Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
33 t
WIND ENERGY CONVERSION SYSTEM
BACI~GROllE~D OF l~E I~T:COI~
1. E'IE~D OF TEIE INVENTION
The present invention relates to wind ~nergy
conversion systems, and more particularly to a control
ystem for a wind machin~ that incorporates a laser Doppler
anemometer to sense wind gust~
2 . DESCRIP~ON OF 1~ PRIOR ART
It ~ w~ll known to u~e a wind mill or wind machine
~or the conver~ion o wind energy to elec ric~l en~rgy. One
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known con truction for ~uch a wind machine incl~des a
rela~ively large propeller havinq a shaft ~hat i9
mechanically csupled to the Yhaft of an electric generator.
Wind inciden~ upon the propeller interacts aerodynamically
wlth the propeller blade~ to impart forces thereon that
rotate the propeller shaft and the generator shaft.
Generally the e~ficiency of a wind energy conversion
machine is dependent upon the :atio of the speed of the wind
to the tip speed of ~he propeller blade which is ba~ed on
the radius of the blade and the angular rotation of the
propeller. The rotational speed of the propeller i~
dependent upon both the wind speed and the magnitude of the
electrical load being powered by the generator.
As is well known, an electrical load impart~ a torque
which counteracts the torque induced by the wind. Thus, the
load torque of the generator tend~ to 510w down the
propeller rotation, while the wind torque tends to increase
the rate of move~ent of the propeller. Consequently, the
rate of rotation of the propeller can be varied for a given
wind ~peed by altering the elec~rical load on the generator.
Such control of ~he propeller rotation rate and according~y
the peed of the propeller blade tip permits adju~tment of
the tip speed ratio for improved efficiency of the wind
energy conversion process.
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~ common problem arising in the use of wind machines
i5 the unpredictable variations of wind speed that result in
corresponding variations in the tip speed ratio. Thi~
pro~lem is of part~cular concern in the case of wind gus~s
wherein the wind speed changes far more rapidly than the
propeller can change its rotational cpeed. A-~ a result, the
efficiency of the wind energy conversion proce~ usually
d~creaRes when there are variable wind speeds and i5
~ignificantly affected by wînd gusts.
One method of dealing with the problem of variable
wind speed and wind gust~ is to employ mechanical
anemometers upwind o~ the wind machine to measure the wind
intensity. Such measurements provide advance information
about wind speed which can then be utilized to alter the
electrical load on the generator so as to optimize the tip
speed ratio.
However, it has been found that the wind receptor
cup~ of mechanical anemome~ers have significant inertia
which retards the transmission of the wind speed
information. Such delay hampers the ability to accurately
control the tip speed ratio.
A further drawback in the u~e of mechanical
anemometers is tbat such anemometers measure wind speed only
at the site of the anemome~er. Since wind speed
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measurements can vary markedly from point to point, the wind
speed data obtained by use of a single mechanical anemometer
may be inadequate and may not properly describe the wind condition
which will be experienced by the propeller of the wind machine.
Thus a mechanical anemometer does not adequately provide the
information needed to attain optimum control of the tip speed
ratio.
SUMMARY OF THE INVENTION
Against the foregoing background the invention seeks to
provide a novel, high efficiency wind energy conversion system.
Further, the invention seeks to effectively control the
propeller tip speed of a wind machine in response to sensed wind
speed without the disadvantages inherent in mechanical anemometers.
Still further, the invention seeks to control the propeller
tip speed of the wind machine by varying the electrical load
of a generator connected to the propeller in response to wind
speed data obtained by laser Doppler measurement of wind speed.
One broad aspect of the invention pertains to a wind energy
conversion system comprising a propeller rotatable by force of
wind, a generator of electricity mechanically coupled to the
propeller for converting power of the wind to electric power for
use by an electric load, and means coupled between the generator
and the electric load for varying the electric power drawn by
the electric load to alter the electric loading of the generator.
Means are provided for electro-optically sensing the speed of
the wind at a location upwind from the propeller and means is
coupled between the sensing means and the power varying means
for operating the power varying means to adjust the electric
load of the generator in accordance with a sensed value of
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wind speed to thereby obtain a desired ratio of wind speed to
the speed of a tip of a blade of the propeller.
