Design Wind Power Turbine - Hybrid Anisotropic Materials for Wind Power Turbine Blades - page 28

is. that. its. power. density. is. higher. than. electrical. equipment. and. it. needs.
fewer.components,.making.for.a.simpler.system..There.are.other.pluses.
Rotating. unions,. such. as. those. from. Deublin,. are precision. mechanical.
devices.for.transferring.luid.from stationary.sources.to.rotating.machinery..
Ball.bearings.in.a.typical.union.support.the.rotating.components.(attached.
to. the. machinery). against. the. stationary. component. (attached. to. the. luid.
supply).and.a.mechanic.seal.prevents.leaks..While.often.found.on.wind.tur-
bines,. rotating. union. devices. work. well. in. other. applications,. such. as. air.
clutches,.gearboxes,.machine.tool.spindles,.and.more.

2.6.3 Stall-Controlled Wind Turbines

Passive.stall.controlled.wind.turbines.have.the.rotor.blades.bolted.onto.the.
hub.at.a.ixed.angle..The.geometry.of.the.rotor.blade.proile,.however,.has.
been.aerodynamically.designed.to.ensure.that.the.moment.the.wind.speed.
becomes.too.high,.it.creates.turbulence.on.the.side.of.the.rotor.blade,.which.
is.not.facing.the.wind..This.stall.prevents.the.lifting.force.of.the.rotor.blade.
from.acting.on.the.rotor. 9
In.Chapter.7,.you.will.see.that.as.the.actual.wind.speed.in.the.area.increases,.
the.angle.of.attack.of.the.rotor.blade.will.increase,.until.at.some.point.it.starts.
to. stall.. If. you. look. closely. at. a. rotor. blade. for. a. stall-controlled. wind. tur-
bine,.you.will.notice.that.the.blade.is.twisted.slightly.as.you.move.along.its.
longitudinal.axis..This.is.partly.done.in.order.to.ensure.that.the.rotor.blade.
stalls.gradually.rather.than.abruptly.when.the.wind.speed.reaches.its.critical.
value..(Other.reasons.for.twisting.the.blade.are.mentioned.in.the.previous.
section.on.aerodynamics.)
The. basic. advantage. of. stall. control. is. that. it. avoids. moving. parts. in. the.
rotor. itself. and. a. complex. control. system.. On. the. other. hand,. stall. control.
represents.a.very.complex.aerodynamic.design.problem,.and.related.design.
challenges. in. the. structural. dynamics. of. the. whole. wind. turbine,. e.g.,. to.
avoid.stall-induced.vibrations..Around.two-thirds.of.the.wind.turbines.cur-
rently.being.installed.in.the.world.are.stall-controlled.machines.

2.6.4 Active Stall-Controlled Wind Turbines

An. increasing. number. of. larger. wind. turbines. (1. MW. and. up). are. being.
developed. with. an. active. stall. power. control. mechanism.. Technically. the.
active. stall. machines. resemble. pitch-controlled. machines,. since. they. have.
pitchable.blades..In.order.to.get.a.reasonably.large.torque.(turning.force).at.
low.wind.speeds,.the.machines.will.usually.be.programmed.to.pitch.their.
blades.much.like.a.pitch-controlled.machine.at.low.wind.speeds..(Often.they.
use.only.a.few.ixed.steps.depending.upon.the.wind.speed.)
When. the. machine. reaches. its. rated. power,. however,. you. will. notice. an.
important. difference. from. the. pitch-controlled. machines.. If. the. generator.
is.about.to.be.overloaded,.the.machine.will.pitch.its.blades.in.the.opposite.