Grid energy storage
Large scale electricity supply management From Wikipedia, the free encyclopedia
Large scale electricity supply management From Wikipedia, the free encyclopedia
Grid energy storage (also called large-scale energy storage) is a collection of methods used for energy storage on a large scale within an electrical power grid. Electrical energy is stored during times when electricity is plentiful and inexpensive (especially from variable renewable energy sources such as wind power and solar power) or when demand is low, and later returned to the grid when demand is high, and electricity prices tend to be higher.
As of 2023[update], the largest form of grid energy storage is pumped-storage hydroelectricity, with utility-scale batteries and behind-the-meter batteries coming second and third.[1] Developments in battery storage have enabled commercially viable projects to store energy during peak production and release during peak demand, and for use when production unexpectedly falls giving time for slower responding resources to be brought online.
Green hydrogen, which is generated from electrolysis of water via electricity generated by renewables or relatively lower carbon emission sources, is a more economical means of long-term renewable energy storage in terms of capital expenditures than pumped-storage hydroelectricity or batteries.[2][3]
Two alternatives to grid storage are the use of peaking power plants to fill in supply gaps and demand response to shift load to other times.
Energy storage can provide multiple benefits to the grid: it can move electricity from periods of low prices to high prices, it can help make the grid more stable (for instance help regulate the frequency of the grid), and help reduce investment into transmission infrastructure.[4] Any electrical power grid must match electricity production to consumption, both of which vary significantly over time. Any combination of energy storage and demand response has these advantages:
Energy derived from solar, tidal and wind sources inherently varies on time scales ranging from minutes to weeks or longer – the amount of electricity produced varies with time of day, moon phase, season, and random factors such as the weather. Thus, renewables in the absence of storage present special challenges to electric utilities. While hooking up many separate wind sources can reduce the overall variability, solar is reliably not available at night, and tidal power shifts with the moon, so slack tides occur four times a day.
How much this affects any given utility varies significantly. In a summer peak utility, more solar can generally be absorbed and matched to demand. In winter peak utilities, to a lesser degree, wind correlates to heating demand and can be used to meet that demand. Depending on these factors, beyond about 20–40% of total generation, grid-connected intermittent sources such as solar power and wind power tend to require investment in grid interconnections, grid energy storage or demand-side management.
In an electrical grid without energy storage, generation that relies on energy stored within fuels (coal, biomass, natural gas, nuclear) must be scaled up and down to match the rise and fall of electrical production from intermittent sources (see load following power plant). While hydroelectric and natural gas plants can be quickly scaled up or down to follow the demand, wind, coal and nuclear plants take considerable time to respond to load. Utilities with less natural gas or hydroelectric generation are thus more reliant on demand management, grid interconnections or costly pumped storage.
The demand side can also store electricity from the grid, for example charging a battery electric vehicle stores energy for a vehicle and storage heaters, district heating storage or ice storage provide thermal storage for buildings.[5] At present this storage serves only to shift consumption to the off-peak time of day, no electricity is returned to the grid.
The need for grid storage to provide peak power is reduced by demand side time of use pricing, one of the benefits of smart meters. At the household level, consumers may choose less expensive off-peak times to wash and dry clothes, use dishwashers, take showers and cook. As well, commercial and industrial users will take advantage of cost savings by deferring some processes to off-peak times.
Regional impacts from the unpredictable operation of wind power has created a new need for interactive demand response, where the utility communicates with the demand. Historically this was only done in cooperation with large industrial consumers, but now may be expanded to entire grids.[6] For instance, a few large-scale projects in Europe link variations in wind power to change industrial food freezer loads, causing small variations in temperature. If communicated on a grid-wide scale, small changes to heating/cooling temperatures would instantly change consumption across the grid.
A report released in December 2013 by the United States Department of Energy further describes the potential benefits of energy storage and demand side technologies to the electric grid: "Modernizing the electric system will help the nation meet the challenge of handling projected energy needs—including addressing climate change by integrating more energy from renewable sources and enhancing efficiency from non-renewable energy processes. Advances to the electric grid must maintain a robust and resilient electricity delivery system, and energy storage can play a significant role in meeting these challenges by improving the operating capabilities of the grid, lowering cost and ensuring high reliability, as well as deferring and reducing infrastructure investments. Finally, energy storage can be instrumental for emergency preparedness because of its ability to provide backup power as well as grid stabilization services".[7]
Energy storage assets are a valuable asset for the electrical grid.[8] They can provide benefits and services such as load management, power quality and uninterruptible power supply to increase the efficiency and supply security. This becomes more and more important in regard to the energy transition and the need for a more efficient and sustainable energy system.
