# Compressed air energy storage

Conceptual representation of the compressed-air energy storage concept. Off-peak (low-cost) electrical power compresses air into an underground air-storage “vessel” (the Norton mine), and later the air feeds a gas-fired turbine generator complex to generate electricity during on-peak (high-price) times. A similar concept [1] uses wind powered air compressors.

Compressed Air Energy Storage (CAES) is a way to store energy generated at one time for use at another time. At utility scale, energy generated during periods of low energy demand (off-peak) can be released to meet higher demand (peak load) periods.[2]

## Types

Compression of air generates a lot of heat. The air is warmer after compression. Decompression requires heat. If no extra heat is added, the air will be much colder after decompression. If the heat generated during compression can be stored and used again during decompression, the efficiency of the storage improves considerably.

There are three ways in which a CAES system can deal with the heat. Air storage can be adiabatic, diabatic, or isothermic:

• Adiabatic storage retains the heat produced by compression and returns it to the air when the air is expanded to generate power. This is a subject of ongoing study, with no utility scale plants as of 2010. Its theoretical efficiency approaches 100% for large and/or rapidly cycled devices and/or perfect thermal insulation, but in practice round trip efficiency is expected to be 70%.[3] Heat can be stored in a solid such as concrete or stone, or more likely in a fluid such as hot oil (up to 300 °C) or molten salt solutions (600 °C).
• Diabatic storage dissipates the extra heat with intercoolers (thus approaching isothermal compression) into the atmosphere as waste. Upon removal from storage, the air must be re-heated prior to expansion in the turbine to power a generator which can be accomplished with a natural gas fired burner for utility grade storage or with a heated metal mass. The lost heat degrades efficiency, but this approach is simpler and is thus far the only system which has been implemented commercially. The McIntosh, Alabama CAES plant requires 2.5 MJ of electricity and 1.2 MJ lower heating value (LHV) of gas for each megajoule of energy output.[4] A General Electric 7FA 2x1 combined cycle plant, one of the most efficient natural gas plants in operation, uses 6.6 MJ (LHV) of gas per kW–h generated,[5] a 54% thermal efficiency comparable to the McIntosh 6.8 MJ, at 53% thermal efficiency.
• Isothermal compression and expansion approaches attempt to maintain operating temperature by constant heat exchange to the environment. They are only practical for low power levels, without very effective heat exchangers. The theoretical efficiency of isothermal energy storage approaches 100% for small and/or slowly cycled devices and/or perfect heat transfer to the environment. In practice neither of these perfect thermodynamic cycles are obtainable, as some heat losses are unavoidable.

A different, highly efficient arrangement, which fits neatly into none of the above categories, uses high, medium and low pressure pistons in series, with each stage followed by an airblast venturi pump that draws ambient air over an air-to-air (or air-to-seawater) heat exchanger between each expansion stage. Early compressed air torpedo designs used a similar approach, substituting seawater for air. The venturi warms the exhaust of the preceding stage and admits this preheated air to the following stage. This approach was widely adopted in various compressed air vehicles such as H. K. Porter, Inc's mining locomotives[6] and trams.[7] Here the heat of compression is effectively stored in the atmosphere (or sea) and returned later on.

Compression can be done with electrically powered turbo-compressors and expansion with turbo 'expanders'[8] or air engines driving electrical generators to produce electricity.

The storage vessel is often an underground cavern created by solution mining (salt is dissolved in water for extraction)[9] or by utilizing an abandoned mine. Plants operate on a daily cycle, charging at night and discharging during the day.

Compressed air energy storage can also be employed on a smaller scale such as exploited by air cars and air-driven locomotives, and also by the use of high-strength carbon-fiber air storage tanks.

## History

City-wide compressed air energy systems have been built since 1870.[10] Cities such as Paris, Birmingham, England, Rixdorf, Offenbach, Dresden in Germany and Buenos Aires, Argentina installed such systems. Victor Popp constructed the first systems to power clocks by sending a pulse of air every minute to change the pointer. They quickly evolved to deliver power to homes and industry.[11] As of 1896, the Paris system had 2.2 MW of generation distributed at 550 kPa in 50 km of air pipes for motors in light and heavy industry. Usage was measured by meters.[10] The systems were the main source of house-delivered energy in these days and also powered the machines of dentists, seamstresses, printing facilities and bakeries. The first utility-scale compressed air energy storage project was the 290 megawatt Huntorf plant in Germany (1978). The second was the 110 megawatt McIntosh plant in Alabama (1991). Both of these projects use salt domes for air caverns. Currently under development is the Iowa Stored Energy Park, a 270 megawatt project which will use aquifer-based air storage. A 300 MW project utilizing depleted gas storage is being developed in California, and a 150 MW salt-based project is under development in upstate New York. The first adiabatic CAES project, a 200 megawatt facility called ADELE, is planned for construction in Germany in 2013.

