Wind Energy Basics

Outline1. What is a wind plant?2. Power production

a. Wind power equationb. Wind speed vs. heightc. Usable speed range

3. Problems with wind; potential solutions

1. What is a wind plant?Overview

1. What is a wind plant? Tower & Blades

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1. What is a wind plant? Towers, Rotors, Gens, Blades

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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

Vestas 1.65 MW 70,80 m 82 m Asynch water cooled 57(52) 47 (43) 138 (105/125)

Vestas 1.8-2.0 MW 80m, 95,105m 90m DFIG/ Asynch 68 38 150/200/225

Enercon 2.0 MW 82 m Synchronous 66 43 232

Gamesa (G90) 2.0 MW 67-100m 89.6m DFIG 65 48.9 153-286

Suzlon 2.1 MW 79m 88 m Asynch

Siemens (82-VS) 2.3 MW 70, 80 m 101 m Asynch 82 54 82-282

Clipper 2.5 MW 80m 89-100m 4xPMSG 113 209

GE (2.5xl) 2.5 MW 75-100m 100 m PMSG 85 52.4 241

Vestas 3.0 MW 80, 105m 90m DFIG/Asynch 70 41 160/285

Acciona 3.0 MW 100-120m 100-116m DFIG 118 66 850/1150

GE (3.6sl) 3.6 MW Site specific 104 m DFIG 185 83

Siemens (107-vs) 3.6 MW 80-90m 107m Asynch 125 95 255

Gamesa 4.5 MW 128 m

REpower (Suzlon) 5.0 MW 100–120 m Onshore90–100 m Offshore

126 m DFIG/Asynch 290 120

Enercon 6.0 MW 135 m 126 m Electrical excited SG 329 176 2500

Clipper 7.5 MW 120m 150m

1. What is a wind plant?Electric Generator

6

generator

full power

PlantFeeders

actodc

dctoac

generator

partia l power

PlantFeeders

actodc

dctoac

generator

Slip poweras heat loss

PlantFeeders

PF controlcapacitor s

actodc

generator

PlantFeeders

PF controlcapacitor s

Type 1Conventional Induction Generator (fixed speed)

Type 2Wound-rotor Induction

Generator w/variable rotor resistance

Type 3Doubly-Fed Induction

Generator (variable speed)

Type 4Full-converter interface

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

7

generator

partia l power

PlantFeeders

actodc

dctoac

• 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

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POI or connection to the grid Collector System

Station

Feeders and Laterals (overhead and/or underground)

Individual WTGs

Interconnection Transmission Line

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

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2. Power productionWind 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 productionWind power equation

ttt

t vAt

xA

t

mQ

3. Mass flow rate at swept area:

22

212

1vvmKE

1. Wind velocity:t

xv

xAm 2. Air mass flowing:

4a. Kinetic energy change:

5a. Power extracted: 2

221

22

21 2

1

2

1vvQvv

t

m

t

KEP t

6a. Substitute (3) into (5a):)()2/1( 2

221 vvvAP tt

4b. Force on turbine blades:

21 vvQvt

m

t

vmmaF t

5b. Power extracted:

21 vvvQFvP ttt

6b. Substitute (3) into (5b):)( 21

2 vvvAP tt

ttttt vvvvvvvvvvvvvvvvv ))(2/1()())(()2/1()()()2/1( 12212

21211222

122

7. Equate

8. Substitute (7) into (6b): ))((4

)()))(2/1(( 2122

2121

221 vvvv

AvvvvAP t

t

9. Factor out v13: )1)()(1(

4 1

22

1

231

v

v

v

vvAP t

2. Power productionWind power equation

10. Define wind stream speed ratio, a: 1

2

v

va

)1)(1(4

231 aavA

P t 11. Substitute a into

power expression of (9):

12. Differentiate and find a which maximizes function:

1,3/10)1)(13(

0123122

0)1()1(24

222

231

aaaa

aaaaa

aaavA

a

P t

This ratio is fixed for a given turbine & control condition.

