Tong W. Wind Power Generation and Wind Turbine Design

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Wind Turbine Cooling Technologies 635

As shown in Figs 14 and 15 [ 26 ], the features of a wind turbine adopting a cooling

system with solar power jets include a solar thermal collector covering on the

nacelle and a jet refrigerating engine with a secondary refrigerant circulation

through heat exchangers of the gearbox, the control converter and the generator.

Figure 14: Schematic diagram of solar power jet cooling wind turbine: 1, saddle

piece; 2, impeller blade; 3, low-speed shaft; 4, gearbox; 5, gearbox heat

exchanger; 6, High-speed shaft; 7, control converter; 8, control converter

heat exchanger; 9, generator; 10, generator heat exchanger; 11, secondary

refrigerant; 12, circulating pump I; 13, carrier pipe of secondary refriger-

ant; 14, heat storage agent; 15, circulating pump II; 16, carrier pipe of heat

storage agent; 17, solar thermal collector; 18, nacelle cover; 19, jet refrig-

erating engine; 20, platform of the pylon; 21, pylon; 30, cooling medium;

31, circulating pump III; 32, carrier pipe of the cooling medium.

636 Wind Power Generation and Wind Turbine Design

Similar to the wind turbine introduced in Section 5.1, during the operation, the

secondary refrigerant sequentially ﬂ ows through the heat exchanger outside the gear-

box, the generator and the control converter, removing the heat produced by these

components. The heated secondary refrigerant is then driven to the jet refrigerating

engine by the circulating pump through the carrier pipes and performs heat exchange

with liquid refrigerant in the evaporator. Finally the cooled secondary refrigerant is

then delivered back to the above three heat exchangers to absorb heat. This circulation

cycles to ensure the durable and secure operation of the wind turbine.

In the operation of the jet refrigerating engine, the solar thermal collector con-

verts the solar energy into heat energy, leading to a rise of the heat storage agent in

it. The heat is then delivered by the heat storage agent and stored in the heat accu-

mulator. The heat-released heat storage agent ﬂ ows back to the heat accumulator

driven by a circulating pump, thus completes a circuit of solar energy conversion.

Meanwhile, the refrigerant absorbs heat in the heat accumulator and gasiﬁ es and

thus supercharges the pressure. The refrigerant is then ejected through the jet

apparatus, leaving a low pressure close to vacuum at the jet tip. The low pressure

steam refrigerant in the evaporator is thus drawn into the jet apparatus due to the

pressure difference. The mixed steam refrigerant from the jet apparatus is then

ejected into the condenser and performs heat exchange with the cooling medium

Figure 15: Schematic diagram of the solar power jet refrigerating engine: 22,

heat accumulator; 23, auxiliary heater; 24, refrigerant; 25, circulating

pump IV; 26, jet apparatus; 27, evaporator; 28, throttle; 29, condenser;

33, ﬁ lter. The remaining symbols have been annotated above.

Wind Turbine Cooling Technologies 637

ﬂ owing through the condenser. The cooled refrigerant is shunted into two parts by

pipelines. One of the streams ﬂ ows back to the heat accumulator to complete the

power cycle while the other passes through the throttle to the evaporator, where it

performs heat exchange with the secondary refrigerant and gasiﬁ es, turning into a

low pressure steam. So far a refrigerating cycle is completed.

It is worth mentioning that sea water or underground water can be used as cool-

ing medium in the condenser. To prevent blockage resulting in bad cooling effect,

a ﬁ lter needs to be installed on the cooling medium carrier pipe on the way to the

condenser. In the area where water is hard to reach, the cooling medium can be the

other liquid processed by the seawater heat exchanger or the underground heat

exchanger. No matter which conditions, for those components and pipelines sub-

merged in water, effective anti-corrosion treatments should be implemented. Those

treatments include choosing the resistant material, protective coating technology

and cathodic protection technology. In the night or the abnormal working condi-

tion of the heat accumulator, an auxiliary heater can be used to heat the refrigerant

ﬂ owing through to offset the energy shortage of the heat accumulator.

