Environmental Engineering Reference
In-Depth Information
CAES facility, the switch from one operating mode to another is completely
automated and requires a minimum of 20 minutes during which time the
system is neither generating power nor compressing air [55]. The switchover
time may have a significant impact for balancing rapid fluctuations in wind
output. It is possible that alternative startup features such as use of an auxil-
iary startup motor could reduce this interval further [52].
Operation switchover time limitations may be eliminated altogether with
new system designs that decouple the compression and turbo expander
trains. By separating these components rather than linking them through a
common shaft via a clutch as in the McIntosh and Huntorf systems, direct
switching between compression and expansion operation is possible. This
change also means compressor size can be optimized independently of the
turbo expander design and permits standard production compressors to be
used in the system configuration [57].
Constant Volume and Constant Pressure
A CAES can operate in a number of ways depending on the type of geol-
ogy utilized for the storage reservoir. The most common mode is to operate
the CAES under constant volume conditions. This means that the storage
volume is a fixed, rigid reservoir operating over an appropriate pressure
range.* This mode of operation offers two design options: (1) it is possi-
ble to design such a system to allow the hp turbine inlet pressure to vary
with the cavern pressure (reducing output), or (2) keep the inlet pressure
of the hp turbine constant by throttling the upstream air to a fixed pres-
sure. Although this latter option requires a larger storage volume (due to
throttling losses), it has been pursued at both the existing CAES facilities
due to the increase in turbine efficiency attained for constant inlet pres-
sure operation. The Huntorf CAES is designed to throttle the cavern air to
46 bar at the hp turbine inlet (with caverns operating at 48 to 66 bar) and
the McIntosh system similarly throttles the incoming air to 45 bar (operat-
ing between 45 and 74 bar).
A third option is to keep the storage cavern at constant pressure through-
out operation by using a head of water applied by an above-ground reservoir
(see Figure 5.11). The use of compensated storage volumes minimizes losses
and improves system efficiency, but care must be taken to manage flow insta-
bilities in the water shaft such as the so-called champagne effect [64].
This technique is incompatible with salt-based caverns since a continual
flow of water would dissolve the cavern walls. Brine cycling with a compen-
sating column connected to a surface pond of saturated brine may be imple-
mented, but biological concerns and groundwater contamination issues
*
Although aquifer bubbles are not rigid bodies, the time scale at which the air-water inter-
faces migrate is much longer than CAES storage cycles and therefore porous rock systems
can be approximated as fixed-volume air reservoirs in this context.
