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 Guyot Hall, (609) / 7715 FAX, 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 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? Three roles for storage: 1.Make wind dispatchable (price arbitrage; even at small wind market share) 2.Offset declining capacity value of wind power as market share expands 3.Facilitate use of remote, high-quality wind resources by reducing transmission costs—role first advanced by Cavallo (1995) Time Power Time Market share Value

Electric storage options Technology Compressed Air Energy Storage (CAES) (≥ 300 MW) Pumped hydroelectric Advanced battery (10 MW) Flywheel (100 MW) Superconductor (100 MW) Cost with 20 hrs. storage ($/kW) Capacity ($/kW) Storage ($/kWh) ~ Source: Schainker, 1999 and EPRI/DOE, 2003 CAES is clear choice for: Several hours (or more) of storage Large capacity (≥ 300 MW)

Compressor trainExpander/generator train Fuel (e.g. natural gas, distillate) CAES system Intercoolers Heat recuperator PCPC PGPG Air Exhaust Air Storage Aquifer, salt cavern, or hard mine h S = Hours of storage (at P G ) P C = Compressor power in P G = Generator power out bar

A wind/CAES model Wind farm Transmission CAES plant Underground air storage For this application CAES is needed to provide baseload power P WF = Wind Farm (WF) max. power out (rated power) P TL = transmission line (TL) max. power P WF P TL CF = TL capacity factor CF

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 Weibull wind speed distribution Wind speed Probability Wind speed time series Autocorrelation time (h A ) Time Wind speed Wind turbine power curve Wind speed Power Time Wind speed Wind power time series Rated power (k 2 > k 1 ) } Power “lost” [1–e –(v/c) k ] d dv  (v)= Cut-out Cut- in Rated speed Turbine C p = 0.39 Array efficiency = 0.86 (below rating) = 1 (above rating)

CAES model Compressor Generator Lost power (if storage full) Fuel P WF Air PGPG HVDC TL Direct output (≤ P TL ) Loss CO 2 Loss PCPC Air storage hShS P TL Spilled power

Base case configuration WF: P WF = 2.5 P TL (5000 MW) Spacing = 50 D 2 v rated = 1.5 v avg Hub height = 84 m TL: P TL = 2000 MW D = 1500 km V = +408 kV DC Comp Gen P C = 0.67 P TL (1330 MW) CAES system Wind resource: k = 2.0, v avg = 8.98 m/s, P wind = 560 W/m 2 (Class 5) h A = 5 hrs. System CF = 0.84 E o /E i = 1.5 P G = 1.00 P TL (2000 MW) h S = 20 hrs. = 4 h A (~700 Mft 3 ) COE = 9.5 ¢/kWh

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-of- way 15% capital charge rate

Wind farm size (P WF /P TL ) Min. COE: 8.8¢/kWh (–8 %) Wind farm size vs. COE * * = Base case Wind farm CAES Transmission Trans. losses 1.7 P WF = varied P C = 0.7 P TL P G = 1.0 P TL h S = 4 h A V = 408 kV

CAES compression size (P C /P TL ) Min. COE: 9.2¢/kWh (–4%) CAES compression vs. COE * * = Base case Wind farm CAES Transmission Trans. losses P WF = 2.5 P TL P C = varied P G = 1.0 P TL h S = 4 h A V = 408 kV

CAES generation size (P G /P TL ) Min. COE: 9.1¢/kWh (–5%) CAES generation vs. COE 0.3 * * = Base case Wind farm CAES Transmission Trans. losses P WF = 2.5 P TL P C = 0.7 P TL P G = varied h S = 4 h A V = 408 kV

Min. COE: 9.5¢/kWh (no change) CAES storage time vs. COE CAES storage time (hours) * * = Base case Wind farm CAES Transmission Trans. losses P WF = 2.5 P TL P C = 0.7 P TL P G = 1.0 P TL h S = varied V = 408 kV

Min. COE: 8.6¢/kWh (–10%) Transmission voltage vs. COE * * = Base case Wind farm CAES Transmission Transmission voltage (kV) P WF = 2.5 P TL P C = 0.7 P TL P G = 1.0 P TL h S = 4 h A V = varied Trans. losses

Optimization Base caseCase A Optimum COE = 9.5¢/kWh P WF = 2.5 P TL P G = 1.0 P TL P C = 0.7 P TL h S = 4 h A V = 408 kV P WF = 1.8 P TL P G = 0.5 P TL P C = 0.8 P TL h S = 10 h A V = 700 kV CF = 0.84CF = 0.81 COE = 7.5¢/kWh (–21%) Trans. losses Wind farm CAES Transmission Trans. losses Wind farm CAES Transmission

