Electrical Equipment - Wind Energy Systems for Electric Power Generation - page 74

be a set of motor-generator coupled to a pump and a turbine, for operation in one
dense of rotation. The other option is to use a motor-generator coupled to a hy-
draulic machine acting as pump or turbine, requiring reverse direction of speed.
In the latter method the resulting efficiency will be lower with synchronous ma-
chines operating at grid frequency. This disadvantage is overcome when by means
of a converter the speed is adjusted in pumping operation where the machine is
running asynchronously [Hin00]. Conversion losses and evaporation losses from
the exposed water are responsible allow in practice for overall energy efficiencies
of 70 ... 85%.
The energy density of pump storage systems is relatively low. The potential
energy of a water volume V in a reservoir with a water head H ( ρ = water den-

sity; g = earth surface acceleration) is described by:

E h = ρ gHV

Note that 1 m 3 of water elevated to 100 m represents a potential energy of
0,277 kWh. Consequently large volumes of water elevation differences are neces-
sary to store significant values of energy. Of course an appropriate geography is
most important for any water pump storage project.
A large number of pump storage systems, exploiting an upper reservoir with
either a lower reservoir or a flowing stream have been built all over the earth,
beginning around 1930. The individual rated power is covering a vast range of
80 ... 1800 MW.
A closer look may be thrown on the Goldisthal plant (2002). The upper reservoir
has a volume of 13 , 5 10 6 m 3 representing 8489 MWh. There are 4 machine sets,

2 of them synchronous for 256 MW each in pump and turbine operation; the other
2 are asynchronous machines for 265 MW each. The total rated power is specified
1060 MW.
Water pump storage is considered a means for equalizing the long time fluctua-
tions of wind energy systems.

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4.4.4.2 Compressed Air Storage Devices

Reversible compression and expansion of air is used in compressed air energy stor-
age (CAES) devices. Air is compressed by a motor driven compressor and stored in
an underground reservoir, preferably using caverns created by salt-deposits. When
discharging the compressed air is used to burn natural gas in combustors. In modern
concepts the heat gained during compression is used for prewarming. The resulting
combustion gas is then expanded in a two-stage gas turbine which drives the gener-
ator. The usual configuration is a one-shaft assembly with a single electric machine
as motor-generator unit. Figure 4.25 shows the concept.
Compressed air storage has been identified as a good combination to supply con-
trol energy in grids with a large percentage of installed wind energy systems. At
present only a few CAES plants are in practical use. Already commissioned in 1978