Wind Energy Basics

Outline What is a wind plant? Power production Wind power equation Wind speed vs. height Usable speed range Problems with wind; potential solutions

1. What is a wind plant? Overview Anemometer: Measures the wind speed and transmits wind speed data to the controller. Blades: Most turbines have either two or three blades. Wind blowing over the blades causes the blades to "lift" and rotate. Brake: A disc brake, which can be applied mechanically, electrically, or hydraulically to stop the rotor in emergencies. Controller: The controller starts up the machine at wind speeds of about 8 to 16 miles per hour (mph) and shuts off the machine at about 55 mph. Turbines do not operate at wind speeds above about 55 mph because they might be damaged by the high winds. Gear box: Gears connect the low-speed shaft to the high-speed shaft and increase the rotational speeds from about 30 to 60 rotations per minute (rpm) to about 1000 to 1800 rpm, the rotational speed required by most generators to produce electricity. The gear box is a costly (and heavy) part of the wind turbine and engineers are exploring "direct-drive" generators that operate at lower rotational speeds and don't need gear boxes. Generator: Usually an off-the-shelf induction generator that produces 60-cycle AC electricity. High-speed shaft: Drives the generator. Low-speed shaft: The rotor turns the low-speed shaft at about 30 to 60 rotations per minute. Nacelle: The nacelle sits atop the tower and contains the gear box, low- and high-speed shafts, generator, controller, and brake. Some nacelles are large enough for a helicopter to land on. Pitch: Blades are turned, or pitched, out of the wind to control the rotor speed and keep the rotor from turning in winds that are too high or too low to produce electricity. Rotor: The blades and the hub together are called the rotor. Tower: Towers are made from tubular steel (shown here), concrete, or steel lattice. Because wind speed increases with height, taller towers enable turbines to capture more energy and generate more electricity. Wind direction: This is an "upwind" turbine, so-called because it operates facing into the wind. Other turbines are designed to run "downwind," facing away from the wind. Wind vane: Measures wind direction and communicates with the yaw drive to orient the turbine properly with respect to the wind. Yaw drive: Upwind turbines face into the wind; the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. Downwind turbines don't require a yaw drive, the wind blows the rotor downwind. Yaw motor: Powers the yaw drive.

1. What is a wind plant? Tower & Blades

1. What is a wind plant? Towers, Rotors, Gens, Blades Manu-facturer Capacity Hub Height Rotor Diameter Gen type Weight (s-tons) Nacelle Rotor Tower 0.5 MW 50 m 40 m Vestas 0.85 MW 44 m, 49 m, 55 m, 65 m, 74 m 52m DFIG/Asynch 22 10 45/50/60/75/95, wrt to hub hgt GE (1.5sle) 1.5 MW 61-100 m 70.5-77 m DFIG 50 31 1.65 MW 70,80 m 82 m Asynch water cooled 57(52) 47 (43) 138 (105/125) 1.8-2.0 MW 80m, 95,105m 90m DFIG/ Asynch 68 38 150/200/225 Enercon 2.0 MW Synchronous 66 43 232 Gamesa (G90) 67-100m 89.6m 65 48.9 153-286 Suzlon 2.1 MW 79m 88 m Asynch Siemens (82-VS) 2.3 MW 70, 80 m 101 m 82 54 82-282 Clipper 2.5 MW 80m 89-100m 4xPMSG 113 209 GE (2.5xl) 75-100m 100 m PMSG 85 52.4 241 3.0 MW 80, 105m 70 41 160/285 Acciona 100-120m 100-116m 118 850/1150 GE (3.6sl) 3.6 MW Site specific 104 m 185 83 Siemens (107-vs) 80-90m 107m 125 95 255 Gamesa 4.5 MW 128 m REpower (Suzlon) 5.0 MW 100–120 m Onshore 90–100 m Offshore 126 m 290 120 6.0 MW 135 m Electrical excited SG 329 176 2500 7.5 MW 120m 150m

