Design Wind Power Turbine - Hybrid Anisotropic Materials for Wind Power Turbine Blades - page 29

direction.from.what.a.pitch-controlled.machine.does..In.other.words,.it.will.
increase.the.angle.of.attack.of.the.rotor.blades.in.order.to.make.the.blades.go.
into.a.deeper.stall,.thus.wasting.the.excess.energy.in.the.wind.
One.of.the.advantages.of.active.stall.is.that.one.can.control.the.power.out-
put.more.accurately.than.with.passive.stall,.so.as.to.avoid.overshooting.the.
rated. power. of. the. machine. at. the. beginning. of. a. gust. of. wind.. Another.
advantage.is.that.the.machine.can.be.run.almost.exactly.at.rated.power.at.all.
high.wind.speeds..A.normal.passive.stall-controlled.wind.turbine.will.usu-
ally.have.a.drop.in.the.electrical.power.output.for.higher.wind.speeds,.as.the.
rotor.blades.go.into.a.deeper.stall.
The.pitch.mechanism.is.usually.operated.using.hydraulics.or.electric.step-
per.motors..As.with.pitch.control,.it.is.largely.an.economic.question.whether.
it. is. worthwhile. to. pay. for. the. added. complexity. of. the. machine. when.the.
blade.pitch.mechanism.is.added.

2.6.5 Individual Pitch Control

The. current. development. trend. is. toward. larger,. higher. power,. wind. tur-
bines. in. order. to. reduce. the. ultimate. generating. cost. per. kWh.. Increasing.
wind. turbine. size. implies. larger. rotor. diameters. and. rotor. sweep. areas..
This.results.in.a.less.uniform.wind.ield.over.the.swept.area,.which.imparts.
uneven.loads.on.the.blades,.drive.shaft,.and.turbine.structure..These.uneven.
loads.increase.component.wear,.reduce.eficiency,.and.increase.the.amount.
of.maintenance.downtime.required.
Individual.pitch.control.helps.address.these.issues.and.provides.accurate,.
real-time.load.information.from.each.blade. 7

2.6.6 Other Power Control Methods

Some. older. wind. turbines. use. ailerons. (laps). to. control. the. power. of. the.
rotor,.just.like.aircraft.use.laps.to.alter.the.geometry.of.the.wings.to.provide.
extra.lift.at.takeoff.
Another.theoretical.possibility.is.to.yaw.the.rotor.partly.out.of.the.wind.to.
decrease.power..This.technique.of.yaw.control.is,.in.practice,.used.only.for.
tiny.wind.turbines.(1.kW.or.less),.as.it.subjects.the.rotor.to.cyclically.varying.
stress,.which.may.ultimately.damage.the.entire.structure.

2.7 WindTurbineComponents

The. yaw. drive. is. an. important. component. of. the. horizontal. axis. turbine's.
yaw.system..To.ensure.the.wind.turbine.is.producing.the.maximal.amount.
of.electric.energy.at.all.times,.the.yaw.drive.is.used.to.keep.the.rotor.facing.