w w w . h o a r e l e a . c o m

Wind Farm Noise Impact Assessment

WIND SHEAR

• what is wind shear ?

• how does wind shear affect wind farm noise ?

• how should wind shear be dealt with ?

Wind shear …….

VL

Vu

What is wind shear ?

What causes wind shear ?

ground roughness

Modelling wind shear due to ground roughness

ground roughness

VU

VL

VU VL=log(HU/z0)

log(HL/z0)x

z0 = ground roughness length

HL

HU

What causes wind shear ?

atmospheric effects

What causes wind shear ?

atmospheric effects

What causes wind shear ?

atmospheric effects

What causes wind shear ?

atmospheric effects

Modelling wind shear due to atmospheric effects

atmospheric effects

VU

VL

m = shear exponent

HL

VU VL= xHU

HL

m

HU

Time of day, hh:mm

Ho

url

y a

ver

ag

ed

win

d s

pe

ed, m

/s

2

4

6

8

00:00 06:00 12:00 24:0018:00

Diurnal variations in wind speeds

10

20

40

80

He

igh

t ab

ov

e g

rou

nd

, m

80

After G.P van den Berg, WTG Noise 2005, Berlin

Diurnal variations in wind shear

Wind shear is generally highest in evening/night which are measured under ETSU-R-97

Wind shear as a function of wind speed

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 2 4 6 8 10 12 14 16 18 20 22 24

Wind Speed, m/s

Sh

ear

Exp

on

ent

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 2 4 6 8 10 12 14 16 18 20 22 240

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 2 4 6 8 10 12 14 16 18 20 22 24

Diurnal and seasonal variations in wind shear

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 2 4 6 8 10 12 14 16 18 20 22 24

Hour of Day

Sh

ear

Ex

po

ne

nt

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Nov

Dec

all wind speeds

Diurnal and seasonal variations in wind shear

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 2 4 6 8 10 12 14 16 18 20 22 24

Hour of Day

Sh

ear

Exp

on

ent

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

3 m/s to 5 m/s wind speeds only

Diurnal and seasonal variations in wind shear

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 2 4 6 8 10 12 14 16 18 20 22 24

Hour of Day

Sh

ear

Ex

po

ne

nt

Jan

Feb

Mar

Apr

May

Jun

Jul

Sep

Nov

Dec

6 m/s to 8 m/s wind speeds only

VH

Why is wind shear important for wind farm noise ?

V10

Wind shear – effect on turbine sound power

80

85

90

95

100

105

110

0 1 2 3 4 5 6 7 8 9 10 11 12

Wind speed at 10m height, m/s

So

un

d P

ow

er o

utp

ut,

dB

(A)

80

85

90

95

100

105

110

0 1 2 3 4 5 6 7 8 9 10 11 12

Wind Farm Noise – setting acceptable limits

ETSU-R-97

ETSU-R-97 sets noise

limits as a function of

10m height wind speed

Calculating wind farm noise at receptors

Noise Propagation Calculation Algorithm

Source Sound Power Level

Wind Turbine Sound Power Output Level

10

15

20

25

30

35

40

45

50

55

60

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Wind Speed at 10m (m/s)

Turb

ine

Sou

nd P

ower

Lev

el, d

B(A

)

Assessment Property - Quiet Day-time Periods

10

15

20

25

30

35

40

45

50

55

60

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Wind Speed at 10m (m/s)

L90

dB

(A)

Wind Farm Noise Immission Level

ETSU-R-97 Noise Limit

Wind turbine noise emissions

are also specified as a function

of 10m height wind speedStandardised from hub height

Using a hub height mast only

VH

V10

- typically requires >50m high mast

+ requires no assumptions about site specific shear profile

+ background noise and turbine SWL’s both related to the same ‘real’ wind speed

Using measurements at two heights

V1

V2

VH

V10

- typically requires up to 50m high mast

+ Lesser measurement height: use shorter mast and shear measured between two heights to extrapolate up to hub height

+ Similar to first method

Using a 10m mast only

VH

V10

- Need to have data or make assumptions on shear profile on the site

+ cheapest and easiest option

Assessment Location 1

10

15

20

25

30

35

40

45

50

55

60

65

70

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Wind Speed at 10m (m/s)

L90

dB

(A)

Wind speed at 10m height (m/s)

10

15

20

25

30

35

40

45

50

55

60

65

70

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

+ IOA GPG provides simplified corrections for use

Wind shear is important for wind farm noise

… and best practice guidance is contained in the IoA GPG