## Isothermal storage physics

The isothermal process maintains constant temperature. The heat that compression generates must flow to the environment for the temperature to remain constant. In practice this is often not the case, because proper intercooling requires a compact internal heat exchanger that is optimized for high heat transfer and low pressure drop. Without an internal heat exchanger, isothermal compression can be approximated only at low flow rates, particularly for small systems. Small compressors have higher inherent heat exchange, due to a higher ratio of surface area to volume. Nevertheless it is useful to describe the limiting case of ideal isothermal compression of an ideal gas:

The ideal gas law for an isothermal process is:

$PV=nRT=\operatorname{constant}$

By the definition of work, where A and B are the initial and final states of the system:

$W_{A\to B}=\int_{V_A}^{V_B} P\,dV =\int_{V_A}^{V_B} \frac{nRT}{V}\,dV = nRT\int_{V_A}^{V_B} \frac{1}{V}\,dV$
$= nRT(\ln{V_B}-\ln{V_A}) = nRT\ln{\frac{V_B}{V_A}} = nRT\ln{\frac{P_A}{P_B}} = PV\ln{\frac{P_A}{P_B}}$

where, PAVA = PBVB , and so, $\frac{V_B}{V_A} = \frac{P_A}{P_B}$

$\ P$ is the absolute pressure,
$\ V$ is the volume of the vessel,
$\ n$ is the amount of substance of gas,
$\ R$ is the ideal gas constant,
$\ T$ is the absolute temperature,
$\ W$ is the energy stored or released.

This amounts to about 2.271 ln(PA/PB) kJ at 0 °C (273 K) or 2.478 ln(PA/PB) kilojoules (kJ) at 25 °C (298 K), per mole, or simply 100 ln(PA/PB) kJ/m³ of gas (at 0.1 megapascal = approx. atmospheric pressure).

Isothermal processes are thermodynamically reversible. To the extent the compression process is isothermal, its efficiency approaches 100%.[12] The above equation represents the maximum energy that can be stored. To be more accurate, that energy isn't stored into compressed air. Instead the energy given to compress the air is converted into heat that is expelled to the ambient. So the energy is stored into the ambient while the air is just the vehicle to do that. This is true because according to thermodynamics ( for the case of a perfect gas ) the internal energy of the gas is given by E=(df/2)*N*k*T where df are the degrees of freedom ( 3 for a monoatomic gas, 5 for a biatomic), N the number of particles, k the Boltzmann constant. So a isothermal compression just diminishes the entropy of air while converting work to heat that is stored in the ambient. Inversely that air may be used at a later time to convert heat into work. But the heat must be taken from the ambient or somewhere else. In practice, no process is perfectly isothermal and the compressors and motors will have heat-related energy losses.

Under adiabatic compression, some of the compression work goes into heating the gas. If this heat is then lost to the surroundings, and the same quantity of heat is not added to the gas upon expansion, efficiency is reduced. Energy storage systems often use large underground caverns. This is the preferred system design, due to the very large volume, and thus the large quantity of energy that can be stored with only a small pressure change. The cavern space can be compressed adiabatically with little temperature change and heat loss.

## Practical constraints in transportation

### Energy density and efficiency

Compressing air heats it and expanding it cools it. Therefore practical air engines require heat exchangers in order to avoid excessively high or low temperatures and even so don't reach ideal constant temperature conditions. Nevertheless it is useful to describe the maximum energy storable using the isothermal case, which works out to about 100 kJ/m3 [ ln(PA/PB)]. Thus if 1.0 m3 of ambient air is very slowly compressed into a 5 L bottle at 20 MPa, the potential energy stored is 530 kJ. A highly efficient air motor can transfer this into kinetic energy if it runs very slowly and manages to expand the air from its initial 20 MPa pressure down to 100 kPa (bottle completely "empty" at ambient pressure). Achieving high efficiency is a technical challenge both due to nonlinear energy storage and thermodynamic considerations.

If the bottle above is emptied to 1 MPa, the extractable energy is about 300 kJ at the motor shaft.

