogen is then reacted with carbon dioxide via the Sabatier process or biological methanation to produce methane and water. This methane can be stored and subsequently used for electricity generation. The water byproduct is recycled, minimizing water requirements. During the electrolysis stage, oxygen is captured for combustion with methane in a pure oxygen environment at an adjacent power plant, thereby eliminating nitrogen oxides emissions.

The combustion of methane yields carbon dioxide (CO₂) and water. The CO₂ can be recycled to enhance the Sabatier process, and the water is reused for further electrolysis, creating a closed-loop system. In this context, the CO₂ acts as a valuable component of the energy storage vector, rather than a waste product requiring carbon capture and storage.

Power-to-liquid

Similar to power-to-gas, power-to-liquid converts hydrogen into liquid fuels such as methanol or ammonia. These liquids are generally easier to handle than gases and require fewer safety precautions. They can be utilized for transportation, including aircraft, as well as for industrial purposes and in the power sector.

Biofuels

A variety of biofuels, including biodiesel, vegetable oil, alcohol fuels, and biomass, can serve as substitutes for fossil fuels. Through various chemical processes, the carbon and hydrogen present in coal, natural gas, plant and animal biomass, and organic wastes can be transformed into short-chain hydrocarbons suitable as replacements for existing hydrocarbon fuels. Examples include Fischer–Tropsch diesel, methanol, dimethyl ether, and syngas. This synthetic diesel production was crucial for Germany during World War II due to limited crude oil access. South Africa relies heavily on coal-derived diesel for similar reasons. A sustained oil price above US$35/bbl could potentially make large-scale synthetic liquid fuels economically viable.

Power-to-Solid

Analogous to power-to-liquid and power-to-gas concepts, energy can be stored in solid materials, such as metals (Iron, Aluminium) and non-metallic materials like 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 employed in Fireworks and Rockets.

Aluminum

Aluminum has been proposed by numerous researchers as an energy storage medium. Its electrochemical equivalent (8.04 Ah/cm³) is nearly four times that of lithium (2.06 Ah/cm³). Energy can be extracted from aluminum by reacting it with water to generate hydrogen. However, this requires removing its natural oxide layer, a process that can involve pulverization, chemical reactions with caustic substances, or the creation of alloys. The byproduct of this hydrogen generation reaction is aluminum oxide, which can be recycled back into aluminum via the Hall–Héroult process, theoretically making the process renewable. If the Hall–Héroult process is powered by solar or wind energy, aluminum could serve as a highly efficient method for storing energy, surpassing direct solar electrolysis in efficiency.

Boron, silicon, and zinc

Boron, silicon, and zinc have also been put forth as potential energy storage solutions.

Other chemical

The organic compound norbornadiene undergoes a reversible transformation to quadricyclane upon exposure to light, effectively storing solar energy within its chemical bonds. A functional system based on this principle, known as a molecular solar thermal system, has been developed in Sweden.

Electrical methods

Capacitor

A capacitor, once known as a 'condenser,' is a passive, two-terminal electrical component designed to store energy electrostatically. While practical capacitors exhibit considerable variation, they all consist of at least two electrical conductors (plates) separated by a dielectric material, which acts as an insulator. When disconnected from its charging circuit, a capacitor can retain stored electric energy, functioning akin to a temporary battery or other types of rechargeable energy storage system. Capacitors are commonly found in electronic devices to provide a stable power supply during brief interruptions, such as when batteries are being changed, preventing the loss of data in volatile memory. Conventional capacitors typically store less than 360 joules per kilogram, significantly less than the approximately 590 kJ/kg density of a conventional alkaline battery.

Capacitors store energy within the electric field established between their plates. When a potential difference is applied across the conductors (for instance, by connecting a capacitor to a battery), an electric field develops across the dielectric. This field causes positive charge (+Q) to accumulate on one plate and negative charge (-Q) on the other. If a battery remains connected to a capacitor for an extended period, direct current cannot flow through it. However, if an alternating or accelerating voltage is applied across the capacitor's terminals, a displacement current can be established. Beyond the conductive plates, charge can also be stored within a dielectric layer.

Capacitance is enhanced by reducing the separation between the conductors and increasing their surface area. In practice, the dielectric material exhibits a small amount of leakage current, and there's a limit to the electric field strength it can withstand, known as the breakdown voltage. Nevertheless, the phenomenon of dielectric recovery after high-voltage breakdown holds promise for developing self-healing capacitors. The conductors and leads introduce undesirable inductance and resistance.

Current research is exploring the quantum effects within nanoscale capacitors for applications in digital quantum batteries.

