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ASSESSMENT TERRAIN CONDITIONS AND TURBULENCE

INTENSITY IN CONDAO FOLLOW THE METHOD OF JAPAN

Le Thi Thuy Hang1, Dr. Tetsuya Kogaki

2, Mr. Nguyen Hai Bac

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1Institute of Energy Science, Vietnam Accademy of Science and Technology (VAST)

2Energy Technology Research Institute, National Institute of Advanced Industrial Science and

Technology (AIST) 3Green resources technology development JSC. (Greenmade Co.)

1. SUMMARY

Selecting wind turbines that conform to the local wind conditions, necessitates the

comparison with international standards on design requirements of wind turbines, such as the

IEC 61400-1 and Germanischer Lloyd Guideline (GL Guideline). However, with the same

nature conditions in Vietnam and Japan, the wind conditions usually are dominated by

complex terrain causes the high turbulence and gusty winds, not fully covered by the existing

IEC standard. In Japan, the researchers have proposed the method to determ the complex of

the terrain and the level of the turbulence intensity for selecting and operating of the wind

turbine systems in here.

With the convenient location, based on the research results on impact by terrain

conditions into wind conditions of Japan, the author will have presented results of the analysis

and assessment on the terrain conditions and the turbulence intensity of the wind conditions in

Condao islands (Vietnam) for developing wind energy in here.

Keywords: wind energy, terrain conditions, turbulence intensity, wind speed

2. TERRAIN AND WIND CONDITIONS IN CONDAO ISLANDS

Condao islands are an archipelago belongs to Ba Ria Vung Tau province in the Southeast region of Vietnam, and coordinates in the latitude of 8.566N to 8.8166N and in the

longitude of 106.6E to 106.75E. Condao is situated about 185 km from Vung Tau and 239 km

from Ho Chi Minh City, includes 16 islands and islets with total land area of 75.15 km.

The topography of Con Dao islands is hilly area, dominated by the granite ranges from

southwest to northeast protecting both sides of the Gulf from strong winds. The highest site is

the top named Thanh Gia of 577m. The central area has the tidal sandy flows is parallel to the

coast, and many the shrubs and small buds. Alternating with the sandy flows, there are some

lowland with the surface mixing sandy with soil (humus). Behind the sand flows are the soil

extending to the foothills, mostly they are the mix of soil, sand and a little red humus. The

surfaced soil is shallow, the below layer is the mix of clay and gravel, and there are some

where the stone rise the surface.

Condao has no rivers, only the small mountain streams flow into the sea. There are

also a number of lakes, the largest lake being Quang Trung (0.2 km2). The coastline is 200

km with the sandy beaches up to 800m when the low tide is under level 0m.

Condao climate features the hot and humid monsoon with 2 monsoons in a year:

- The southwest monsoon: from May to September. In this season, the climate is quite

humid; the rainfall averages 2200mm/year, the least rain in the January, almost rain in August

and September.

- The northeast monsoon: from October to next April; this season has many the gusts

up to level 6, level 7 and over level 7. In this season, the climate is cool, the annual average

temperature is 27oC, February is the coolest month with the average temperature of 22

oC, and

May is the hottest months with the average temperature reached 34oC.

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3. METHOD TO DETERM THE TERRAIN COMPLEXITY AND THE

TURBULENCE INTENSITY OF JAPAN

3.1 Method to determ the terrain complexity

These 5 indices should be evaluated for each wind sector, and the maximum value of

them should be used in the evaluation. At least 8 wind sectors must be used in this evaluation.

(1) Maximum slope within 5 km radius is equal to or more than 10 degrees.

(2) Standard deviation of the topographic cross-section in the upwind part within 5 km

radius is equal to or more than 1.5 times of the hub height.

(3) Standard deviation of the topographic cross-section in the upwind part within

distance of 5, 10 and 20 times of the hub height are more than 0.5, 0.8 and 1.4 times of the

hub height, respectively.

(4) Speed up ratio caused by local geography is more than 1.2.

(5) More 30% of slopes between neighboring contour lines are steeper than 30%.