Additional aspects, advantages and novel features of the
invention shall be set forth in part in the description that
follows, and in part will become apparent to those skilled in
the art upon examination of the following, or may be learned by
the practice of the invention. The aspects and the advantages
of the invention may be realized and attained by means of the
instrumentalities and combinations particularly pointed out
in the appended claims.
To achieve the foregoing and other aspects, and in accord-
ance with the purpose of the present invention, as embodied and
broadly described herein, the wind energy conversion system
comprises a propeller rotatable by the force of wind, and a
generator of electricity mechanically coupled to the propeller.
The generator serves to convert the power of the wind to electric
power for use by an electric load. Circuitry for varying the
power applied to the load is coupled between the generator and
the load, the circuitry permitting adjustment of the loading of
the generator to attain a desired ratio of wind speed to the
tip speed of the propeller blade. Also included in the invention
is an electro-optic subsystem for the sensing of the wind speed
at various locations upwind of the propeller, an output of the
subsystem being used to operate the circuitry for varying the
loading on the generator. The response time of the electro-
optic sensing is significantly faster than that associated with
mechanical anemometers so as to provide the capability of
maintaining a desired tip speed ratio even in the presence of
gusty winds.
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8RIEF DB5CRIPTIO~ OF T~B DRA~I~GS
The accompanying drawings of the wind energy
conver~ion sy~tem whl~h are incorporated in and form a part
o~ the -~pecification illustrate a preferred embodimen~ of
the present invention, and together with the description,
serve to explain the prlnciples of the invention. In the
drawing~:
F~G. 1 is a bloc~ diagram of a wind energy conversion
system with electro-optic sensing of wind speed for co~trol
of the.tip speed ratio that incorporates one embodiment of
the invention; and,
FIG. 2 is a block diagram of a wind measuring
subsy tem thereof comprising laser beam transmission and
reception equipment, and digital storag~ and correlation
circuitry for mea~urement of Doppler frequency shift
a~sociated with moving particles in air.
Corresponding reference characters indicate
corresponding part~ throughout the several views of the
drawings.
20D~TAIL~N D~SCRIP~IO~ OF T~E PR~F~RR~D ~MBODI~T
A wind energy conver ion ~ystem incorporat~ng a
preferred embodiment o~ the inven~ion i~ generally indica~ed
- by the reference numb~r 20 in Fig. 1.
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The wind energy conversion ~ys~em 20 includes a wind
mill or machine 22 which ~n~eracts wlth the wind, indicated
by an arrow 24, for driving an electric generator 26. ~he
wînd machine 22 i-~ hown ln stylized view, and includes a
propeller 28 moanted on a propeller shaft 30 set on top o~ a
tower 32. ~he shaft 30 i~ mechanically coupledt via dashed
llne 34, to the generator 26, as well a~ to a sensor 36 of
the angular velocity of the shat 3Q0 The genera~or 26,
which may be an alternator providing three-pha~e current,
10~ via wire~ 38, connects with a load 40 via power-control
circuitry 42.
A wind measuring sub~ystem 44 is included within the
energy conversion system 20 for sensing the pre~ence of wind
gusts, as indicated by the arrow 46, upwind of the machine
22. The subsy~tem 44 provides a measure of the wind speed
associa~ed with such gusts and, with reference to Fig. 2~
includes a laser beam transmitter 48, a laser beam receiver
50 and a digital data processor 5~.
The proce~sor 52 i~ coupled to the transmitter 48 and
the receiver 50 for the extraction of wind speed data
a~ociated wlth Doppler frequency shift imparted to a la~er
beam by moving particle aerosols 53 in the gu ting air upwind
of the machine 22, the aerosole 53 acting a~ scatterer~0
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The system 20 further compri3es electrical circuitry
54 for the determination of the actual tip ~peed ratio, an
ad~ustable source 56 of a reference voltage s$gn~1 serving
a~ a reference tip speed, a ~umming device 58 and an
integrator 60. The ~ip peed circuitry 54 can include well-
known circul~ry, ~uch as that ~ound in a microproces~or, for
the division of one electrical signal by a second electrical
8~ gnal o prov$de an output electrical signal on ~he line 62
repre~enting the ratio of the speed of the wind to the ~peed
of the blade tip of the propeller 28~
In operation, the wind measuring subsyqtem 44
provide~ an output signal on the line 64 representing the
mea~ured speed of a wind gust 46 prior to the time when the
gu3t 46 reaches the propelle~ 28. The shaft speed qensor
36, which can include a known shaft angle encoder or
tachometer, provides an output electrical ~ignal on line 66
which i3 proportional to the angular rate of rotation of the
shaft 30 and the propeller 28, the signal on the line 66
also being proportional to the tangential speed of each
blade tip of the propeller 28.