Superconducting magnetics

Superconducting magnetic energy storage (SMES) systems store energy within a magnetic field generated by the flow of direct current through a superconducting coil cooled below its superconducting critical temperature. A typical SMES system comprises the superconducting coil, a power conditioning system, and refrigeration equipment. Once the superconducting coil is charged, the current remains constant, allowing the magnetic energy to be stored indefinitely.

The stored energy can be discharged back into the grid. The associated inverter/rectifier systems introduce an energy loss of approximately 2–3% in each direction of energy transfer. SMES systems exhibit the lowest energy loss during the storage process compared to other energy storage methods, achieving round-trip efficiency exceeding 95%.

Due to the significant energy requirements for refrigeration and the high cost of superconducting wire, SMES systems are primarily employed for short-duration storage applications, such as improving power quality and grid balancing.

Applications

Mills

Before the advent of the Industrial Revolution, a primary application of energy storage involved the controlled management of waterways to power water mills for tasks such as grain processing or operating machinery. Elaborate systems of reservoirs and dams were constructed to store and release water, and thus its contained potential energy, as needed.

Homes

Home energy storage is poised to become increasingly prevalent, driven by the growing importance of distributed renewable energy generation, particularly from photovoltaics, and the significant share of energy consumption attributed to buildings. To achieve a household self-sufficiency exceeding 40% with photovoltaic installations, energy storage becomes indispensable. Numerous manufacturers now produce rechargeable battery systems designed to store surplus energy generated by home solar or wind systems. Currently, Li-ion batteries are favored over lead-acid for home energy storage due to their comparable cost and significantly superior performance.

Tesla Motors offers two models of its Tesla Powerwall: a 10 kWh weekly cycle version for backup applications and a 7 kWh version for daily cycling. In 2016, the cost of a limited version of the Tesla Powerpack 2 was reported at $398(US)/kWh. Considering the average US grid price of 12.5 cents/kWh, the return on investment was questionable unless electricity prices exceeded 30 cents/kWh.

RoseWater Energy produces two models of its "Energy & Storage System," the HUB 120 and SB20. Both versions provide 28.8 kWh of output, capable of powering larger homes or light commercial premises, and offering protection for custom installations. The system integrates five key functions: a clean 60 Hz sine wave output, zero transfer time, industrial-grade surge protection, optional renewable energy grid sell-back, and battery backup.

Enphase Energy has introduced an integrated system that allows homeowners to store, monitor, and manage their electricity. This system provides 1.2 kWh of energy storage with a power output of 275W/500W.

While less flexible than batteries, storing wind or solar energy using thermal energy storage is considerably more economical. A standard 52-gallon electric water heater can store approximately 12 kWh of energy, useful for supplementing hot water or space heating needs.

For purely financial considerations in regions with net metering policies, homeowners can sell surplus electricity generated at home back to the grid through a grid-tie inverter, obviating the need for battery storage.

Grid electricity and power stations

Renewable energy sources, particularly solar and wind, are characterized by variable power. Storage systems play a crucial role in leveling out the imbalances between supply and demand that this variability creates. Electricity must either be used as it is generated or immediately converted into storable forms.

The predominant method for electrical grid storage remains pumped-storage hydroelectricity. Regions like Norway, Wales, Japan, and the US have ingeniously utilized elevated geographic features to create reservoirs. Electrically powered pumps are used to fill these reservoirs. When electricity is needed, the water is released through generators, converting the gravitational potential energy of the falling water into electricity. Norway, which derives almost all its electricity from hydro, currently possesses 1.4 GW of pumped storage capacity, with the potential for significant expansion given its total installed capacity of nearly 32 GW, 75% of which is regulable.

Various storage technologies exist that can generate electricity, including pumped-storage hydroelectric dams, rechargeable batteries, thermal storage (particularly using molten salts for efficient heat storage and release), and compressed air energy storage, flywheels, cryogenic systems, and superconducting magnetic coils.

Surplus power can also be converted into methane through processes like the Sabatier process and stored within the existing natural gas network.

In 2011, the Bonneville Power Administration in the northwestern United States launched an experimental initiative to absorb excess wind and hydro power generated during periods of high wind or at night. Under centralized control, household appliances were utilized to consume this surplus energy. This involved heating ceramic bricks in special space heaters to high temperatures and increasing the temperature of modified hot water heater tanks. Once charged, these appliances then provided heating and hot water as needed. This program was prompted by a severe storm in 2010 that resulted in such a significant overproduction of renewable energy that all conventional power sources had to be shut down or reduced to their minimum operational levels, leaving a large region powered almost entirely by renewables.

Another advanced technique, employed at the former Solar Two project in the United States and the Solar Tres Power Tower in Spain, involves using molten salt to store thermal energy captured from sunlight. This stored heat is then converted and dispatched as electrical power. The system circulates molten salt through a tower or specialized conduits to be heated by the sun. Insulated tanks store the heated salt. Electricity is generated by using the heat to produce steam, which drives turbines.

