wind power systems historical development of wind power

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Wind Power Systems

Energy Systems Research Laboratory, FIU

Historical Development of Wind Power

• In the US - first wind-electric systems built in the late 1890’s

• By 1930s and 1940s, hundreds of thousands were in use in rural areas not yet served by the grid

• Interest in wind power declined as the utility grid


• Interest in wind power declined as the utility grid expanded and as reliable, inexpensive electricity could be purchased

• Oil crisis in 1970s created a renewed interest in wind until US government stopped giving tax credits

• Renewed interest again since the 1990s

Global Installed Wind Capacity


Source: Global Wind Energy Council

Annual Installed Wind Capacity



Growth in US Wind Power Capacity


Source: AWEA Wind Power Outlook 2nd Qtr, 2010For more info: http://www.windpoweringamerica.gov/pdfs/wpa/wpa_update.pdf

Top 10 Countries - Installed Wind Capacity (as of the end of 2009)



Total Capacity 2009 Growth

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US Wind Resources

50 meters

Energy Systems Research Laboratory, FIUhttp://www.windpower.org/en/pictures/lacour.htmhttp://www.windpoweringamerica.gov/pdfs/wind_maps/us_windmap.pdf

US Wind Resources


80 meters

http://www.windpoweringamerica.gov/pdfs/wind_maps/us_windmap_80meters.pdf

Cape Windoff-shore wind farm

• For about 10 years Cape Wind Associates has been attempting to build an off-shore 170 MW wind farm in Nantucket Sound, Massachusetts. Because the closest turbine would be more than three miles from shore (4.8 miles) it is subject to federal, as opposed to state jurisdiction


opposed to state, jurisdiction.

– Federal approval was given on May 17, 2010

– Cape Wind would be the first US off-shore wind farm

• There has been significant opposition to this project, mostly out of concern that the wind farm would ruin the views from private property, decreasing property values.

Massachusetts Wind Resources


Cape Wind Simulated View, Nantucket Sound, 6.5 miles Distant

Energy Systems Research Laboratory, FIUSource: www.capewind.org

State Wind Capacities (7/20/2010)State Existing Under

ConstructionRank

(Existing)

Texas 9,707 370 1

Iowa 3,670 0 2

California 2,739 443 3

Oregon 1,920 614 4


Oregon 1,920 614 4

Washington 1,914 815 5

Illinois 1,848 437 6

Minnesota 1,797 673 7

New York 1,274 95 8

Colorado 1,248 552 9

North Dakota 1,222 37 10

http://www.awea.org/projects/

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Types of Wind Turbines

• “Windmill”- used to grind grain into flour

• Many different names - “wind-driven generator”, “wind generator”, “wind turbine”, “wind-turbine generator (WTG)”, “wind energy conversion system


(WECS)”

• Can have be horizontal axis wind turbines (HAWT) or vertical axis wind turbines (VAWT)

• Groups of wind turbines are located in what is called either a “wind farm” or a “wind park”

Vertical Axis Wind Turbines• Darrieus rotor - the only vertical axis

machine with any commercial success

• Wind hitting the vertical blades, called aerofoils, generates lift to create rotation

• No yaw (rotation about vertical axis)

Energy Systems Research Laboratory, FIUhttp://www.reuk.co.uk/Darrieus-Wind-Turbines.htm

• No yaw (rotation about vertical axis) control needed to keep them facing into the wind

• Heavy machinery in the nacelle is located on the ground

• Blades are closer to ground where windspeeds are lower

http://www.absoluteastronomy.com/topics/Darrieus_wind_turbine

Horizontal Axis Wind Turbines

• “Downwind” HAWT – a turbine with the blades behind (downwind from) the tower

• No yaw control needed- they naturally orient themselves in line with the wind


themselves in line with the wind

• Shadowing effect – when a blade swings behind the tower, the wind it encounters is briefly reduced and the blade flexes

Horizontal Axis Wind Turbines

• “Upwind” HAWT – blades are in front of (upwind of) the tower

• Most modern wind turbines are this type

• Blades are “upwind” of the tower


Blades are upwind of the tower

• Require somewhat complex yaw control to keep them facing into the wind

• Operate more smoothly and deliver more power

Number of Rotating Blades

• Windmills have multiple blades

– need to provide high starting torque to overcome weight of the pumping rod

– must be able to operate at low wind speeds to provide nearly continuous water pumping


y p p g

– a larger area of the rotor faces the wind

• Turbines with many blades operate at much lower rotational speeds - as the speed increases, the turbulence caused by one blade impacts the other blades

