Chapter 6
Compressed Air Energy Storage in Underground Formations
Sabine Donadei
Gregor-Sönke Schneider KBB Underground Technologies GmbH, Hannover, Germany
Abstract
Unlike fossil energy carriers, renewables are characterized by short-term and long-term fluctuations and therefore cannot supply energy upon demand. The increased use of fluctuating renewable energy sources strengthens the significance of the storage of electrical energy at the grid scale. In addition to pumped hydro technology which has been used successfully for many decades, and future hydrogen systems, there is increasing interest in a storage technology that was developed well over 50 years ago: compressed air energy storage. This process uses electrical energy to compress air and store it under high pressure in underground geological storage facilities. This compressed air can be released on demand to produce electrical energy via a turbine and generator. This chapter describes various plant concepts for the large-scale storage of compressed air and presents the options for underground storage and their suitability in accordance with current engineering practice. Compressed air energy storage projects which are currently in operation, construction, or planning are also presented.
Keywords
CAES
compressed air energy storage
large-scale energy storage
underground storage
electrical energy storage
1. Introduction
Unlike fossil fuels, renewable energy sources such as wind and solar are characterized by short- and long-term seasonal fluctuations and cannot deliver energy on demand. Moreover, only a very small amount of the electrical energy they generate has so far been stored. Compared with the storage of fossil fuels, the storage of electrical energy at the grid scale has only played a very subordinate role so far. This is highlighted by the fact that although the storage capacities in the European Union for liquid and gaseous hydrocarbons guarantee a statutorily regulated supply security for weeks and months, the storage capacity for electrical energy currently corresponds to 5% of total installed generation capacity. In Germany, for instance, electrical storage can only maintain the power in the whole of the German grid for less than one hour. Storage of electrical energy at the grid scale is gaining in significance because of the increased use of fluctuating renewables. In addition to pumped hydro technology, which has proven its worth over many decades, and future hydrogen systems (power-to-gas), attention is again being focused on a storage technology which was developed over 50 years ago: compressed air energy storage (CAES) [1–4].
The use of compressed air to store energy is currently deployed in applications ranging from very small outputs up to triple-figure megawatt installations. In this chapter the focus is on underground energy storage at the grid scale comparable to conventional pumped hydro power plants, which means that the following chapters are restricted to investigation of the megawatt class.
The concept of large-scale compressed air storage was developed in the middle of the last century. The first patent for compressed air storage in artificially constructed cavities deep underground, as a means of storing electrical energy, was issued in the United States in 1948. Frazer W. Gay, the patent holder, described his invention as follows: “In the present invention, I propose to provide equivalent storage space for gas relatively close to the earth’s surface and, furthermore, to make this storage space available for the storing of compressed air to be used for power generation purposes during periods of heavy power load, as well as for natural gas or manufactured gas, butane, propane or other fluids. The invention in general comprises the construction of huge caverns located comparatively close to the earth’s surface.” [5]
In Germany, a patent for the storage of electrical energy via compressed air was issued in 1956 whereby “energy is used for the isothermal compression of air; the compressed air is stored and transmitted long distances to generate mechanical energy at remote locations by converting heat energy into mechanical energy” [6]. The patent holder, Bozidar Djordjevitch, is sometimes quoted as the inventor of CAES technology. His concept was supplemented by the former Federal Institute for Geology—today’s BGR (Bundesanstalt für Geowissenschaften und Rohstoffe), which chose cavities in underground rock salt as the storage facilities for the air because these were considered to be the most economical and the safest [7,8].
CAES power plants attracted interest in the United States at the end of the 1960s with the focus in this case on air storage. This led to the discussion and patenting of CAES systems with salt caverns and aquifer structures [9,10].
Several studies and projects on CAES arose in Europe in the 1970s. Salt caverns, aquifer structures, and mines were investigated and taken into consideration as potential storage spaces.