Since the early 21st century, batteries have been increasingly deployed for utility-scale load leveling and frequency regulation purposes.

In the context of vehicle-to-grid storage, electric vehicles connected to the energy grid can discharge stored electrical energy from their batteries back into the grid when required.

Air conditioning

Thermal energy storage (TES) finds significant application in air conditioning systems, particularly for cooling large single buildings or clusters of smaller ones. Commercial air conditioning systems are major contributors to peak electrical loads. By 2009, thermal storage systems were in use in over 3,300 buildings across more than 35 countries. The principle involves chilling a medium at night, when electricity is cheaper, and then utilizing this chilled medium for cooling during the hotter daytime hours.

The most prevalent technique is ice storage, which requires less space and is more cost-effective than fuel cells or flywheels. In this approach, a standard chiller operates overnight to produce a pile of ice. During the day, water circulates through the ice pile, chilling the water that would normally be produced directly by the chiller.

A partial storage system minimizes initial capital investment by operating chillers for nearly 24 hours a day. At night, they produce ice for storage, and during the day, they simultaneously chill water and melt ice to augment chilled water production. This system typically produces ice for 16 to 18 hours and melts ice for six hours. Capital expenditures can be reduced as the chillers may only need to be 40%–50% of the size required for a conventional, non-storage design. Storage capacity sufficient to cover half a day's cooling needs is often adequate.

A full storage system, conversely, completely shuts off the chillers during peak load hours. This necessitates higher capital costs due to the requirement for larger chillers and a more substantial ice storage system.

The ice is produced during off-peak hours when electricity rates are lower. This strategy of utilizing off-peak cooling systems can lead to significant reductions in energy costs. The U.S. Green Building Council has developed the Leadership in Energy and Environmental Design (LEED) program to promote the design of buildings with reduced environmental impact. Off-peak cooling can contribute towards LEED Certification.

Thermal storage for heating purposes is less common than for cooling. One example involves storing solar heat for use during nighttime heating.

Latent heat can also be stored using technical phase change materials (PCMs). These materials can be encapsulated within wall and ceiling panels to help regulate room temperatures.

Transport

Liquid hydrocarbon fuels remain the dominant form of energy storage for transportation, followed by a growing adoption of Battery Electric Vehicles and Hybrid Electric Vehicles. Alternative energy carriers, such as hydrogen, are being explored to mitigate greenhouse gas emissions.

Public transportation systems, like trams and trolleybuses, require a steady supply of electricity. However, their dynamic movement patterns can pose challenges for a consistent supply from renewable energy sources. Photovoltaic systems installed on building rooftops could potentially power public transportation during periods of high electricity demand when other energy sources may be less readily available. Future transitions in the transportation sector are also investigating electric power solutions for ferries and airplanes.

Electronics

Capacitors are fundamental components in electronic circuits, serving to block 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 specific frequencies. In electric power transmission systems, they contribute to voltage stabilization and power flow management.

Use cases

The United States Department of Energy International Energy Storage Database (IESDB) is a valuable, freely accessible repository of information on energy storage projects and policies, supported by the United States Department of Energy Office of Electricity and Sandia National Labs.

Capacity

Storage capacity refers to the quantity of energy that can be extracted from an energy storage device or system. It is typically measured in joules or kilowatt-hours and their multiples. It can also be expressed in terms of the number of hours of electricity production at a power plant's nameplate capacity. When the primary storage type is inherent to the system (e.g., thermal or pumped water), the output is solely derived from the embedded storage system.

Economics

The economic viability of energy storage is intrinsically linked to the specific reserve service required, and numerous factors introduce uncertainty that can impact profitability. Consequently, not all storage methods are technically and economically suitable for storing large quantities of energy (several MWh), and the optimal size of an energy storage system is highly dependent on market conditions and location.

Furthermore, Energy Storage Systems (ESS) are subject to various risks, including:

  • Techno-economic risks: These are associated with the specific technology employed.
  • Market risks: Factors that influence the overall electricity supply system.
  • Regulation and policy risks: Changes in government policies and regulations.

Given these complexities, traditional investment appraisal methods like deterministic Discounted Cash Flow (DCF) may not fully capture the risks and uncertainties involved. Therefore, the literature often recommends employing Real Option Analysis (ROA) to assess the value of risks and uncertainties, as it provides a more robust framework for decision-making in uncertain environments.

The economic valuation of large-scale applications, including pumped hydro storage and compressed air, considers benefits such as the avoidance of curtailment, mitigation of grid congestion, price arbitrage opportunities, and the delivery of carbon-free energy. One technical assessment by the Carnegie Mellon Electricity Industry Centre suggested that batteries could meet economic goals if their capital cost was between 30and30 and 50 per kilowatt-hour.