Competition with Coal IGCC with CCS (CO 2 Capture and Storage) 1500 km 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 IGCCWind/CAES IGCC ($1635/kW e,  = 30%) in Portland, Oregon Wind/CAES in E. Wyoming Fuel prices: $1.36/MBtu (coal); $4.64/MBtu (natural gas) Assume:

Wind power density vs. COE Wind farm CAES Transmission Trans. losses Wind power density (W/m 2 ) Wind power class: * 7.5 ¢/ kWh 560 Coal IGCC with CCS 6.2 ¢/ kWh 930 * = Case A P WF = 1.8 P TL P G = 0.5 P TL P C = 0.8 P TL h S = 10 h A V = 700 kV P wind = varied C turb = $923/kW

Turbine cost vs. COE Turbine cost ($/kW) * * = Case A Current: 7.5 ¢/ kWh Future: 6.2 ¢/ kWh P WF = 1.8 P TL P G = 0.5 P TL P C = 0.8 P TL h S = 10 h A V = 700 kV P wind = 560 W/m 2 C turb = varied Coal IGCC with CCS

Production tax credit Coal IGCC with CCS Wind/CAES Case A 6.2 ¢/kWh6.4 ¢/kWh7.5¢/kWh P WF = 1.8 P TL P G = 0.5 P TL P C = 0.8 P TL h S = 10 h A V = 700 kV P wind = 560 W/m 2 C turb = $923/kW Expired Dec. 31, 2003; extension through 2006 in pending energy bill (H.R. 6) 10-year 1.8 ¢/kWh renewable energy Levelized credit = 1.1 ¢/kWh (assume 25-year lifetime, 89% renewable content of wind/CAES) Wind/CAES with PTC

Carbon tax vs. COE Assume: Turbine cost: $923/kW Class 5 winds (560 W/m 2 ) Production tax credit of 1.1 ¢/kWh Coal IGCC w/ CCS Break-even ~$140/tC Wind/CAES: tC/MWh; 0.26 ¢/kWh per $100/tC Coal IGCC w/ CCS: tC/MWh; 0.42 ¢/kWh per $100/tC Wind/CAES w/ PTC Wind/CAES will compete at $140/tC but is sensitive to technology cost; essentially a dead heat!

Other competing technologies Assume: Turbine cost: $923/kW Class 5 winds (560 W/m 2 ) Production tax credit of 1.1 ¢/kWh NGCC Coal SC steam Coal IGCC w/ CCS Wind/CAES w/ PTC Coal IGCC CO 2 vented Non-decarbonized electricity will have trouble competing in carbon- constrained 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-to- Oregon 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 (v rated /v avg ) Min. COE: 9.5¢/kWh (no change) Wind turbine rating vs. COE * * = Base case Wind farm CAES Transmission Trans. losses P WF = 2.0 P TL v rated = varied P C = 0.7 P TL P G = 1.0 P TL h S = 4 h A V = 408 kV

Transmission distance (km) Transmission distance vs. COE * * = Base case Wind farm CAES Transmission Trans. losses P WF = 2.5 P TL P C = 0.7 P TL P G = 1.0 P TL h S = 4 h A V = 408 kV D = varied 9.5¢/kWh

Storage vs. autocorrelation time Storage time (h S ) (hrs. log scale) Autocorrelation time (h A ) (hrs. log scale) Base case CF = 70% CF = 79% CF = 74% CF = 65% No improvement in CF if h S >> h A or vice-versa h A (hrs. log scale) CF Cut along constant h S : Base case h S = h A case

Compressor/generator ratio P C /P TL P G /P TL CF = 81% CF = 76% CF = 68% CF = 72% Base case Slope ~ 1.7 For given CF, least cost configuration appears to lie along slope line Minimal increase in CF for P G /P TL = 0.5  Max. CF = 85%

kW 770 kW v rate/ v avg = 1.2 Rated power Power probability Full range Power derating v rate /v avg kW MWh/y CF # turbines $M $/kW ¢/kWh* *15% CCR v avg = 7.9 m/s P wind = 560 W/m 2 (Class 5)

Exploiting lower wind classes Wind farm CAES Transmission Trans. losses Wind power density (W/m 2 ) Wind power class: * 7.5 ¢/ kWh ¢/ kWh 450 * = Base case P WF = 1.8 P TL P G = 0.5 P TL P C = 0.8 P TL h S = 10 h A V = 700 kV P wind = varied C turb = $923/kW