Tong W. Wind Power Generation and Wind Turbine Design

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New Small Turbine Technologies 405

7.2 Passive yaw control with downwind system

With downwind WTs, the rotor functions as a tail wing and it has passive yawing-

function to make WTs follow the wind direction. A typical commercial downwind

wind turbine is shown in Figure 9. No power source is necessary for yaw control,

but stability of yaw motion needs to be carefully investigated, because when one

more freedom of the system is added, instability could happen under accelerated

or decelerated conditions of rotor speeds.

8 Power/speed control

Power control/rotor speed control is an essential part of whole system, upon which

both performance and safety depend. As described in the previous sessions, an

optimal aerodynamic performance design can start, provided a feasible power/

speed control is available.

8.1 Initial start-up control

A WT with pitch control system are capable to make start-up at lower wind speed by

regulating the pitch angle of the blades. However, SWTs of ﬁ xed pitch rotor blades

Figure 8 : Swing rudder (Courtesy of Zephyr Corporation).

Figure 9 : Yaw control system with downwind WT (Courtesy of Proven Energy Ltd.).

406 Wind Power Generation and Wind Turbine Design

must wait until stronger wind starts to rotates the rotor into operation. Power assist

system can catch the chance to bring the WT into operation when wind speed is

near cut-in wind speed. It works just the same as variable pitch system. The power

assist system is driven by a motor, which means it needs additional power source.

This may lose certain energy but such energy loss would be just marginal since the

start-up mechanism will not be activated during constant rotation operation, and

that if improved rate of operation is considered, the gain will be larger.

8.2 Power/speed control

In the whole operation wind speed range, power/speed control must be reliable

and fail-safe. Therefore, the control system provides more than two independent

control or brake mechanisms.

8.2.1 Aerodynamic methods

There are two aerodynamic control methods. One is traditional stall regulation. In

this method, when a gust attacks the rotor, a separation will occur on a blade which

decreases the torque resulting in power/speed regulation, preventing over speed of the

rotor as well. It is considered stall regulation method is more suitable in the condition

of relatively stable wind of small turbulence. However, active stall system has been

developed that is more reliable and available for higher turbulent wind conditions.

According to fail-safe concept, a stall regulation is usually supported by other

systems such as mechanical brake. Collapsible rotor blade tip is an aerodynamic

device for over-speed limitation.

Variable pitch control or pitch regulation is also aerodynamic method. The gen-

erated torque of the rotor as well as the lift on the blades is controlled by the attack

angles by changing the pitch angle. Hydraulic power or motor(s) are necessary to

change the pitch angle(s) by way of some movable mechanical elements such as

links. Variable pitch control is reliable and works also at starting-up at cut-in state

and shutdown at cut-out state.

8.2.2 Mechanical methods

All kind of mechanical brakes are possible. A mechanical method need consider-

able power source to shut down the system without any aerodynamic method. Dur-

ing the shutdown, all the kinetic energy lost from the rotor is converted to thermal

energy that will be absorbed into the brake materials.

8.2.3 Electro-magnetic methods

Electro-magnetic method is also possible. The lost kinetic energy from the rotor is

converted to thermal energy and temperature monitoring/control is very important.

One 1 kW SWT with such an electro-magnetic regulation has an experience that

the WT operates during high wind under a typhoon attack. The turbine was generat-

ing power at 300–650 W under the wind speed range of 27–37 m/s until just before

5 s when the maximum wind speed of 47 m/s blew through the site as illustrated

in Fig. 10 . The turbine immediately responded to the extreme wind by falling into

New Small Turbine Technologies 407

idling motion. Thus the WT can resist against extreme winds under typhoons as

well as capture energy instead of shutting down. This feature of high wind opera-

tion is especially very important for remote sites like offshore locations or high

mountain uninhabited sites.

9 Tests and veriﬁ cation

9.1 Safety requirements

Safety requirements are described in IEC Standard 81400-2. Basic procedure of

certiﬁ cation is also indicated. Particularly for SWTs, key items for demonstration

or veriﬁ cation of technical integrity of a SWT are introduced. Informative Annex

A gives the structure of type certiﬁ cation of SWTs as shown in Fig. 11 .

9.2 Laboratory and ﬁ eld tests of a new rotor

After system design including aerodynamic design and structural design, then

proto type tests and veriﬁ cation are required.

Regarding to rotor design, at least three methods of rotor performance tests are

possible.

