Wind energy storage technologies - Wind Power Generation and Wind Turbine Design - page 671

required is so small that a GT working simultaneously with CAES can produce
three times more electricity than a GT operating on its own, using the same amount
of natural gas.
The reservoir can be man-made but this is expensive so CAES locations are
usually decided by identifying natural geological formations that suit these facili-
ties. These include salt-caverns, hard-rock caverns, depleted gas fi elds or an aqui-
fer. Salt-caverns can be designed to suit specifi c requirements. Fresh water is
pumped into the cavern and left until the salt dissolves and saturates the fresh
water. The water is then returned to the surface and the process is repeated until the
required volume cavern is created. This process is expensive and can take up to
2 years. The costs associated with hard-rock caverns are likely to be more than
50% higher. Finally, aquifers cannot store the air at high pressures and therefore
have a relatively lower energy capacity.
CAES uses both electrical energy and natural gas so its effi ciency is diffi cult to
predict. It is estimated that the effi ciency of the entire cycle is in the region of 64%
[11] to 75% [3]. Typical plant capacities for CAES are in the region of 50-300 MW.
The life of these facilities is proving to be far longer than existing GTs and the
charge/discharge ratio is dependent on the size of the compressor used, as well as
the size and pressure of the reservoir.
4.3.1 Applications of CAES
CAES is the only very large-scale storage technique other than PHES. CAES
has a fast reaction time with plants usually able to go from 0 to 100% in less
than 10 min, 10 to 100% in approximately 4 min and 50 to 100% in less than
15 s [2]. As a result, it is ideal for acting as a large sink for bulk energy supply
and demand and also, it is able to undertake frequent start-ups and shutdowns.
Furthermore, traditional GT suffer a 10% effi ciency reduction from a 5°C rise
in ambient temperatures due to a reduction in the air density. CAES use com-
pressed air so they do not suffer from this effect. Also, traditional GTs suffer
from excessive heat when operating on partial load, while CAES facilities do
not. These fl exibilities mean that CAES can be used for ancillary services such
as frequency regulation, load following, and voltage control [3]. As a result,
CAES has become a serious contender in the wind power energy storage mar-
ket. A number of possibilities are being considered such as integrating a CAES
facility with a number of wind farms within the same region. The excess off-
peak power from these wind farms could be used to compress air for a CAES
facility. Iowa Association of Municipal Utilities is currently planning a project
of this nature [12].
4.3.2 Cost of CAES
The cost of CAES facilities are $425/kW [2] to $450/kW [3]. Maintenance is esti-
mated between $3/kWh [13] and $10/kWh [14]. Costs are largely dependent on
the reservoir construction. Overall, CAES facilities expect to have costs similar to
or greater than conventional GT facilities. However, the energy cost is much lower
for CAES systems.