Energy storage offers a range of socio-economic benefits, environmental benefits, energy market support, and is an important technology for intermittently generated energy such as wind and photovoltaic.
Energy storage may be accomplished in a variety of methods, including pumped hydro storage, superconducting magnetic energy storage, and the use of capacitors.
Each technology has unique qualities.
The quantity of energy that each technology can store is an important differentiating feature of the various energy storage systems, as is the speed with which this energy may be released by each system.
This article aims to introduce, explain and promote Compressed Air Energy Storage Systems.
What is Compressed Air Energy Storage (CAES) technology and how does it work?
The technical concept of compressed air energy storage (CAES) has been around for more than 50 years. In the Nuclear Power Industry, compressed air energy storage was substantially examined in the 1970s as a way of providing load, following, and meeting peak demand while maintaining constant generation capacity.
Since the late 1970s, compressed air energy storage (CAES) technology has been commercially accessible. Commercial CAES facilities have been working extremely well for decades, with the oldest one known to be in Germany currently operating for over 44 years, one in the US for 31 years, and so on….
The phrase “Compressed Air Energy Storage” essentially describes how the CAES technology works. In times of excess electricity (on the grid, for example, due to high power delivery during periods of low demand), a compressed air energy storage facility can compress and store the compressed air in a cavern or a pressure vessel – either underground or above ground.
When energy demand is high, the stored energy (in the form of pressured air) may be released and recovered as electricity.
Because electricity is stored at low demand or low-cost periods, and electricity is generated at high demand or high price by releasing the stored energy (pressure).
The primary motivation for storing energy is not environmental conservation, but rather the economic gains provided by CAES technology. Furthermore, this technology supports the energy market and gives socioeconomic advantages.
The compressed air can be held underground in suitable mines or caverns formed under salt rocks, or above ground in pressure vessels.
The storage facility, on the other hand, must be as airtight as possible to minimize energy loss through leakage. For very large-scale utility-sized CAES systems, storage in mined caverns (caverns dug expressly for compressed air energy storage) is utilized. However, compressed air energy storage may be converted for usage above ground in distributed, small-scale operations employing high-pressure vessels, in addition to large-scale installations (APS, 2007)
In conventional CAES plants, natural gas is consumed during plant discharge, much like in a turbine plant. During discharge, the combustion turbine in a standard CAES plant, on the other hand, utilizes all of its mechanical energy to create electricity, making the system exceedingly efficient. 2001 (Schoenung).
Generally, when a CAES plant discharges, the pressured medium powers a generator, allowing the energy to be recovered in the form of electricity.
In further advanced stages we are today ready to supply instead of electricity in the same technology energies in form of cooling- or heating energy. Considering that almost 40% of all energies consumed today is cooling and heating this is an amazing step further in advancing HICAES technology.
Conventional CAES plants as well as so-called “Hydro-pneumatic Isothermal Compressed Air Energy Systems” or “HICAES” – reach non degrading round trip efficiencies above 85 percent and design lives exceeding 50 years. Both are amazing aspects that distinguish technological supremacy and place this technology beyond most other storage systems on the market today.
The application of compressed air energy storage technology on a small scale (e.g., domestic, residential, …) is almost identical to that of large-scale compressed air energy storage technology, except that the pressure vessel is generally smaller and can also be installed above ground. It may be installed practically anywhere that is sheltered from weather, does not require extensive building work, and is available in very small sizes, even considered movable. The size of the reservoir determines the amount of electricity that may be stored in these semi-industrial and residential plants.
Concluding, CAES (HICAES) plants are used in a broad variety of sizes, from very small-scale residential plants to very big utility storage facilities, allowing for highly efficient yet project-specific applications.
Feasibility of Compressed Air Energy Storage and operational necessities
As previously stated, the CAES technological concept dates back more than fifty (50) years. The world’s longest-running CAES plant is located in Huntorf, Germany.
The 290-MW Huntorf facility has been in service since 1978, principally serving as a cyclic duty, ramping duty, and hot spinning reserve for industrial clients in northwest Germany. This facility has recently leveled the fluctuating power from a number of wind turbine generators in Germany. In the United States, a 110 MWe facility in McIntosh, Alabama, has been in operation since 1991.
In Norton, Ohio, the United States, another very large scale CAES plant is being progressed. With a 2.700 MWe capacity, this plant will compress air to 1500 pounds per square inch (psi) in an existing limestone mine 2200 feet deep. A number of additional CAES plants have been constructed and/or studied (EPRI, 2002).