The ratio circu$try 54 divides the ~ignal on the line
64 by the ~ignal on the line 66 to provide a ratio ~ignal
via the line 62, which 18 coupled to a negative input
terminal of the summing device 58. The reference signal of
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the -qource 56 is applied to a positive input terminal of the
~umming device 58~ Thu~ the summing device 58 applies an
error signal to the integrator 60, the error 9ignal being a
mea~ure of the difference between a desired or re~erence
Yalue of the tip speed ratio and the anticipated ratic at
the time the propeller 28 receives the wind gust 46.
The int~grator 60 integrates the error signal of th~
summing device 5~ in accordance with well-known principle~
of feedback control theory, to produce a drive signal on the
line 68 for operating the power control circuitry 42. The
circuitry 42 can include a known silicon control rectifier
(SCR) circuit connected to each o~ the wires 38, the control
terminal of each of the SCRs being driven by the drive
~ignal on the line 6~. Tbe drive signal on the line 68
establishes the duty cycle in the extraction of alternating
current from the wire~ 38 and hence, the average power
delivered by the generator 26 via the circuitry 42 to the
load 40.
It should be noted that the coupling of electric
power from ~he generator 26 to the load 40 introduces a
retarding torque which tend~ to 910w down the rotational
speed of ~he generator and, a3 a consequence, the rota ional
~peed of the propeller shaft~ Thu~, the re~arding ~orque of
the generator 26 tends to counteract the torque induced on
the propeller 28 by action o~ the wind impinging thereon.
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rt can also be appreciated that a reduction in ~he
duty cycle of the output current of the generator 26 permit~
the propeller shaft 30 to speed up, while an increase in ths
duty cycle of the output curren~ of the generator 26 causes
a decrease in the speed of the propeller shaft 30.
Consequently, variations in the magnitude of the drive
signal on the line 68 can produce changes in the rotational
cpeed of the shaft 30. In thiR manner, the drive signal i~
used to adjust the speed of the shaft 30 so a~ to provid~
the desired ratio between the wind speed and the tip speed
o the propeller b~ades. A ratio of approximately unity is
desirable in that it maximizes the efficiency of the
conversion o~ the wind energy or power to the electric
energy or power.
The response o~ the propeller 28 to a change in load
torque accomplished by the power-control circuitry 42
depends on the inertia of the propeller 28, the generator 26
and the connecting mechanical apparatus~ The integrator 60,
which may be fabricated as a low pass filter slows down the
re~ponse of the feedback loop configura~ion of the system
20. This slowdown ta~es into account the response time of
the propeller 28, so as to in~ure a controlled oscillation
free transition in propeller speed and the proce~ of
ad~u tment in the tip speed ratio. Such closed-loop ~peed
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control concept3 are known and accordingly need not be
de~cribed herein. The sy3~em 20 therefore adjusts the tip-
speed ratio by alteration of the electrical load on the
generator 26.
The correction of the tip speed ratio i~ based on the
anticipated wind peed, as measured at a time a~d location
prior to the arrival of a wind gust at the propeller 28.
The prior knowledge o~ the wina speed i3 attained with the
aid of the wind measuring sub~ystem 44 which operate3 by
mean~ of photo~electric circuitry including the laser
transmitter 48 and the receiver S0.
Referring now to Fig. 2, the details of the
construction and operation of the subsystem 44 will now be
described in connection with the laser tran mitter 48, the
laser receiver 50 and the data proce~sor 52 previou~ly
described with reference to Fig. 1.