The world’s first underground CAES power plant was constructed in Huntorf (Germany) in the middle of the 1970s and was primarily aimed at storing the electrical energy produced by less flexible coal and nuclear power plants during low periods of demand, and to feed this energy back into the grid again during periods of high demand. Other motives for constructing the plant were the cold-start capacity, its ability to regulate the grid frequency, and the phase shifter operation [11]. Another CAES power plant was constructed in McIntosh, Alabama (United States) in 1991.
After the expected demand for additional CAES power plants evaporated as a result of the merger of smaller grids to form larger shared grids, interest was reawakened at the beginning of this century by the transition from fossil fuels to renewable energy. This was initially stimulated by the growing demand for “minute and hour” reserves in the power grid to balance out deviations between forecast and actual wind energy generation. Growing demand for flexibility again focused attention on underground CAES power plants because of their analogous properties to pumped hydro plants. Attention now concentrates particularly on the development of new power plant components.
2. Mode of operation
In underground CAES power plants, electrical energy from the power grid drives a compressor to inject large volumes of air under high pressure into a storage facility. When electricity is required, this air can be released from the storage and passed through a turbine and generator to regenerate electrical power, which can be fed back into the grid. The heat energy generated by compression is either lost to the environment or made available to other users, or stored for later deployment. Before being fed back into the turbine and generator installation the air is warmed back up again by using heat from this thermal storage, or a different source, or from combustion gases (for more on mode of operation see chapter: “Compressed Air Energy Storage”).
A CAES power plant consists of a storage space for the air and a power plant with motor compressor and turbine generator units. Although the storage of compressed air on the surface is possible, for example, in spherical and pipe storage systems, or in gasometers, these have much lower storage capacities than underground storage systems. Installation concepts at the grid scale therefore usually depend on the underground storage system. Since these underground geological storage systems must be injected with cool air (<10 °C), the air has to be cooled down before injection. In the case of diabatic CAES storage (Fig. 6.1) the extracted heat is not stored. During the generation phase, it is therefore necessary to reheat the cold air flowing out of the storage with the help of combustion gas to prevent the turbine icing up and to compensate for the energy lost during the preceding cooling process. This results in a relatively low level of efficiency of only (42–54)%. This is significantly lower than that obtained from a pumped hydro power plant [12].
Figure 6.1 Diagram of a diabatic CAES power plant.
© KBB Underground Technologies GmbH.
© KBB Underground Technologies GmbH.
In adiabatic CAES systems (Fig. 6.2) the heat of compression is stored in one or more separate storage facilities so that it can be reused to heat up the air when it is withdrawn from the storage. Since this dispenses with the addition of combustion gas, this can be considered a pure power-to-power storage system. The level of efficiency of adiabatic CAES power plants is reported to be up to 70% [12]. This concept has not yet been put into practice.
Figure 6.2 Diagram of an adiabatic CAES power plant.
© KBB Underground Technologies GmbH.
© KBB Underground Technologies GmbH.
CAES power plants are primarily suitable for balancing out short-term fluctuations in the power grid during periods of peak demand in the megawatt range over a period of hours to a few days (Fig. 6.3). Compared with long-term storage systems the use of energy storage of this kind is characterized by much higher cyclicity and relatively low storage volumes. CAES power plants are mechanical storage facilities and therefore have a relatively low volumetric energy density—depending on the dimensions—of around twice the value of pumped hydro storage. Long-term storage therefore requires a large amount of storage volume and thus high total costs for a similar output. In contrast, the high efficiency in part of CAES power plants has a positive effect on the economics, particularly when frequent cycles are involved.
Figure 6.3 Storage layout diagram.
© KBB Underground Technologies GmbH.
© KBB Underground Technologies GmbH.
3. Plant concept
3.1. Diabatic Concept
The basic idea of diabatic CAES was to transfer off-peak energy produced by base nuclear or coal-fired units for high-demand periods, using only a fraction of the gas or oil that would be used by a standard peaking machine, such as a conventional gas turbine. There are only two CAES plants in the world in operation at present: the 321 MW plant belonging to E.ON Kraftwerke, Huntorf (Germany) (Fig. 6.4), and the 110 MW plant of PowerSouth Energy Cooperative in Alabama (United States). Both use underground salt caverns for storing the air (Section 4.7).