• Most modern wind turbines have two or three blades

Power in the Wind (for reference solar is about 600 w/m2 in summer)

• Power increases like the cube of wind speed

• Doubling the wind speed increases the power by eight


power by eight

• Energy in 1 hour of 20 mph winds is the same as energy in 8 hours of 10 mph winds

• Nonlinear, so we cannot use average wind speed

Figure 6.5

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Power in the Wind

• Power in the wind is also proportional to A

• For a conventional HAWT, A = (π/4)D2, so wind

31P A (6.4)

2W v


For a conventional HAWT, A (π/4)D , so wind power is proportional to the blade diameter squared

• Cost is roughly proportional to blade diameter

• This explains why larger wind turbines are more cost effective

Nikola Tesla: Inventor of Induction Motor (and many other things)

• Nikola Tesla (1856 to 1943) is one of the key inventors associated with the development of today’s three phase ac system. His contributions include the induction motor and polyphase ac systems.– Unit of flux density is named after him


• Tesla conceived of the inductionmotor while walking through a park in Budapest in 1882.

• He emigrated to the US in 1884

World’s Largest Offshore Wind Farm OpensTurbinesare locatedin waterdepth of 20-25m.Rows


• “Thanet” located off British coast in English Channel

• 100 Vestas V90 turbines, 300 MW capacity

http://edition.cnn.com/2010/WORLD/europe/09/23/uk.largest.wind.farm/?hpt=Sbinhttp://www.vattenfall.co.uk/en/thanet-offshore-wind-farm.htm

are800mapart; 500mbetweenturbines

Off-shore Wind• Offshore wind turbines currently need to be in

relatively shallow water, so maximum distance from shore depends on the seabed

• Capacityfactors tend


to increaseas turbinesmove furtheroff-shore

Image Source: National Renewable Energy Laboratory

Maximum Rotor Efficiency

Rotor efficiency CP vs. wind speed ratio λ


Figure 6.10

Tip-Speed Ratio (TSR)

• Efficiency is a function of how fast the rotor turns

• Tip-Speed Ratio (TSR) is the speed of the outer tip of the blade divided by windspeed

Rotor tip speed rpm D


Rotor tip speed rpm DTip-Speed-Ratio (TSR) = (6.27)

Wind speed 60v

• D = rotor diameter (m) • v = upwind undisturbed windspeed (m/s) • rpm = rotor speed, (revolutions/min)• One meter per second = 2.24 miles per hour

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Tip-Speed Ratio (TSR)

• TSR for various rotor types

• Rotors with fewer blades reach their


blades reach their maximum efficiency at higher tip-speed ratios

Figure 6.11

Synchronous Machines

• Spin at a rotational speed determined by the number of poles and by the frequency

• The magnetic field is created on their rotors

• Create the magnetic field by running DC through


windings around the core

• A gear box is needed between the blades and the generator

• 2 complications – need to provide DC, need to have slip rings on the rotor shaft and brushes

Asynchronous Induction Machines

• Do not turn at a fixed speed

• Acts as a motor during start up as well as a generator

• Do not require exciter, brushes, and slip rings


• The magnetic field is created on the stator instead of the rotor

• Less expensive, require less maintenance

• Most wind turbines are induction machines

The Induction Machine as a Generator

• Slip is negative because the rotor spins faster than synchronous speed

• Slip is normally less than 1% for grid-connected generator


connected generator

• Typical rotor speed

(1 ) [1 ( 0.01)] 3600 3636 rpmR SN s N

Speed Control

• Necessary to be able to shed wind in high-speed winds

• Rotor efficiency changes for different Tip-Speed Ratios (TSR), and TSR is a function of windspeed

• To maintain a constant TSR blade speed should


• To maintain a constant TSR, blade speed should change as windspeed changes

• A challenge is to design machines that can accommodate variable rotor speed and fixed generator speed

Blade Efficiency vs. Windspeed


Figure 6.19At lower windspeeds, the best efficiency is achieved at a lower rotational speed

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Power Delivered vs. Windspeed


Figure 6.20Impact of rotational speed adjustment on delivered power, assuming gear and generator efficiency is 70%

Variable Slip Example: Vestas V80 1.8 MW

• The Vestas V80 1.8 MW turbine is an example in which an induction generator is operated with variable rotor resistance (opti-slip).