The laser beam transmitter 48 includes a laser 70, a
modulator 72 for pulsing the beam of light from the laser
70t and a beam deflector 74 which deflecks the pulsed beam
of light from the modulator 72 over a predetermined ~can
angle.
The light @xiting the beam deflector 74 i3 directed
to the reg~on of the aerosol~ 53. ~he beam deflector 74 ca~
include an oscillating mirror (not shown) but preferably
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comprises an acousto-optic Bragg cell 76 driven by a
piezoelectric transducex 78 activated by a radio frequency
(RF) drive~ 80. The transducer 78 i5 located at one end of
the cell 76 and a sonic ab~orber 82 is located at the
opposite end of the cell 76. ~coustic waves are generated
within the ~ranspare~t material of the cell 76 and are
absorbed by ~he absorber 82.
Interaction of the light wave from the laser 70 with
the acoustic wave from the tran~ducer 78 results in a known
deflection of the light beam. A generator 84 applies an R~
~ignal to the driver 80, the R~ signal being of suitable
frequecy for deflecting the light beam in a desired
direction. A timing unit 86 in the data proces~or 52
provides timing ~ignals for synchronizing the operatlons of
the generator 84 and the modulator 72 with operation of the
processor 52.
The beam receiver 50 comprises a lens assembly 88
having a field lens 90 and an objective lens 92, a line
filter 94 and a four-quadrant detector 96. Light from the
laser 70 impinges upon the aerosols 53 and is reflected back
to the lens assembly 88, which direct~ the received light
through the filter 94 to focus upon the detector 96.
The len~ a~sembly 88 pas3es light at ~he original
~requency of the laser 70, as W211 as light which has been
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shifted in frequency by a Doppler frequency shift associatéd
with motion of the aerosols 53. While there i~ a measure of
random movement as ociated with the aerosols 53, there i
al30 a bulk movement a~sociated with air movement or wind.
Accordingly, the Doppler shift contains data as to the wind
speed.
The region of focus of the received light upon the
detector 96 varies in accordance with ~he region of the air
illuminated by the scanned beam from the deflector ~4. The
~canning beam illuminates a much larger region of space than
would a stationary beam, and thereby provides speed data
~rom a much larger region of space than could be provlded by
a single mechanical anemometer. The location of the region
of focu~ of the received light on the detector 96
corresponds to the region of space which has been
illuminated by the scanned beam from the deflector 74.
The four quadrants o~ the detector 96 are numbered 1,
2, 3, and 4, each of the quadrants being connected to
corresponding channel~ of a four-channel amplifier 98. T~e
detector 96 has x and y axe~ ~uperimpo4ed to identify a two-
dimensional display of dat~ from the scanned reg~on. The .
detector 96 and the amplifler 98 can include a known photo-
electron emis-qive material and a set of photo multipliers
(not shown) whereby a set of four output signal~ are
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12a~5z~
obtained corresponding to tbe illumination of each o~ the
four quadrants. The output signals of the amplifier 98 are
applied to a channel ccmbiner 100 of the data proces~or 52.
The line filter 9~ is of known con~truction and
filters oot or attenua~e~ optical energy at the tran~misslon
frequency of the laser 70. In th~ absence of any Doppler
shift, a minimum intenqity of light is reoeived at the
detector 96 becau e substan~ially all of ~he optîcal energy
is filtered out by the filter 94.
The optical configuration of the laser beam receiver
50 mitigates the effect~ of received beam wander induced by
atmospheric variation~ in the refractive index due to
temperature gradients in the air.
In the presence of Doppler shift, the energy
associated with the optical spectrum of the shifted
frequencies passes through the filter 94 to impinge upon the
detector 96. Consequently, the four output signals of the
amplifier 98 provide information both as to the location of
sources of Doppler ~requency shift as well as the amount of
such hift. In addition, the use of the filter 94 prevents
overloading of the detector 96 and the amplifier 98 so as to
perm~t these COmpOQent~ to be adjusted for a maximum
sensitivity at the anticipated Doppler frequencies,
The data processor 52 combines the aforementioned
~ timing u~i~ 86 and the channel combiner 100, and further
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includes a pair o ~ilters 102 and a pair of ~ignal samplers
104 connected to output channel~ of the combiner 100. Also
included within the data processor 52 are a storage unit
106, a correlator 108, a Fourier transformer 110 and a
di criminator 112. The ~amplers 104~ the skorage unit 106
and the correlator 108 are operated by clock signals of a
clock (not shown~ within the timing unit 86.