1. What is a wind plant? Electric Generator Type 1 Conventional Induction Generator (fixed speed) Type 2 Wound-rotor Induction Generator w/variable rotor resistance Type 3 Doubly-Fed Induction Generator (variable speed) Type 4 Full-converter interface Plant Feeders ac dc generator to to dc ac full power

Type 3 Doubly Fed Induction Generator 1. What is a wind plant? Type 3 Doubly Fed Induction Generator Most common technology today Provides variable speed via rotor freq control Converter rating only 1/3 of full power rating Eliminates wind gust-induced power spikes More efficient over wide wind speed Provides voltage control

1. What is a wind plant? Collector Circuit Distribution system, often 34.5

1. What is a wind plant? Offshore About 600 GW available 5-50 mile range About 50 GW available in <30m water Installed cost ~$3000/MW; uncertain because US cont. shelf deeper than N. Sea

2. Power production Wind power equation v1 vt v2 v x Swept area At of turbine blades: The disks have larger cross sectional area from left to right because v1 > vt > v2 and the mass flow rate must be the same everywhere within the streamtube. Therefore, A 1 < At < A 2

2. Power production Wind power equation 1. Wind velocity: 2. Air mass flowing: 3. Mass flow rate at swept area: 4a. Kinetic energy change: 4b. Force on turbine blades: 5b. Power extracted: 5a. Power extracted: 6a. Substitute (3) into (5a): 6b. Substitute (3) into (5b): 7. Equate 8. Substitute (7) into (6b): 9. Factor out v13:

2. Power production Wind power equation 10. Define wind stream speed ratio, a: This ratio is fixed for a given turbine & control condition. 11. Substitute a into power expression of (9): 12. Differentiate and find a which maximizes function: 13. Find the maximum power by substituting a=1/3 into (11):

2. Power production Wind power equation 14. Define Cp, the power (or performance) coefficient, which gives the ratio of the power extracted by the converter, P, to the power of the air stream, Pin. power extracted by the converter power of the air stream 15. The maximum value of Cp occurs when its numerator is maximum, i.e., when a=1/3: The Betz Limit!

2. Power production Cp vs. a

2. Power production Cp vs. λ and θ u: tangential velocity of blade tip Tip-speed ratio: ω: rotational velocity of blade R: rotor radius v1: wind speed Pitch: θ GE SLE 1.5 MW

2. Power production Cp vs. λ and θ u: tangential velocity of blade tip Tip-speed ratio: ω: rotational velocity of blade R: rotor radius v1: wind speed Pitch: θ GE SLE 1.5 MW

2. Power production Wind Power Equation So power extracted depends on Design factors: Swept area, At Environmental factors: Air density, ρ (~1.225kg/m3 at sea level) Wind speed v3 2. Control factors: Tip speed ratio through the rotor speed ω Pitch θ

2. Power production Control In Fig. a, a dotted curve is drawn through the points of maximum torque. This curve is very useful for control, in that we can be sure that as long as we are operating at a point on this curve, we are guaranteed to be operating the wind turbine at maximum efficiency. Therefore this curve, redrawn in Fig. b, dictates how the machine should be controlled in terms of torque and speed.

Effects on wind speed: Location 2. Power production Effects on wind speed: Location

Effects on wind speed: Location 2. Power production Effects on wind speed: Location

Effects on wind speed: Height 2. Power production Effects on wind speed: Height “In the daytime, when 10 m temperature is greater than at 80 m, the difference between the wind speeds is small due to solar irradiation, which heats the ground and causes buoyancy such that turbulent mixing leads to an effective coupling between the wind fields in the surface layer. During nighttime the temperature DIFFERENCE changes sign because of the cooling of the ground. This inversion dampens turbulent mixing and, hence, decouples the wind speed at different heights, leading to pronounced differences between wind speeds.” T80m < T10m Ground heatingAir rise Turbulent mixingCoupling  v80m ~ v10m Source: M. Lange and U. Focken, “Physical approach to Short-Term Wind Power Prediction,” Springer, 2005.