Adj ti th t i t


• Adjusting the rotor resistance changes the torque-speed curve

• Operates between 9 and 19 rpm

Source: Vestas V80 brochure

Vestas V80 1.8 MW


Doubly-Fed Induction Generators

• Another common approach is to use what is called a doubly-fed induction generator in which there is an electrical connection between the rotor and supply electrical system using an ac-ac


pp y y gconverter

• This allows operation over a wide-range of speed, for example 30% with the GE 1.5 MW and 3.6 MW machines

GE 1.5 MW and 3.6 MW DFIG Examples

Energy Systems Research Laboratory, FIUSource: GE Brochure/manual

GE 1.5 MW turbines are the best selling wind turbines in the US with 43% market share in 2008

Indirect Grid Connection Systems

• Wind turbine is allowed to spin at any speed

• Variable frequency AC from the generator goes through a rectifier (AC-DC) and an inverter (DC-AC) to 60 Hz for grid-connection

• Good for handling rapidly changing wind speeds


Good for handling rapidly changing wind speeds

Figure 6.21

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Example: GE 2.5 MW Turbines


Wind Turbine Gearboxes

• A significant portion of the weight in the nacelle is due to the gearbox

– Needed to change the slow blade shaft speed into the higher speed needed for the electric machine

• Gearboxes require periodic maintenance (e.g., change the oil),


q p ( g , g ),and have also be a common source of wind turbine failure

• Some wind turbine designs are now getting rid of the gearbox by using electric generators with many pole pairs (direct-drive systems)

• Enercon is the leader in this area, with others considering direct drives

Enercon E126, World’s Largest Wind Turbine at 6 MW (7.5 MW Claimed)

This turbine uses direct drivetechnology. The hub height is 135m while the rotor diameter is 126


Source: en.wikipedia.org/wiki/File:E_126_Georgsfeld.JPG

126m.

Average Power in the Wind

• How much energy can we expect from a wind turbine?

• To figure out average power in the wind, we need to know the average value of the cube of velocity:


• This is why we can’t use average windspeed vavg to find the average power in the wind

3 31 1 (6.29)

2 2avg avgavg

P Av A v

Example Windspeed Site Data

Energy Systems Research Laboratory, FIU Figure 6.22

Wind Probability Density Functions

Windspeed probability density function (p.d.f) –between 0 and 1, area under the curve is equal to 1


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Altamont Pass, CA• Old windfarm with

various-sized turbines

• 576 MW total capacity

• Average output is


g p125 MW

• Wind turbines are on hilltop ridges

http://en.wikipedia.org/wiki/File:Altamont_Wind_Turbines_7-11-09.JPG

http://xahlee.org/Whirlwheel_dir/livermore.html

Wind Power Classification Scheme


Table 6.5

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

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

Wind PowerDensity (W/m2)

Speed(b)

m/s (mph)Wind PowerDensity (W/m2)

Speed(b)

m/s (mph)

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

2 100 - 150 4.4 (9.8)/5.1 (11.5) 200 - 3005.6 (12.5)/6.4 (14.3)


3 150 - 200 5.1 (11.5)/5.6 (12.5) 300 - 4006.4 (14.3)/7.0 (15.7)

4 200 - 250 5.6 (12.5)/6.0 (13.4) 400 - 5007.0 (15.7)/7.5 (16.8)

5 250 - 300 6.0 (13.4)/6.4 (14.3) 500 - 6007.5 (16.8)/8.0 (17.9)

6 300 - 400 6.4 (14.3)/7.0 (15.7) 600 - 8008.0 (17.9)/8.8 (19.7)

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

http://www.awea.org/faq/basicwr.html

Wind Power Classification Scheme

• Table 6.550 meters

Energy Systems Research Laboratory, FIUhttp://www.windpoweringamerica.gov/pdfs/wind_maps/us_windmap.pdf

• Not all of the power in the wind is retained - the rotor spills high-speed winds and low-speed winds are too slow to overcome losses

• Depends on rotor, gearbox, generator, tower, controls, terrain, and the wind

Estimates of Wind Turbine Energy


• Overall conversion efficiency (Cp·ηg) is around 30%

WPBP EP

Power in the Wind

Power Extracted by Blades

Power to Electricity

PCRotor Gearbox &

Generator

g

Wind Farms

• Normally, it makes sense to install a large number of wind turbines in a wind farm or a wind park

• Benefits

– Able to get the most use out of a good wind site

Reduced development costs


– Reduced development costs

– Simplified connections to the transmission system

– Centralized access for operations and maintenance

• How many turbines should be installed at a site?

