Compressed-air-accumulating power plant

The invention:



Plants that can be used to store energy in the form

of compressed air when electric power demand is low and use it

to produce energy when power demand is high.



The organization behind the invention:



Nordwestdeutsche Kraftwerke, a Germany company









Power, Energy Storage, and Compressed Air



Energy, which can be defined as the capacity to do work, is essential

to all aspects of modern life. One familiar kind of energy, which

is produced in huge amounts by power companies, is electrical energy,

or electricity. Most electricity is produced in a process that consists

of two steps. First, a fossil fuel such as coal is burned and the resulting

heat is used to make steam. Then, the steam is used to

operate a turbine system that produces electricity. Electricity has

myriad applications, including the operation of heaters, home appliances

of many kinds, industrial machinery, computers, and artificial

illumination systems.

An essential feature of electricity manufacture is the production

of the particular amount of electricity that is needed at a given time.

If moment-to-moment energy requirements are not met, the city or

locality involved will experience a “blackout,” the most obvious

feature of which is the loss of electrical lighting. To prevent blackouts,

it is essential to store extra electricity at times when power production

exceeds power demands. Then, when power demands exceed

the capacity to make energy by normal means, stored energy

can be used to make up the difference.

One successful modern procedure for such storage is the compressed-

air-accumulation process, pioneered by the Nordwestdeutsche

Kraftwerke company’s compressed-air-accumulating power

plant, which opened in December, 1978. The plant, which is

located in Huntorf, Germany (at the time, West Germany), makes

compressed air during periods of low electricity demand, stores the

air in an underground cavern, and uses it to produce extra electricity

during periods of high demand.



Plant Operation and Components



The German 300-megawatt compressed-air-accumulating power

plant in Huntorf produces extra electricity from stored compressed

air that will provide up to four hours per day of local peak electricity

needs. The energy-storage process, which is vital to meeting very

high peak electric power demands, is viable for electric power

plants whose total usual electric outputs range from 25 megawatts

to the 300 megawatts produced at Huntorf. It has been suggested,

however, that the process is most suitable for 25- to 50-megawatt

plants.

The energy-storage procedure used at Huntorf is quite simple.

All the surplus electricity that is made in nonpeak-demand periods

is utilized to drive an air compressor. The compressor pumps air

from the surrounding atmosphere into an airtight underground

storage cavern. When extra electricity is required, the stored compressed

air is released and passed through a heating unit to be

warmed, after which it is used to run gas-turbine systems that produce

electricity. This sequence of events is the same as that used in

any gas-turbine generating system; the only difference is that the

compressed air can be stored for any desired period of time rather

than having to be used immediately.

One requirement of any compressed-air-accumulating power

plant is an underground storage chamber. The Huntorf plant utilizes

a cavern that was hollowed out some 450 meters below the surface

of the earth. The cavern was created by drilling a hole into an

underground salt deposit and pumping in water. The water dissolved

the salt, and the resultant saltwater solution (brine) was

pumped out of the deposit. The process of pumping in water and removing

brine was continued until the cavern reached the desired

size. This type of storage cavern is virtually leak-free. The preparation

of such underwater salt-dome caverns has been performed

roughly since the middle of the twentieth century. Until the Huntorf

endeavor, such caves were used to stockpile petroleum and natural

gas for later use. It is also possible to use mined, hard-rock caverns

for compressed-air accumulation when it is necessary to compress

air to pressures higher than those that can be maintained effectively

in a salt-dome cavern.

The essential machinery that must be added to conventional

power plants to turn them into compressed-air-accumulating power

plants are motor-driven air compressors and gas turbine generating

systems. This equipment must be connected appropriately so that

in the storage mode, the overall system will compress air for storage

in the underground cavern, and in the power-production mode, the

system will produce electricity from the stored compressed air.

Large compressed-air-accumulating power plants require specially

constructed machinery. For example, the compressors that

are used at Huntorf were developed specifically for that plant by

Sulzer, a Swiss company. When the capacity of such plants is no

higher than 50 megawatts, however, standard, readily available

components can be used. This means that relatively small compressed-

air-accumulating power plants can be constructed for a reasonable

cost.





Consequences



The development of compressed-air-accumulating power plants

has had a significant impact on the electric power industry, adding to

its capacity to store energy. The main storage methods available prior

to the development of compressed-air-accumulation methodology

were batteries and water that was pumped uphill (hydro-storage). Battery

technology is expensive, and its capacity is insufficient for major,

long-term power storage. Hydro-storage is a more viable technology.

Compressed-air energy-storage systems have several advantages

over hydro-storage. First, they can be used in areas where flat terrain

makes it impossible to use hydro-storage. Second, compressedair

storage is more efficient than hydro-storage. Finally, the fact that

standard plant components can be used, along with several other

factors, means that 25- to 50-megawatt compressed-air storage plants

can be constructed much more quickly and cheaply than comparable

hydro-storage plants.

The attractiveness of compressed-air-accumulating power plants

has motivated efforts to develop hard-rock cavern construction

techniques that cut costs and make it possible to use high-pressure

air storage. In addition, aquifers (underground strata of porous rock

that normally hold groundwater) have been used successfully for

compressed-air storage. It is expected that compressed-air-accumulating

power plants will be widely used in the future, which will

help to decrease pollution and cut the use of fossil fuels.





See also : Alkaline storage battery; Breeder reactor; Fuel cell; Geothermal

power; Heat pump; Nuclear power plant; Tidal power plant.