These indices should be evaluated for each wind direction sector, and then the

maximum value should be chosen for the assessment. The evaluation of the terrain

complexity with these indices should be done with the following criteria.

a) None of (1), (2), (3), (4), (5) are true : Low Complexity

b) At least one of (1), (2), (3), (4) is true, AND (5) is false : Medium Complexity

c) At least one of (1), (2), (3), (4) is true, AND (5) is true : High Complexity

The evaluation criteria shown here are for reference purpose on the assessment of

terrain complexity, and however, the actual turbulence intensity and/or the extreme wind

speed at the site can be high if the terrain complexity is evaluated as low or medium.

3.2 Method to determ the turbulence intensity

Turbulent intensity should be evaluates from the field measurement data at the site, as

the turbulence intensity corresponding to the non-exceeded level of 90% of the 10 minute

average wind speed and the expected value Iref* at the wind speed of 15m/s. Iref

* should be

estimated by the appropriate method according to the terrain complexity.

(1) Low terrain complexity: IEC standard wind model.

(2) Medium terrain complexity: simple terrain model and linear simulation model.

(3) High terrain complexity: non-linear simulation model and wind tunnel test.

IEC61400-1 Ed.3 (2005) defines an equation for the turbulence intensity (NTM

model) in the longitudinal direction relative to the 10 minute average wind speed as a normal

wind model.

Table 1: Basic parameters for wind turbine classes (IEC 61400-1 Ed.3 2005)

WT class I II III S

Vref (m/s) 50 42.5 37.5

Values

specified by

designer

Vave (m/s) 10 8.5 7.5

Iref A 0.16

B 0.14

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C 0.12

IEC61400-1 Ed.3 (2005) categorizes the expected value of the turbulence intensity at

15 m/s wind speed, Iref, into 3 turbulence categories A, B and C. The turbulence intensities in

the transverse and vertical directions are given as ratios relative to the value in the

longitudinal direction.

4. ASSESSING THE TERRAIN COMPLEXITY AND THE TURBULENCE

INTENSITY IN CONDAO

4.1 Selecting the site

The selected site is located at the Telecommunication station of the post-office, near

the meteorological station in Condao (Fig. 1).

Fig. 1: Site is being installed anemometers in Condao (red mark)

The wind conditions are measured at the site in a year, from March 20, 2012 to March

19 2013 at 3 heights (30m, 50m and 60m). Wind condition measurements were done at high

speed data sampling rate with three cup anemometer and vane sensor, and then other data

such as the 10 minute average wind speed and standard deviation, up-flow angle were

calculated from the measured data.

The below is the wind conditions at 60m height.

Fig. 2: Chart of wind speed and wind rose in Condao.

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Table 2: Wind speed at 60m height

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Vave(m/s) 9.31 9.50 7.27 4.77 5.72 6.36 7.31 8.27 6.68 4.86 6.52 9.17

Vmax(m/s) 30.3 26.0 24.5 19.5 20.6 26.4 27.8 28.9 21.4 19.0 25.5 31.6

Vmin(m/s) 0.47 0.47 0.06 0.09 0.01 0.07 0.03 0.06 0.05 0.47 0.47 0.47

4.2 Determing the terrain complexity in Condao

a) Index 1

This index enables assessing the possibility of local geographical feature influencing

on the local wind flow.

In Condao islands, the slope of fitted plane within radius 5km is from 0o to 9.76

o, and

so the max slope within radius 5km from the mast site is less than 10o.

Table 3: Max slopes within radius 5 km from the mast site

Above sea level (m) Distance from

wind turbine (m)

Slope from wind

turbine

577 3160 9.37o

520 2700 9.76o

500 4000 6.30o

440 3600 6.05o

390 3230 5.85o

320 2000 7.45o

b) Index 2

This index is to assess the terrain complexity around the site.

In Condao islands, the standard deviation of the topographic cross-section in the

upwind part within radius 5 km is 0 m. And so, the standard deviation of the topographic

cross-section in the upwind part within 5 km radius is less than 1.5 times of the mast height.

Table 4: Indicators for assessing index 2 at site

60 m height of the mast

1.5*Zhub (m) 90

c) Index 3

The same index 2, this index is to assess the terrain complexity around the site.

Within radius of 300 m to 1200 m from the mast site, the standard deviation of the

topographic cross-section in the upwind part is 0 m. And so, the standard deviation of the

topographic cross-section in the upwind part within distance of 5, 10 and 20 times of the hub

height are less than 0.5, 0.8 and 1.4 times of the hub height, respectively.