A key metric for evaluating energy efficiency in storage is the energy storage on energy invested (ESOI). This ratio represents the amount of energy a technology can store relative to the energy required for its construction. Higher ESOI values indicate more energetically efficient storage technologies. For lithium-ion batteries, this ratio is approximately 10, while for lead-acid batteries, it is around 2. Other storage forms, such as pumped hydroelectric storage, generally exhibit higher ESOI values, with figures around 210.

Pumped-storage hydroelectricity remains the most widely deployed storage technology globally. However, its application is constrained by the requirement for specific terrain with significant elevation differences and its considerable land use for relatively modest power output. In locations lacking suitable natural geography, underground pumped-hydro storage offers a potential alternative. Despite advancements, the high costs and limited lifespan of batteries continue to make them a less than ideal substitute for dispatchable power sources, and they struggle to compensate for variable renewable power gaps lasting for extended periods (days, weeks, or months). In grid models with a high proportion of Variable Renewable Energy (VRE), the excessive cost of storage tends to dominate the overall system costs. For instance, California's goal of an 80% VRE share would necessitate 9.6 TWh of storage, while a 100% VRE target would require 36.3 TWh. As of 2018, the state possessed only 150 GWh of storage, primarily in pumped storage facilities and a small fraction in batteries. A separate study suggested that supplying 80% of US demand from VRE would require either a nationwide smart grid or battery storage capable of supporting the entire system for 12 hours, with estimated costs around $2.5 trillion for both scenarios. Similarly, several studies indicate that relying solely on VRE and energy storage would result in costs 30–50% higher than comparable systems that integrate VRE with nuclear plants or facilities with carbon capture and storage.

Research

Germany

In 2013, the German government committed €200 million (approximately US$270 million) to research initiatives and an additional €50 million to subsidize battery storage for residential rooftop solar installations, according to a representative of the German Energy Storage Association.

Siemens AG established a production research facility in 2015 at the Zentrum für Sonnenenergie und Wasserstoff (ZSW – the German Center for Solar Energy and Hydrogen Research in the State of Baden-Württemberg). This facility, staffed by approximately 350 scientists, researchers, engineers, and technicians, focuses on developing near-production manufacturing materials and processes (NPMM&P) utilizing a computerized Supervisory Control and Data Acquisition system. The objective is to enhance the quality and reduce the cost of rechargeable battery production.

Since 2023, a new project funded by the German Research Foundation is investigating molecular photoswitches for storing solar thermal energy. Professor Dr. Hermann A. Wegner is the spokesperson for these so-called molecular solar thermal (MOST) systems.

United States

In 2014, the United States saw the opening of several research and testing centers dedicated to evaluating energy storage technologies. Among these was the Advanced Systems Test Laboratory at the University of Wisconsin at Madison in Wisconsin State, which collaborated with battery manufacturer Johnson Controls. This laboratory was established as part of the university's newly inaugurated Wisconsin Energy Institute. Its primary goals include the assessment of state-of-the-art and next-generation electric vehicle batteries, including their potential role as grid supplements.

The State of New York inaugurated its New York Battery and Energy Storage Technology (NY-BEST) Test and Commercialization Center at Eastman Business Park in Rochester, New York. This facility, costing $23 million, houses a laboratory spanning nearly 1,700 m². It includes the Center for Future Energy Systems, a collaborative effort between Cornell University in Ithaca, New York and Rensselaer Polytechnic Institute in Troy, New York. NY-BEST is dedicated to testing, validating, and independently certifying diverse forms of energy storage intended for commercial deployment.

On September 27, 2017, Senators Al Franken of Minnesota and Martin Heinrich of New Mexico introduced the Advancing Grid Storage Act (AGSA), proposing an allocation of over $1 billion for research, technical assistance, and grants aimed at promoting energy storage development in the United States.

As previously noted, grid models incorporating a high share of VRE face significant cost challenges due to the necessity for extensive storage. California's ambition for an 80% VRE penetration necessitates 9.6 TWh of storage, escalating to 36.3 TWh for a 100% VRE scenario. Projections suggest that achieving 80% VRE penetration across the entire US would require either a nationwide smart grid or battery storage capable of sustaining the system for 12 hours, both with estimated costs around $2.5 trillion.

United Kingdom

In the United Kingdom, a collaborative effort involving approximately 14 industry and government agencies and seven British universities led to the establishment of the SUPERGEN Energy Storage Hub in May 2014. This initiative aims to foster coordination in energy storage technology research and development.

There. A rather thorough dissection of how we manage to bottle lightning, wouldn't you say? It's not always pretty, this hoarding of energy, but it’s undeniably necessary. Do you have any other… inquiries? Or are you content to simply absorb this information, like a sponge left out in the rain?