Being in operation for almost 50 years, CAES technology has been demonstrating that it is a proven storage system, both technologically as well as economically. As a result, this technology is suitable for both decentralized household storage and centralized very large utility scale energy storage in capacities of up to several 1.000 MWe.
Apart from a hot spinning reserve operation, as shown in Figure 2, CAES technology, like certain batteries and pumped hydro storage technologies, can be somewhat sluggish in discharging the stored power capacity.
Hence, the quantity of energy that this technology can store is among the greatest of any energy storage technology known.
Operational necessities of Compressed Air Energy Storage (CAES)
Also illustrated in Figure 1, for a conventional CAES plant cycle the major components include (EPRI, 2002):
- A motor/generator with clutches on both ends (to engage/disengage it from the compressor, expander, or both trains).
- Multistage air compressors equipped with intercoolers to minimize power needs during the compression cycle and an aftercooler to reduce storage capacity requirements.
- A high- and low-pressure turboexpander train with combustors between stages.
- System of control (to regulate and control the off-peak energy storage and peak power supply, to switch from the compressed air storage mode to the electric power generation mode, or to operate the plant as a synchronous condenser to regulate VARS on the grid).
- Auxiliary apparatus (fuel storage and handling, cooling system, mechanical systems, electrical systems, heat exchangers).
- Compressed air storage, including piping and fittings, can be underground or aboveground.
- Auxiliary apparatus (fuel storage and handling, cooling system, mechanical systems, electrical systems, heat exchangers).
- Compressed air storage, including piping and fittings, can be underground or aboveground. Underground air storage is frequently carried out in aquifers or mined caves, whilst aboveground air storage is carried out in specially built holding tanks.
Several key features of CAES and HICAES
- CAES technology are simple to tailor to unique site circumstances and economics.
- CAES are proven technology that can be offered competitively.
- CAES plants have the ability to black start (further discussed below). Both the Huntorf and McIntosh facilities feature utility scale black start capability, which is used on occasion.
- CAES plants have a quick start-up time.
- CAES plant may be used as a hot spinning reserve, reaching maximum capacity in a matter of seconds.
- The emergency restart time of a CAES plant from cold temperatures is around 5 minutes. Normal starting times might range from 5 to 10 minutes.
According to numerous research, the major cause for this technology’s market position is utility planners’ lack of understanding of this alternative. Furthermore, utilities are likely to regard subterranean geology as a risk problem in the case of utility-sized traditional CAES facilities.
Furthermore, very few engineers are aware that CAES locations are rather frequent and that HICAES does not even require subsurface caves or caverns anymore. Because of these factors, the commercial potential for CAES is enormous and has yet to be fully realized.
Table 1. Key features of a CAES plant
Feature | Parameter range |
Space requirement | 1 MWe requires less than 350m2 area |
Roundtrip efficiency | ~ 85% |
Design life of plant | ~ 50 years |
Maintenance requirement | ~ US$0.30/MWh generated |
Plant storage degradation | None |
Plant storage recyclability | Yes |
Plant hazards | None |
Plant poisions | None |
Rare materials needed | None |
Environmental impact | None |
Auxiliary equipment needs | None |
Power conditioning needs | None |
Grid – network support | Yes |
Ramp up duty | Yes |
Black Start capability | Yes |
Hot spinning duty | Yes |
Status of Compressed Air Energy Storage (CAES, HICAES) technology and its future market potential
After almost half a century of successful operation, CAES technologies are very well established and proven technologies, and because renewable energies must bridge the intermittent energy production gap, the market potential is large and therefore poised to achieve a rising market share.
Because renewable energies constitute an intermittent energy source, energy storage is critical for high market penetration rates of both wind and solar energy technologies to meaningfully contribute to the COP26 “near-zero emission” aim.
According to the APS (2007) research, CAES is a natural but also necessary partner for renewable energy facilities. This is due to CAES’s characteristics in that it can operate on a short enough time scale to smooth out fluctuations in the energy grid caused by intermittent generator fluctuations, that it meets the storage capacity required by generation plants, and that the CAES technology is more uniformly applicable than pumped hydro storage (which requires elevation) or most other energy storage technologies today.
Furthermore, according to the APS (2007) research, CAES and pumped hydro storage technologies are ideal for load control and load management.
What Compressed Air Energy Storage technology contributes to socio-economic development and environmental protection
A 2010 report by the California Public Utilities Commission (CPUC) highlights the economic benefits of CAES technology. According to the report, considerable economic gains result from energy bill savings, cheaper future energy storage costs as grid power tariffs rise over time, employment, and other prospects for economic growth.