In operation9 the combiner 100 sums together the
quadrant signals of the right and the left sides of the
detector 96 and outputs their di~ference a~ the output x
channel signal. Similarly, the combiner 100 su~s together
the signals of the upper and lower quadrants of the detector
96, and outputs their difference as the y channel signal.
The filters 102 filter the a~and y ou~put signal of the
comblner 100, the filters 102 being band-pas~ filters which
limit the spectrum of the received signals to the band of
interest, thereby improving the signal-to-noise ratio. The
samplers 104 provide digital sampling of the x and y channel
signals from the filters 102, the digital samples being
stored in sections 114 of the memory 106.
Ag portrayed in a graph 116 in the.block of the
correlator 108, a sucoession of samples of the Doppler
~hifted ~ignal ls taken by each of the samplers 104. These
sample~ ~re then correlated agains~ themselves, an
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autocorrela~ion by ~he correlator 108 to produce an output
correlation function containing the desired wind speed data.
The correlation may be done separately for the x and the y
channel~, and may al50 be done a~ a cross correlation
be ween the x and the y channels.
The extraction of Doppler data by means of the
correlation operation i~ de cribed in the article
~Measuremenk of Cro~ Wind Velocity~ by F. Durst et al,
Applied Optics, Vol. ~1, No. 14, July lSr 1982~ page~ 2596-
2607.
The spectral components of the correlation functions
are obtained by the transformer 110 which may be implemented
as a well-known fast-Fourier transformation. An amplitude
discriminator 112 selects the frequency components having
the largest amplitudes as being representative of the wind
speed. The amplitude of the wind speed appears as an output
signal of the discriminator 112 on the line 64 to be applied
to the circuitry 54 (Fig. 1) for calculation of the tip
speed ratio.
In the selection of sample~ for the correlation, it
may be desirable to u~e only those amples appearing within
a preselected time interval after the transmis~ion of a
pulse o~ laser light. Such a gelection of 3amples provides
the function of range gating for mea~urement of wind cpeed
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at a pre~elected range ~rom the propeller 28. As a
practical ma~ter, in 'che implementation of the data
proces or 52, the function3 of the correlator 108, the
transmitter 110, and the descriminator 112 can readily be
attained by use of a microprocessor which is suitably
programmed to provide the fore~oing functions.
~ umerous programs for correlation, Fourier
transformation and selection of signal~ based on amplitude
are available for use with commercially available
10 mlcroprocessorY.
By means of the foregoing system, the loadiny of a
wind operated electrical generator can be altered in
accordance with infor~ation of future wind speed, the
information being obtained before a gust of wind reaches the
propeller of a wind machine. Such alteration of the loading
of the generator, and thus of the propeller itself, permits
adjustment of the tip speed ratio of the propeller blades so
as to operate the wind machine efficiently by adapting the
speed of the propeller to match the wind speed. Thereby,
there i~ a more efficient conversion of wind energy to
electrical energy.
The use of electro-op~ic Doppler measurement
technique~ of a volume of air upwind of the wind machine,
in combination with data processing of received Doppler
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slgnal echoes permit~ a rapid measurement o the wind speed.
~he rapidity of measurement permits adjustment sf the
electrlcal loading to be accompli~hed before any wind gu~t
reache~ the propeller. Al~o the employment of electro-optic
scanning of the la~er beam permits the gathering of data
over a much larger area of space than can be obtained by
mechanical anemometers.
The foregoing 1~ considered as illustrative only of
the principles of the invention. ~urther, since numerouq
modi~lcations and change~ will readily occur to ~ho~e
skilled in the art, it i5 not desirable to limit the
invention to the exact construction and operation shown and
de3cribed, and accordingly all suitable modifications and
equivalents may be resorted to falling within the scope o
the invention as defined by the claims which follow.
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