Numerous energy storage technologies (pumped-storage hydroelectricity, electric battery, flow battery, flywheel energy storage, supercapacitor etc.) are suitable for grid-scale applications, however their characteristics differ. For example, a pumped-hydro station is well suited for bulk load management applications due to their large capacities and power capabilities. However, suitable locations are limited and their usefulness fades when dealing with localized power quality issues. On the other hand, flywheels and capacitors are most effective in maintaining power quality but lack storage capacities to be used in larger applications. These constraints are a natural limitation to the storage's applicability.
Several studies have developed interest and investigated the suitability or selection of the optimal energy storage for certain applications. Literature surveys comprise the available information of the state-of-the-art and compare the storage's uses based on current existing projects.[9][10] Other studies take a step further in evaluating energy storage with each other and rank their fitness based on multiple-criteria decision analysis.[11][12] Another paper proposed an evaluation scheme through the investigation and modelling of storage as equivalent circuits.[13][14] An indexing approach has also been suggested in a few studies, but is still in the novel stages.[15] In order to gain increased economic potential of grid connected energy storage systems, it is of interest to consider a portfolio with several services for one or more applications for an energy storage system. By doing so, several revenue streams can be achieved by a single storage and thereby also increasing the degree of utilization.[16] To mention two examples, a combination of frequency response and reserve services is examined in,[17] meanwhile load peak shaving together with power smoothing is considered in.[18]
Italian firm Energy Dome uses supercritical (liquified by compression) CO
2 drawn from an atmospheric gasholder. Energy is accessed by evaporating and expanding the CO
2 into a turbine. The gas is returned to the atmospheric gasholder, until the next charging cycle. The system can be run in a closed loop, avoiding emissions. In July, 2024, the US DOE Office of Clean Energy Demonstrations awarded $7 million to an Energy Dome test project hosted by US gas and electricity supplier Alliant Energy.[19]
Compressed air energy storage (CAES) stores electricity by compressing air. The compressed air is typically stored in large underground caverns. The expanding air can be used to drive turbines, converting the energy back into electricity. As air cools when expanding, some heat needs to be added in this stage to prevent freezing. This can be provided by heat stored from a low-carbon source, or in the case of advanced CAES, from reusing the heat that is released when air is compressed. As of 2023[update], there are three advanced CAES project in operation in China.[20] Typical efficiencies of advanced CAES are between 60% and 80%.[21]
Another electricity storage method is to compress and cool air, turning it into liquid air, which can be stored, and expanded when needed, turning a turbine, generating electricity. This is called liquid air energy storage (LAES).[22] The air would be cooled to temperatures of −196 °C (−320.8 °F) to become liquid. Like with compressed air, heat is needed for the expansion step. In the case of LAES, low-grade industrial heat can be used for this.[23] Energy efficiency for LEAS lies between 50% and 70%. As of 2023[update], LAES is moving from pre-commercial to commercial.[24]
Lithium-ion batteries are the most commonly used batteries for grid applications, as of 2024[update], following the application of batteries in electric vehicles (EVs). In comparison with EVs, grid batteries require less energy density, meaning that more emphasis can be put on costs, the ability to charge and discharge often and lifespan. This has led to a shift towards lithium iron phosphate batteries (LFP batteries), which is cheaper and has a longer lifespan than traditional lithium-ion batteries.[28]
Costs of batteries are declining rapidly; from 2010 to 2023 costs fell by 90%.[29] As of 2024[update], utility-scale systems account for two thirds of added capacity, and home applications (behind-the-meter) for one third.[30] Lithium-ion batteries is highly suited to short-duration storage (<8h), but unlikely to become the cheapest form of electricity storage for longer-duration storage.[28]
The electric vehicle fleet has a large overall battery capacity, which can potentially be used for grid energy storage. This could be in the form of vehicle-to-grid (V2G), where cars store energy when they are not in use, or by repurposing batteries from cars at the end of the vehicle's life. Car batteries typically range between 33 and 100 kWh;[31] for comparison, a typical upper-middle-class household in Spain might use some 18 kWh in a day.[32] As of 2024[update], there have been more than 100 V2G pilot projects globally.[33] The effect of V2G charging on battery life can be positive or negative. Increased cycling of batteries can lead to faster degradation, but due to better management of the state of charge and gentler charging and discharing, V2G might instead increase the lifetime of batteries.[33][34] Second-hand batteries may be useable for stationary grid storage for roughly 6 years, when their capacity drops from roughly 80% to 60% of the initial capacity. LFP batteries are particularly suitable to second-use application, as they degrade less than other lithium-ion batteries and recycling is less attractive as their materials are not as valuable.[33]
