Toward optimization of a wind/
compressed air energy storage
(CAES) power system
Jeffery B. Greenblatt
Samir Succar
David C. Denkenberger
Robert H. Williams
Princeton University, Princeton, NJ 08544
Guyot Hall, (609) 258-7442 / 7715 FAX, jgreenbl@princeton.edu
Electric Power Conference, Baltimore, MD, 30 March – 1 April 2004
Session 11D (Wind Power II), 1 April 2004
Foote Creek Rim, Wyoming
Energy in a Greenhouse World
• U.S. 2001 electricity use: 3,600 TWh. Only 0.5% was
wind-generated.
• U.S. wind potential: ~10,600 TWh/y Possible major
role in climate-change mitigation…under carbon
constraint, can wind compete with coal?
• Resource concentrated in sparsely populated Great
Plains—exploitation requires getting wind power to
distant population centers
Sources: AWEA, 2003; EIA, 2003; EWEA, 2001, Wind Force 12
Does wind power need storage?
2. Offset declining capacity
value of wind power as
market share expands
Time
Time
Value
1. Make wind dispatchable
(price arbitrage; even at
small wind market share)
Power
Three roles for storage:
Market share
3. Facilitate use of remote, high-quality
wind resources by reducing
transmission costs—role first advanced
by Cavallo (1995)
Electric storage options
Cost with 20
Capacity Storage hrs. storage
Technology
($/kW) ($/kWh) ($/kW)
~1
Compressed Air Energy
440
460
Storage (CAES) (≥ 300 MW)
10
Pumped hydroelectric
900
1100
100
Advanced battery (10 MW) 120
2100
300
Flywheel (100 MW)
150
6200
300
Superconductor (100 MW) 120
6100
Source: Schainker, 1999 and EPRI/DOE, 2003
CAES is clear choice for:
• Several hours (or more) of storage
• Large capacity (≥ 300 MW)
CAES system
Compressor train
Expander/generator train
Air
Exhaust
PC
PG
Intercoolers
PC = Compressor
power in
PG = Generator
power out
Aquifer,
salt cavern,
or hard mine
Heat recuperator
Fuel (e.g. natural gas, distillate)
70-100 bar
Air
Storage
hS = Hours of storage
(at PG)
A wind/CAES model
PWF
PTL
CF
CAES plant
Wind farm
PWF = Wind Farm (WF)
max. power out
(rated power)
Transmission
Underground
air storage
PTL = transmission line
(TL) max. power
CF = TL capacity factor
For this application CAES is needed to provide baseload power
Research objectives
• What are the important parameters that affect
capacity factor (CF) and cost of energy (COE) at
end of TL? How do these parameters interact?
• What is the lowest cost wind/CAES configuration
for baseload power (e.g., CF > 0.80)?
• What combination of parameters (including cost
improvements) are required to make wind/CAES
competitive with coal at end of TL?
Wind farm simulation
Wind turbine
power curve
Cut-out
Power
Probability
Weibull wind speed
distribution
k
d
–(v/c)
]
(v)= [1–e
dv
(k2 > k1)
Rated power
Cutin
Rated
speed
Wind speed
Wind speed
Autocorrelation
time (hA)
Time
Wind power time series
Wind speed
Wind speed time series
Wind speed
Turbine Cp = 0.39
Array efficiency
= 0.86 (below rating)
= 1 (above rating)
} Power “lost” Rated power
Time
CAES model
Lost power
(if storage full)
PC
CO2
Compressor
Air
Loss
Generator
Loss
Air
storage
Spilled
power
PWF
PG
Fuel
hS
Direct output
(≤ PTL)
PTL
HVDC TL
Loss
Base case configuration
Wind resource:
System
CF = 0.84
k = 2.0, vavg = 8.98 m/s,
Pwind = 560 W/m2 (Class 5)
hA = 5 hrs.
WF:
PWF = 2.5 PTL (5000 MW)
Spacing = 50 D2
vrated = 1.5 vavg
Hub height = 84 m
PC = 0.67 PTL
(1330 MW)
PG = 1.00 PTL
(2000 MW)
Comp
Gen
hS = 20 hrs.
