Tong W. Wind Power Generation and Wind Turbine Design

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New Small Turbine Technologies 395

its scope is mostly limited within the design safety parameters of SWTs. The pri-

mary goal of the new task is to give incentives to this industrial sector to improve

the technical reliability of SWTs and therefore their performance.

4 Wind conditions

4.1 External conditions

SWTs are subjected to external conditions, which are divided into wind condi-

tions, other environmental conditions and electrical conditions [ 1 ].

4.1.1 Wind turbine class

Wind conditions are the primary external conditions and SWT classiﬁ cation is

given in IEC standard IEC 61400-2 depending on wind conditions as shown in

Table 2 .

Reference wind speed is a basic parameter which is deﬁ ned as an extreme

10 min averaged wind speed with a recurrence period of 50 years. For standard

SWT classes I–IV, annual average wind speed is given by the formula:

ave ref

0.2VV=

(5 )

4.1.2 S class

Offshore conditions and conditions experienced in tropical storms (hurricanes,

cyclones and typhoons) are excluded from Standard classes I–IV.

The SWTs that will be driven by the winds whose characteristics exceed those

of standard classes in Table 2 is classiﬁ ed to special S classes.

Both for designers and owners of SWTs, site selection will be quite important,

although mini- or micro-WTs are easily installed at any place or on a building. It

is because wind conditions considerably vary depending on the site. For large

WTs, the sites are mostly open free ﬂ ow ﬁ elds, while for SWTs the sites may often

gardens, parks, roofs or rooftops of urban houses/high buildings, etc. At these

sites, ﬂ ow characteristics are often quite turbulent and those SWTs installed there

should be also classiﬁ ed into S.

Table 2: Basic parameters for SWT classes (IEC 61400-2).

SWT class

I II III IV S

Reference wind speed,

ref

(m/s)

50 42.5 37.5 30 Values to be speciﬁ ed

by the designer

Annual averaged wind

speed, V

ave

(m/s)

10 8.5 7.5 6

Turbulence intensity at

V = 15 m/s

0.18

396 Wind Power Generation and Wind Turbine Design

4.2 Normal wind conditions and external wind conditions

Wind conditions are usually divided into two wind conditions: normal wind

conditions and extreme wind conditions. The former will occur frequently during

normal operation of a WT through its lifetime and very much related to fatigue,

while the latter will occur very rare after their deﬁ nition of 1-year or 50-year

recurrence period and relate to ultimate loads.

4.3 Models of wind characteristics

Wind speed distribution, wind proﬁ le (wind shear model), turbulence model,

extreme wind speed model, gust model and model of wind direction change are to

be used for calculations of loads.

5 D esign of SWTs

A wind turbine is designed to capture maximum energy from the wind and to ensure

the safety under any external and internal condition. Safety and performance are the

main technical requirements in the design procedure.

Design procedure is given in IEC 16400-2 to attain the engineering and techni-

cal requirements to ensure the safety of the structural, mechanical, electrical and

control systems.

5.1 Conceptual design

5.1.1 HAWT or VAWT

Wind turbines are classiﬁ ed into two types: HAWT and VAWT. They have some

different technical features as shown in Table 3 .

It is well known that lift-driven wind turbines have much higher performance

than drag-driven types from the aerodynamic theory.

Table 3: Technical features of HAWT and VAWT.

Item HAWT VAWT

Performance Lift-driven and high Lift-driven and high (Darrieus

rotor), drag-driven and low

(Savonius rotor)

Power/speed

control

Variable pitch control is

possible

Usually, variable pitch control

is not possible

Yaw control Needed Free from yaw control

Structural

arrangement

Nacelle is on the top of the

tower

All the components except

rotor are on the ground level

Support structure Support structure like gay

wire will not induce vibration

Support structure like gay wire

may induce vibration

New Small Turbine Technologies 397

Structural simplicity is reliability as well as beauty. From this point of view, no

pitch control and/or no yaw control is desirable. However, power control/rotational

speed control is vital with such a rotating machine in the free ﬁ eld under all weather

conditions. This results in that power/speed control system including brake system

is very important.