Figure 6.4 Huntorf CAES power plant.
© KBB Underground Technologies GmbH.
© KBB Underground Technologies GmbH.
During low-cost off-peak load periods a motor uses power to compress and store air in an underground facility. During peak load periods, the process is reversed: the compressed air is returned to the surface to burn natural gas in the combustion chambers. The resulting combustion gas is then expanded in the two-stage gas turbine to spin the generator and produce electricity.
In a pure gas turbine power station, around two-thirds of the output are needed for compressing the combustion air (100 MW net output + 200 MW compressor consumption equal to 300 MW gross output). In a CAES power station no compression is needed during turbine operation because the required energy is already included in the compressed air. This has two advantages:
• using cheaper (excess) power for compression during off-peak periods;
• generation of all the output instead of one-third by the gas turbine [11].
In addition, the compressor and generator operations can be operated at different times. The diabatic CAES concept is not a “pure” energy storage system, but rather a gas turbine power plant in which compression of the combustion air and depressurization of the heated gases take place at different places and at different times. The major advantage of this technology is that cheap (excess) energy can be used for compression.
The efficiency of the diabatic storage plant is determined by the deployment of energy in the form of electricity, as well as the natural gas, and the generated output. Taking all this into account, the efficiency of the Huntorf plant is 42%:
If the waste heat of the gas turbine exhaust gas is recovered in a recuperator used to preheat the combustion air for the gas turbine, it is possible to increase the efficiency and reduce the amount of fossil energy required. This results in an enhanced efficiency of 54%:
3.2. Single-Stage Adiabatic Concept
Due to the low efficiency compared with modern pumped hydro plants and the desire to construct a “pure” power storage without any additional combustion gas, an increasing number of concepts for CAES power plants have been developed since the beginning of the millennium to incorporate the storage and deployment of compression heat. In these concepts a key element is dimensioning of the heat storage in combination with the compression process. The single-stage adiabatic concept corresponds to compression from atmospheric pressure up to storage pressure in one step, without any interim cooling, and subsequent cooling down of the compressed storage gas by several hundred degrees in one heat storage phase. The basis for this concept was developed in the EU project “Advanced Adiabatic Compressed Air Energy Storage” [13]. This was followed up by establishment of a consortium operated by RWE AG and realization of the ADELE project comprising additional research and development aimed at developing the components and/or the plant to such a level that it would lead to the construction and operation of a pilot plant (Section 4.7).
When air is compressed to pressures of well over 50 × 105 Pa (50 bar) the temperature of the air increases by several hundred degrees centigrade. Since the material properties of underground geological storage are not designed to handle such temperatures the compressed air has to be cooled down considerably prior to injection. If this heat of compression is to be used to heat up the stored air again immediately before it is used to regenerate electricity, it is necessary to cool down the compressed air in a thermal energy storage (TES) unit until it reaches ambient temperature. When the air in the storage is withdrawn the cold compressed air is heated back up again in the TES so that no natural gas needs to be added in the turbine.
The TES has to satisfy a number of challenging specifications:
• The capacity to store heat in the high-megawatt range.
• Ability to rapidly cool down air with temperatures of over 600 °C to ambient temperature and vice versa.
• Resistance to pressures of up to 100 × 105 Pa (100 bar) and temperatures exceeding 600 °C.
• Low-pressure losses with very high mass flow in the several 100 kg s−1 range.
• Minor heat losses when on standby.
The compressor also has to satisfy very high specifications, not only because of high input temperatures, but also due to the high temperature–time gradient—because the installation has to generate full capacity within only a few minutes.
An efficiency of 70% is targeted for future adiabatic CAES power plants.