9.2.1 Wind tunnel test

Provided a wind tunnel facility is available in scale and in wind speed range, the

rotor performance vs. arbitrary wind speed as desired can be captured at any time by

wind tunnel testing. However, the stable performances obtained under stable winds

100

200

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1000

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Power(W), RPM, Wind direction (deg)

Wind Speed (m/s)

Time (MM/YY/HH/MM/SS)

RPM

Wind Direction

Max Wind speed: 47m/s

Power W

Wind speed

Figure 10 : An operation data of a SWT under a typhoon attack.

408 Wind Power Generation and Wind Turbine Design

in a wind tunnel seriously differ from the dynamic rotor performances obtained in

the turbulent ﬂ ow, sometimes gusty ﬂ ow over a natural open test ﬁ eld.

A wind tunnel test is capable of verifying the basic stable design power perfor-

mance. If proper corrections of the wind tunnel wall effect are not done, the power

curve might be higher than the actual because of the channel ﬂ ow effect of the

wind tunnel. If the tested WT is scale-down model, Reynolds number effect shall

be carefully analysed.

9.2.2 Track test

Track test is an alternative method with wind tunnel test, in which relative wind

speed is realized by driving a vehicle with a test turbine on it along a test course

of a vehicle. The relative wind speed is as high as that of running vehicle. If the

maximum speed is 180 km/h, even an extreme wind speed of 50 m/s can be cre-

ated. Since an extreme wind speed, which is a maximum 10-min averaged wind

speed with 50 years recurrence period for IEC Class I turbines, is a very rare

case to occur and cannot usually happen during a few years ﬁ eld test. Therefore,

it is quite useful test method for structural safety tests and power regulation or

brake tests.

Figure 12 shows a vehicle for track test. A SWT is ﬁ xed on a support structure

on the vehicle. An anemometer is also mounted on the vehicle at a suitable

position where ﬂ ow distortion by the vehicle is the least.

Design

Evaluation

Type

Testing

Manufacturing

Evaluation

Foundation

Design

Evaluation

(optional)

Type

Characteristic

Measurements

(optional)

Type

Certification

Safety and

Function Tests

PowerPerforman

Measurements

Duration Tests

Other Tests

(Environmental impacts, etc.)

Control

and

Protection

System

Test

and

Design

Data

Load

and

Load

Cases

All

Compo-

nents

Static

Blade

Tests

Componets

Test

Foundation

Design

Require-

ments

Design

Control

Manufactur

-ing

Plan

Installation

Plan

Mainten-

ance

Plan

Personnels

Safety

(Evaluation of)

Figure 11: Structure of IEC type certiﬁ cation of SWTs .

New Small Turbine Technologies 409

If the natural wind does not affect seriously, a series of steady performance

tests and structural tests including extreme conditions can be conducted. Highest

attentions shall be given for personal safety and structural safety.

Another valuable task is development of control strategy, which relates power

control, speed control, load regulation, break adjustment and yaw control.

Figure 13 is a sample of track test data of a Micro WT of 1.8 m rotor diameter.

Power output data are shown for two values of volt of battery load. By conducting

parametric tests, optimal control parameters can be found.

9.2.3 Field test

Final evaluation and validation of a SWT’s power performance must be

determined by ﬁ eld tests. It is because actual energy products from a WT are

obtained under free wind ﬂ ow varying with time both in speed and in direction.

Figure 12 : A vehicle for track test.

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

0246810121416182022

Wind speed (m/s)

Power Output (W)

28 Volkt

26 Volt

Figure 13: A sample of track test, wind speed vs. power.

410 Wind Power Generation and Wind Turbine Design

The power performance is statistically expected values of power output and

Bin method is applied.

Figure 14 shows a ﬁ eld test data of a 1 kW SWT. The sampling period is 1 s and

the period is continuous 8 h. Wind speed is varying from 5 to 15 m/s. Due to vari-

ous uncertainties in measurement, data scatter widely. Therefore, a statistically

expected power curve represents the performance of a WT.

A power performance curve is decided by applying Bin-averaging method (Bin

method).

In Fig. 15 , 1-min averaged data are plotted with symbol ×.

For SWTs, data shall be collected continuously at a sampling rate of 1 Hz or faster

and every 1-min data set gives a 1-min mean value, 1-min standard deviation, 1-min

maximum value and 1-min minimum value for wind speed and power output. Then

plenty of mean values are further averaged by Bin method as follows:

Bin i i j

∑

( 19 )

Bin i i j

∑

(20 )

where i is the Bin number (when using 0.5 m/s Bins, 0.5 i – 0.25 ≤ V

i,j

< 0.5 i – 0.25);

i,j

is the normalized wind speed of data set j in Bin i ; P

i,j

is the normali zed power

output of data set j in Bin i ; V

Bin, j

is the Bin-averaged wind speed in Bin i ; P

Bin, j

is the

Bin-averaged power output in Bin i ; N

is the number of data sets in Bin i .