As shown in Table 3 , in general, a HAWT can easily be equipped with both

aerodynamic regulation system (pitch control or stall regulation) and mechanical

brake. However, with SWTs, especially with micro-WTs, pitch regulation is pref-

erably not engaged in order to avoid the structural complexity. This point is very

special with SWTs and a designer must ﬁ rst decide the method how to control the

power/speed of the WT together with the type of WT.

5.1.2 Wind characteristics

Design wind turbine class is decided according to Table 2 depending on the wind

characteristics of the site where the wind turbine is to be installed.

The wind characteristics of the site are expressed in a series of mathematical

models in IEC standard so that all important load cases that act on the wind turbine

can be evaluated for the structural design. A full combination of aerodynamic design,

structural design and design of control system will give the engineering integrity.

5.2 Aerodynamic design

5.2.1 Annual energy production

Design methods for SWTs are basically same as those for LWTs. However, vari-

able pitch control system is often avoided with mini- and micro-WTs. Yaw system

is also passive to avoid additional power sources for control. Therefore, simpliﬁ ed

systems in structures or in control will bring some special problems to SWTs.

With a SWT with ﬁ xed pitch control system, aerodynamic design must be

completed in combination with power/speed control system and brake system.

Once technical solutions to start or stop the turbine as designed are found, the

aerodynamic design is proceed.

Let f ( V ) and P ( V ) be a probability density function of wind speed at a site and

a power curve of a wind turbine, respectively. Then the expected annual energy

production (AEP) is:

out

AEP ( ) ( )d ( )(0.5 )d

fVPV V fV VAC Vr

∞

∫∫

(6 )

where V

and V

out

are cut-in and cut-out wind speed respectively, and A and C

are

the rotor swept area and power coefﬁ cient.

f ( V ) is usually well ﬁ tted by Weibull distribution function expressed in the

following formula:

() exp

kV V

CC C

−

⎛⎞

⎛⎞ ⎛⎞

=−

⎜⎟

⎜⎟ ⎜⎟

⎝⎠ ⎝⎠

⎝⎠

(7 )

where k and C are shape and scale parameters of Weibull distribution, respectively.

398 Wind Power Generation and Wind Turbine Design

5.2.2 Optimization for maximum energy yield

An optimization is given by the condition that AEP will be maximum, i.e.:

AEP Max→

(8 )

A rotor designer must ﬁ nd the optimal power curve P ( V ) together with WT

operation modes. An optimal rotor geometry which comprises of chord and twist

distributions and aerofoil sections is found in combination with operation modes;

cut-in, cut-out and rated wind speed in principle. Thus an optimal rotor design is

site-dependent. Further, since every reliable operation mode depends on power/

speed control methods, conceptual design of control methods are also needed at

the initial stage.

5.2.3 One-point optimal rotor design

Simplest optimization of rotor design is “One-point design method”, in which an

optimal rotor geometry is determined for one operation point: one combination of

a design wind speed V

and a design rotor speed Ω

by using BEM theory [ 2 ].

The optimization is given by the following condition for V = V

with selected rotor

speed Ω

Max. At every axial position of rotor blade

C →

(9 )

Algebraic solution determines the chord and twist distributions along its axis of

the blade. However, it must be noticed that “One-point design method” does not

always give the maximum AEP, because a maximum AEP is obtained under the best

combination of decided WT’s operation mode and wind characteristics at the site.

The question is how to decide design wind speed V

The simplest design by BEM theory is conducted under the following

assumptions:

Low Reynolds number effect is negligible •

Power curve is a function of tip speed ratio • l only.

Then eqn (6) is expressed as:

out

in R

AEP ( ) ( )d ( ) ( )d

AfVVC VVC fVVrll

⎧⎫

⎪⎪

⎨⎬

⎪⎪

⎩⎭

∫∫

(10 )

where V

is rated wind speed and

(11 )

Max(AEP) is derived as follows:

In case of variable-speed operation WT, an optimal l

opt

≈ l

is deﬁ ned and the

optimal value can be possibly in the manner:

opt in R

for VVVll≈≤≤

(12 )

New Small Turbine Technologies 399

Then the optimal value of C

Popt

= C

( l

opt

) is kept and by choosing l

opt

= l

AEP → Max in eqn (10). The second term of in the integral of the right side of eqn

(10) is determined by the wind speed probability function f ( V ).