= 4 hA
(~700 Mft3)
Eo/Ei = 1.5
CAES system
COE = 9.5
¢/kWh
TL:
PTL = 2000 MW
D = 1500 km
V = +408 kV DC
Base case cost assumptions
• Wind turbines: $923/kW (Malcolm & Hansen, NREL,
2002)
– 1500 kW, 70 m diameter, 84 m hub height
• CAES system: $460/kW (EPRI/DOE, 2003)
– $155/kW compressor, $170/kW generator, $170/kW
BOP, $1/kWh storage (solution-mined salt cavity)
• Transmission: $345/kW, $460k/km (Hauth et al.,
ORNL, 1997)
– $215/kW line, $100/kW converters, $30/kW right-ofway
• 15% capital charge rate
Wind farm size vs. COE
Min. COE:
8.8¢/kWh
(–8 %)
PWF = varied
PC = 0.7 PTL
PG = 1.0 PTL
hS = 4 hA
V = 408 kV
*
* = Base case
1.0
1.5 1.7 2.0
2.5
Wind farm size (PWF/PTL)
3.0
CAES compression vs. COE
Min. COE:
9.2¢/kWh
(–4%)
PWF = 2.5 PTL
PC = varied
PG = 1.0 PTL
hS = 4 hA
V = 408 kV
*
* = Base case
0.0
0.2
0.4
0.6
0.8
1.0
CAES compression size (PC/PTL)
1.2
CAES generation vs. COE
Min. COE:
9.1¢/kWh
(–5%)
PWF = 2.5 PTL
PC = 0.7 PTL
PG = varied
hS = 4 hA
V = 408 kV
Wind farm
*
* = Base case
0.0
0.2 0.3
0.4
0.6
0.8
1.0
CAES generation size (PG/PTL)
1.2
CAES storage time vs. COE
Min. COE:
9.5¢/kWh
(no change)
PWF = 2.5 PTL
PC = 0.7 PTL
PG = 1.0 PTL
hS = varied
V = 408 kV
*
* = Base case
1
10
100
CAES storage time (hours)
1000
Transmission voltage vs. COE
Min. COE:
8.6¢/kWh
(–10%)
PWF = 2.5 PTL
PC = 0.7 PTL
PG = 1.0 PTL
hS = 4 hA
V = varied
Transmission
CAES
Wind farm
*
* = Base case
400
600
800
1000
Transmission voltage (kV)
1200
Optimization
Base case
CF = 0.84
PWF = 2.5 PTL
PG = 1.0 PTL
PC = 0.7 PTL
hS = 4 hA
V = 408 kV
COE =
9.5¢/kWh
Trans. losses
Transmission
CAES
Wind farm
Case A Optimum
COE =
7.5¢/kWh
CF = 0.81
(–21%)
Trans. losses
PWF = 1.8 PTL
Transmission
PG = 0.5 PTL
CAES
PC = 0.8 PTL
hS = 10 hA
Wind farm
V = 700 kV
Competition with Coal IGCC with
CCS (CO2 Capture and Storage)
IGCC Wind/CAES
Coal IGCC: 6.2 ¢/kWh
Wind/CAES: 7.5 ¢/kWh
What does it take to make
wind/CAES competitive?
Need some combination of:
• Better winds
• Cheaper turbines
• Production tax credit
• Carbon tax
Assume: IGCC ($1635/kWe,  = 30%) in Portland, Oregon
Wind/CAES in E. Wyoming
Fuel prices: $1.36/MBtu (coal); $4.64/MBtu (natural gas)
Wind power density vs. COE
7.5 ¢/
kWh
PWF = 1.8 PTL
PG = 0.5 PTL
PC = 0.8 PTL
hS = 10 hA
V = 700 kV
Pwind = varied
Cturb = $923/kW
6.2 ¢/
kWh
Coal IGCC with CCS
*
* = Case A
400
Wind power class:
4
560 600
800
930 1000
Wind power density (W/m2)
5
6
7+
Turbine cost vs. COE
PWF = 1.8 PTL
PG = 0.5 PTL
PC = 0.8 PTL
hS = 10 hA
V = 700 kV
Pwind = 560 W/m2
Cturb = varied
Future:
6.2 ¢/
kWh
Coal IGCC with CCS
Current:
7.5 ¢/
kWh
*
* = Case A
200
400
600650
800 9231000
Turbine cost ($/kW)
Production tax credit
• Expired Dec. 31, 2003; extension through 2006 in pending
energy bill (H.R. 6)
• 10-year credit @ 1.8 ¢/kWh renewable energy
• Levelized credit = 1.1 ¢/kWh (assume 25-year lifetime, 89%
renewable content of wind/CAES)
PWF = 1.8 PTL
PG = 0.5 PTL
PC = 0.8 PTL
hS = 10 hA
V = 700 kV
Pwind = 560 W/m2
Cturb = $923/kW
Wind/CAES
Case A
Wind/CAES
with PTC
Coal IGCC
with CCS
7.5¢/kWh
6.4 ¢/kWh
6.2 ¢/kWh
Carbon tax vs. COE
Assume:
Turbine cost:
$923/kW
Class 5 winds
(560 W/m2)
Production
tax credit of
1.1 ¢/kWh
Wind/CAES will compete at $140/tC
but is sensitive to technology cost;
essentially a dead heat!