Comparing with the current wind turbine cooling system, the jet cooling system

with solar power assistant possesses many advantages, such as lower power con-

sumption, better cooling performance and environment friendly. Only the circulat-

ing pump and auxiliary heater need electricity support in the cooling system,

which enables low electricity consumption of the whole system; while the solar

power jet cooling method can obtain temperature below the ambient temperature,

which will ensure the temperature in the optimal working condition. And the

refrigerant in this system can be chosen from the non- chloroﬂ uorocarbons sub-

stances (NON-CFC) which will be of great help to the environment protection.

Besides, since the jet refrigerating engine has a simpler and lighter structure than

other ones in the current technology, this merit will enhance the total anti-fatigue

property of the nacelle; also because it is usually installed in the nacelle, which

will avoid the corrosion caused by blown sand and rain, the working performance

and life span will be effectively secured. Therefore, it can satisfy the highly

efﬁ cient and dependable working requirements of the high-power wind turbine.

5.4 Heat pipe cooling gearbox

The three solutions mentioned above mainly focus on the entire cooling effect of all

the heat producing components. On the other hand, considering different structural

conditions and cooling demands of various components and using corresponding

combinatorial solutions will possibly further enhance the operating economical

efﬁ ciency, which is also a future trend for wind turbine cooling systems. Since the

gearbox has a simple structure and an ample space for installation, a cooling solu-

tion adopting gravity heat pipe is proposed in this section while the generator and

the control converter remain adopting a traditional liquid cooling system.

As shown in Figs 16 and 17 [ 27 ], the feature of a wind turbine system adopting

a heat pipe cooling gearbox is that it has several apparatuses, including a gravity

heat pipe connected with the gearbox to refrigerate the lubricating oil in it, a

638 Wind Power Generation and Wind Turbine Design

Figure 16: Schematic diagram of the wind turbine system adopting heat pipe

cooling gearbox: 1, saddle piece; 2, impeller blade; 3, low-speed shaft;

4, gearbox; 5, lubricating oil; 6, gravity heat pipe; 10, temperature con-

troller; 11, electric heater; 12, high-speed shaft; 13, control converter;

14, generator; 15, nacelle cover.

Figure 17: Schematic diagram of gravity heat pipe: 7, working substance; 8,

shell of pipe; 9, radiating rib; a, evaporation zone; b, insulation zone;

c, condensation zone.

temperature controller and an electric heater aiming at preventing extreme cold of

the lubricating oil in winter. During the operation of the wind turbine, the heat

produced by the gearbox is ﬁ rstly absorbed by lubricating oil, and then delivered

to the evaporation zone of the heat pipe. The working substance in the heat pipe

evaporates because of heat absorption. The vapor ascends in the axial direction

along the heat pipe, passing the thermal insulation zone, and then releases heat and

Wind Turbine Cooling Technologies 639

condenses in the condensation zone outside the nacelle. Finally the liquor condensate

returns to the evaporation zone with the assistance of gravity, which begins the

next evaporation procedure and so forth to perform effective cooling on the gear-

box. In winter, in order to avoid getting extremely cold lubricating oil caused by

the large cooling capacity of heat pipes, which is common in the northern cold

area, the gearbox is installed with a temperature controller, performing real-time

monitoring of the gearbox lubricating oil temperature. In addition, the heat pipe in

the system can adopt inclination arrangement to ensure the liquor condensate

returning to the evaporation zone smoothly, by which to ensure continuity and

validity of the cooling system. Besides, in the operation, the number and distribution

of heat pipes can be selected according to the real heat transfer capacity.

Compared with the current cooling system with a wind turbine gearbox and the

above three solutions, the wind turbine system adopting a heat pipe cooling gear-

box has many advantages to satisfy the wind turbine cooling demands, such as

simple structure, self-driven, high heat transfer efﬁ ciency, low cost, easy to install

and repair, small need on lubricating oil and so forth. Besides, it can keep the gear-

box operating under the condition of optimum working temperature by selecting

proper working substance in the heat pipe.

The wind turbine cooling systems introduced above are only limited examples

of the future possible systems. With the development of the wind turbine technol-

ogy, various cooling solutions will emerge. However, only those cooling systems

with better cooling effect, higher economical efﬁ ciency and lower power

consumption will be widespread in the future development.