5.2.4 In case of constant-speed WT

In case of ﬁ xed-speed WT, l varies with V in proportion to 1/ V . Therefore, an

optimal value of design tip-speed ratio l

opt

may vary from l

. It will be soon

understood if eqn (10) is rewritten as:

out

in R

AEP ( ) ( )d ( ) ( )d

A fVVCV V VCV fV Vr

⎧⎫

⎪⎪

⎨⎬

⎪⎪

⎩⎭

∫∫

(13 )

Since C

is a function of V , AEP is also a function of V , but depends on C

( V )

and f ( V ). This means an optimal rated wind speed V

and an optimal design wind

speed V

will be obtained through making AEP → Max. taking into account the

characteristics of WT, i.e. C

( V ), and wind, i.e. f ( V ), which are independent of

each another.

An empirical formula is known as

R mean

(1.2 1.5)VV≈−

(14 )

where V

mean

is the annual mean wind speed at the site and is statistically given as

follows when the wind characteristics follow Weibull distribution:

[]

mean

()d 1VEVVfVVC

∞

⎛⎞

==⋅ =Γ+

⎜⎟

⎝⎠

∫

( 15 )

where Γ denotes the Gamma function.

5.2.5 Multi-point optimal rotor design

A goal is to give AEP a maximum value of the integration in eqn (10). One of the

simplest methods is “Multi-points design method”, with which some desirable

power curve is delivered by combination of plural design points. Various original

designs are possible, but most important thing is to ﬁ nd a solution that the operation

modes together with the power curve of a SWT will most ﬁ t to the characteristics of

the wind at the site where to install the WT.

It is very often experienced that under low wind speed conditions just around

cut-in wind speed, a ﬁ xed-pitch SWT will hard to start rotation or generation if it

stands still, however it will soon do if it is in idling state. Let assume that cut-in

and rated wind speed are 2.5 and 10 m/s, respectively. The ratio of the power out-

put is 1:64. Although the power output at cut-in wind speed is less than 2% of that

at rated wind speed, fast start-up performance will result in better availability in

time and higher energy production.

400 Wind Power Generation and Wind Turbine Design

5.3 Selection of aerofoil sections

In general, C

is a function of several parameters:

(, ,, )

CC Relbe=

(16 )

where l , b , e and Re tip-speed ratio, pitch angle, drag-lift ratio and Reynolds num-

ber, respectively. All these parameters relate to the ﬂ ow condition on the rotor

blade. Typically, l and b deﬁ ne the ﬂ ow angle at every location along the blade and e

and Re directly relate to the performance of the aerofoil section at the location. Thus,

selection of aerofoil section is very important.

5.3.1 Mechanism of performance reduction

Wind turbines of high performance are driven by lift.

According to BEM theory, local power produced by a blade element has a

expression:

Local power : (1 cot )PrejΔ∞ − Δ

(17 )

where j is ﬂ ow angle on a blade element of length Δ r and e is drag lift ratio of

the blade element. At an ideal ﬂ ow angle, e ≈ 0.01–0.02, but if a stall occurs on

the blade element e > 1.0, which suggests both selection of aerofoil sections and

control of relative low condition between a blade and air ﬂ ow are keen to the rotor

aerodynamic performance.

Thin aerofoil sections have usually smaller drag coefﬁ cients than thick sec-

tions. However, they will soon fall into stall if the angle of attack varies from the

ideal condition. At such laminar separations with SWTs, the performance reduc-

tion is considerably large. Thick aerofoil sections have high structural stiffness

than thin aerofoil sections, but usually have higher values of e which induce

higher energy loss.

5.3.2 High lift devices

To avoid the laminar separations, and also to regulate the power output and rotor

speed, several techniques are used.

Pitch regulation is the most effective active method, but is sometimes difﬁ cult to

apply to SWTs with less space or capacity for control power.

Vortex generator (VG) is simple and useful high lift device. Even a large WT

beneﬁ ts from this device. Longitudinal vortexes generated by VGs prevent the

boundary layer from inducing large scale laminar separations, but drag coefﬁ cient

will usually slightly increase.