3.3. Multistage Adiabatic Concept
Due to the high technical specifications and the associated costs of realizing the single-stage adiabatic CAES concept, a great deal of interest has been shown in recent years in isothermal or quasiisothermal CAES concepts [14,15]. The focus here is on subdividing compression and decompression into several stages so that each stage is only associated with minor temperature increases (Fig. 6.5).
Figure 6.5 Sketch of the principle behind the isothermal concept [17].
© KBB Underground Technologies GmbH.
© KBB Underground Technologies GmbH.
Each stage is followed by a heat exchanger. Due to the low temperatures, heat can be stored in either conventional storage facilities or will only require much less development input. The major requirement here is the size of the heat exchanger surface because only minor temperature differences are available for the heat exchange process, at the same time as involving very large volumes of heat. No project has yet been implemented to apply this concept.
3.4. Performance Metrics
Typical performance metrics of various underground CAES concepts are summarized in Table 6.1 [16–19].
Table 6.1
Performance Metrics of Various CAES Concepts
| Concept | E.ON Kraftwerke/Huntorf | PowerSouth Energy Cooperative/McIntosh | RWE/ADELE | Fraunhofer/ LTA-CAES | u&í—umwelt-technik und ingenieure GmbH |
| Diabatic | Diabatic with waste heat utilization | Single-stage adiabatic | Multistage adiabatic | Multistage adiabatic | |
| Status | In operation | In operation | In planning | Concept | Concept |
| pmin/105 Pa | 46 | 46 | – | – | – |
| pmax/105 Pa | 72 | 75 | 65 | 150 | 70 |
| mair/kg s–1 | 455 | 154 | 300 | – | 840 |
| Power/Generator/MW | 321 | 110 | 260 | 30 | 290 |
| Full load after time/min | 6–9 | 14 | – | <5 | – |
| Efficiency factor/% | 42 | 54 | c. 70 | 52– 60 | 72.5 |
| Heat output/MW | – | – | – | – | 70 |
| Vcavity/m3 | 310 000 | 538 000 | – | – | – |
| Wcavity/MWh | 200–642 | 240–2 640 | 1 040 | – | – |
LTA, low-temperature approach.
4. Underground Storage
A great deal of experience has been gained worldwide over several decades with the storage of gas in deep underground geological formations—primarily involving natural gas. Underground storage generally has a number of crucial advantages over surface storage in tanks:
• Achieving high storage capacity as a result of large geometrical volumes and high operating pressures of up to 200 × 105 Pa (200 bar).
• Considerable protection against external influences since the only surface features are the connection valves.
• Very low footprint compared with surface pressure tanks.
• Low specific costs with respect to storage capacity.
There are several options available for the storage of compressed air in underground geological formations at the grid scale: in natural pore storages such as depleted oil and gas fields, aquifer formations, artificially constructed cavities such as salt and rock caverns, and abandoned mines. The following specific aspects have to be taken into consideration when storing compressed air in underground geological formations:
• The high reactivity of oxygen in compressed air, for example, forming compounds with the mineral constituents of the storage rock, and thus leading to oxygen depletion.
• Suitability/dimensioning of the storage for frequent, rapid operation cycles, and high injection and withdrawal rates, because CAES power plants are typically operated in an extremely fluctuating mode.
• Possibility of operating the storage for a short period of time at atmospheric pressure, for example, during repairs and maintenance measures.
The following describes the options available for the underground storage of compressed air, and their suitability in light of today’s standard engineering practice. It also evaluates compressed air storage projects currently in operation, construction, or at the planning stage.
4.1. Depleted Oil and Gas Fields
Depleted oil fields, and particularly gas fields, can be used for the storage of gases because their imperviousness over geological time periods has already been proven. Another advantage is that these reservoirs have already been very well researched as part of the preceding exploration and production activities. Moreover, it may also be possible to use existing wells for storage operations after relevant conversion activities have been carried out. However, not all depleted oil and gas fields are suitable for conversion into a gas storage facility. The suitability criteria include depth and adequate permeability and porosity of the reservoir rock. Due to the reservoir engineering properties of depleted oil and gas fields, they tend to have low flexibility for injection and withdrawal, and the gas storage facilities constructed using them therefore tend to be best used for seasonal applications [20].