References

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640 Wind Power Generation and Wind Turbine Design

[ 10] Tang, Y.Q. & Shi, N., Electric Machinery , Beijing: China Machine Press,

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[13] Sun, M.L., Optimization design of a 600 kW/125 kW wind turbine. Shanghai

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Refrigeration and Air-Conditioning , 8 , Beijing: China Machine Press,

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in china. Process Equipment & Piping , 44(6) , pp. 9–12, 2007.

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and resistance of various heads of the plate-ﬁ n heat exchangers. Chemical

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No. 200810124198.9, 2008.

CHAPTER 20

Wind turbine noise measurements

and abatement methods

Panagiota Pantazopoulou

BRE, Watford, UK.

This chapter presents an overview of the types, the measurements and the potential

acoustic solutions for the noise emitted from wind turbines. It describes the fre-

quency and the acoustic signature of the sound waves generated by the rotating

blades and explains how they propagate in the atmosphere. In addition, the avail-

able noise measurement techniques are being presented with special focus on the

technical challenges that may occur in the measurement process. Finally, a discus-

sion on the noise treatment methods from wind turbines referring to a range of the

existing abatement methods is being presented.

1 Introduction

Wind turbines generate two types of noise: aerodynamic and mechanical. A tur-

bine’s sound power is the combined power of both. Aerodynamic noise is gener-

ated by the blades passing through the air. The power of aerodynamic noise is

related to the ratio of the blade tip speed to wind speed. The mechanical noise is

associated with the relevant motion between the various parts inside the nacelle.

The compartments move or rotate in order to convert kinetic energy to electricity

with the expense of generating sound waves and vibration which is transmitted

through the structural parts of the turbine.

Depending on the turbine model and the wind speed, the aerodynamic noise

may seem like buzzing, whooshing, pulsing, and even sizzling. Downwind tur-

bines with their blades downwind of the tower cause impulsive noise which can

travel far and become very annoying for people. The low frequency noise gener-

ated from a wind turbine is primarily the result of the interaction of the aerody-

namic lift on the blades and the atmospheric turbulence in the wind. High frequency

noise is also generated due to the interaction of the air turbulence and the blades

during their rotation, constantly changing angle of attack.

642 Wind Power Generation and Wind Turbine Design

Depending on the rotational speed of the turbine, the size and the airﬂ ow wind

turbines emit infrasound, low and high frequency sound waves. Large wind tur-

bines produce infrasound of 8–12 Hz range. Small turbines can achieve higher

blade tip velocities that can give low frequency noise of 20–20 kHz range.

To get a better feeling of the frequency ranges that are audible and are produced

by wind turbines, we can see Table 1.

The levels of infrasound radiated by the large wind turbines are very low in

comparison to other sources of acoustic energy in this frequency range. However,

the annoyance is often connected with the periodic nature of the emitted sounds

rather than the frequency of the acoustic energy. Because low frequencies travel

farther than the high frequencies due to their long wavelengths, they become a

cause of irritation for residents living not so close to wind farms.

Sound is a series of waves that travel through air in the form of disturbances

and reaches our eardrums. Any natural or artiﬁ cial obstacles such as hills and

buildings play a shielding effect role, reﬂ ecting the sound, while most of the

times absorbing some of the acoustic energy. Trees and ground vegetation also

attenuate sound and change its directivity patterns. The distance and obstacles

that are located between the source and the receiver have a signiﬁ cant impact on

the acoustic ‘line of sight’.

The ways to reduce noise from wind turbines are mostly focused on blade design

optimisation and choice of wind farm location as it still a new ﬁ eld and new tech-

niques are under development. A major challenge that needs to be addressed before

abatement techniques are put in action; is to establish accurate measurement meth-

ods of wind turbine noise in order to deﬁ ne the parameters of the problem and ﬁ nd

an appropriate acoustic solution for it. Noise measurements from wind turbines is a

complex task because the background noise levels are comparable to the noise levels

from wind turbine when it starts operating at certain wind speeds. This is the reason

a commonly used approach that overcomes the above issue needs to be established.

Current acoustic treatment methods range from designing quieter wind turbine

compartments to placing wind turbines as far as possible from residential areas.