CAES energy storage solutions provide consumers with flexibility when drawing electricity from the grid to satisfy demand. CAES systems enable energy arbitrage opportunities for consumers on dynamic rates. In other words, energy storage helps customers who pay variable rates based on the time of purchase of power to save money on their energy bills by altering the timing when energy is obtained from the grid at high rates. When the cost of energy is low, the energy storage system charges, and when the cost of energy is high, it discharges.
As a result, CAES systems deliver immediate economic benefits to their owners.
CAES technologies are, beyond any doubts, the required proven technology, particularly given the predicted growth in energy expenditures over time. These market developments will need policymaker, investor and consumer to take into account.
CAES Energy storage technologies create jobs and give other chances for economic progress. CAES storage solutions are not yet widely used, but as the technology’s market penetration grows, employment in production and installation will be created. Furthermore, CAES technologies are intended to promote and safeguard economic growth objectives through electricity grid stabilization and smoothening.
The technological advantages provided by CAES energy storage systems contribute to socioeconomic growth. National Energy Technology Laboratory (NETL) research on the market of electric energy storage systems published in 2008 provides significant data on the technological benefits afforded by energy storage technologies in general, but especially CAES technology. This section concentrates on the technological advantages offered by CAES technologies, while certain advantages may also be applicable to other energy storage methods.
Energy storage systems help to stabilize the grid by extracting energy from and giving energy to it at predetermined periods. After a disruption, the energy system can become unstable, and stored energy can greatly aid in stabilization efforts. For example, in the event of a high-power surge in the grid, CAES energy storage devices retain the energy and gradually release it back into the grid. Alternatively, in the event of a power outage on the grid, the CAES energy storage technologies give the necessary energy to the system. In the case of CAES technology, this benefit is a realistic reality if the power loss or surge is predicted to last for an extended length of time.
According to the NETL (2008) study, energy storage systems in general can address three types of grid instability: rotor angle instability, voltage instability, and frequency excursions.
CAES technology and grid operations
CAES energy storage technology may be utilized to assist grid functioning and hence provide grid operational support. Grid operational assistance is classified into four categories of operations:
- Energy storage can be used to inject and absorb electricity in order to maintain grid frequency in the face of changes in generation and load.
- Contingency reserves include spinning and supplementary reserve units at the transmission level, which supply power for up to two hours in the event of a sudden loss of generating or a transmission outage.
- Voltage support: Energy storage can help with the injection or absorption of reactive power into the grid to keep system voltages within an acceptable range. Power-conditioning electronics are used in energy storage systems to convert the power output of the storage technology to the proper voltage and frequency for the grid.
- Black start: black start units may start up from a shutdown state without grid assistance and then re-energize the grid to allow additional units to start up. A correctly scaled energy storage device can enable black start.
Furthermore, CAES technologies have the potential to increase power quality and dependability. However, because the bulk of grid-related power quality events are voltage sags and interruptions lasting barely a fraction of a second, CAES technologies are less suitable for this role if not used in hot spinning mode.
LOAD SHIFTING is a function that Compressed Air Energy Storage systems excel at. Load shifting is accomplished by storing energy during low demand times and releasing the stored energy during high demand periods.
According to the NETL (2008) study, load shifting can take numerous forms. PEAK SHAVING is the most prevalent of these kinds, in which CAES technology can be quite useful. Peak shaving is the use of energy storage to minimize a region’s peak supply and/or peak demand. It is frequently advocated when the peak demand for a system is substantially higher than the average load, and when the peak demand occurs seldom.
As a result, capacity expansions are quite expensive since they permanently increase the capacity of the grid in order to handle infrequent peak demand situations.
Peak shaving allows a utility to postpone the expenditure necessary to expand the grid’s capacity. According to the EPRI (2003) research, the economic feasibility of energy storage for peak shaving is dependent on various parameters, most notably the pace of load increase. Because unusual occurrences are expected to become more regular as load climbs, rapid load growth enhances the economic feasibility of capacity additions.
Low load growth, on the other hand, promotes the economic feasibility of energy storage technologies since uncommon occurrences must be handled, and energy storage is appropriate for treating rare and occasional peak demand through peak shaving.
The ability to facilitate the integration of renewable energy sources is a significant feature of CAES energy storage technology. This is an important quality given that renewable energy is now the largest and fastest increasing power source. In the context of wind power, the following uses for energy storage technologies are presented. Other renewables, including PV, have similar applications.
