Regulation is one of the ancillary services for which storage is especially well-suited. Regulation involves managing interchange flows with other control areas to match closely the scheduled interchange flows and momentary variations in demand within the control area. The primary reasons for including regulation in the power system are to maintain the grid frequency and to comply with the North American Electric Reliability Council’s (NERC’s) Real Power Balancing Control Performance (BAL001) and Disturbance Control Performance (BAL002) Standards. Regulation is used to reconcile momentary differences caused by fluctuations in generation and loads. Regulation is used for damping of that difference.
Spinning, Non-Spinning, and Supplemental Reserves
Spinning Reserve (Synchronized) – Generation capacity that is online but unloaded and that can respond within 10 minutes to compensate for generation or transmission outages. ‘Frequency- responsive’ spinning reserve responds within 10 seconds to maintain system frequency. Spinning reserves are the first type used when a shortfall occurs.
Non-Spinning Reserve (Non-synchronized) – Generation capacity that may be offline or that comprises a block of curtailable and/or interruptible loads and that can be available within 10 minutes.
Supplemental Reserve – Generation that can pick up load within one hour. Its role is, essentially, a backup for spinning and non-spinning reserves. Backup supply may also be used as backup for commercial energy sales. Unlike spinning reserve capacity, supplemental reserve capacity is not synchronized with grid frequency. Supplemental reserves are used after all
spinning reserves are online. Importantly for storage, generation resources used as reserve capacity must be online and operational (i.e., at part load). Unlike generation, in almost all circumstances, storage used for reserve capacity does not discharge at all; it just has to be ready and available to discharge when needed.
Voltage Support is a requirement for electric grid operators is to maintain voltage within specified limits. In most cases, this requires management of reactance, which is caused by grid-connected equipment that generates, transmits, or uses electricity and often has or exhibits characteristics like those of inductors and capacitors in an electric circuit. To manage reactance at the grid level, system operators need voltage support resources to offset reactive effects so that the transmission system
can be operated in a stable manner.
Normally, designated power plants are used to generate reactive power (VAR) to offset reactance in the grid. These power plants could be displaced by strategically placed energy storage within the grid at central locations or taking the distributed approach and placing multiple VAR-support storage systems near large loads.
Black Start is a mechanism to utilize storage systems to provide active reserves of power and energy within the grid and can be used to energize transmission and distribution lines and provide station power to bring power plants on line after a catastrophic failure of the grid. Storage can provide similar startup power to larger power plants, if the storage system is suitably sited and there is a clear transmission path to the power plant from the storage system’s location.
Other Related Uses
Load Following/Ramping Support for Renewables
Electricity storage is eminently suitable for damping the variability of wind and PV systems and is being widely used in this application. Technically, the operating requirements for a storage system in this application are the same as those needed for a storage system to respond to a rapidly or randomly fluctuating load profile. Most renewable applications with a need for storage will specify a maximum expected up- and down-ramp rate in MW/minute and the time duration of the ramp. This design guidance for the storage system is applicable for load following and renewable ramp support; this Handbook therefore treats them as the same application.
Load following is characterized by power output that generally changes as frequently as every several minutes. The output changes in response to the changing balance between electric supply and load within a specific region or area. Output variation is a response to changes in system frequency, timeline loading, or the relation of these to each other that occurs as needed to maintain the scheduled system frequency and/or established interchange with other areas within predetermined limits.
Conventional generation-based load following resources’ output increases to follow demand up as system load increases. Conversely, load following resources’ output decreases to follow demand down as system load decreases. Typically, the amount of load following needed in the up direction (load following up) increases each day as load increases during the morning. In the evening, the amount of load following needed in the down direction (load following down) increases as aggregate load on the grid drops.
Frequency response is very similar to regulation, described above, except it reacts to system needs in even shorter time periods of seconds to less than a minute when there is a sudden loss of a generation unit or a transmission line. As shown in Figure 10, 13 various generator response actions are needed to counteract this sudden imbalance between load and generation to maintain the system frequency and stability of the grid.
