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development cost estimates for natural gas storage in similar formations [50].
This implies a capital cost of $2 to $7/kWh of storage capacity, depending on
the site characteristics and assuming a 5:1 base gas-to-working gas volume
ratio [51]. These costs are somewhat lower than those estimated for salt cav-
ern storage ($6 to $10/kWh of storage capacity)—the next most economical
option (see Table 5.1).
Aquifer CAES has the further advantage that the cost of incremental addi-
tions to storage capacity is significantly lower than for alternative geologies.
Assuming sufficient wells are in place to ensure adequate air flow to the sur-
face turbo machinery, the cost of increasing the storage capacity of an aqui-
fer is simply the compression energy required to increase the volume of the
bubble [52]. This cost (~$0.11/kWh) is an order of magnitude lower than the
equivalent marginal costs of solution mining salt and more than two orders
smaller than excavating additional cavern volume from hard rock [1].
Despite these low development costs and apparent widespread availabil-
ity, extensive characterization of candidate formations is required to deter-
mine project feasibility. Detailed measurements of permeability, porosity,
and structure are required to determine a formation's suitability for storage
operation [52]. Prior industrial experience with natural gas storage will be
valuable as many of the methodologies used to characterize formations and
develop projects are directly applicable to CAES development in an aqui-
fer [53]. The industry's extensive experience with natural gas storage pro-
vides a theoretical and practical framework for describing underground
storage media and assessing candidate sites for seasonal storage of natural
gas. Storage capacity assessments for CO 2 storage may be helpful as well,
although minimum depth required for CO 2 to become supercritical (~800 m)
is typically at the high end of acceptable limits for CAES due to high pres-
sure turbine inlet limitations.
TABLE 5.1
Capital Costs for Energy Storage Options
CapitalCost:
Capacity($/kW)
CapitalCost:
Energy($/kWh)
Hoursof
Storage
TotalCapital
Cost($/kW)
Technology
CAES (300 MW)
580
1.75
40
650
Pumped hydroelectric
(1,000 MW)
600
37.5
10
975
Sodium sulfur battery
(10 MW)
1720 to 1860
180 to 210
6 to 9
3100 to 3400
Vanadium redox
battery (10 MW)
2410 to 2550
240 to 340
5 to 8
4300 to 4500
Sources: Electric Power Research Institute and U.S. Department of Energy. 2003. Handbook of
Energy Storage for Transmission and Distribution Applications . Palo Alto, CA and
Washington; Electric Power Research Institute and U.S. Department of Energy. 2004.
Energy Storage for Grid-Connected Wind Generation Applications . Palo Alto, CA and
Washington; Electric Power Research Institute. 2005. Wind Power Integration: Energy
Storage for Firming and Shaping , Palo Alto, CA.
 
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