In redox flow batteries, energy is stored in liquids, which are placed in two separate tanks. When charging or discharging, the liquids are pumped into a cell with the electrodes. The amount of energy stored (as set by the size of the tanks) can be adjusted separately from the power output (as set by the speed of the pumps).[35] Flow batteries have the advantages of low capital cost for charge-discharge duration over 4 h, and of long durability (many years). Flow batteries are inferior to lithium-ion batteries in terms of energy efficiency, averaging efficiencies between 60 and 75%. Vanadium redox batteries is most commercially advanced type of flow battery, with roughly 40 companies making them as of 2022[update].[36]
Sodium-ion batteries are possible alternative to lithium-ion batteries, as they rely on cheaper materials and less on critical materials. It has a lower energy density, and possibly a shorter lifespan. If produced at the same scale as lithium-ion batteries, they may become 20% to 30% cheaper.[35] Iron-air batteries may be suitable for even longer duration storage than flow batteries (weeks), but the technology is not yet mature.[37]
| Technology | Less than 4h | 4h to 8h | Days | Weeks | Seasons |
|---|---|---|---|---|---|
| Li-ion | Yes | Yes | No | No | No |
| Sodium-ion | Yes | Yes | No | No | No |
| Vanadium flow | Maybe | Yes | Yes | No | No |
| Iron-air | No | No | Maybe | Yes | No |
Flywheels store energy in the form of mechanical energy. They are suited to supplying high levels of electricity over minutes and can also be charged rapidly. They have a long lifetime and can be used in settings with widely varying temperatures. The technology is mature, but more expensive than batteries and supercapacitors and not used frequently.[23]
Powercorp in Australia have been developing applications using wind turbines, flywheels and low load diesel (LLD) technology to maximize the wind input to small grids. A system installed in Coral Bay, Western Australia, uses wind turbines coupled with a flywheel based control system and LLDs. The flywheel technology enables the wind turbines to supply up to 95 percent of Coral Bay's energy supply at times, with a total annual wind penetration of 45 percent.[38]
Hydrogen can be used as a long-term storage medium.[39] Green hydrogen is produced from the electrolysis of water, and converted back into electricity in an internal combustion engine, or a fuel cell, with a round-trip efficiency of roughly 41%.[40] It is expected to be a more economical means of long-term renewable energy storage than pumped-storage hydroelectricity or batteries.[2][3]
The low efficiency of hydrogen storage imposes economic constraints.[41][42] The price ratio between purchase and sale of electricity must be at least proportional to the efficiency in order for the system to be economic. Whether hydrogen can use natural gas infrastructure depends on the network construction materials, standards in joints, and storage pressure.[43]
Underground hydrogen storage is the practice of hydrogen storage in caverns, salt domes and depleted oil and gas fields.[44][45] Large quantities of gaseous hydrogen have been stored in caverns by Imperial Chemical Industries (ICI) for many years without any difficulties.[46] The European project Hyunder[47] indicated in 2013 that for the storage of wind and solar energy an additional 85 caverns are required as it cannot be covered by PHES and CAES systems.[48]
Power-to-gas is a technology which converts electrical power to a gas fuel. There are 2 methods, the first is to use the electricity for water splitting and inject the resulting hydrogen into the natural gas grid. The second less efficient method is used to convert carbon dioxide and water to methane, (see natural gas) using electrolysis and the Sabatier reaction. The excess power or off peak power generated by wind generators or solar arrays is then used for load balancing in the energy grid. Using the existing natural gas system for hydrogen, fuel cell maker Hydrogenics and natural gas distributor Enbridge have teamed up to develop such a power to gas system in Canada.[42]
Pipeline storage of hydrogen where a natural gas network is used for the storage of hydrogen. Before switching to natural gas, the German gas networks were operated using towngas, which for the most part consisted of hydrogen. The storage capacity of the German natural gas network is more than 200,000 GW·h which is enough for several months of energy requirement. By comparison, the capacity of all German pumped-storage power plants amounts to only about 40 GW·h. The transport of energy through a gas network is done with much less loss (<0.1%) than in a power network (8%)[clarification needed].