The first response within the initial seconds is the primary frequency control response of the governor action on the generation units to increase their power output as shown in the lower portion of the figure. This is followed by the longer duration secondary frequency control response by the AGC that spans the half a minute to several minutes shown by the dotted line in the lower portion of Figure 10. It is important to note that the rate at which the frequency decays after the triggering event – loss of generator or
transmission – is directly proportional to the aggregate inertia within the grid at that instant. The rotating mass of large generators and/or the aggregate mass of many smaller generators collectively determines this inertia.
The combined effect of inertia and the governor actions determines the rate of frequency decay and recovery shown in the arresting and rebound periods in the upper portion of Figure 10. This is also the window of time in which the fast-acting response of flywheel and battery storage
systems excels in stabilizing the frequency. The presence of fast-acting storage assures a smoother transition from the upset period to normal operation if the grid frequency is within its normal range. The effectiveness of fast-acting storage in this application has been successfully utilized by utilities14 and also described in other reports and papers.
The size of storage systems to be used in frequency response mode is proportional to the grid or balancing area in which they are needed. Generally, storage systems in the 20 MW and greater size can provide effective frequency response due to their fast action; some studies16 have shown that the response is twice as effective as a conventional fossil-fueled generator, including combustion turbines (CTs) and coal units. However, location of the storage system within the grid with respect to other generation, transmission corridors, and loads plays a crucial role in the effectiveness as a frequency response resource.
- Battery storage systems that are interconnected behind-the-meter (BTM) can provide services for individual electricity consumers as well as services ‘upstream’ at the distribution- and transmission-levels. ‘Customer-facing’ services can broadly be categorized as (1) Bill savings; (2) Increased PV self-consumption; and (3) Backup power.
- Bill savings: retail tariff elements determine how a customer is charged for electricity consumed from the grid and consequently determine the extent to which energy storage systems can help to reduce their electricity bills. Flat volumetric tariff elements that charge the same rate for energy consumption from the grid ($/kWh) regardless of when the energy is consumed provide little to no opportunity for energy storage to help customers reduce their bills. Time-of-use energy charges, which charge different rates for consumption during different parts of the day, and demand charge elements, which charge customers based on their maximum instantaneous consumption ($/kW) during a given period, offer opportunities to reduce bills with energy storage by shifting demand to different periods.
- Increased PV self-consumption: Production from customer-sited solar PV systems and energy demand may be poorly aligned depending on customer demand patterns. This may mean solar PV energy that exceeds customer demand is either curtailed or exported to the power system, depending on restrictions on the customer’s interconnection agreement. Depending on how solar PV exports are compensated, this may represent a lost financial opportunity for the customer. Energy storage can help customers address the mismatch between their demand and PV generation by storing excess PV energy and discharging to meet demand after PV generation has tapered off.
- Backup power: Energy storage, especially if combined with a generating source like solar PV or when interconnecting with multiple distributed energy resources (DER) in a micro-grid setting, can meet the energy needs of customers in the case of grid outages. This can be critical for essential infrastructure by, for example, ensuring power to an emergency shelter or hospital during a storm. Uninterrupted power can also be critical for sensitive industries that would suffer significant consequences from even brief interruptions.
When starting up, large generators need an external source of electricity to perform key functions before they can begin generating electricity for the grid. During normal system conditions, this external electricity can be provided by the grid. After a system failure, however, the grid can no longer provide this power, and generators must be started through an on-site source of electricity. On-site energy storage such as a lithium-ion battery storage system can provide this service and avoid fuel costs and emissions from conventional black-start generators. As system-wide outages are rare, on-site energy storage can provide additional services when not performing black starts.
The U.S. Department of Energy defines community solar as any solar project or purchasing program, within a geographic area, in which the benefits of a solar project flow to multiple customers such as individuals, businesses, nonprofits, and other groups. Community solar is a form of solar energy generation that allows community members of all types to access meaningful benefits of renewable energy, including reduced energy costs, low- to moderate-income household access, increased resilience, community ownership, and equitable workforce development and entrepreneurship.
