Wind Power Systems
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
Global Installed Wind Capacity Source: Global Wind Energy Council
Annual Installed Wind Capacity Source: Global Wind Energy Council
Growth in US Wind Power Capacity Source: AWEA Wind Power Outlook 2 nd Qtr, 2010 For 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 Source: Global Wind Energy Council
US Wind Resources 50 meters Energy http://www.windpoweringamerica.gov/pdfs/wind_maps/us_windmap.pdf Systems Research Laboratory, FIU http://www.windpower.org/en/pictures/lacour.htm
US Wind Resources 80 meters http://www.windpoweringamerica.gov/pdfs/wind_maps/us_windmap_80meters.pdf
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.
Massachusetts Wind Resources
Cape Wind Simulated View, Nantucket Sound, 6.5 miles Distant Source: www.capewind.org
State Wind Capacities (7/20/2010) State Existing Under Construction Rank (Existing) Texas 9,707 370 1 Iowa 3,670 0 2 California 2,739 443 3 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/
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) 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 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 d 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 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 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/m 2 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 Figure 6.5
Power in the Wind 1 2 3 PW A v (64) (6.4) Power in the wind is also proportional o to A For a conventional HAWT, A = (π/4)d 2, so wind power is proportional p 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 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. 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
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
Maximum Rotor Efficiency Rotor efficiency C P 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 Tip-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
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
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 gridconnected generator Typical rotor speed N R (1 s ) N [1 ( 0.01)] 01)] 3600 3636 rpm S
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
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%
Variable Slip Example: Vestas V8018MW 1.8 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 V8018MW 1.8
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
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 Energy Systems Research Laboratory, Source: FIU GE Brochure/manual
Indirect Grid Connection Systems Wind turbine is allowed to spin at any speed Variable frequency enc AC from the generator goes through a rectifier (AC-DC) and an inverter (DC- AC)to60Hzforgrid-connection Good for handling rapidly changing wind speeds Figure 6.21
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), 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 drive technology. The hub height is 135m while the rotor diameter is 126m. Source: en.wikipedia.org/wiki/file:e_126_georgsfeld.jpg
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 P avg Av A v 2 2 3 3 avg This is why we can t use average windspeed v avg to find the average power in the wind avg (6.29)
Example Windspeed Site Data 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 Figure 6.23
Altamont Pass, CA Old windfarm with various-sized 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
Wind Power Classification Scheme Table 6.5
Wind Power Class Classes of Wind Power Density at 10 m and 50 m (a) 10 m (33 ft) 50 m (164 ft) Wind Speed (b) Wind Speed (b) Power m/s (mph) Power m/s (mph) Density Density (W/m 2 ) (W/m 2 ) 1 <100 <4.4 (9.8) <200 <5.6 (12.5) 2 100-150 4.4 (9.8)/5.1 (11.5) 200-300 5.6 (12.5)/6.4 (14.3) 3 150-200 5.1 (11.5)/5.6 (12.5) 300-400 6.4 (14.3)/7.0 (15.7) 4 200-250 5.6 (12.5)/6.0 (13.4) 400-500 7.0 (15.7)/7.5 (16.8) 5 250-300 6.0 (13.4)/6.4 (14.3) 500-600 7.5 (16.8)/8.0 (17.9) 6 300-400 6.4 (14.3)/7.0 (15.7) 600-800 8.0 (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 50 meters Table 6.5 http://www.windpoweringamerica.gov/pdfs/wind_maps/us_windmap.pdf
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 P W Power in the Wind C P Rotor P B Power Extracted by Blades g Gearbox & Generator Overall conversion efficiency (C p η g ) is around 30% P E Power to Electricity
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 Simplified connections to the transmission i system Centralized access for operations and maintenance How many turbines should be installed at a site?
Wind Farms We know that wind slows down as it passes through the blades. Recall the power extracted by the blades: 1 2 2 P b m v vd (6.18) 2 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?
Wind Farms Figure 6.28
Wind Farms Optimum Spacing Ballpark figure for GE 1.5 MW in Midwest is one per 80 acres Optimum spacing is estimated to be Figure 3-6.29 5 rotor diameters between towers and 5-9 between rows 5 D to 9D 3 D to 5D
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, 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 Graphs show the mean, and then the 75% and 90% probability values; note for August the 90% probability is zero. Source: www.uwig.org/xcelmndocwindcharacterization.pdf
California ISO Daily Wind Energy 700 600 500 400 300 200 100 0 hour
How Rotor Blades Extract Energy from the Wind Airfoil could be the wing of an airplane or the blade of a wind turbine Figure 6.30 (a) Bernoulli s Principle - air pressure on top is greater than air pressure on bottom because it has further to travel, creates lift
How Rotor Blades Extract Energy Air is moving towards the wind turbine blade from the wind but also from the relative blade motion The blade is much faster at the tip than at the hub, so the blade is twisted to keep the angles correct from the Wind Figure 6.30 (b)
Angle of Attack, Lift, and Drag Increasing angle of attack increases lift, but it also increases drag Figure 6.31 (a) If the angle of attack is too great, stall occurs where turbulence destroys the lift Energy Systems Research Laboratory, Figure 6.31 FIU (b) - Stall
Idealized Power Curve Cut in windspeed, rated windspeed, cut-out windspeed Figure 6.32
Idealized Power Curve 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 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
Idealized Power Curve 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
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) Source: 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 than 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)
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 Source: 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 Capacity factors tend to increase as turbines move further off-shore Image Source: National Renewable Energy Laboratory
Cape Wind Simulated View, Nantucket Sound, 6.5 miles Distant Source: 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 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)
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 din power system analysis 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 Would go in shallow trenches Four connection points to ac grid First stage would go into service in 2016. Cost is estimated at $5 billion Source: Google Blog; NYTimes also 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 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 stuato