The power-to-ammonia concept offers a carbon-free energy storage route with a diversified application palette. At times when there is surplus low-carbon power, it can be used to create ammonia fuel. Ammonia may be produced by splitting water into hydrogen and oxygen with electricity, then high temperature and pressure are used to combine nitrogen from the air with the hydrogen, creating ammonia. As a liquid it is similar to propane, unlike hydrogen alone, which is difficult to store as a gas under pressure or to cryogenically liquefy and store at −253 °C.
Just like natural gas, the stored ammonia can be used as a thermal fuel for transportation and electricity generation or used in a fuel cell.[49] A standard 60,000 m³ tank of liquid ammonia contains about 211 GWh of energy, equivalent to the annual production of roughly 30 wind turbines. Ammonia can be burned cleanly: water and nitrogen are released, but no CO2 and little or no nitrogen oxides. Ammonia has multiple uses besides being an energy carrier, it is the basis for the production of many chemicals, the most common use is for fertilizer.[50] Given this flexibility of usage, and given that the infrastructure for the safe transport, distribution and usage of ammonia is already in place, it makes ammonia a good candidate to be a large-scale, non-carbon, energy carrier of the future.
In 2023, world pumped hydroelectric storage (PHS) was the largest storage technology, with a capacity of 181 GW, compared to some 55 GW of storage in utility-scale batteries and 33 GW of behind-the-meter batteries.[51] PHS is well suited to evening out daily variations, pumping water to a high storage reservoir during off-peak hours, and using this water during peak times for hydroelectric generation.[52] The efficiency of PHS ranges between 75% and 85%, and the response time is fast, between seconds and minutes.[53]
PHS systems can only be built in limited locations. Pumped storage systems may also be possible by using deep salt caverns or building a hollow deposit at the seabed, and using the sea itself as the higher reservoir.[52] PHS construction can be costly, takes relatively long and can be disruptive for the environment and people living nearby.[52] The efficiency of pumped hydro can be increased by placing floating solar panels on top, which prevent evaporation. This also improves the efficiency of the solar panels, as they are constantly cooled.[54]
Hydroelectric dams with large reservoirs can also be operated to provide peak generation at times of peak demand. Water is stored in the reservoir during periods of low demand and released through the plant when demand is higher. The net effect is the same as pumped storage, but without the pumping loss. Depending on the reservoir capacity the plant can provide daily, weekly, or seasonal load following.
Many existing hydroelectric dams are fairly old (for example, the Hoover Dam was built in the 1930s), and their original design predated the newer intermittent power sources such as wind and solar by decades. A hydroelectric dam originally built to provide baseload power will have its generators sized according to the average flow of water into the reservoir. Uprating such a dam with additional generators increases its peak power output capacity, thereby increasing its capacity to operate as a virtual grid energy storage unit.[55][56] The United States Bureau of Reclamation reports an investment cost of $69 per kilowatt capacity to uprate an existing dam,[55] compared to more than $400 per kilowatt for oil-fired peaking generators. While an uprated hydroelectric dam does not directly store excess energy from other generating units, it behaves equivalently by accumulating its own fuel – incoming river water – during periods of high output from other generating units. Functioning as a virtual grid storage unit in this way, the uprated dam is one of the most efficient forms of energy storage, because it has no pumping losses to fill its reservoir, only increased losses to evaporation and leakage.
A dam which impounds a large reservoir can store and release a correspondingly large amount of energy, by controlling river outflow and raising or lowering its reservoir level a few meters. Limitations do apply to dam operation, their releases are commonly subject to government regulated water rights to limit downstream effect on rivers. For example, there are grid situations where baseload thermal plants, nuclear or wind turbines are already producing excess power at night, dams are still required to release enough water to maintain adequate river levels, whether electricity is generated or not. Conversely there's a limit to peak capacity, which if excessive could cause a river to flood for a few hours each day.[57]
In Denmark the direct storage of electricity is perceived as too expensive for very large scale usage, albeit significant usage is made of existing Norwegian Hydro. Instead, the use of existing hot water storage tanks connected to district heating schemes, heated by either electrode boilers or heat pumps, is seen as a preferable approach. The stored heat is then transmitted to dwellings using district heating pipes.
Molten salt is used to store heat collected by a solar power tower so that it can be used to generate electricity in bad weather or at night.[58]
Building heating and cooling systems can be controlled to store thermal energy in either the building's mass or dedicated thermal storage tanks. This thermal storage can provide load-shifting or even more complex ancillary services by increasing power consumption (charging the storage) during off-peak times and lowering power consumption (discharging the storage) during higher-priced peak times.[59] For example, off-peak electricity can be used to make ice from water, and the ice can be stored. The stored ice can be used to cool the air in a large building which would have normally used electric AC, thereby shifting the electric load to off-peak hours. On other systems stored ice is used to cool the intake air of a gas turbine generator, thus increasing the on-peak generation capacity and the on-peak efficiency.
