chapter 6 renewable energy systems. wind energy

CHAPTER 6RENEWABLE ENERGY SYSTEMS.WIND ENERGY

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Dr. Anwar Abu-Zarifa . Islamic University Gaza . Department of Industrial Engineering

Introduction Use of wind energy for human purposes entails the

conversion of the kinetic energy that is presentintermittently in the wind into mechanical energy, usuallyin the form of rotation of a shaft.

From there, the energy can be applied to mechanical workor further converted to electricity using a generator.

No location is continuously windy, and the power in thewind is highly variable, requiring provision both foralternative energy supplies during times of little or nowind, and means of protecting the wind energy conversiondevice from damage in times of extremely high wind.

World use of wind power has been growing rapidly as wellin recent years.

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Early Wind Generators An early example:

Brush Electric Generator, Cleveland, OH, 1880s, 12kW Early adaptation of wind-powered mechanical pump to generatingelectricity

First attempt at > 1 MW turbine Smith-Putnam Turbine, Vermont, 1940s, 1.25 MW Failed prematurely, not repeated

Development of modern utility-scale turbine California, Denmark in 1970s and 1980s Experimentation with vertical axis turbines, but eventually settled onhorizontal axis design.

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Total installed capacity, 2005 and 2010

Source: Global Wind Energy Consortium

Total = 59.1 GW Total = 195.0 GW

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Classification of wind resource by wind speed range in m/s and mph at hub height of turbine

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Dr. Anwar Abu-Zarifa . Islamic University Gaza . Department of Industrial Engineering 6

Madison utility-scale wind farm with seven 1.5-MW turbines near Utica, New York.


Not to scale

Main parts of a utility-scale wind turbine

Wind Turbines

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“Windmill”- used to grind grain into flour (or pumpwater in Holland)

Can have be horizontal axis wind turbines (HAWT) orvertical axis wind turbines (VAWT)

Groups of wind turbines are located in what is calledeither a “wind farm” or a “wind park”

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How Lift Based Turbines Extract Energy from Fluid

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

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

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Angle of Attack, Lift, and DragIncreasing angle of attack increases lift, but it also increases drag

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

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Wind Turbines extract energy from the force of the wind on anaerofoil, in this case a turbine blade.

The relative motion between the air flow and the turbine blade,is the same as for the aircraft wing, but in this case the wind isin motion towards the turbine blades and the blades are passiveso that the external thrust provided by the moving air flow is inthe opposite direction to the thrust provided by the aircraftwing.

The turbine blades thus experience lift and drag forces, similarto the aircraft wing, which set the blades in motion transferringthe wind energy into the kinetic energy of the blades.

The turbine blades are connected to a single rotor shaft and theforce of the wind along the length of the blades creates a torquewhich turns the rotor.

As with aircraft wings, the magnitudes of the lift and drag onthe turbine blade are dependent on the angle of attack betweenthe apparent wind direction and the chord line of the blade.

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More energy can be extracted from wind using lift rather than drag, but this requires specially shaped airfoil surfaces, like those used on airplane wings.


Lots of ideas, only a few good…

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Vertical Axis Wind TurbinesDarrieus rotor - the only vertical axis machine with any

commercial successWind hitting the vertical blades (airfoils) generates lift

to create rotation

Advantages• No yaw (rotation about vertical axis)

control needed to keep facing into wind• Heavy machinery located on the ground

Disadvantage• Blades are closer to ground where

windspeeds are lower

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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

Because blades are “upwind” of the tower Require active yaw control to keep facing into wind Operate more smoothly and deliver more power

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Power in the WindConsider the kinetic energy of a “packet” of air with

mass m moving at velocity v

Divide by time and get power

The mass flow rate is

21KE2mv

21 passing though APower through area A 2m v

t

passing though A = = A m m vt

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Power in the WindCombining we get

21Power through are A A 2

a v v

31P A2

v

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

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Power increases as (wind speed)3

Doubling the wind speed increases the power by eight 1h x 20mph wind is same energy as 8h x 10 mph wind

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Power in the Wind (cont.)Power in the wind is also proportional to A

For a conventional HAWT, A = (π/4)D2, so wind power isproportional to the blade diameter squared

Cost is roughly proportional to blade diameter

31P A 2

v

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Maximum Rotor EfficiencyConsider wind passing though turbine: as energy extracted, air

slows down

2 212 dP m v v

ṁ = mass flow rate of air within stream tubev = upwind windspeedvd = downwind windspeed

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Mass Flow RateAt the rotor with area A and, mass flow rate is

If velocity through the rotor vb is the average of upwindvelocity v and downwind velocity vd

bm Av

= 2 2

d dbv v v vv m A

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Power Extracted by the BladesThen power relationship at the rotor could be

Define new parameter such that

We can rewrite the power relationship as

2 21 2 2

db d

v vP A v v

dvv

2 2 21 2 2b

v vP A v v

3 21 1 1 1 2 2bP Av

Power in the wind Rotor efficiency (CP) 23


Maximum Rotor Efficiency

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Number of Rotating BladesWindmills have multiple blades

need to provide high starting torque to overcome weight ofthe pumping rod

must be able to operate at low windspeeds to provide nearlycontinuous water pumping

a larger area of the rotor faces the wind

Turbines with many blades must operate at lowerrotational speeds – as speed increases, turbulencecaused by one blade impacts other blades

Most modern wind turbines have two or three blades

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Estimates of Wind Turbine EnergyNot all of the power in the wind is retained - the rotor spills high-

speed winds and low-speed winds are too slow to overcomelosses,

Depends on rotor, gearbox, generator, tower, controls, and thewind

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

WPBP EP

Power inthe Wind

Power Extracted by Turbine

Electric Power

PCRotor Gearbox &

Generator

g

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Power Generated by H-Wind Turbine

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Air density is lower at higher elevation. For 1000 feet above sea level, ρ is about 1.16 kg/m3

Power = ½ (ρ)(A)(V)3 (η)

= 0.5(1.16)(502)(12)3(0.4)

= 3.15 x 106 Watt

= 3.15 MW

where we assumed the turbine efficiency is 40%.

How much power a wind turbine with 50 meters long blade can generate with a wind speed of 12 m/s? The site of the installation is about 1000 feet above sea level. Assume 40% efficiency (η).


Wind FarmsIt 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 system Centralized access for operations and maintenance

How many turbines should be installed at a site?What is a sufficient distance between wind turbines so thatwindspeed has recovered enough before it reaches the nextturbine?

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Wind Farms Rectangular arrays with only a few long rows are better Recommended spacing is 3-5 rotor diameters between

towers in a row and 5-9 diameters between rows. (to avoidnegative effects of turbulence)

Offsetting or staggering the rows is common

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Using Software to Optimize Wind Farm Layout Data requirements

Topographical data Wind data Technology characteristics of turbines

Given number of devices, wind, optimizes location to maximize output.


The Gansu Wind Farm in China is the largest wind farm in the world, with a target capacity of 20,000 MW by 2020.

chapter 6 renewable energy systems. wind energy

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