Community solar programs make solar more accessible to all Americans, particularly to those with low-to-moderate incomes, renters, and other community members for whom traditional rooftop solar is unavailable. Rather than putting solar on their own home or building, community solar allows energy users to subscribe to a shared system of solar panels, often located within their community.
Amount of time or number of cycles a battery storage system can provide regular charging and discharging before failure or significant degradation
|The decrease in the solar PV production or battery’s capacity over time and through use
Depth of charge
|The battery capacity that has been discharged as a percentage of its maximum capacity
Electric Energy Time-shift (Arbitrage)
Electric energy time-shift involves purchasing inexpensive electric energy, available during periods when prices or system marginal costs are low, to charge the storage system so that the stored energy can be used or sold at a later time when the price or costs are high. Alternatively, storage can provide similar time-shift duty by storing excess energy production, which would otherwise be curtailed, from renewable sources such as wind or photovoltaic (PV). The functional operation of the storage system is similar in both cases, and they are treated interchangeably in this discussion.
Electricity is the flow of electrical power or charge. Electricity is both a basic part of nature and one of the most widely used forms of energy.
The electricity that we use is a secondary energy source because it is produced by converting primary sources of energy such as coal, natural gas, nuclear energy, solar energy, and wind energy into electrical power. Electricity is also referred to as an energy carrier, which means it can be converted to other forms of energy such as mechanical energy or heat. Primary energy sources are renewable or nonrenewable energy, but the electricity we use is neither renewable nor nonrenewable.
Despite its great importance in daily life, few people probably stop to think about what life would be like without electricity. Like air and water, people tend to take electricity for granted. However, people use electricity to do many jobs every day—from lighting, heating, and cooling homes to powering televisions and computers.
Before electricity became widely available, about 100 years ago, candles, whale oil lamps, and kerosene lamps provided light; iceboxes kept food cold; and wood-burning or coal-burning stoves provided heat.
Scientists and inventors have worked to decipher the principles of electricity since the 1600s. Benjamin Franklin, Thomas Edison, and Nikola Tesla made notable contributions to our understanding and use of electricity.
Before 1879, direct current (DC) electricity was used in arc lights for outdoor lighting. In the late 1800s, Nikola Tesla pioneered the generation, transmission, and use of alternating current (AC) electricity, which reduced the cost of transmitting electricity over long distances. Tesla’s inventions brought electricity into homes to power indoor lighting and into factories to power industrial machines.
Maximum amount of stored energy (in kilowatt-hours [kWh] or megawatt-hours [MWh]) a battery can hold
Energy storage mediates between variable sources and variable loads. Without storage, energy generation must equal energy consumption. Energy storage works by moving energy through time. Energy generated at one time can be used at another time through storage. Electricity storage is one form of energy storage. Other forms of energy storage include oil in the Strategic Petroleum Reserve and in storage tanks, natural gas in underground storage reservoirs and pipelines, thermal energy in ice, and thermal mass/adobe.
Measure of the energy or power capacity of a battery relative to its volume (kW/L, kWh/L)
Environmental Impact of Electrification
Although electricity is a clean and relatively safe form of energy when it is used, the generation and transmission of electricity affects the environment. Nearly all types of electric power plants have an effect on the environment, but some power plants have larger effects than others.
The United States has laws that govern the effects that electricity generation and transmission have on the environment. The Clean Air Act regulates air pollutant emissions from most power plants. The U.S. Environmental Protection Agency (EPA) administers the Clean Air Act and sets emissions standards for power plants through various programs such as the Acid Rain Program. The Clean Air Act has helped to substantially reduce emissions of some major air pollutants in the United States.
The effect of power plants on the landscape
All power plants have a physical footprint (the location of the power plant). Some power plants are located inside, on, or next to an existing building, so the footprint is fairly small. Most large power plants require land clearing to build the power plant. Some power plants may also require access roads, railroads, and pipelines for fuel delivery, electricity transmission lines, and cooling water supplies. Power plants that burn solid fuels may have areas to store the combustion ash.