A pumped-heat electricity storage system uses a highly reversible heat engine/heat pump to pump heat between two storage vessels, heating one and cooling the other. The UK-based engineering company Isentropic that is developing the system claims a potential electricity-in to electricity-out round-trip efficiency of 72–80%.[60]
A Carnot battery is a type of energy storage systems that stores electricity in heat storage and converts the stored heat back to electricity via thermodynamics cycles. This concept has been investigated and developed by many research projects recently.[61] One of the advantage of this type of system is that the cost at large-scale and long-duration of thermal storage could be much lower than other storage technologies.
Alternatives include storing energy by moving large solid masses upward against gravity. This can be achieved inside old mine shafts[62] or in specially constructed towers where heavy weights are winched up to store energy and allowed a controlled descent to release it.[63][64]
The levelized cost of storing electricity depends highly on storage type and purpose; as subsecond-scale frequency regulation, minute/hour-scale peaker plants, or day/week-scale season storage.[65][66][67]
Using battery storage is said to have a levelized cost of $120[68] to $170[69] per MWh. This compares with open cycle gas turbines which, as of 2020, have a cost of around $151–198 per MWh.[70]
Generally speaking, energy storage is economical when the marginal cost of electricity varies more than the costs of storing and retrieving the energy plus the price of energy lost in the process. For instance, assume a pumped-storage reservoir can pump to its upper reservoir a volume of water capable of producing 1,200 MW·h after all losses are factored in (evaporation and seeping in the reservoir, efficiency losses, etc.). If the marginal cost of electricity during off-peak times is $15 per MW·h, and the reservoir operates at 75% efficiency (i.e., 1,500 MW·h are consumed and 1,200 MW·h of energy are retrieved), then the total cost of filling the reservoir is $22,500. If all of the stored energy is sold the following day during peak hours for an average $40 per MW·h, then the reservoir will see revenues of $48,000 for the day, for a gross profit of $25,500.
However, the marginal cost of electricity varies because of the varying operational and fuel costs of different classes of generators.[71] At one extreme, base load power plants such as coal-fired power plants and nuclear power plants are low marginal cost generators, as they have high capital and maintenance costs but low fuel costs. At the other extreme, peaking power plants such as gas turbine natural gas plants burn expensive fuel but are cheaper to build, operate and maintain. To minimize the total operational cost of generating power, base load generators are dispatched most of the time, while peak power generators are dispatched only when necessary, generally when energy demand peaks. This is called "economic dispatch".
Demand for electricity from the world's various grids varies over the course of the day and from season to season. For the most part, variation in electric demand is met by varying the amount of electrical energy supplied from primary sources. Increasingly, however, operators are storing lower-cost energy produced at night, then releasing it to the grid during the peak periods of the day when it is more valuable.[72] In areas where hydroelectric dams exist, release can be delayed until demand is greater; this form of storage is common and can make use of existing reservoirs. This is not storing "surplus" energy produced elsewhere, but the net effect is the same – although without the efficiency losses. Renewable supplies with variable production, like wind and solar power, tend to increase the net variation in electric load, increasing the opportunity for grid energy storage.
It may be more economical to find an alternative market for unused electricity, rather than try and store it. High Voltage Direct Current allows for transmission of electricity, losing only 3% per 1000 km.
The demand for electricity from consumers and industry is constantly changing, broadly within the following categories:
There are currently three main methods for dealing with changing demand:
The problem with standby gas turbines is higher costs; expensive generating equipment is unused much of the time. Spinning reserve also comes at a cost; plants running below maximum output are usually less efficient. Grid energy storage is used to shift generation from times of peak load to off-peak hours. Power plants are able to run at their peak efficiency during nights and weekends.
Supply-demand leveling strategies may be intended to reduce the cost of supplying peak power or to compensate for the intermittent generation of wind and solar power.
Virtually all devices that operate on electricity are adversely affected by the sudden removal of their power supply. Solutions such as UPS (uninterruptible power supplies) or backup generators are available, but these are expensive. Efficient methods of power storage would allow for devices to have a built-in backup for power cuts, and also reduce the impact of a failure in a generating station. Examples of this are currently available using fuel cells and flywheels.
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