Natural gas has been stored for many decades in depleted oil and gas fields. A depleted gas field was used for the storage of gas for the first time 100 years ago in Welland County, Ontario (Canada) [21]. They are the main storage option for underground geological storage worldwide primarily because of their capacity. Of the natural gas currently stored underground worldwide, 81% is held in depleted oil and gas fields [22,23].
Even if depleted oil and gas fields play the most important role in the storage of natural gas worldwide, there are problems with their use for storing compressed air because they always contain residual quantities of hydrocarbons, which can lead to the formation of ignitable and explosive mixtures when coming into contact with injected compressed air. The suitability of depleted oil and gas fields for the storage of compressed air is currently being looked at in scientific studies [24–26]. No depleted oil and gas fields have been used so far for compressed air storage.
4.2. Aquifers
Aquifers are porous, permeable, water-bearing underground rock layers, which are in principle suitable for the storage of gas. The injected gas displaces the water to more distant parts of the reservoir to create a gas cap, which acts as the storage. Crucial conditions for the suitability of an aquifer as gas storage are the adequate porosity and permeability of the reservoir rock, and the presence of an upper seal to the aquifer structure formed by a gas-tight rock layer. This is why a large amount of exploration effort is required before constructing a gas storage facility in an aquifer, unlike the situation with depleted oil and gas fields. Aquifers are also less flexible with respect to injection and withdrawal and are therefore more suitable for seasonal storage. Several wells need to be drilled to be able to achieve the rates required for the storage operations.
The first aquifer storage world-wide was constructed in the United States in 1931 [21]. The use of aquifers for the storage of natural gas is now standard engineering practice worldwide and accounts for 13% of the natural gas stored underground globally [22,23].
The positive experience gained from underground storage of natural gas cannot be directly extrapolated to compressed air storages because of the risk of reactions between the oxygen in the air and the minerals and microorganisms in the reservoir rock. This can lead to the loss of oxygen as a result of oxygen depletion, or blockage of flow paths—which can make storage unusable.
Industry has been looking at the use of aquifer structures for the storage of compressed air for many decades [27,28]. Only one pilot plant for the storage of compressed air has been carried out in an aquifer in the megawatt range (Section 4.7). Furthermore, the injection of compressed air into an aquifer was tested over a period of a few years and then withdrawn to investigate the feasibility. Although the feasibility was confirmed from a reservoir engineering point of view, considerable oxygen depletion was a serious problem [29,30].
4.3. Salt Caverns
Salt caverns are artificial cavities in underground salt formations, which are created by the controlled dissolution of rock salt by injection of water during the solution mining process. Geometrical volumes of a few 100 000 m3, and up to 1 000 000 m3 and more in individual cases, can be achieved depending on technical specifications and geological conditions. In Germany the caverns lie at depths of around (500–2000) m, with cavern heights of up to 400 m. Depending on the depth, these caverns can be operated with a pressure of up to 200 × 105 Pa (200 bar) and thus allow the storage of very high volumes of gas. The favorable mechanical properties of the salt enable the construction and operation of extremely large cavities stable for long periods of time, which are also completely impervious to gases. In addition, salt is inert with respect to gases and liquid hydrocarbons. The amount of exploration work required is also usually much lower than the aforementioned aquifer storages because many salt structures are already known from oil and gas exploration and the investigation of salt as a raw material itself. Salt caverns are primarily used for the storage of seasonal reserves, trading storages, and as strategic reserves. Moreover, because they are very flexible with respect to injection and withdrawal cycles, they can also be used to cover daily demand peaks.