Many power plants are large structures that alter the visual landscape. In general, the larger the structure, the more likely it is that the power plant will affect the visual landscape.
The two coal-fired power plants of the Crystal River North Steam Complex in Crystal River, Florida
Source: Ebyabe, Wikimedia Commons author (GNU Free Documentation License) (public domain)
Fossil fuel, biomass, and waste burning power plants
In the United States, about 61% of total electricity generation in 2021 was produced from fossil fuels (coal, natural gas, and petroleum), materials that come from plants (biomass), and municipal and industrial wastes. The substances that occur in combustion gases when these fuels are burned include:
- Carbon dioxide (CO2)
- Carbon monoxide (CO)
- Sulfur dioxide (SO2)
- Nitrogen oxides (NOx)
- Particulate matter (PM)
- Heavy metals such as mercury
Nearly all combustion byproducts have negative effects on the environment and human health:
- CO2 is a greenhouse gas, which contributes to the greenhouse effect.
- SO2 causes acid rain, which is harmful to plants and to animals that live in water. SO2 also worsens respiratory illnesses and heart diseases, particularly in children and the elderly.
- NOx contribute to ground-level ozone, which irritates and damages the lungs.
- PM results in hazy conditions in cites and scenic areas and coupled with ozone, contributes to asthma and chronic bronchitis, especially in children and the elderly. Very small, or fine PM, is also believed to cause emphysema and lung cancer.
- Heavy metals such as mercury are hazardous to human and animal health.
Power plants reduce air pollution emissions in various ways
Air pollution emission standards limit the amounts of some of the substances that power plants can release into the air. Some of the ways that power plants meet these standards include:
- Burning low-sulfur-content coal to reduce SO2 emissions. Some coal-fired power plants cofire wood chips with coal to reduce SO2 emissions. Pretreating and processing coal can also reduce the level of undesirable compounds in combustion gases.
- Different kinds of particulate emission control devices treat combustion gases before they exit the power plant:
- Bag-houses are large filters that trap particulates.
- Electrostatic precipitators use electrically charged plates that attract and pull particulates out of the combustion gas.
- Wet scrubbers use a liquid solution to remove PM from combustion gas.
- Wet and dry scrubbers mix lime in the fuel (coal) or spray a lime solution into combustion gases to reduce SO2 emissions. Fluidized bed combustion also results in lower SO2 emissions.
- NOx emissions controls include low NOx burners during the combustion phase or selective catalytic and non-catalytic converters during the post combustion phase.
Hunter Power Plant, a coal-fired power plant south of Castle Dale, Utah
Source: Tricia Simpson, Wikimedia Commons author (GNU Free Documentation License) (public domain)
Many U.S. power plants produce CO2 emissions
The electric power sector is a large source of U.S. CO2 emissions. Electric power sector power plants that burned fossil fuels or materials made from fossil fuels, and some geothermal power plants, were the source of about 32% of total U.S. energy-related CO2 emissions in 2021.
Some power plants also produce liquid and solid wastes
Ash is the solid residue that results from burning solid fuels such as coal, biomass, and municipal solid waste. Bottom ash includes the largest particles that collect at the bottom of the combustion chamber of power plant boilers. Fly ash is the smaller and lighter particulates that collect in air emission control devices. Fly ash is usually mixed with bottom ash. The ash contains all the hazardous materials that pollution control devices capture. Many coal-fired power plants store ash sludge (ash mixed with water) in retention ponds. Most of these ponds are unlined and pose risks to ground water. Several of these ponds have burst and caused extensive damage and pollution downstream. Some coal-fired power plants send ash to landfills or sell ash for use in making concrete blocks or asphalt.
Nuclear power plants produce different kinds of waste
Nuclear power plants do not produce greenhouse gases or PM, SO2, or NOx, but they do produce two general types of radioactive waste:
- Low-level waste, such as contaminated protective shoe covers, clothing, wiping rags, mops, filters, reactor water treatment residues, equipment, and tools, is stored at nuclear power plants until the radioactivity in the waste decays to a level safe for disposal as ordinary trash, or it is sent to a low-level radioactive waste disposal site.