13. Find the maximum power by substituting a=1/3 into (11):

27

8

3

4

9

8

4)

3

4)(

9

11(

4

31

31

31 vAvAvA

P ttt

2. Power productionWind 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.

)1)(1(4

231 aavA

P t

31

211

211

21 2

1

2

1

2

10

2

1vAvvAvQv

t

m

t

KEP ttin

power extracted by the converter

power of the air stream

)1)(1(2

1

21

)1)(1(4 2

31

231

aavA

aavA

P

PC

t

t

inp

15. The maximum value of Cp occurs when its numerator is maximum, i.e., when a=1/3:

5926.027

16)

3

4)(

9

8(

2

1

inp P

PC

The Betz Limit!

312

1vACPCP tPinp

2. Power productionCp vs. a

2. Power productionCp vs. λ and θ

Tip-speed ratio:11 v

R

v

u u: tangential velocity of blade tip

ω: rotational velocity of blade

R: rotor radiusv1: wind speed

Pitch: θ

GE SLE 1.5 MW

2. Power productionCp vs. λ and θ

Tip-speed ratio:11 v

R

v

u u: tangential velocity of blade tip

ω: rotational velocity of blade

R: rotor radiusv1: wind speed

Pitch: θ

GE SLE 1.5 MW

2. Power productionWind Power Equation

31),(

2

1vACPCP tPinp

So power extracted depends on 1.Design factors:

• Swept area, At 2.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 productionControl

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 productionEffects on wind speed: Location

Wind Wind Speed(b) Wind Speed(b)

Power Power m/s (mph) Power m/s (mph)

Class Density Density

(W/m2) (W/m2)

1 <100 <4.4 (9.8) <200 <5.6 (12.5)

2 100 - 1504.4 (9.8)/5.1 200 - 300

5.6 (12.5)/6.4

3 150 - 2005.1 (11.5)/5.6 300 - 400

6.4 (14.3)/7.0

4 200 - 2505.6 (12.5)/6.0 400 - 500

7.0 (15.7)/7.5

5 250 - 3006.0 (13.4)/6.4 500 - 600

7.5 (16.8)/8.0

6 300 - 4006.4 (14.3)/7.0 600 - 800

8.0 (17.9)/8.8

7 >400 >7.0 (15.7) >800 >8.8 (19.7)

Classes of Wind Power Density at 10 m and 50 m(a)

        10 m (33 ft)         50 m (164 ft)

2. Power productionEffects on wind speed: Location

2. Power productionEffects 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, 2005.

T80m < T10m Ground heatingAir riseTurbulent mixingCoupling v80m ~ v10m

2. Power productionEffects on wind speed: Height

Source: M. Lange and U. Focken, “Physical approach to Short-Term Wind Power Prediction,” Springer, 2005.

“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 productionEffects on wind speed: Height

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1

Height Hub

refref H

UUWind shear exponent differs locationallyU: wind speed estimate at Hub HeightHref is height at which reference data was takenUref is wind speed at height of Href

“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 productionEffects on wind speed: Contours

Wind profile at top of slope is fuller than thatof approaching wind.

2. Power productionEffects on wind speed: Roughness

2. Power productionUsable speed range

Cut-in speed (6.7 mph) Cut-out speed (55 mph)

3. Problems with wind; potential solutionsDay-ahead forecast uncertainty

• Fossil-generation is planned day-ahead• Fossil costs minimized if real time same as plan• Wind increases day-ahead forecast uncertainty

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Hourly Load Variability and Load-Wind Variability When Wind Penetration is 10%

0

500

1000

1500

2000

2500

3000

3500

4000

-800

-700

-600

-500

-400

-300

-200

-100

0 100

200

300

400

500

600

700

800

Load and Load-Wind Hourly Variability (MW)

Fre

qen

cy

Load Hourly Variability Load-Wind Hourly Variability

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 solutionsDaily, annual wind peak not in phase w/load

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Solutions:• “Spill” wind• Shift loads in time• Storage

• Pumped storage• Pluggable hybrid vehicles• Batteries• H2, NH3 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 solutionsWind Power Movies

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JULY2006JANUARY2006

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 solutionsCost

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3. Problems with wind; potential solutionsCost

•$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 MW20-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

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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, 2009.

• 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.