The white circles in Fig. 15 are Bin-averaged values.

A power performance curved is completed if sufﬁ cient data sets cover all the

operation range of wind speed. Then an uncertainty analysis follows.

Figure 14: A sample of ﬁ eld test data of a 1 kW SWT.

200

400

600

800

1000

1200

1400

1600

1800

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

P (W)

V (m/s)

T (sec)

V (m/s)

P (W)

IWAKI DATA 2007.05.27

New Small Turbine Technologies 411

10 Captureability

There are various parameters that should be optimized such as power coefﬁ cient,

capacity factor, cost of energy, etc.

Parameter “captureability” is a new indicator which shows how much energy

has been captured from the nature in a certain period [ 3 ]. Now assume a WT has a

power curve function P ( V ) and the wind at the site has probability density function

f ( V ) vs. wind speed V .

Choosing a continuous time period T , the following formulas are obtained:

Wind power generation • G during period T :

() ()dGTPVfVV

∞

=⋅

∫

( 21)

Cumulative wind energy ﬂ ux • E

past through the rotor swept area A during

period T :

() ()d

ETEVfVV

∞

=⋅

∫

(22 )

where E ( V ) = r AV

r V

p R

Designed maximum WT generation G

under the assumption that WT has

constantly operated at rated power P

rated

during the period T is given as:

R rated

GP T=⋅ ( 23)

200

400

600

800

1000

1200

1400

1600

0 2 4 6 8 10 12 14 16

P, Pbin (W)

V (m/s)

P1m-ave

Pbin

Figure 15 : Sample of 1-day ﬁ eld test data at 1-s sampling rate and Bin-averaged

data.

412 Wind Power Generation and Wind Turbine Design

Then capacity factor C

is:

(24 )

Captureability C

is deﬁ ned as:

(25 )

Captureability is an absolute indicator of the wind turbine plant performance

reﬂ ecting both WT performance and wind characteristics at the site. But capacity

factor is not absolute indicator because rated power can be changed on purpose. For

example, if a WT of very low rated wind speed is built at an excellent wind site, its

capacity factor could be very high although captureability would be quite poor.

When the wind speed probability function is expressed by Weibull function and

the system efﬁ ciency at rated condition is given, the following equation is obtained:

ave

PT rated rated rated

() ()

1(13/)

(1 1 / )

EV fV

CP V

⋅

⎛⎞

Γ+

⎜⎟

⎝⎠

Γ+

∑

(26 )

where V

ave

is the annual mean wind speed, V

rated

is the rated wind speed, h

rated

is the

system efﬁ ciency at rated condition and Γ is the Gamma function.

Figure 16 shows general relations between C

/ C

and V

ave

/ V

rated

for h

rated

= 0.4

and for different Weibull parameter k .

0.8 1 1.2 1.4 1.6 1.8 2 2.2

CF/CPT

Vrated/Vave

CF/CPT(k=2)

CF/CPT(k=1.8)

CF/CPT(k=1.6)

CF/CPT(k=1.4)

Figure 16 : General relations between C

/ C

and V

ave

/ V

rated

( h

rated

= 0.4, Weibull

shape parameter k = 1.4–2.0).

New Small Turbine Technologies 413

Using the power curve of a 1 kW SWT, expected captureability C

and capacity

factor C

are shown for different wind Weibull shape parameter k in Fig. 17 .

As a conclusion, an optimal design concept is to design a WT and choose a site

so that the captureability is maximized. This is the same as traditional optimal

design concept to maximize the annual energy product. In spite of this fact, there

are wide tendency to evaluate WT plants by capacity factor.

Therefore, a performance evaluation should be done by captureability rather

than by capacity factor.

References

[1] IEC 61400-2 2

Ed., Wind turbines – Part 2: Design requirements for small

wind turbines, 2006.

[ 2] Hansen, Martin O.L., Aerodynamics of Wind Turbine , Earthscan, 2008.

[ 3] Matsumiya, H., et al. , Field Operation and Track Tests of SHWT “Airdolphin”

under High Wind Conditions, EWEC2008, 2008.

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 2 4 6 8 10 12 14

CPT, CF

Vave (m/s)

CPT(k=1.4) CF(k=1.4)

CPT(k=1.6) CF(k=1.6)

CPT(k=1.8) CF(k=1.8)

CPT(k=2.0) CF(k=2.0)

Figure 17: A sample calculation of captureability and capacity factor of a SWT

with parameters annual mean wind speed and Weibull parameter k .

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