Artificially constructed salt caverns have been used for the storage of energy carriers for over 50 years—primarily to store fossil fuels such as natural gas, oil, and petroleum products (refined fuels, liquefied gas), but also for the storage of hydrogen and compressed air. Liquefied petroleum gas (LPG) as well as oil were stored in the first caverns in the United States and Europe in the 1950s. The first natural gas cavern was constructed in Marysville, Michigan (United States) in 1961 [31]. The first hydrogen cavern was constructed on Teesside in the United Kingdom in 1971–72 and is still in operation [32]. Today, there are more than 2000 salt caverns in North America and over 300 salt caverns in Germany used to store energy carriers [33,23].
The distribution of salt deposits worldwide is very localized (Fig. 6.6). In Europe, for instance, countries such as Germany, Denmark, and the Netherlands have a large amount of salt, with a great deal of additional expansion and enlargement potential for salt caverns, while other countries have very minor or no potential at all. However, it is feasible that countries with large potential for the construction of salt caverns can create capacities beyond their own national needs as part of an international storage system and thus make them available to other countries.
Figure 6.6 Map of underground salt deposits worldwide.
© KBB Underground Technologies GmbH.
© KBB Underground Technologies GmbH.
4.4. Rock Caverns
Rock caverns are mined underground using conventional mining techniques and consist of a system of shafts or ramps and drifts, forming cavities in solid rock deep underground, for example, in granite. Although just as stable, unlike rock salt, solid rock is not impervious to liquids, and especially gases, because of fractures within the rock. Sealing therefore has to be achieved by engineering measures.
The most widespread technology to create impervious rock caverns is hydrodynamic sealing, which is primarily used for liquid storage. Since the caverns are located deep underneath aquifers, small amounts of water flow into the storage space. The pressure of the water column prevents the stored medium from leaking out of the caverns through the fractures. However, the water has to be continuously pumped out of the lower part of unlined rock caverns. Since hydrodynamic technology can only build up limited operational pressures at economically acceptable depths for engineering the caverns, a concept using lined rock caverns was developed. Put simply, the rock caverns here are lined with thin sheets of stainless steel [20]. The rock then provides the structural stability of the storage against the gas pressure, while the metal lining ensures that the caverns are sealed.
Unlined rock caverns have been used to store liquid hydrocarbons, mainly in the United States and in Europe since the 1950s. Most of these have been realized in Scandinavia because of the presence of suitable geological formations in these countries. Other rock caverns have been constructed in the United States, Saudi Arabia, and East Asia. The feasibility of the lined rock cavern concept was verified in Skallen (Sweden) in 2004 in a relatively small pilot cavern [20].
Unlined and lined rock caverns have not been used so far for the storage of compressed air. They have, however, been the subject of scientific analysis for a long time analogous to other storage options [34–36]. A pilot plant for the adiabatic storage of compressed air is currently being constructed in Switzerland (Section 4.7).
Compressed air storage in rock caverns—particularly in lined rock caverns—could be interesting in future for countries which are not able to construct salt caverns but have adequate hard-rock potential. In a similar way to salt caverns, no reactions between the oxygen and the in situ constituents of the rock are expected in rock caverns, and flexible operations at high rates are also considered feasible.
4.5. Abandoned Mines
Abandoned mines, which were previously used for the extraction of commodities such as salt, ores, coal, or limestone can sometimes be used for storage of gases and liquids, depending on the local geological situation. Numerous abandoned mines with appropriate volumes and suitable depths exist worldwide. However, whether a mine is actually suitable for the construction of a storage facility largely depends on the imperviousness of the surrounding rock mass and the expense of creating a technical seal. The position and quality of the mined minerals are the priorities in conventional mining so that the resulting underground workings often only have limited suitability later on as compressed air storage facilities.
An abandoned mine was converted for the first time in Sweden at the end of the 1940s for storing liquid hydrocarbons. The storage of gaseous hydrocarbons followed in the 1960s. Compared with other storage options in deep underground geological formations, mines have only been used very rarely for the storage of gas. For instance, only one former mine in Europe is currently used for the storage of natural gas—the former Burggraf-Bernsdorf salt mine in Germany [20].