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Table 5: Indicators for assessing index 3 at site

60 m height of the mast

5*Zhub (m) 300 0.5*Zhub (m) 30

10*Zhub (m) 600 0.8*Zhub (m) 48

20*Zhub (m) 1200 1.4*Zhub (m) 84

d) Index 4

This index is related to the flow acceleration due to local geographic feature indicated

by index 1. At the mast site in Condao, the average speed-up ratios at 60 m height caused by

local geography are 1.01 (less than 1.2).

Table 6: Indicators of the speed up ratio at site

60 m height of the mast

Average ratio 1.01

Max. ratio 5.78

Min. ratio 0.15

e) Index 5

This index related to RIX index used in the WAsP software.

As showed in section 2.3, RIX is designed to show the ratio of the slopes steeper than

the threshold for generating flow separation, and the threshold value for the judging the

possibility of flow separation on slopes is 0.3 (if the slope angle is about 17 degrees, the slope

is 30%). And for reference, typical values for the RIX are given as follows.

- Flat or hilly terrain : 0%

- More complex than hills : 10%

- Mountains : 10% to 50%

In this report, due to the mast site is off-shore, and so the value for slopes between

neighboring contour lines are 0%.

f) Conclusion

As showed in section 3.1 for 5 indices to evaluate of the terrain complexity.

a) None of (1), (2), (3), (4), (5) are true : Low Complexity

b) At least one of (1), (2), (3), (4) is true, AND (5) is false : Medium Complexity

c) At least one of (1), (2), (3), (4) is true, AND (5) is true : High Complexity

Following analysis in section 4.2, none of (1), (2), (3), (4), (5) are true, and so the

terrain complexity at site is low.

4.3 Determing the turbulence intensity in Condao

Because the terrain complexity is low, and so, the turbulence intensity could be

evaluated following IEC standard wind model follow as:

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VI

q

q

90

90

vi )75.0(90 bVI refq

In which: b - 5.6, m/s; Iref - expected value of turbulence intensity at 15 m/s;

V - 10 minute average wind speed, m/s; - standard deviation, m/s;

90q - non-exceedance level of 90%;

The calculated results are showed below.

+ Standard deviation at 60m height, = 1.42 m/s

+ Turbulence intensity at 60m height, Iref = 0.16

Fig. 3: Standard deviation and turbulence intensity at the selected site

The calculation results indicate that the standard deviation and turbulence intensity at

the site suits with the IEC wind standards.

5. CONCLUSION AND RECOMMENDATION

As showed in section 4, terrain condition at the site is low and the mast site is on

coastline, the annual wind speed at 60m height is 7.13 m/s, the turbulence intensity at the site

is 0.16. And so, the conditions of terrain, turbulence intensity and average wind speed fixs

IEA standards, are feasible to deploying and applying the wind power in Condao islands, the

wind turbines are selected in accordance with the technical standards of IIIB class.

However, the relationship between the extreme wind speeds and the annual average

wind speeds at the mast in Condao is 0.18 (less than 0.2), and so it should be noted that the

wind conditions in Condao is different from the standard wind conditions defined in the IEC

standard. And so, the wind turbines should be selected in accordance with the technical

standards of S class, class specified by designer.

Acknowledgement: This work was financially supported by grant from Institute of

Energy Science VAST (Vietnam), Energy Technology Research Institute - AIST (Japan), New Energy Foundation (NEF, Japan) and Green Resources Technology Development JSC.

(Vietnam). We are grateful for Mr. Nagao (NEF), Dr. Kogaki (AIST), Dr. Duong Duy Hoat

(IES) and Dr. Nguyen Thuy Nga (IES), Mr. Nguyen Hai Bac (Greenmade Co.) for supporting

me done this work.

6. REFERENCES

1. New Energy and Industrial Technology Development Organization (2008), Wind Power Guideline for Japan.

2. Green resources technology development JSC. (2013 - 2015), Condao island windpower project.

3. Le Thi Thuy Hang (2013-2014), Study of the assessment method of wind conditions in Japan and Application to Condao island in Vietnam, Research report of Renewable Energy Researchers Invitation Program 2013-2014 of New Energy Foundation (Japan).

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