A standard 20 MPa, 5 L steel bottle has a mass of 7.5 kg, a superior one 5 kg. High-tensile strength fibers such as carbon-fiber or Kevlar can weigh below 2 kg in this size, consistent with the legal safety codes. One cubic meter of air at 20°C has a mass of 1.225 kg.[13] Thus, theoretical energy densities are from roughly 70 kJ/kg at the motor shaft for a plain steel bottle to 180 kJ/kg for an advanced fiber-wound one, whereas practical achievable energy densities for the same containers would be from 40 to 100 kJ/kg.

### Comparison with batteries

Advanced fiber-reinforced bottles are comparable to the rechargeable lead-acid battery in terms of energy density. Advanced battery systems are several times better.[2] Batteries also provide nearly constant voltage over their entire charge level, whereas the pressure varies greatly while using a pressure vessel from full to empty. It is technically challenging to design air engines to maintain high efficiency and sufficient power over a wide range of pressures. Compressed air can transfer power at very high flux rates, which meets the principal acceleration and deceleration objectives of transportation systems, particularly for hybrid vehicles.

Compressed air systems have advantages over conventional batteries including longer lifetimes of pressure vessels and lower material toxicity. Newer battery designs such as those based on Lithium Iron Phosphate chemistry suffer from neither of these problems. Compressed air costs are potentially lower; however advanced pressure vessels are costly to develop and safety-test and at present are more expensive than mass-produced batteries.

As with electric storage technology, compressed air is only as "clean" as the source of the energy that it stores. Life cycle assessment addresses the question of overall emissions from a given energy storage technology combined with a given mix of generation on a power grid.

### Safety

As with most technologies, compressed air has safety concerns, mainly catastrophic tank rupture. Safety codes make this a rare occurrence at the cost of higher weight. Codes may limit the legal working pressure to less than 40% of the rupture pressure for steel bottles (safety factor of 2.5), and less than 20% for fiber-wound bottles (safety factor of 5). Commercial designs adopt the ISO 11439 standard.[14] High pressure bottles are fairly strong so that they generally do not rupture in vehicle crashes.

## Vehicle applications

### History

Air engines have been used since the 19th century to power mine locomotives, pumps, drills and trams, via centralized, city-level, distribution.

A compressed air locomotive by H. K. Porter, Inc., in use at the Homestake Mine between 1928 and 1961.

Racecars use compressed air to start its internal combustion engine (ICE).

Many people have been working on the idea of compressed air vehicles with renewed interest since the 1990 oil price shock.[citation needed]

### Engine

A compressed air engine uses the expansion of compressed air to drive the pistons of an engine, turn the axle, or to drive a turbine.

The following methods can increase efficiency:

• A continuous expansion turbine at high efficiency
• Multiple expansion stages
• Use of waste heat, notably in a hybrid heat engine design
• Use of environmental heat

A highly efficient arrangement uses high, medium and low pressure pistons in series, with each stage followed by an airblast venturi that draws ambient air over an air-to-air heat exchanger. This warms the exhaust of the preceding stage and admits this preheated air to the following stage. The only exhaust gas from each stage is cold air which can be as cold as −15 °C (5 °F); the cold air may be used for air conditioning in a car.[7]

Additional heat can be supplied by burning fuel as in 1904 for Whitehead's torpedoes.[15] This improves the range and speed available for a given tank volume at the cost of the additional fuel.

As an alternative to pistons or turbines, the Quasiturbine is also capable of running on compressed air, and is thus also a compressed air engine.

#### Cars

Since about 1990 several companies have claimed to be developing compressed air cars, but none are available. Typically the main claimed advantages are: no roadside pollution, low cost, use of cooking oil for lubrication, and integrated air conditioning.

The time required to refill a depleted tank is important for vehicle applications. "Volume transfer" moves pre-compressed air from a stationary tank to the vehicle tank almost instantaneously. Alternatively, a stationary or on-board compressor can compress air on demand, possibly requiring several hours. The cost of driving such car is typically projected to be around €0.75 per 100 kilometres (62 mi) with a complete refill at a service station costing about US\$3.[citation needed]

## Types of systems

### Cryogenic systems

A special CAES system has been created that uses liquid air as an energy carrier. This system is called Highview Power Storage's CryoEnergy System (CES).[16]

### Hybrid systems

Brayton cycle engines compress and heat air with a fuel suitable for an internal combustion engine. For example, natural gas or biogas heat compressed air, and then a conventional gas turbine engine or the rear portion of a jet engine expands it to produce work.