4.6. Conclusions
Unlike the storage space available in depleted oil and gas field, or an aquifer, which consists of a large number of microscopic interconnected pore spaces, caverns, and mines consist of one large open space. This means that caverns can be filled or emptied without having to counteract the resistance of a pore matrix to the flow of gases and are therefore particularly suitable for flexible storage operations with high flow rates and frequent cycles. Combined with lower specific costs a high level of imperviousness and large realizable volumes, this aspect makes salt caverns the best choice for the construction of future CAESs to balance out fluctuating wind and solar power. Existing CAES power plants in Germany and the United States therefore use salt caverns for the storage space. However, because suitable salt formations are highly localized [40] further research is needed to look at other options for the geological storage of compressed air.
4.7. Existing and Proposed Plants
Although only two CAES power plants are currently operated (discussed later), a great deal of work is being done on the realization of additional CAES power plants. The following list provides an overview of some of the plants in operation, in the planning stage, and those that have not come to fruition.
4.7.1. Huntorf (Germany)—in Operation
The world’s first CAES power plant began operations in Huntorf (Germany), approximately 40 km northwest of Bremen, in 1978—and is still in operation today. The company E.ON Kraftwerke GmbH controls and monitors the fully automatic plant from the Wilhelmshaven coal-fired power plant 50 km away [41]. When it first began operations the diabatic power plant had an output of 290 MW. This was raised to 321 MW in 2006 after comprehensive upgrading [41]. Two salt caverns with a total geometrical volume of 310 000 m3, lying at depths between (650 and 800) m, are used to store the compressed air [11]. A third salt cavern with a volume of 300 000 m3 is used to store the natural gas, which is required to heat up the air when it is released from the storage caverns [11]. The turbine can operate at full power within (6–9) min [41]. The CAES power plant in Huntorf has an efficiency of 42% [12].
4.7.2. McIntosh, Alabama (United States)—in Operation
The world’s second diabatic CAES power plant was commissioned in McIntosh, Alabama (United States) in 1991. One salt cavern with a volume of 538 000 m3 is used to store the compressed air [42,43]. An output of 110 MW is achievable within 14 min, adequate to provide electricity to approximately 110 000 homes [44]. By using the waste heat from the turbine the power plant has an efficiency of 54% [12].
4.7.3. Sesta (Italy)—Shut Down
4.7.4. Larne (United Kingdom)—under Construction
A diabatic CAES power plant with an output of (140–270) MW is being planned in Larne (United Kingdom). The current phase of the project involves geological exploration of the salt deposit [48].
4.7.5. Pollegio (Switzerland)—under Construction
ALACAES in Switzerland is currently constructing an adiabatic CAES pilot power plant in an abandoned tunnel in the Alps. Tests are scheduled to begin in the third-quarter of 2015 [49].
4.7.6. Bakersfield, California (United States)—in Planning
The project in Bakersfield, California (United States) involves storing compressed air in a depleted gas field. The aim is to construct a power plant with a capacity of 300 MW by 2020–21. A feasibility study will be completed by the end of 2015 and will form the basis for a decision on whether to continue the project [50].
4.7.7. Tennessee Colony, Texas (United States)—in Planning
The construction of a diabatic CAES power plant with an initial output of 317 MW is being planned in Tennessee Colony, Texas (United States). The power plant will have capacity to provide electricity to 300 000 households. Construction of the power plant, which uses a salt cavern to store the compressed air, is scheduled to begin in 2015. Commissioning is scheduled in 2018 [51].
4.7.8. Alberta (Canada)—in Planning
A CAES power plant with an output of (125–150) MW is being planned in Canada as part of the Alberta Saskatchewan Intertie Storage project (ASISt). Compressed air is scheduled to be stored in salt caverns [52].
