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.