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||This article may be confusing or unclear to readers. (October 2010)|
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.
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.
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 and trams. Here the heat of compression is effectively stored in the atmosphere (or sea) and returned later on.
The storage vessel is often an underground cavern created by solution mining (salt is dissolved in water for extraction) or by utilizing an abandoned mine. Plants operate on a daily cycle, charging at night and discharging during the day.
City-wide compressed air energy systems have been built since 1870. 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. 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. 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.
In order to achieve a near thermodynamic reversible process so that most of the energy is saved in the system and can be retrieved, and losses are kept negligible, a near reversible isothermal process or an isentropic process is desired.
In an isothermal compression process, the gas in the system is kept at a constant temperature throughout. This necessarily requires removal of heat from the gas, which otherwise would experience a temperature rise due to the energy that has been added to the gas by the compressor. This heat removal can be achieved by heat exchangers (intercooling) between subsequent stages in the compressor. To avoid wasted energy, the intercoolers must be optimised for high heat transfer and low pressure drop. Naturally this is only an approximation to an isothermal compression, since the heating and compression occurs in discrete phases. Some smaller compressors can approximate isothermal compression even without intercooling, due to the relatively ratio of surface area to volume and the resulting improvement in heat dissipation from the compressor body itself.
To obtain a perfect isothermal storage process, we need the process to be reversible. This requires that the heat transfer between the surroundings and the gas occur over an infinitesimally small temperature difference. In that case, there is no exergy loss in the heat transfer process, and so the compression work can be completely recovered as expansion work: 100% storage efficiency. However, in practise, there is always a temperature difference in any heat transfer process, and so all practical energy storage obtains efficiencies significantly lower than 100%.
To estimate the compression/expansion work in an isothermal process, we can assume that the compressed air obeys the ideal gas law,
From a process from an initial state A to a final state B, with absolute temperature constant, we find that the work required for compression (negative) or done by the expansion (positive), to be
How much energy can we store in a 1m³ storage vessel at a pressure of 7 000 000 Pa (=70 bar), if the ambient pressure is 100 000 Pa (=1 bar) In this case, the process work is
The negative sign means that work is done on the gas by the surroundings. Process irreversibilities (such as in heat transfer) will result in less energy being recovered from the expansion process than is required for the compression process. If the environment is at a constant temperature, for example, the thermal resistance in the intercoolers will mean that the compression occurs at a temperature somewhat higher than the ambient temperature, and the expansion will occur at a temperature somewhat lower than ambient temperature. So a perfect isothermal storage system is impossible to achieve.
An adiabatic process is one where there is no heat transfer between the fluid and the surroundings: the system is insulated against heat transfer. If the process is furthermore internally reversible (smooth, slow and frictionless, to the ideal limit) then it will additionally be isentropic.
An adiabatic storage system does away with the intercooling during the compression process, and simply allows the gas to heat up during compression, and likewise to cool down during expansion. This is attractive, since the energy losses associated with the heat transfer are avoided, but the downside is that the storage vessel must be insulated against heat loss. It should also be mentioned that real compressors and turbines are not isentropic, but instead have an isentropic efficiency of around 85%, with the result that round-trip storage efficiency for adiabatic systems is also considerably less than perfect.
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 easily insulated, compressed adiabatically with little temperature change (approaching a reversible isothermal system) and heat loss (approaching an isentropic system). This advantage is in addition to the low cost of constructing the gas storage system, using the underground walls to assist in containing the pressure.
Recently there have been developed undersea insulated air bags, with similar thermodynamic properties to large underground cavern storage.
In order to use air storage in vehicles or aircraft for practical land or air transportation, the energy storage system must be compact and lightweight. Energy density is the engineering term that defines these desired qualities.
As explained in the thermodynamics of gas storage section above, 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, or ideal thermal insulation.
Nevertheless, as stated above, 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 (200 bar), 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 heat loss to the ambient and to unrecoverable internal gas heat. 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. 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.
Advanced fiber-reinforced bottles are comparable to the rechargeable lead-acid battery in terms of energy density. 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.
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. High pressure bottles are fairly strong so that they generally do not rupture in vehicle crashes.
The following methods can increase efficiency:
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.
Additional heat can be supplied by burning fuel as in 1904 for Whitehead's torpedoes. 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.
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.
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.
Huntorf, Germany in 1978, and McIntosh, Alabama in 1991 (USA) commissioned hybrid power plants. 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.
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." Power output of the McIntosh and Iowa systems is in the range of 2–300 MW.
Deep water in lakes and the ocean can provide pressure without requiring high-pressure vessels or drilling into salt caverns or aquifers. 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.
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.  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.