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Wind Farms

• We know that wind slows down as it passes through the blades. Recall the power extracted by the blades:

2 21 (6.18)

2b dP m v v


• Extracting power with the blades reduces the available power to downwind machines

• What is a sufficient distance between wind turbines so that windspeed has recovered enough before it reaches the next turbine?

( )2b d

Wind Farms


Figure 6.28

Wind Farms – Optimum Spacing

Ballparkfigure for GE 1.5 MW in Midwestis one per80


Figure 6.29Optimum spacing is estimated to be 3-5 rotor diameters between towers and 5-9 between rows

5 D to 9D

3 D to 5D

80 acres

Time Variation of Wind

• We need to not just consider how often the wind blows but also when it blows with respect to the electric load.

• Wind patterns vary quite a bit with geography,


Wind patterns vary quite a bit with geography, with coastal and mountain regions having more steady winds.

• In the Midwest the wind tends to blow the strongest when the electric load is the lowest.

Upper Midwest Daily Wind Variation


August April

Source: www.uwig.org/XcelMNDOCwindcharacterization.pdf

Graphs show the mean, and then the 75% and 90% probability values; note for August the 90% probabilityis zero.

California ISO Daily Wind Energy 700

600

500

400


hour

300

200

100

0

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How Rotor Blades Extract Energy from the Wind

Airfoil – could be the wing of an airplane or the blade of a wind turbine


Bernoulli’s Principle - air pressure on top is greater than air pressure on bottom because it has further to travel, creates lift

Figure 6.30 (a)

How Rotor Blades Extract Energy from the Wind

• Air is moving towards the wind turbine blade from the wind but also from the relative blade motion


motion

• The blade is much faster at the tip than at the hub, so the blade is twisted to keep the angles correct

Figure 6.30 (b)

Angle of Attack, Lift, and Drag

• Increasing angle of attack increases lift, but it also increases


drag

• If the angle of attack is too great, “stall” occurs where turbulence destroys the lift

Figure 6.31 (a)

Figure 6.31 (b) - Stall

Idealized Power Curve

Cut –in windspeed, rated windspeed, cut-out windspeed



• Before the cut-in windspeed, no net power is generated

• Then, power rises like the cube of windspeed

• After the rated windspeed is reached, the wind turbine operates at rated power (sheds excess wind)

• Three common approaches to shed excess wind


• Three common approaches to shed excess wind

– Pitch control – physically adjust blade pitch to reduce angle of attack

– Stall control (passive) – blades are designed to automatically reduce efficiency in high winds

– Active stall control – physically adjust blade pitch to create stall


• Above cut-out or furling windspeed, the wind is too strong to operate the turbine safely, machine is shut down, output power is zero

• “Furling” –refers to folding up the sails when winds


are too strong in sailing

• Rotor can be stopped by rotating the blades to purposely create a stall

• Once the rotor is stopped, a mechanical brake locks the rotor shaft in place

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Current Prices for Small Wind

• The Amazon is selling a 900W wind turbine for $1739; inverter (maybe $250), tower and batteries are extra (65’ tower goes for about $1000 plus installation) (Whisper 100; designed for 100 kWh per month)

Energy Systems Research Laboratory, FIUSource: www.homedepot.com; www.kansaswindpower.net

Government Credits

• Federal government provides tax credits of 30% of cost for small (household level) solar, wind, geothermal and fuel cells (starting in 2009 the total cap of $4000 was removed)


• I don’t think Illinois has a wind credit, but they do have a solar credit (30% of cost)

• For large systems the Federal Renewable Electricity Production Tax Credit pays 1.5¢/kWh (1993 dollars, inflation adjusted, currently 2.1¢) for the first ten years of production

Source for federal/state incentives: www.dsireusa.org

Small Wind Turbine Cost

• Assume total cost is $3000– Federal credit reduces cost to $2100

• With an assumed lifetime of 15 years and simple payback, the annual cost is $140. .


• Say unit produces 100 kWh per month, or 1200 per year.

• This unit makes economic sense if electricity prices are at or above 100/1200 = $0.083/kWh.

• With modest annual O&M, say $50, this changes to $0.125/kWh.