Start-up assist method is a smart technology to improve the ﬂ ow condition on

rotor blades by accelerating the rotor. When a rotor condition is far from design

point, such as rotor speed is very low or wind is very low, for example. An instan-

taneous acceleration of the rotor by additional power source for certain seconds

will bring the ﬂ ow angle into regular conditions capable of generation and the

operation becomes more ideal condition. Although this technology does not work

as a brake device, it works as an alternate method with pitch regulation.

New Small Turbine Technologies 401

5.3.3 Blade material

Aerodynamic performance of a rotor depends on blade structures and materials.

If rotor stiffness is not enough, deformation of rotor blade will move the opera-

tion point from the designed point, thereby the power performance will decrease.

As long as such an aero-elastic effect cannot be reﬂ ected in the rotor design, stiff

materials shall be chosen.

On the other hand, an advanced design may utilize the aero-elastic effect. A

thought-out design of a ﬂ exible rotor may obtain an ability of regulating power

although it is a ﬁ xed pitch system.

5.4 Structural design

Most important works in the structural design is to verify that all the limit states

are not exceeded for the WT design [ 1 ]. Therefore, all the loads that will act on the

WT are determined ﬁ rst and then structural design of the WT and its components

will follow in combination with the WT conditions.

Loads are classiﬁ ed as:

Vibration, internal and gravitation loads •

Aerodynamic loads •

Operational loads caused by yawing, braking, furling, pitching, etc. •

Other loads such as wave loads, wake loads, transport, etc. •

Three different methods for the determination of design loads are described in

IEC Standard 61400-2. Simpliﬁ ed load equation methods are simple but conserva-

tive. The method using aero-elastic modelling is ﬁ ner and more complicated than

the ﬁ rst one. This method has common base with that for general wind turbines.

Design loads can be derived from load measurements.

A designer of SWTs shall refer the IEC standard to understand general design

methodology and conduct design by using equations for loads, table of load cases

and safety factors.

6 Control strategy of SWTs

Decision of the control strategy of a SWT is also important element in the initial

system design. Both of its power output control and yaw control are usually quite

different if compared with those of LWTs, particularly with mini- and micro-WTs.

The reasons are:

The representative system speed • Ω

is proportional to 1/ D as shown in eqn (4),

which means SWTs are higher rotational speed machines than LWTs and the

control rate shall be faster in proportion to 1/ D .

SWTs are more apt to be inﬂ uenced by various kinds of local turbulence than •

LWTs.

402 Wind Power Generation and Wind Turbine Design

Thus it should be understood that both speed control and yaw control perfor-

mances are requested to be as high as or higher than LWTs.

Upon this nature, the ﬁ rst issue to decide on the control strategy is whether to adopt

active control or passive control concept. The former is a concept to control the system

using a yaw drive with any kind of active power source such as electric or hydraulic

power actuators, while the latter is to control by natural forces generated by wind

or mechanical forces such as centrifugal force without using any active power source.

The ﬁ ner control system based on active control concept is desirable at one

hand, however, there are another reasons that passive control concept is attractive

at another. The passive control concept gives us the advantage of structural sim-

plicity, which would give greater reliability in general, provides as smart control

measures using natural force without extra control power sources and supplies us

at more economical cost.

In principle both concepts exist in LWT design and large turbines, but the status

of technical tendency is that more complete active control concept with LWTs and

more passive control concept or less active control concept with SWTs. Should

“smart” technologies be really smart, the passive control concept could be also

popular with LWTs in future.

There is an evidence for this. For long, a constant-speed operation system was

technically thought to be most suitable for grid connected systems simply because the

electrical grid has constant frequency and the rotor should be regulated to rotate at

constant speed. But wind will seldom keep blowing constantly at the rated wind speed

at most of the time during the operation of wind turbine. It is much more natural for a

WT to work at any given wind speed with higher efﬁ ciency but with less extra active

power source. The solution was given by “variable operation systems” which employ

a power convertor system such as AC–DC–AC generator system. Electronically

ﬁ ner and faster while mechanically less loaded, is modern technical tendency.