- High-level waste, which includes the highly radioactive spent (used) nuclear fuel assemblies, must be stored in specially designed storage containers and facilities (see Interim storage and final disposal in the United States).
Electric power lines and other distribution infrastructure also have a footprint
Electricity transmission lines and the distribution infrastructure that carries electricity from power plants to customers also have environmental effects. Most transmission lines are above ground on large towers. The towers and power lines alter the visual landscape, especially when they pass through undeveloped areas. Vegetation near power lines may be disturbed and may have to be continually managed to keep it away from the power lines. These activities can affect native plant populations and wildlife. Power lines can be placed underground, but it is a more expensive option and usually not done outside of urban areas.
Last updated: November 23, 2022
Grid parity happens when our use of alternative energies like solar costs less than, or equal to, the price of using power from conventional sources such as coal, oil and natural gas (i.e., fossil fuel).
Grid Services: Benefits, Opportunity, and Value-Staking
Grid services provide dynamic load control to support the electric grid including shedding, shifting, and modulating loads. Modulating loads provide ancillary services (e.g., frequency regulation) and voltage control. Grid services support the generation, transmission, and distribution of electricity by providing value through avoided electricity system costs (generation and/or delivery costs).
Grid Services include the value-stacking of a range of power generation, energy efficiency and load management solutions including but not limited to the following:
- Providing capacity services and energy shifting: System operators must ensure they have an adequate supply of generation capacity to reliably meet demand during the highest-demand periods in a given year. This peak demand is typically met with higher-cost generators which are almost exclusively used to serve peak demand, such as open cycle natural gas turbines; however, energy storage systems can also be used to ensure adequate peaking generation capacity. System operators can also improve the ability of variable renewable energy (VRE) plants to reliably contribute to peaking capacity by pairing VRE with energy storage, which can enable these resources to shift their generation to times when they are most needed. Storage systems may not need to be sited with VRE generators (known as co-location) in order to provide such benefits, and there are pros and cons to such co-location that must be carefully considered before siting storage systems.
- Providing fast-response ancillary services: Many forms of energy storage, most notably batteries, are capable of rapidly and accurately changing their charging and discharging rates in response to external signals. By quickly changing their output, these storage resources can provide valuable ancillary services that system operators use to help balance short-term differences between demand and supply. These ancillary services are particularly important in systems with large amounts of variable renewable energy generation, as system operators must be able to respond to unexpected changes in energy supply. Currently, ancillary services are predominantly provided by conventional generators. Using cost-effective and system-appropriate energy storage projects to align supply and demand through the provision of ancillary services increases the flexibility of the power system and helps reduce both the curtailment of renewable energy resources and spinning reserve requirements from conventional resources.
- Transmission and Distribution Upgrade Deferral: The electricity grid’s transmission and distribution infrastructure must be sized to meet peak demand, which may only occur over a few hours of the year. When anticipated growth in peak electricity demand exceeds the grid’s existing capacity, new investments are needed to upgrade equipment and expand network infrastructure. Deploying energy storage can help defer or avoid the need for new grid investments by meeting peak demand with energy stored from lower-demand periods, reducing congestion during periods of stress on network infrastructure and improving overall transmission and distribution asset utilization.
- Black Start: When starting up, large generators need an external source of electricity to perform key functions before they can begin generating electricity for the grid. During normal system conditions, this external electricity can be provided by the grid. After a system failure, however, the grid can no longer provide this power, and generators must be started through an on-site source of electricity. On-site energy storage such as a lithium-ion battery storage system can provide this service and avoid fuel costs and emissions from conventional black-start generators. As system-wide outages are rare, on-site energy storage can provide additional services when not performing black starts.