Wind turbine blades are aerodynamically shaped to avoid causing abrupt air ﬂ ow

disturbances and the turbine is placed upwind to eliminate interference of the ﬂ ow

between the tower and the wind turbine blades. Apart from aerodynamic solutions

other treatments include sound prooﬁ ng of the nacelle to reduce mechanical

noise and careful selection of the wind turbine site to attenuate noise until it

reaches the receiver.

Table 1: Important frequency ranges.

Description Frequency range

Normal hearing 20 Hz to 20 kHz

Normal speech 100 Hz to 3 kHz

Low frequency 20–200 Hz

Infra sound <16 Hz

Wind Turbine Noise Measurements and Abatement Methods 643

It is common sense that the extensive use of wind turbines is included in the

current and the future environmental plans of every country, fact which will

certainly place a strong demand for dealing noise issues in the near future.

2 Noise types and patterns

2.1 Sources of wind turbine sound

The sources of sounds emitted from operating wind turbines can be divided into

two categories: (1) mechanical sounds, from the interaction of turbine compo-

nents, and (2) aerodynamic sounds, produced by the ﬂ ow of air over the blades.

A summary of each of these sound generation mechanisms follows, and a more

detailed review is included in [ 1 ].

More speciﬁ cally, wind turbines produce energy by the rotational motion of the

blades due to the wind ﬂ ow. Rotating blades are known to emit three different

types of acoustic signature:

tonal noise •

broadband noise •

mechanical noise •

Tonal noise is characterised by discrete frequencies and it is generated by the peri-

odical rotation of the turbine blades. An example of tonal noise history is shown in

Fig. 1 . Tonal noise is caused by the unsteady air velocity due to the blade rotation which

disturbs the ﬂ ow on the blade surface. It is directional as it is produced at the direction

the blades meet the airﬂ ow and therefore is dependent on the observer position.

As the blades rotate the load on the blade surface changes periodically causing

analogous changes in the unsteady pressure on the blade surface inducing sound

waves. Depending on the position of the observer in relation to the turbine while

Figure 1: Tonal noise time history.

644 Wind Power Generation and Wind Turbine Design

the blade is in operation the observer receives variable acoustic signals. Also the

sound component in the direction of the observer varies with time and a sound

wave is generated. Normally, the ﬂ ow through the blades is distorted (non-

uniform) and therefore the angle of attack of each blade varies continuously as the

turbine rotates causing the sound generation to be highly directional and more

frequent. This change in the angle of attack can be very abrupt especially when

velocity discontinuities occur in the inﬂ ow proﬁ le resulting in rapid changes in the

blade loading, ﬂ ow disturbance and therefore generation of acoustic waves.

Such type of acoustic waves can be produced when the mounting tower of the

wind turbine interferes with the ﬂ ow passing through the wind turbine blades. This

causes local velocity instabilities in the ﬂ ow ﬁ eld which moves through the blades

producing pulsing low frequency noise. A downwind design is shown in Fig. 2a .

It becomes self-explanatory that when the reverse order occurs, the disturbances

ease as the blades encounter only the free ﬁ eld ﬂ ow disturbances. An upwind

turbine is shown in Fig. 2b .

The noise generated by downwind designs has low frequency in the range of

20–100 Hz and it is caused when the turbine blade encounters localised ﬂ ow deﬁ -

ciencies due to the ﬂ ow around a tower. It can also be impulsive described by short

acoustic impulses or thumping sounds that vary in amplitude with time. It is caused

by the interaction of wind turbine blades with disturbed air ﬂ ow around the tower

of a downwind machine.

2.2 Infrasound

A special category of the tonal noise released by wind turbines is infrasound. As we

have already mentioned while low frequency ranges at the bottom of human perception

(10–200 Hz), the infrasound is below the common limit of human perception. Sound

with frequency below 20 Hz is generally considered infrasound, even though there

may be some human perception in that range. A distinctive characteristic of infrasound

is that it can travel very far because of its long wavelength that dissipates with a low

rate and therefore it makes it easier to ‘survive’ and be present in our everyday life.

(a) (b)

Figure 2 : (a) Downwind turbine; (b) upwind turbine.