Economies of Scale

• Presently large wind farms produce electricity more economically than small operations

• Factors that contribute to lower costs are

– Wind power is proportional to the area covered by the blade (square of diameter) while tower costs vary with a value less


( q ) ythan the square of the diameter

– Larger blades are higher, permitting access to faster winds

– Fixed costs associated with construction (permitting, management) are spread over more MWs of capacity

– Efficiencies in managing larger wind farms typically result in lower O&M costs (on-site staff reduces travel costs)

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Environmental Aspects of Wind Energy

• US National Academies issued report on issue in 2007

• Wind system emit no air pollution and no carbon dioxide; they also have essentially no water requirements

• Wind energy serves to displace the production of energy


from other sources (usually fossil fuels) resulting in a net decrease in pollution

• Other impacts of wind energy are on animals, primarily birds and bats, and on humans

Environmental Aspects of Wind Energy, Birds and Bats

• Wind turbines certainly kill birds and bats, but so do lots of other things; windows kill between 100 and 900 million birds per year

Estimated Causes of Bird Fatalities, per 10,000

Energy Systems Research Laboratory, FIUSource: Erickson, et.al, 2002. Summary of Anthropogenic Causes of Bird Mortality

Environmental Aspects of Wind Energy, Human Aesthetics, Offshore

• Offshore wind turbines currently need to be in relatively shallow water, so maximum distance from shore depends on the seabed

• Capacityf t t d


factors tendto increaseas turbinesmove furtheroff-shore

Image Source: National Renewable Energy Laboratory

Cape Wind Simulated View, Nantucket Sound, 6.5 miles Distant

Energy Systems Research Laboratory, FIUSource: www.capewind.org

In the News: NREL Report on US Offshore Wind Potential

• NREL just issued a report discussing US off-shore wind potential, with a key conclusion being that we could get about 54 GW of new off-shore wind by 2030.

• Offshore wind has a significant advantage that the ti i l t d l ti l l l t th hi h l d


generation is located relatively closely to the high load urban areas. Offshore wind is also more constant.

• Offsetting are the higher costs of locating in water.

• Report claims 43,000 permanent jobs but doesn’t discuss loss of jobs in other areas.

• World off-shore wind UK (1041 MW), Denmark (664)Source (full report) http://www.nrel.gov/docs/fy10osti/40745.pdf

Wind Turbines and Radar• “Wind Turbines interfere with radar. This has led the

FAA, DHS and DOD to contest many proposed wind turbine sites.”– Either through radar shadows, or doppler returns that look

like false aircraft or weather patterns


• No fundamental constraint with respect to radar interference, but mitigation might require either upgrades to radar or regulation changes to require, for example, telemetry from wind farms to radar– For Cape Wind project the developer agreed to pay $1.5

million to upgrade radar at a nearby military base, with an escrow of $15 million.

Source: www.fas.org/irp/agency/dod/jason/wind.pdf (2008)

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Power Grid Integration of Wind Power

• Wind power had represented a minority of the generation in power system interconnects, so its impact of grid operations was small, but now the impact of wind needs to be considered in power system analysis

L t i d f i ld i R Wi d F i T


– Largest wind farm in world is Roscoe Wind Farm in Texas with a total capacity of 781 MW, which matches the size of many conventional generators.

• Wind power has impacts on power system operations ranging from that of transient stability (seconds) out to steady-state (power flow)– Voltage and frequency impacts are key concerns

In the News: Off-shore Transmission System Proposed

• Several companies, including Trans-Elect and Google are proposing a 6000 MW, 350 MW long off-shore “superhighway for clean energy.” – It would be located between 15 to 20 miles offshore

W ld i h ll t h


– Would go in shallow trenches

– Four connection points to ac grid

– First stage would go into servicein 2016.

– Cost is estimated at $5 billion

Source: Google Blog; NYTimesalso thanks to Pallav Pathak

Wind Power, Reserves and Regulation

• A key constraint associated with power system operations is pretty much instantaneously the total power system generation must match the total load plus losses– Excessive generation increases the system frequency, while

excessive load decreases the system frequency


excessive load decreases the system frequency

• Generation shortfalls can suddenly occur because of the loss of a generator; utilities plan for this occurrence by maintaining sufficient reserves (generation that is on-line but not fully used) to account for the loss of the largest single generator in a region (e.g., a state)

Wind Power, Reserves and Regulation, cont.

• A fundamental issue associated with “free fuel” systems like wind is that operating with a reserve margin requires leaving free energy “on the table.”– A similar issue has existed with nuclear energy, with the fossil

fueled units usually providing the reserve margin


• Because wind turbine output can vary with the cube of the wind speed, under certain conditions a modest drop in the wind speed over a region could result in a major loss of generation– Lack of other fossil-fuel reserves could exacerbate the

situation

wind power systems historical development of wind power

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