COMPRESSED-AIR ENERGY STORAGE
Compressed-air energy storage (CAES) is a system whereby energy is stored in the form of air pressurized above atmospheric pressure. Compressed air has a long history as a means of both storing and distributing energy. Systems based on this energy distribution medium were installed during the late 19th century in cities as various as Paris (France), Birmingham (U.K.), Dresden (Germany), and
Buenos Aries (Argentina) to supply power for industrial motors and commercial use in a variety of applications including the textile and printing industries.
The use of compressed-air storage as an adjunct to the power grid began with the construction of the Huntorf power plant that was built in Germany in 1978 but only operated commercially for 10 years. A second CAES plant was built by the Alabama Electric Cooperative in the United States and entered service in 1991. The latter facility has continued to provide storage services ever since. Details of these two plants are shown in Table 10.2.
In spite of being championed by organizations such as the U.S. Electric Power Research Institute, no further commercial project has ever been built, although others have been proposed and work even started on two. Even so, CAES remains of interest because it is the only other very large-scale energy storage system after pumped storage hydropower. Individual CAES plants are generally smaller than typical pumped storage plants but sites suitable for their construction are much more widespread than those for the hydropower storage plants. They could, therefore, provide a more widely distributed large- scale energy storage network.
Compressed-Air Energy Storage Principle
A CAES plant requires two principal components: a storage vessel in which compressed air can be stored without loss of pressure, and a compressor/ expander to charge the storage vessel and then extract the energy again (Figure 10.1). (The latter might, in fact, be a compressor and a separate expander.) In operation the plant is broadly analogous to the pumped storage hydropower plant. Surplus electricity is used to compress air with the compressor and the higher-pressure air is stored within the storage chamber. This stored energy can then be retrieved by allowing it to escape through the expander, an air turbine that is essentially a compressor operating in reverse. The expanding air drives the air turbine, which turns a generator to provide electrical power.
The compression and expansion functions of the CAES plant can be per- formed by the two primary components of a standard gas turbine. As seen in Chapter 4, the gas turbine comprises three components: a compressor, combustion chamber, and turbine. If the combustion chamber is removed and the two rotary components separated, then these can alternately use electricity to
compress air for storage and extract energy from it again to regenerate the stored power (Figure 10.2).
In practice, a slightly different arrangement is preferred that is closer still to the gas turbine. A rotary gas turbine compressor is used to compress air that is then stored in the storage chamber. When power is required, compressed air is extracted again and fed into a combustion chamber where it is mixed with fuel and ignited, generating a higher-pressure, higher-temperature thermodynamic fluid that is then used to drive the turbine stage of the plant.
Since a plant operated in this way requires natural gas or another fuel, it is not a straightforward energy storage system. However, the economics of this mode of operation appear to be the most attractive because it can generate more electricity than was used to store the compressed air. Additional generation is between 25% and 60% depending on the plant design. A further advantage is that the turbine stage of the plant does not have to drive the compressor as it would in a conventional gas turbine, so it can generate up to three times more power than it would when coupled to a compressor. Therefore, turbines for CAES plants are relatively smaller than for a similar generating capacity gas turbine.
Compressed-Air Storage Facilities
The most important part of a CAES plant is somewhere to store the compressed air. Small-scale CAES plants—with storage capacities up to 100 MWh and out- puts up to 20 MW—can use aboveground storage tanks built with steel pressure vessels but large, utility-scale plants need underground caverns in which to store the air. The natural gas industry has used underground storage caverns for years to store gas; these same caverns can provide ideal storage facilities for a CAES plant. However, the demand for such a cavern limits the development of CAES to places where such storage caverns are available.
A number of different types of underground caverns can be exploited. The most expensive is a human-made rock cavern excavated in hard rock or created by expanding existing underground mine workings. Such a site must be located in an impervious rock formation if it is to retain the compressed air without loss so the suitability of underground coal mines and limestone mines will depend on whether they are air-tight.
Salt caverns are another type of storage site, one that has been commonly used for gas storage. These are created within naturally occurring underground salt domes by drilling into the dome and pumping in water to dissolve and remove the salt to create an enclosure. Salt deposits suitable for such caverns occur in many parts of the world.
It is another type of geological structure, however, an underground porous
rock formation, which offers the cheapest underground storage facility. Structures of this type suitable for gas storage are found where a layer of porous rock is covered by an impervious rock barrier. Examples can be found in water- bearing aquifers, or in porous underground strata from which oil or gas have been extracted. Aquifers can be particularly attractive as storage media because the compressed air will displace water within the porous rock, setting up a constant-pressure storage system. With rock and salt caverns, in contrast, the pressure of the air will vary as more is added or released.
All three types of underground storage structures require sound rock formations to prevent the air from escaping. They also need to be sufficiently deep and strong to withstand the pressures imposed on them. It is important, particularly in porous-rock storage systems, that there are no minerals present that can deplete the oxygen in the air by reacting with it. Otherwise, the ability of the air to react with the fuel during combustion will be affected, reducing the power available during the generation phase of the storage generation cycle.
Underground rock structures capable of storing compressed air are often widely available. For example, a survey in the United States found accessible sites of different types across 80% of the country.
Turbine Technology and CAES Cycles
A CAES plant generally exploits standard gas turbine compressor and turbine technology, but because the two units operate independently, they can be sized differently to match the requirements of the plant. The larger the compressor compared to the turbine, the less time it requires to charge the cavern with a given amount of energy. The Hundorf plant that was built in Germany required 4 hours of compression to provide 1 hour of power generation, whereas the McIntosh plant in Alabama needs only 1.7 hours of compression for 1 hour of generation.
As a consequence of compression and generation being separated, a CAES plant turbine can operate well at part load as well as full load. More complex operation is also possible. The Alabama plant, for example, uses two turbine
stages with the exhaust from the last turbine is used to heat air from the cavern before it enters the first turbine. Fuel is not actually burned in the compressed air until it enters a combustion chamber between the first and second chambers.
Many of the refinements used to improve gas turbine performance outlined in Chapter 4 can be used in CAES plants too. For example, the compressor can be divided into two sections with air cooling between the stages to reduce its volume (intercooling) and heat from the turbine exhaust can be recovered and used to heat the compressed air extracted from the storage chamber (recuperation). Reheating, where the turbine is divided into two stages with an additional combustion chamber between stage one and stage two can also be applied versions of these latter two are used in the McIntosh plant, as noted before.
Mechanical components are never 100% efficient so there are consequential energy losses during compression and expansion in a CAES plant. There is also an additional source of energy loss. When air is compressed it generates heat and this heat energy is lost in a conventional CAES plant. A proposed additional refinement to the conventional mode of operation involves capturing this heat and storing it for use to heat the pressured air as it exits the storage chamber, before entering the turbine. This adiabatic cycle could theoretically be used to design a CAES plant that has no need for additional fossil fuel and that could achieve a roundtrip efficiency of 65%.
In principle a CAES plant could be of virtually any size and one proposed project would have had a generating capacity of 2400 MW. However in practice most schemes are likely to be smaller than this, in the tens or hundreds of megawatts range. Startup for the two plants that have operated was around 12 min but both could be brought into service in 5 min if necessary. Round trip efficiency without the use of additional fuel will be low for conventional CAES plants such as the two that have operated but, as noted, refinements could improve this.
Costs
There is little experience with CAES so any cost estimates must be considered tentative. However, it would appear to be an economically attractive option for energy storage. Proposals within the last 10 years or so for conventional CAES plants in the United States have had installed costs of $400–900/kW depending on size and storage type. An adiabatic plant is likely to be much more expensive, with potential costs as high as $1700/kW.