Design Wind Power Turbine - Hybrid Anisotropic Materials for Wind Power Turbine Blades - page 18

where. the. pitch. control. system. begins. to. limit. power. output. and. prevent.
generator. and. drive. train. overload.. At. around. 22. to. 25. m/sec.,. the. control.
system.pitches. the.blades.to.stop.rotation,.feathering.the.blades.to.prevent.
overloads.and.damage.to.the.turbine's.components.
The. job. of. the. rotor. is. to. operate. at. the. absolute. highest. eficiency. pos-
sible.between.cut-in.and.rated.wind.speeds,.to.hold.the.power.transmitted.
to.the.drive.train.at.the.rated.power.when.the.winds.go.higher,.and.to.stop.
the.machine.in.extreme.winds..Modern.utility-scale.wind.turbines.generally.
extract.about.50.percent.of.the.energy.in.this.stream.below.the.rated.wind.
speed. compared. to. the. maximum. energy. that. a. device. could. theoretically.
extract,.which.is.59.percent.of.the.energy.stream.(see.The.Betz.Limit).
Most.of.the.rotors.on.today's.large-scale.machines.have.an.individual.mech-
anism.for.pitch.control,.i.e.,.the.mechanism.rotates.the.blade.around.its.long.
axis.to.control.the.power.in.high.winds..This.device.is.a.signiicant.improve-
ment. over. the. irst. generation. of. ixed-pitch. or. collective-pitch. linkages.
because.the.blades.can.now.be.rotated.in.high.winds.so.they.can.be.feathered.
out. of. the. wind.. This. reduces. the. maximum. load. on. the. system. when. the.
machine.is.parked..Pitching.the.blades.out.of.high.winds.also.reduces.operat-
ing.loads,.and.the.combination.of.pitchable.blades.with.a.variable-speed.gen-
erator. allows. the. turbine. to. maintain. generation. at. a. constant,. rated-power.
output..The.older.generation.of.constant-speed.rotors.sometimes.had.instan-
taneous. power. spikes. up. to. twice. the. rated. power.. Additionally,. this. pitch.
system.operates.as.the.primary.safety.system.because.any.one.of.the.three.
independent.actuators.is.capable.of.stopping.the.machine.in.an.emergency.
The.Betz.Limit
Not.all.of.the.energy.present.in.a.stream.of.moving.air.can.be.extracted;.some.air.must.
remain.in.motion.after.extraction..Otherwise,.no.new,.more.energetic.air.can.enter.the.device..
Building.a.wall.would.stop.the.air.at.the.wall,.but.the.free.stream.of.energetic.air.would.just.
low.around.the.wall..On.the.other.end.of.the.spectrum,.a.device.that.does.not.slow.the.air.
is.not.extracting.any.energy,.either..The.maximum.energy.that.can.be.extracted.from.a.luid.
stream.by.a.device.with.the.same.working.area.as.the.stream.cross.section.is.59.percent.of.
the.energy.in.the.stream..This.maximum.is.known.as.the.Betz.Limit.because.wind.turbine.
pioneer.Albert.Betz.irst.derived.it..
The.No..1.target.for.advancement.is.the.means.by.which.the.energy.is.ini-
tially.captured:.the.rotor..No.indicators.currently.suggest.that.rotor.design.
novelties.are.on.their.way,.but.there.are.considerable.incentives.to.use.bet-
ter.materials.and.innovative.controls.to.build.enlarged.rotors.that.sweep.a.
greater. area. for. the. same. or. lower. loads.. Two. approaches. are. being. devel-
oped.and.tested.to.either.reduce.load.levels.or.create.load-resistant.designs..
The. irst. approach. is. to. use. the. blades. themselves. to. attenuate. both. grav-
ity-.and.turbulence-driven.loads.(see.section.2.5)..The.second.approach.lies.
in.an.active.control.that.senses.rotor.loads.and.actively.suppresses.the.loads.
transferred.from.the.rotor.to.the.rest.of.the.turbine.structure..These.improve-
ments.will.allow.the.rotor.to.grow.larger.and.capture.more.energy.without.