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TONGA ISLANDS

Kaushik. Sharma and M.Rafiuddin. Ahmed

The University of the South Pacific, Division of Mechanical Engineering The paper presents an overview of the wind resource assessments carried out in Tonga Islands in the South Pacific region. The assessment was carried out in the mainland Tongatapu. The mean wind speed at 34m AGL of the Tongatapu site is analyzed as 4.5m/s and 4.54 m/s, respectively and the mean speed at 20m AGL is analyzed to be 3.63m/s and 3.45m/s respectively. The prevailing wind direction recorded for both the NRG 34m towers is south-east, which corresponds to the south-east trade winds. The WAsP analysis indicates a low wind speed regime.

Keywords: resource, prevailing, NRG, Tonga, WAsP

Introduction

Tonga is a Polynesian country which is located to the south of Samoa and south-east of Fiji Islands. Tongatapu is the main island of Tonga and the location of its capital

km2. Tonga has no mineral resources and relies on agriculture and tourism. The main exports of the country include fish, pumpkin, coconut products and vanilla beans. The population of the country amounts to 105,000 [1]. The total electricity production is 41 million KWh, all of which is catered by fossil fuels [2]. Fig. 1 displays the two towers situated at the University of thcampus approximately 125m apart and aligned in the south-east direction. The two towers were closely located to analyse the wind speed variations for different heights between the two locations. This gives a better idea of the flow

Fig. 1 Overview of Tongan Campus along with the two NRG tower locations

In comparison to other RETs wind energy may be a better choice in terms of land space requirements and deployment issues. Availability and quality of roads, electrical transmission and shipping infrastructure are some issues associated with installing turbines in the Pacific. A major factor in terms of the lifetime of the

system would be the periodical tropical cyclones which devastate the PICs. Wind turbines that can withstand strong weather conditions or designed to be lowered may help in maintaining the lifetime of the system. Due to the greater availability of the resource offshore, the trend towards moving offshore would benefit the PICs as space issues would be catered for by moving away from onshore turbines. To have islands that are independent of mainland resources, wind power needs to be used in combination with other RETs for power generation to ensure energy security.

Methodology

Preliminary Site-Selection

Siting of monitoring locations is the first step in any wind resource assessment plan. The site must have desirable characteristics for future wind farm development. Candidate sites are chosen through Google Earth images and advice from locals is taken into consideration. The local topography and the surrounding area are critically assessed for future development. Preliminary sites are chosen for short term assessments with a 10m mast. The candidate sites are ranked in terms of available land area and obstructions like buildings and trees. Qualitative indicators such as deformed trees by strong winds are considered.

The data transfer method specific to each individual island via cellular phone companies is considered. This is an important aspect as some islands such as Tongatapu (Tonga) are quite remote and the area is not covered by all cellular networks. After the candidate sites are assessed using the 10 m mast, the recorded data is then critically analysed for its wind potential. The best overall candidate site is chosen for the 34m NRG tower installation to assess the wind resource for a period of 1 year.

Wind Tower On-Site Installation

On site installations are carried out with the help of technicians and locals. The installation involves a specific

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method to install the tower along with the data logger and the sensors. Fig. 2 presents the tower layout used for each installation. The position of the tower is carefully marked along with the 18.29m radius for the guy anchors bolts.

Fig. 2 34m NRG tower layout overview

A winch is used to raise the tower. Once the tower is raised a few meters above the ground it is wired with the anemometers, wind vane and other sensors mounted on the boom after calibration. A copper rod is also mounted for lightning protection. The gin-pole in conjunction with a winch aids in lifting the tower along with the sensors. The wires are continuously slackened and given a tension during the lifting process. The winch is used to fully raise the tower in conjunction with the guy wires. The guy wires are continuously adjusted as the tower is raised.

Data Collection, Validation and Processing

The isolated 34 m NRG towers has a GSM modem, SIM card and 2 GB SD card for data transmission to the USP-KOICA data-server based at the ICT centre. The SD card ensures there is no risk of data loss. Suspicious sets of data are checked and validated, improving the quality of the resource assessment. The data is analysed using Microsoft Excel taking into account the average wind speed and direction for a period of a year.

Numerical Procedures with WASP 10.2

WAsP is a PC program developed by the DTU Wind Energy [6]. The number of points used to digitize the map was 311,885 giving a resolution of 50m. The sites are then analysed for its wind energy potential using resource grids available in WAsP 10.2. The Map editor tool of WAsP allows digitization of new geographical maps. Fig. 3 displays the manually digitized map of Tongatapu using the Map editor tool.

Fig. 3 Manual digitization of the Tongatapu Island map

Results and Discussion:

The raw data is presented and discussed in this section. The data is analysed for the respective sites wind potential. The WAsP results are also discussed. The mean wind speed over the data collection period was calculated from:

where is the wind speed which is averaged over a time interval t, N is the number of recorded observations[7].

Tonga Wind Data Analysis

Tower A (Daily Average)

Fig. 4 Graph of Daily Average Wind Speeds for the Tongatapu site (tower A) The 10 minute-interval wind speed observations of the 34m NRG tower was used as a basis for daily average calculations. Fig. 4 displays the daily-averaged data for the first 12 months. The analysis indicates average wind speeds of 4.64 m/s and 4.70m/s at 34m AGL. The average wind speed at 20m AGL is 3.50 m/s. The highest mean wind speed recorded is 12.56 m/s for the 34m AGL anemometer. The low average wind speeds is attributed to the thermal inversion effect highly prevalent in the islands[8] hence, the lull periods are generally during the night-time.

