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# definition - Compressed_air_energy_storage

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# Compressed air energy storage

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.[1]

## Types

Compression of air generates heat; the air is warmer after compression. Expansion requires heat. If no extra heat is added, the air will be much colder after expansion. If the heat generated during compression can be stored and used during expansion, 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, but a German project ADELE is planned to enter development in 2013[2]. The theoretical efficiency of adiabatic storage approaches 100% with perfect 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 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, France; Birmingham, England; Rixdorf, Germany; Offenbach, Germany; Dresden, 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.

• 1978— The first utility-scale compressed air energy storage project was the 290 megawatt Huntorf plant in Germany using a salt dome.
• 1991— A 110 megawatt plant with a capacity of 26 hours was built in McIntosh, Alabama (1991). The Alabama facility's $65 million cost works out to$550 per Kilowatt hour of capacity, using a 19 million square foot solution mined salt cavern to store air at up to 1100 psi. Although the compression phase is approximately 82% efficient, the expansion phase requires combustion of natural gas at one third the rate of a gas turbine producing the same amount of electricity.[12][13][14]
• November 2009— The US Department of energy awards $24.9 million in matching funds for phase one of a 300 MW,$356 million Pacific Gas and Electric CAES installation utilizing a saline porous rock formation being developed near Bakersfield in Kern County, California. Goals of the project is to build and validate an advanced design.[15]

## 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).[22]

### 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].[23]

#### Existing hybrid systems

Huntorf, Germany in 1978, and McIntosh, Alabama in 1991 (USA) commissioned hybrid power plants.[8][24] 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."[24] Power output of the McIntosh and Iowa systems is in the range of 2–300 MW.[25]

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.[26]

### 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.[27] 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.[28]

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.[29] [30] 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.[31]

## References

1. ^ Wild, Matthew, L. Wind Drives Growing Use of Batteries, New York Times, July 28, 2010, pp.B1.
2. ^
3. ^ "German AACAES project information". Retrieved February 22, 2008.
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 August 27, 2006.
9. ^
10. ^ a b Chambers's Encyclopaedia: A Dictionary of Universal Knowledge. W. & R. Chambers, LTD. 1896. pp. 252–253. Retrieved January 7, 2009.
11. ^ Technische Mislukkingen by Lex Veldhoen & Jan van den Ende
12. ^ (pdf) Compressed Air Storage (CAES), Dresser-Rand Corporation, 2010, brochure form# 85230
13. ^ Wald, Matthew (September 29, 1991), Using Compressed Air To Store Up Electricity, New York Times
14. ^ CAES:McIntosh Power Plant, PowerSouth Energy Cooperative, 2010, retrieved April 15, 2012
15. ^ a b (pdf) ARRA Energy Storage Demonstrations, Sandia National Laboratories, retrieved April 13, 2012
16. ^ NYSEG considering Compressed Air Energy Storage, Energy Overviews Publishing, retrieved April 13, 2012
17. ^ Energy bags under the sea to be tested in 2011(Cleantechnica website). See in sections below.
18. ^ Heat loss of practical systems is explained in the #Thermodynamics of heat storage section.
19. ^ Air – Density and Specific Weight, The Engineering Toolbox
20. ^ Gas cylinders – High pressure cylinders for the on-board storage of natural gas as a fuel for automotive vehicles
21. ^ A History of the Torpedo The Early Days
22. ^ CryoEnergy System
23. ^ Energine PHEV-system schematic
24. ^ a b Pendick, Daniel (November 17, 2007). "Squeeze the breeze: Want to get more electricity from the wind? The key lies beneath our feet". New Scientist 195 (2623): 4. Retrieved November 17, 2007.
26. ^ http://www.firstenergycorp.com/NewsReleases/2009-11-23%20Norton%20Project.pdf
27. ^ "Wind plus compressed air equals efficient energy storage in Iowa proposal". Energy Services Bulletin website. Western Area Power Administration. Retrieved April 29, 2008.
28. ^ Prior art. Oliver Laing et al. Energy storage for off peak electricity. United States Patent No. 4873828.
29. ^ "Energy bags and super batteries". Nottingham University. June 18, 2008.
30. ^ "The man making 'wind bags'". BBC. March 26, 2008.
31. ^

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