- Intermittency and unpredictability in wind production output owing to unexpected variations in wind patterns can lead to considerable imbalances between generation and load, resulting in fluctuations in grid frequency in grids with a major proportion of wind power. Such imbalances are often managed at the transmission level by spinning reserve, but energy storage can enable quick reaction to such imbalances without the emissions associated with most current systems.
- Reduced Transmission Curtailment: Wind power generating is frequently located in rural places that are underserved by transmission and distribution facilities. As a result, wind operators are sometimes forced to reduce their output, resulting in missed energy production opportunities, or system operators are obliged to spend in extending transmission capabilities. An energy storage unit positioned near wind production can store extra energy and then deliver it when the transmission line is not crowded. CAES technology are excellent for this application.
- Time shifting: Wind turbines are categorized as non-dispatchable resources. CAES energy storage can be utilized to store energy generated during low demand times and provide it during high demand periods. This technique is frequently referred to as “firming and shaping” when used to wind generating since it affects the power profile of the wind to allow for more control over dispatch.
The above-mentioned socioeconomic advantages result in environmental benefits.
Load shifting, peak shaving, and other technical benefits reduce greenhouse gas and other emissions; energy market support functions delay energy grid expansions, saving resources; and the storage capacity provided by CAES technology delays energy power supply expansions, saving both natural resources and reducing emissions from power production.
Furthermore, if renewables such as wind and PV expand as a share of the off-peak power mix, the carbon advantages of energy storage will grow.
Financial requirements and costs
CAES technologies are the only ones capable of providing considerable energy storage (tens of thousands of MWhs) at relatively moderate costs (about $400 to $700/kW). CAES technology offer virtually limitless potential for major load control at the utility or regional levels.
Because of the high per-kWh operational cost, the CAES cost curve is not straight. While the prices of other energy storage systems are almost entirely determined by installed capacity, the costs of CAES are determined by both installed capacity and the quantity of energy that travels through storage (Schwyzer, 2006).
Capital costs
CAES systems’ capital costs are determined by the storage medium, plant capacity (power), and the amount of energy stored in the storage. (EPRI, 2002).
Storage medium for CAES plant | Size (MWe) | Cost for power-related plant components ($/kW) | Cost for the energy storage components ($/kWh) | Typical hours of storage for a plant | Total capital costs ($/kWe) |
Salt/ Porous media | 200 | 350 | 1 | 10 | 351 |
Above Ground | 200 | 350 | 200 | 4 | 550 |
Hard rock (new cavern) | 200 | 350 | 300 | 10 | 650 |
Table 2. CAES Plant Costs For Various Storage Media and Plant Configurations. Source: EPRI, 2002
References and sources
- CTCN, United Nations Climate Technology Centre and Network, Compressed Air Energy Storage at: https://www.ctc-n.org/technologies/compressed-air-energy-storage-caes
- EPRI, 2002. Handbook for Energy Storage for Transmission or Distribution Applications. Report No. 1007189. Technical Update December 2002. Document can be found at: www.epri.com
- NETL, 2008. Market Analysis of Emerging Electric Energy Storage Systems. National energy technology laboratory and Department of Energy report with code DOE/NETL-2008/1330 of July 31, 2008. Document can be found online at: http://www.netl.doe.gov/energy-analyses/refshelf/PubDetails.aspx?Action=View&PubId=212
- Schoenung, 2001. Characteristics and technologies for long vs short term energy storage. A study by the U.S. department of energy (DOE) Energy Storage Systems Program. A SANDIA report with code SAND2001-0765. Document can be found online at: http://www.doe.gov/bridge
- CPUC, 2010. Electric Energy Storage: An Assessment of Potential Barriers and Opportunities. A Policy and Planning Division White Paper of the California Public Utilities Commission. Document can be found online at: www.cpuc.ca.gov/PUC/energy/reports.htm
- Schwyzer, 2006. “The Potential of Wind Power and Energy Storage in California,” Diana Schwyzer, Masters Thesis for Energy and Resources Group at UC Berkeley. November 2006. Document can be found online at: www.iaee.org/en/students/best_papers/Diana_Schwyzer.pdf
- APS, 2007. Challenges of Electric Energy Storage Technologies: A Report from the APS Panel on Public Affairs Committee on Energy and Environment. Document can be found online at: www.aps.org/policy/reports/popa-reports/upload/Energy_2007_Report_ElectricityStorageReport.pdf