Compressed air engines can recharge an electric battery. The apparently defunct Energine promoted its Pne-PHEV or Pneumatic Plug-in Hybrid Electric Vehicle-system)[citation needed].[17]

#### Existing hybrid systems

Huntorf, Germany in 1978, and McIntosh, Alabama in 1991 (USA) commissioned hybrid power plants.[8][18] Both systems use off-peak energy for air compression. The McIntosh plant achieves its 24-hour operating cycle by burning a natural gas/compressed air mix.

#### Future hybrid systems

The Iowa Stored Energy Park (ISEP) will use aquifer storage rather than cavern storage. The displacement of water in the aquifer results in regulation of the air pressure by the constant hydrostatic pressure of the water. A spokesperson for ISEP claims, "you can optimize your equipment for better efficiency if you have a constant pressure."[18] Power output of the McIntosh and Iowa systems is in the range of 2–300 MW.[19]

Additional facilities are under development in Norton, Ohio. FirstEnergy, an Akron, Ohio electric utility obtained development rights to the 2,700 MW Norton project in November, 2009.[20]

### Lake or ocean storage

Deep water in lakes and the ocean can provide pressure without requiring high-pressure vessels or drilling into salt caverns or aquifers.[21] The air goes into inexpensive, flexible containers such as plastic bags below in deep lakes or off sea coasts with steep drop-offs. Obstacles include the limited number of suitable locations and the need for high-pressure pipelines between the surface and the containers. Since the containers would be very inexpensive, the need for great pressure (and great depth) may not be as important. A key benefit of systems built on this concept is that charge and discharge pressures are a constant function of depth. Carnot inefficiencies can thereby be reduced in the power plant. Carnot efficiency can be increased by using multiple charge and discharge stages and using inexpensive heat sources and sinks such as cold water from rivers or hot water from solar ponds. Ideally, the system must be very clever—for example, by cooling air before pumping on summer days. It must be engineered to avoid inefficiency, such as wasteful pressure changes caused by inadequate piping diameter.[22]

A nearly isobaric solution is possible if the compressed gas is used to drive a hydroelectric system. However, this solution requires large pressure tanks located on land (as well as the underwater air bags). Also, hydrogen gas is the preferred fluid, since other gases suffer from substantial hydrostatic pressures at even relatively modest depths (such as 500 meters).

The University of Nottingham is one centre of research on seabed–anchored energy bags. E.ON, one of Europe's leading power and gas companies, has provided €1.4 million (£1.1 million) in funding to develop undersea air storage bags.[23] [24] Hydrostor in Canada is developing a commercial system of underwater storage "accumulators" for compressed air energy storage, starting at the 1 to 4 MW scale. [25]

## References

1. ^ http://www.energytower.org/cawegs.html
2. ^ a b Wild, Matthew, L. Wind Drives Growing Use of Batteries, New York Times, July 28, 2010, pp.B1.
3. ^ "German AACAES project information". Retrieved 2008-02-22.
4. ^ http://my.epri.com/portal/server.pt?Abstract_id=TR-101751-V2
5. ^ http://www.westgov.org/wieb/electric/Transmission%20Protocol/SSG-WI/pnw_5pp_02.pdf
6. ^ Compressed-Air Propulsion
7. ^ a b 3-stage propulsion with intermediate heating
8. ^ a b "Distributed Energy Program: Compressed Air Energy Storage". United States Department of Energy. Retrieved 2006-08-27.
10. ^ a b Chambers's Encyclopaedia: A Dictionary of Universal Knowledge. W. & R. Chambers, LTD. 1896. pp. 252–253. Retrieved 2009-01-07.
11. ^ Technische Mislukkingen by Lex Veldhoen & Jan van den Ende
12. ^ http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node33.html
13. ^ Air - Density and Specific Weight, The Engineering Toolbox
14. ^ Gas cylinders -- High pressure cylinders for the on-board storage of natural gas as a fuel for automotive vehicles
15. ^ A History of the Torpedo The Early Days
16. ^ CryoEnergy System
17. ^ Energine PHEV-system schematic
18. ^ a b Pendick, Daniel (2007-11-17). "Squeeze the breeze: Want to get more electricity from the wind? The key lies beneath our feet". New Scientist 195 (2623): 4. Retrieved 2007-11-17.
20. ^ http://www.firstenergycorp.com/NewsReleases/2009-11-23%20Norton%20Project.pdf
21. ^ "Wind plus compressed air equals efficient energy storage in Iowa proposal". Energy Services Bulletin website. Western Area Power Administration. Retrieved 2008-04-29.
22. ^ Prior art. Oliver Laing et al. Energy storage for off peak electricity. United States Patent No. 4873828.
23. ^ "Energy bags and super batteries". Nottingham University. 2008-06-18.
24. ^
25. ^

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