Effects on wind speed: Height 2. Power production Effects on wind speed: Height “The mean values of the wind speed show a pronounced dirunal cycle. At 10 m, the mean wind speed has a maximum at noon and a minimum around midnight. This behavior changes with increasing height, so that at 200 m, the dirunal cycle is inverse, with a broad minimum in daytime and maximum wind speeds at night. Hence, the better the coupling between the atmospheric layers during the day, the more horizontal momentum is transferred downwards from flow layers at large heights to those near the ground.” Nighttime peak occurs at 200 m. Almost flat at 80 m. Daytime peak occurs at 10 m. Average wind speed increases with height. Source: M. Lange and U. Focken, “Physical approach to Short-Term Wind Power Prediction,” Springer, 2005.

Effects on wind speed: Height 2. Power production Effects on wind speed: Height “The atmosphere is divided into several horizontal layers to separate different flow regimes. These layers are defined by the dominating physical effects that influence the dynamics. For wind energy use, the troposphere which spans the first five to ten km above the ground has to be considered as it contains the relevant wind field regimes.” Source: M. Lange and U. Focken, “Physical approach to Short-Term Wind Power Prediction,” Springer, 2005. Wind shear exponent differs locationally U: wind speed estimate at Hub Height Href is height at which reference data was taken Uref is wind speed at height of Href

Effects on wind speed: Contours 2. Power production Effects on wind speed: Contours Wind profile at top of slope is fuller than that of approaching wind.

Effects on wind speed: Roughness 2. Power production Effects on wind speed: Roughness

2. Power production Usable speed range Cut-in speed (6.7 mph) Cut-out speed (55 mph)

3. Problems with wind; potential solutions Day-ahead forecast uncertainty Fossil-generation is planned day-ahead Fossil costs minimized if real time same as plan Wind increases day-ahead forecast uncertainty Solutions: Pay increased fossil costs from fossil energy displaced by wind Use fast ramping gen Distribute wind gen widely Improve forecasting Smooth wind plant output On-site regulation gen Storage

3. Problems with wind; potential solutions Daily, annual wind peak not in phase w/load Daily wind peaks may not coincide w/ load Annual wind peaks occur in winter Solutions: “Spill” wind Shift loads in time Storage Pumped storage Pluggable hybrid vehicles Batteries H2, NH3 with fuel cell Compressed air …others Midwestern Region

3. Problems with wind; potential solutions Wind Power Movies JULY2006 JANUARY2006 Notice January has a lot more high-wind power than July. Also notice how the waves of wind power move through the entire EI.

3. Problems with wind; potential solutions Cost

3. Problems with wind; potential solutions Cost •$1050/kW capital cost • 34% capacity factor • 50-50 capital structure • 7% debt cost; 12.2% eqty rtrn • 20-year depreciation life • $25,000 annual O & M per MW 20-year levlzd cost=5¢/kWhr • Existing coal: <2.5¢/kWhr • Existing Nuclear: <3.0¢/kWhr • New gas combined cycle: >6.0¢/kWhr • New gas combustion turbine: >10¢/kWhr Solution: Cost of wind reduces with tower height Tower designs, nacelle weight reduction, innovative constructn Carbon cost makes wind good (best?) option

3. Problems with wind; potential solutions Wind is remote from load centers Transmission cost: a small fraction of total investment & operating costs. …And it can pay for itself: Assume $80B provides 20,000 MW delivery system over 30 years, 70% capacity factor, for Midwest wind energy to east coast. This adds $21/MWh. Cost of Midwest energy is $65/MWh. Delivered cost of energy would then be $86/MWh. East coast cost is $110/MWh.

Conclusions High penetration levels require solution to cost, variability, and transmission. Wind economics driven by wind speed, & thus by turbine height. Solutions to variability and transmission problems could increase growth well beyond what is not being predicted. Source: European Wind Energy Association, “Wind Energy – The Facts,” Earthscan, 2009.