Wind Energy Basics

Outline 1.What is a wind plant? 2.Power production a.Wind power equation b.Wind speed vs. height c.Usable speed range 3.Problems with wind; potential solutions

1. What is a wind plant? Overview

1. What is a wind plant? Tower & Blades 4

1. What is a wind plant? Towers, Rotors, Gens, Blades 5 Manu- facturer CapacityHub HeightRotor Diameter Gen typeWeight (s-tons) NacelleRotorTower 0.5 MW50 m40 m Vestas0.85 MW44 m, 49 m, 55 m, 65 m, 74 m 52mDFIG/Asynch221045/50/60/75/95, wrt to hub hgt GE (1.5sle)1.5 MW m mDFIG5031 Vestas1.65 MW70,80 m82 mAsynch water cooled57(52)47 (43)138 (105/125) Vestas MW80m, 95,105m90mDFIG/ Asynch /200/225 Enercon2.0 MW82 mSynchronous Gamesa (G90)2.0 MW67-100m89.6mDFIG Suzlon2.1 MW79m88 mAsynch Siemens (82-VS)2.3 MW70, 80 m101 mAsynch Clipper2.5 MW80m89-100m4xPMSG GE (2.5xl)2.5 MW75-100m100 mPMSG Vestas3.0 MW80, 105m90mDFIG/Asynch /285 Acciona3.0 MW m mDFIG /1150 GE (3.6sl)3.6 MWSite specific104 mDFIG18583 Siemens (107-vs)3.6 MW80-90m107mAsynch Gamesa4.5 MW128 m REpower (Suzlon)5.0 MW100–120 m Onshore 90–100 m Offshore 126 mDFIG/Asynch Enercon6.0 MW135 m126 mElectrical excited SG Clipper7.5 MW120m150m

1. What is a wind plant? Electric Generator 6 generator full power Plant Feeders ac to dc to ac 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

1. What is a wind plant? Type 3 Doubly Fed Induction Generator 7 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

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 9

2. Power production Wind power equation v1v1 vtvt v2v2 v xx Swept area A t of turbine blades: The disks have larger cross sectional area from left to right because v 1 > v t > v 2 and the mass flow rate must be the same everywhere within the streamtube. Therefore, A 1 < A t < A 2

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

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

2. Power production Wind power equation 14. Define C p, the power (or performance) coefficient, which gives the ratio of the power extracted by the converter, P, to the power of the air stream, P in. power extracted by the converter power of the air stream 15. The maximum value of C p 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 Tip-speed ratio: u: tangential velocity of blade tip ω: rotational velocity of blade R: rotor radius v 1 : wind speed Pitch: θ GE SLE 1.5 MW

2. Power production Cp vs. λ and θ Tip-speed ratio: u: tangential velocity of blade tip ω: rotational velocity of blade R: rotor radius v 1 : wind speed Pitch: θ GE SLE 1.5 MW

2. Power production Wind Power Equation So power extracted depends on 1.Design factors: Swept area, A t 2.Environmental factors: Air density, ρ (~1.225kg/m 3 at sea level) Wind speed v 3 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.

2. Power production Effects on wind speed: Location

2. Power production Effects on wind speed: Location

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.” Source: M. Lange and U. Focken, “Physical approach to Short-Term Wind Power Prediction,” Springer, T 80m < T 10m  Ground heating  Air rise  Turbulent mixing  Coupling  v 80m ~ v 10m

2. Power production Effects on wind speed: Height Source: M. Lange and U. Focken, “Physical approach to Short-Term Wind Power Prediction,” Springer, “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.” Daytime peak occurs at 10 m. Nighttime peak occurs at 200 m. Almost flat at 80 m. Average wind speed increases with height.

2. Power production Effects on wind speed: Height  Wind shear exponent differs locationally U: wind speed estimate at Hub Height H ref is height at which reference data was taken U ref is wind speed at height of H ref “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.

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

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 27 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 28 Solutions: “Spill” wind Shift loads in time Storage Pumped storage Pluggable hybrid vehicles Batteries H 2, NH 3 with fuel cell Compressed air …others Daily wind peaks may not coincide w/ load Annual wind peaks occur in winter Midwestern Region

3. Problems with wind; potential solutions Wind Power Movies 29 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 30

31 3. Problems with wind; potential solutions Cost $1050/kW capital cost 34% capacity factor 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 32 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 Source: European Wind Energy Association, “Wind Energy – The Facts,” Earthscan, 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.