- Behind-the–Meter Applications: Battery storage systems that are interconnected behind-the-meter (BTM) can provide services for individual electricity consumers as well as services ‘upstream’ at the distribution- and transmission-levels. ‘Customer-facing’ services can broadly be categorized as (1) Bill savings; (2) Increased PV self-consumption; and (3) Backup power.
1) Bill savings: retail tariff elements determine how a customer is charged for electricity consumed from the grid and consequently determine the extent to which energy storage systems can help to reduce their electricity bills. Flat volumetric tariff elements that charge the same rate for energy consumption from the grid ($/kWh) regardless of when the energy is consumed provide little to no opportunity for energy storage to help customers reduce their bills. Time-of-use energy charges, which charge different rates for consumption during different parts of the day, and demand charge elements, which charge customers based on their maximum instantaneous consumption ($/kW) during a given period, offer opportunities to reduce bills with energy storage by shifting demand to different periods.
2) Increased PV self-consumption: Production from customer-sited solar PV systems and energy demand may be poorly aligned depending on customer demand patterns. This may mean solar PV energy that exceeds customer demand is either curtailed or exported to the power system, depending on restrictions on the customer’s interconnection agreement. Depending on how solar PV exports are compensated, this may represent a lost financial opportunity for the customer. Energy storage can help customers address the mismatch between their demand and PV generation by storing excess PV energy and discharging to meet demand after PV generation has tapered off.
3) Backup power: Energy storage, especially if combined with a generating source like solar PV or when interconnecting with multiple distributed energy resources (DER) in a micro-grid setting, can meet the energy needs of customers in the case of grid outages. This can be critical for essential infrastructure by, for example, ensuring power to an emergency shelter or hospital during a storm. Uninterrupted power can also be critical for sensitive industries that would suffer significant consequences from even brief interruptions.
Investment-based tax credits
- Allows storage system owner to claim a tax credit equal to a fixed percentage of eligible project costs, for example, 10% or 30%.
- Percentage can adjust downward over time
- Accelerated depreciation can lower your tax bill as well.
The systematic application of evidence based principles and methods from educational technology and the learning sciences to create engaging and effective learning experiences, support the difficulties and challenges of learners as they learn, and come to better understand learners and learning.
- A contract between the creditors (public or private) and a borrower such as banks or other commercial loan institutions that the Federal government will cover the borrower’s debt obligation in the event that the borrower defaults.
A local electrical grid with defined electrical boundaries, acting as a single and controllable entity. It can operate in grid connected and in island mode. A ‘Stand alone microgrid’ or ‘isolated microgrid’ only operates off the grid and cannot be connected to a wider electric power system.
A miniaturized version of a microgrid that supplies power to a single house or even a single load. It connects generating sources and provides distribution in the structure in an island or grid-like mode. By contrast, microgrids are larger versions that serve more than one building or much larger buildings.
- Provides ongoing payments to system owner based on the system’s actual kilowatt-hour production
Production-based tax credits
- Tax credit for every kWh of renewable energy produced
- Earned over time with energy production (e.g., 10 years)
A prosumer is someone who not only consumes energy, but also produces it themselves using distributed energy sources such as solar. Prosumers also take energy monitoring into their own hands and can optimize their energy usage and reduce costs by becoming as efficient as possible.
Rated power capacity
Total possible instantaneous discharge capability (in kilowatts [kW] or megawatts [MW]) of the battery energy storage system (BESS), or the maximum rate of discharge that the BESS can achieve, starting from a fully charged state
Ratio of the energy charged to the battery to the energy discharged from the battery
State of charge
The battery capacity as a percentage of its maximum capacity at a given time
Amount of time storage can discharge at its power capacity before depleting its energy capacity
- Eliminate or reduce the following taxes
- Sales Taxes
- Taxes on imported equipment
- Property taxes on value of solar system
- Reduces initial cost of installing a renewable energy generation, storage or energy efficient system
Virtual Power Plants
Virtual power plants, generally considered a connected aggregation of distributed energy resource (DER) technologies, offer deeper integration of renewables and demand flexibility, which in turn offers more Americans cleaner and more affordable power.