Decision of choosing either of constant-speed operation or variable-speed

operation is also important task.

Figure 5 shows an example of a basic performance of an optimal rotor design.

The rotor is designed by BEM theory at design wind speed of 10 m/s, tip-speed

ratio of 7 m for 10 m of rotor diameter using a thin wing section of Illinois Univer-

sity. Once aerofoil section(s) selected, Reynolds number effect neglected and pitch

angle ﬁ xed, then C

is a function of l = r Ω / V only.

Maximum C

is 0.475 for l = 7 at the design point. When neglecting the effect

of low Reynolds number, as long as l = 7 is kept, the rotor produces power with

maximum aerodynamic efﬁ ciency. As shown in Fig. 5 , C

( l ) will decrease with

constant-speed system ( Ω = Ω

= Const.) as l departs from the design point while

keeping pitch angle constant. As a result,

0.4 only for 7.5 m/s < < 14 m/s

CV≥

(18 )

On the other hand, with a variable-speed operation rotor, optimal C

can be real-

ized if the optimal rotor speed is properly regulated. In this case, under the varying

natural wind speed, it is desirable to keep l = l

opt

= 7, or to vary rotor speed in the

manner: Ω = ( V / R ) l

opt

New Small Turbine Technologies 403

7 Yaw control

Yaw control is a function to face the rotor to the natural wind so that the WT can

capture maximum energy from the wind. Therefore, a WT shall face perpendicular

to the wind directions as much as possible at all times. Because of the dimensional

effect, a SWT will much more susceptible to be caught in local smaller eddies than

LWTs and as a result, yaw control is technically more complicated than LWTs.

One thing most desirable is stable wind, however, the people often like to install

SWTs rather close to or directly upon buildings where the wind tends to be more

turbulent.

It might be better to provide a ﬁ ne yaw controls system but actually almost all

mini- and micro-WT have passive control systems.

Yaw motion is governed not only by yaw control device but also inertia and gyro-

scopic moment of the nacelle and yawing dynamic ability of the rotor itself. Some

typical yaw control systems with mini- and micro-WTs are described below.

7.1 Tail wing

An upwind oriented rotor could have yaw instability. Most of LWTs except

downwind type is equipped with a yaw control system driven by a hydraulic

or electric power. With SWTs of mini- or micro-classes, however, a tail wing is

mostly used as a passive control system. Tail wing is a simple and classic tool

since the middle aged.

7.1.1 Variety of tail wings

The performance of a tail wing as an aerofoil as well as the performance of

yaw control, both in static and dynamic response, is decided by the aerody-

namic performance of the tail wing. Then, there are various types and shapes

of tail wings.

0 5 10 15 20 25

Wind Speed V m/s

= RΩ/V

-0.1

0.1

0.2

0.3

0.4

0.5

0.6

Power Coefficent C

Figure 5 : A sample of power coefﬁ cient obtained by BEM theory .

404 Wind Power Generation and Wind Turbine Design

Figure 6 shows a various shapes of tail wings. There is no ultimate shape of the

tail wing at all, because there are many yaw control strategies at one hand, there

are many wind characteristics depending on the site at another.

The yaw control moment is the product of aerodynamic force on the aerody-

namic centre of the wing and the distance from yaw axis to the centre. The tail

wing (a) has much small aspect ratio than the tail wing (b). Then wing type (a) has

smaller lift coefﬁ cient at small attack angles, i.e. less yaw control force than type

(b). However, under high turbulent wind conditions, too strong control force will

sometimes bring unstable yaw motion.

7.1.2 New types of tail wings

Some unique and advanced types have been developed. The tail wing design shown

in Fig. 7 has an elegant shape.

A swing ladder shown in Fig. 8 is a swinging tail wing. The ladder which

works as a wing is pin-jointed to the tail-end of the nacelle free in rotation around

the pin-axis. This system responds ﬂ exible to varying wind speed and directions

and one of the large advantages is the regulative effect that prevents the excessive

response and oscillating motion of the tail.

(a) (b) (c) (d)

Figure 6 : Various types of tail wings.

Figure 7: Modern tail wing (Courtesy of Anna Estanqueiro, IEA WIND).