Wind Power Systems

2/1/2013

Alternate and Renewable Energy Sources

Energy Systems Research Laboratory, FIU

Professor O. A. Mohammed, EEL5285 Lecture Notes, Spring 2013

Wind Power Systems



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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 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 Energy Systems Research Laboratory, FIU


Global Installed Wind Capacity

Source: Global Wind Energy Council Energy Systems Research Laboratory, FIU


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Annual Installed Wind Capacity



Growth in US Wind Power Capacity

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


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

Total Capacity

2009 Growth



US Wind Resources 50 meters

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


http://www.windpower.org/en/pictures/lacour.htm

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US Wind Resources

80 meters

http://www.windpoweringamerica.gov/pdfs/wind_maps/us_windmap_80meters.pdf Energy Systems Research Laboratory, FIU


Cape Wind off-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. – 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.



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Massachusetts Wind Resources



Cape Wind Simulated View, Nantucket Sound, 6.5 miles Distant

Source: www.capewind.org



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State Wind Capacities (7/20/2010) State

Existing

Texas Iowa California Oregon Washington Illinois Minnesota New York Colorado North Dakota

9,707 3,670 2,739 1,920 1,914 1,848 1,797 1,274 1,248 1,222

Under Rank Construction (Existing) 370 1 0 2 443 3 614 4 815 5 437 6 673 7 95 8 552 9 37 10

http://www.awea.org/projects/ Energy Systems Research Laboratory, FIU


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” Energy Systems Research Laboratory, FIU


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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) 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.reuk.co.uk/Darrieus-Wind-Turbines.htm Professor O. A. Mohammed, EEL5285 Lecture Notes, Spring 2013 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 • Shadowing effect – when a blade swings behind the tower, the wind it encounters is briefly reduced and the blade flexes



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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 • Require somewhat complex yaw control to keep them facing into the wind • Operate more smoothly and deliver more power Energy Systems Research Laboratory, FIU


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 – 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



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Power in the Wind • Consider the kinetic energy of a “packet” of air with mass m moving at velocity v 1 KE  mv 2 (6.1) 2 • Divide by time and get power 1  m passing though A  2 Power through area A   v 2 t  • The mass flow rate is (r is air density) m passing though A m = =  Av (6.3) t Energy Systems Research Laboratory, FIU

(6.2)


Power in the Wind Combining (6.2) and (6.3), Power through area A  PW 

1  Av3 2

(6.4)

1   Av  v 2 2 Power in the wind

PW (Watts) = power in the wind ρ (kg/m3)= air density (1.225kg/m3 at 15˚C and 1 atm) A (m2)= the cross-sectional area that wind passes through v (m/s)= windspeed normal to A (1 m/s = 2.237 mph)



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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 • 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 Energy Systems Research Laboratory, FIU

Figure 6.5 Professor O. A. Mohammed, EEL5285 Lecture Notes, Spring 2013

Power in the Wind PW 

1  Av3 2

(6.4)

• Power in the wind is also proportional to A • For a conventional HAWT, A = (π/4)D2, 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



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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 induction motor while walking through a park in Budapest in 1882. He emigrated to the US in 1884



World’s Largest Offshore Wind Farm Opens Turbines are located in water depth of 20-25m. Rows are 800m apart; 500m between turbines • “Thanet” located off British coast in English Channel • 100 Vestas V90 turbines, 300 MW capacity http://www.vattenfall.co.uk/en/thanet-offshore-wind-farm.htm http://edition.cnn.com/2010/WORLD/europe/09/23/uk.largest.wind.farm/?hpt=Sbin Energy Systems Research Laboratory, FIU


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Off-shore Wind • Offshore wind turbines currently need to be in relatively shallow water, so maximum distance from shore depends on the seabed • Capacity factors tend to increase as turbines move further off-shore


Image Source: National Renewable Energy Laboratory Professor O. A. Mohammed, EEL5285 Lecture Notes, Spring 2013

Maximum Rotor Efficiency Rotor efficiency CP vs. wind speed ratio λ

Figure 6.10 Energy Systems Research Laboratory, FIU


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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 Tip-Speed-Ratio (TSR) 

• • • •

Rotor tip speed rpm   D = (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



Tip-Speed Ratio (TSR) • TSR for various rotor types • Rotors with fewer blades reach their maximum efficiency at higher tip-speed ratios Figure 6.11 Energy Systems Research Laboratory, FIU


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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 Energy Systems Research Laboratory, FIU


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 Energy Systems Research Laboratory, FIU


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The Induction Machine as a Generator • Slip is negative because the rotor spins faster than synchronous speed • Slip is normally less than 1% for gridconnected generator • Typical rotor speed N R  (1  s ) N S  [1  (0.01)]  3600  3636 rpm



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



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Blade Efficiency vs. Windspeed

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



Power Delivered vs. Windspeed

Figure 6.20 Impact of rotational speed adjustment on delivered power, assuming gear and generator efficiency is 70% Energy Systems Research Laboratory, FIU


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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). • Adjusting the rotor resistance changes the torque-speed curve • Operates between 9 and 19 rpm

Source: Vestas V80 brochure



Vestas V80 1.8 MW



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


GE 1.5 MW and 3.6 MW DFIG Examples

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

Source: Energy Systems Research Laboratory, FIU

GE Brochure/manual


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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 (DCAC) to 60 Hz for grid-connection • Good for handling rapidly changing wind speeds

Figure 6.21 Energy Systems Research Laboratory, FIU


Example: GE 2.5 MW Turbines



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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), 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 Energy Systems Research Laboratory, FIU


Enercon E126, World’s Largest Wind Turbine at 6 MW (7.5 MW Claimed) This turbine uses direct drive technology. The hub height is 135m while the rotor diameter is 126m.

Source: en.wikipedia.org/wiki/File:E_126_Georgsfeld.JPG Energy Systems Research Laboratory, FIU


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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:

1 1  Pavg    Av3    A  v 3  avg 2 avg 2

(6.29)

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



Example Windspeed Site Data


Figure 6.22


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


Figure 6.23


Altamont Pass, CA • Old windfarm with various-sized turbines • 576 MW total capacity • Average output is 125 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 Energy Systems Research Laboratory, FIU


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Wind Power Classification Scheme

Table 6.5 Energy Systems Research Laboratory, FIU


Wind Power Classification Scheme Classes of Wind Power Density at 10 m and 50 m(a) 10 m (33 ft) 50 m (164 ft) (b) Wind Speed Speed(b) Wind Wind m/s (mph) Power m/s (mph) Power Power Density Class Density (W/m2) (W/m2) 8.8 (19.7)

http://www.awea.org/faq/basicwr.html Energy Systems Research Laboratory, FIU


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Wind Power Classification Scheme 50 meters•

Table 6.5

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


Estimates of Wind Turbine Energy • 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

PW Power in the Wind

CP Rotor

PB

Power Extracted by Blades

g Gearbox & Generator

PE

Power to Electricity

• Overall conversion efficiency (Cp·ηg) is around 30% Energy Systems Research Laboratory, FIU


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