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Wind Direction Analysis

Fig. 5 Wind Direction Analysis for Tongatapu (tower A)

The wind rose shown in Fig. 5 displays the wind direction distribution in space with respect to percentage of time. The prevailing wind direction is 91 -120 from north. Significant contribution of 58.77% is available for the 61 -150 sector which is desirable for wind turbines. The south easterly winds dominate the island due to its small

Tower B (Daily Average)

Fig. 6 Graph of Daily Average Wind Speeds for the Tongatapu site (tower B)

Fig. 6 Graph of Daily Average Wind Speeds for the Tongatapu site (tower A)

Fig. 6 displays the daily-averaged wind speeds from tower B recorded for a period of 12 months. The analysis indicates average wind speeds of 4.67 m/s for both the anemometers at 34m AGL. The average wind speed at 20m AGL is 3.58 m/s. The current assessment indicates low-medium wind power density for the site.

Wind Direction Analysis

Fig. 7 Wind Direction Analysis for Tongatapu (tower B)

Fig. 7 displays the wind rose plot of tower B. The prevailing wind direction is 121 -150 from north. Significant contribution of 60.42% is available for the 91 -180 sector. The assessment indicates a dominant wind direction ideal for wind turbines.

Effects of Thermal Inversion

Wind speed increases with height, therefore wind turbines are more efficient and has the capacity to produce greater power as the hub height increases[9]. When wind speed measurements are carried out at different heights the wind shear coefficient (WSC) may be calculated[9]. The WSC can be calculated using the following equation by Farrugia [10]:

where and are the wind speeds (m/s) at heights (m) and , respectively.

Fig. 8 Diurnal WSC variation between 20m and 34m AGL for Tongatapu site

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The temperature inversion is a condition when a stable layer of cold, dense air is trapped close to the surface of the ground by warm air masses moving above it. The destruction of the inversion is caused after a few hours after sunrise due to the build-up of convective boundary layers over the floor following sunrise [11]. The temperature inversion effect occurs in flat open terrains and valleys and at night due to prevalence of significant temperature differences[8]. The net effect resulting in significant lower wind speeds at 20m AGL during the morning period (12:00:00 am 4:00:00am). Tarakia[8] mentions that the thermal inversion effect greatly reduces wind speeds in the South Pacific countries due to the small and open nature of the landscape.

WAsP Results: Tongatapu site

Fig. 9 presents the WAsP analysis for the Tongatapu site. The power density ranges from 74-174 W/m2. The resource map indicates greater potential for wind power generation for near shore and offshore. The WAsP analysis indicates that the Bonus 300kW Mk III wind turbine generator would have an annual energy production of 282.90 MWh for the assessed site. Using eqn. (3) this would correspond to a capacity factor (CF) of 10.74%. The CF would further drop due to periodical cyclone seasons and maintenance downtimes.

Fig. 9 High Resolution Wind Map for Tongatapu site at 34m AGL

Conclusions:

The wind resource assessment of two sites on the island of Tongatapu, Tonga has been presented. The annual average wind speed at 20m AGL is 3.50m/s and 4.64m/s and 4.70m/s at 34m AGL for tower A. The annual average wind speed at 20m AGL is 3.58m/s and 4.6m/s and 4.70m/s at 34m AGL for tower B. This range pre-dominantly falls in the low-medium wind speed regime.

The dominant wind direction for most of the sites in the region is south-east, corresponding to the trade winds. Due to the small land area and flat lands, south-east is the common wind direction in the South Pacific. Due to space scarcity in small islands, wind farms are recommended to be implemented offshore to utilize the higher power density The next phase of assessments would be for

offshore wind energy potential.

References

[1] (2012, 12/12/2013). Tonga Profile. Available: http://www.bbc.co.uk/news/world-asia-pacific-16197014

[2] (June 5, 2013). The World Factbook. Available: https://www.cia.gov/library/publications/the-world-factbook/

[3] M. Miller, P. Voss, A. Warren, I. Baring-Gould, and M. Conrad, "Strategies for International Cooperation in Support of Energy Development in Pacific Island Nations " National Renewable Energy Laboratory2012.

[4] M. Faizal and M. R. Ahmed, "Experimental studies on a closed cycle demonstration OTEC plant working on small temperature difference," Renewable Energy, vol. 51, pp. 234-240, 2013.

[5] R. K. Singh, "Design, Fabrication And Performance Testing Of A Twisted 2-Bladed Horizontal Axis Wind Turbine," Master of Science in Engineering Scientific, Mechanical Engineering, University of the South Pacific, 2012.

[6] N. G. Mortensen, D. N. Heathfield, L. Myllerup, L. Landberg, and O. Rathmann, "Getting Started with WAsP 9 " Risø National Laboratory Technical University 2009.

[7] J. F. Manwell, J. G. McGowan, and A. L. Rogers, Wind Energy Explained: Theory, Design and Application: Wiley, 2010.

[8] T. T. Tarakia, "Feasibility Study of a Hybrid Energy System for Sustainable Energy production in Kiribati," Energy Studies, Murdoch University 2009.

[9] S. Rehman and N. M. Al-Abbadi, "Wind shear coefficient, turbulent intensity and wind power potential assessment for Dhulom, Saudi Arabia," Renewable Energy, vol. 33, pp. 2653-2660, 2008.

[10] R. N. Farrugia, "The wind shear exponent in a Mediterranean island climate," Renewable Energy, vol. 28, pp. 647-653, 2003.

[11] C. D. Whiteman, "Breakup of Temperature Inversions in Colorado Valleys," Atmospheric Science, Colorado State University, 1980.



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