4.7.9. Wesel (Germany)—in Planning
The Energy Storage Niederrhein project involves a CAES power plant in Germany, which is currently at the concept stage. Three salt caverns in Wesel, North Rhine-Westphalia are to be used for the storage of diabatic compressed air in a power plant, which is hoped to achieve an efficiency of (55–60)% [53].
4.7.10. Cheshire (United Kingdom)—in Planning
There are plans to erect a CAES power plant in Cheshire (United Kingdom) between 2014 and 2019. The first phase involves a pilot plant with an output of between (25 and 40) MW, which is then scheduled to be expanded to 500 MW for commercial operations. The efficiency of the power plant, which uses salt caverns as storages, is reported to be (70–75)% [54].
4.7.11. Millard County, Utah (United States)—in Planning
An ambitious CAES project is planned for commissioning in Millard County, Utah (United States) in 2023. Four salt caverns are planned as the (interim) storages for 2100 MW from a wind farm in the neighboring state of Wyoming. The state of California will be the final user of the energy. Power will be transmitted along power lines several hundreds of kilometers long [55].
4.7.12. Norton, Ohio (United States)—in Planning
A project that has been on the cards for several years aims to use a former limestone mine in Norton, Ohio (United States) to store compressed air with a volume of 10 × 106 m3 and an output of 2700 MW [46,43]. The project was postponed in 2013 according to reports in the media [56]. However, the project still remains unrealized even up to the present time.
4.7.13. Staßfurt (Germany)—in Planning
The company RWE AG, together with various partners from industry and science, has been working on the ADELE project since 2010. This involves development of an adiabatic CAES power plant, including the conception and realization of the world’s first pilot plant in Germany [57].
4.7.14. Iowa (United States)—Planning Suspended
A project for the construction of a CAES power plant using an aquifer structure to store the air has been pursued in Iowa (United States) for several years as part of the Iowa Stored Energy Park project (ISEP). The project was abandoned in 2011 after 8 years of planning activity when the results of geological investigations revealed that the planned capacity could not be achieved in the selected aquifer formations [58,59].
4.7.15. Donbas (Russia)—Construction Abandoned
Construction of a CAES power plant began in the Donbas region—650 km southwest of Volgograd—shortly before the collapse of the Soviet Union. The plant was scheduled to have an output of 1050 MW and was to use salt caverns to store compressed air. The work was abandoned with the collapse of the Soviet Union [47,60].
4.7.16. Gaines, Texas (United States)—Status Unclear
The Texas Dispatchable Wind Project was commissioned in late 2012. This adiabatic CAES power plant was scheduled to use a salt cavern [61]. The current status of the project remains unclear.
4.7.17. Columbia Hills, Washington (United States)—Status Unclear
A diabatic CAES power plant with a capacity of 207 MW has already been through the conceptual stage and was planned for Columbia Hills in the US state of Washington. Compressed air was scheduled to be stored in four wells drilled into a flood basalt formation. The status of the project is currently unclear [62,63].
4.7.18. Selah, Washington (United States)—Status Unclear
This project in Yakima Canyon in Selah, Washington (United States) brings together compressed air storage and geothermal energy. During the injection process of the adiabatic power plant the heat generated by compression is to be stored in molten salt, which is then used again to heat up the air when it is released from the underground—together with the energy from the geothermal plant, which is part of the overall power plant facility. The total output of the CAES plant is scheduled to be 83 MW [62,63]. The status of this project is also unclear.
5. Conclusions
The application of air compression to decouple energy absorption from the grid and energy consumption is known and has been practiced for decades. Two underground CAES power plants are in operation today, using salt caverns as storage. With the transition from a fossil fuel–based energy system to a system integrating more and more renewable energies, interest in this technology has been reawakened. Here the main focus is the development of new concept designs and the usage of additional underground formations beside salt caverns. Currently, no further underground CAES power plants are in operation, mainly for economic reasons or due to the need for more investigation into the suitability for proposed underground storage. The technology is available, and with the growth of renewable energy we can expect underground CAES power plants to be developed in suitable situations.
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