Coal IGCC w/ CCS: 0.042 tC/MWh;
0.42 ¢/kWh per $100/tC
Break-even
~$140/tC
Wind/CAES: 0.026 tC/MWh;
0.26 ¢/kWh per $100/tC
Other competing technologies
Assume:
Turbine cost:
$923/kW
Class 5 winds
(560 W/m2)
Production
tax credit of
1.1 ¢/kWh
CO2 vented
Non-decarbonized electricity will have trouble competing in carbonconstrained market, with exception of natural gas (NGCC). However,
diversity will require competition with decarbonized energy.
Conclusions
• Explored wind/CAES sensitivity of transmission
capacity factor and cost of energy to multiple
configuration parameters.
• Optimal configuration (with today’s technology
and no subsidy) gives 7.5 ¢/kWh for 2 GW
wind/CAES system with 81% CF and 1500 km
transmission line
• Break-even cost with coal IGCC/CCS achievable
with at least one of the following: better wind
resources, lower turbine cost, production tax credit
with carbon tax.
Future research
• Explore model sensitivities, particularly
cost assumptions, in more detail
• Develop more detailed case studies for
configurations such as the Wyoming-toOregon wind/CAES system depicted here
• Develop better synthetic wind algorithms
for general use
Acknowledgments
• Dennis Elliott, Michael Milligan, Marc Schwarz,
and Yih-Wei Wan, NREL
• Al Dutcher, HPRCC
• Marc Kapner, Austin Energy
• Nisha Desai, Ridge Energy Storage
• Bob Haug, Iowa Municipal Utilities District
• Paul Denholm, University of Wisconsin, Madison
• Joseph DeCarolis, Carnegie Mellon University
• Al Cavallo, NIST
Extra material
Wind turbine rating vs. COE
Min. COE:
9.5¢/kWh
(no change)
PWF = 2.0 PTL
vrated = varied
PC = 0.7 PTL
PG = 1.0 PTL
hS = 4 hA
V = 408 kV
*
* = Base case
0.5
1.0
1.5
Wind turbine rating (vrated/vavg)
2.0
Transmission distance vs. COE
9.5¢/kWh
PWF = 2.5 PTL
PC = 0.7 PTL
PG = 1.0 PTL
hS = 4 hA
V = 408 kV
D = varied
Trans. losses
Transmission
CAES
Wind farm
*
* = Base case
500
1000
1500
2000
Transmission distance (km)
2500
Storage vs. autocorrelation time
Cut along constant hS:
Base case
10
CF
Storage time (hS)
(hrs. log scale)
100
1
Base
case hS = hA
case
hA (hrs. log scale)
0.1
0.1
No improvement in
1
10
100 CF if hS >> hA or
Autocorrelation time (hA)
vice-versa
(hrs. log scale)
Compressor/generator ratio
1.5
Max. CF
= 85%
Slope ~ 1.7
Base case
PC/PTL
1
Minimal increase in CF
for PG/PTL = 0.5  1
CF = 81%
0.5
CF = 76%
CF = 72%
CF = 68%
0
0.5
1
PG/PTL
For given CF, least cost
configuration appears
to lie along slope line
1.5
Power derating
Rated
power
1500 kW
770 kW
Power
probability
Full range
vavg = 7.9 m/s
Pwind = 560 W/m2 (Class 5)
vrate/vavg
kW
MWh/y
CF
# turbines
$M
$/kW
¢/kWh*
*15% CCR
vrate/vavg = 1.2 1.5
1.2
770
3400
0.51
2600
1.17
1500
5.2
1.5
1500
4700
0.36
1300
1.39
920
4.4
Exploiting lower wind classes
8.4 ¢/
kWh 7.5 ¢/
kWh
PWF = 1.8 PTL
PG = 0.5 PTL
PC = 0.8 PTL
hS = 10 hA
V = 700 kV
Pwind = varied
Cturb = $923/kW
*
* = Base case
400450
Wind power class:
4
560 600
800
Wind power density (W/m2)
5
6
7+
1000