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Wind is an important source of renewable energy, as we all knows. Bangladesh being a tropical country does have a lot of wind flow at different seasons of the year. However, there are some windy locations in which wind energy projects could be feasible. We analyzed the wind speed data of eight different sites collected from the Meteorological Department of Bangladesh by choosing the proper sites at best possible seasons of the year at different heights to show the prospect of utilizing the wind power by installing wind turbine effectively. This study can be implemented to make the best use of reasonable wind speed available in these sites throughout the year to extract power.
Bangladesh is a developing country lying in the northeastern part of South Asia and is situated between 200 30Â´ to 260 38Â´ north latitude and 880 04Â´to 920 44Â´ east longitudes. The vast Bay of Bengal on the south of the country along with the V-shaped coastal belt is the main source of the southwesterly wind blowing over the terrain. This country is experiencing a severe electric power capacity crisis that is only likely to worsen over the next 15 years. Further, over fifty percent of Bangladesh's population still lives with no electricity, and the rate of grid expansion to connect rural villages is threatened by the looming capacity shortage. There are a number of underlying reasons for the crisis, but ultimately the country lacks the fossil fuel resources i.e. natural gas, required to conduct a large scale grid expansion program. Alternative approaches to electrifying the country must be found and thus the importance of utilizing renewable energy has come into concern where wind energy is one of the most promising sectors in Bangladesh.
Recently a project on wind energy discovered that the average wind speed in sea shore is 4 m/s. This speed is enough to generate electricity requiring some design change from the wind mill of other countries. Already a wind farm of 1 MW has been setup in Kutubdia spending 110 Million Taka. There is another wind farm of 0.9 MW in Feni. These wind farms proves the potential of producing huge amount of electricity from wind. The main problem is the multi directional wind during cyclone season. If some technical change can be made then our next wind generators will be much more reliable.
In this paper, we analyze on some collected wind speed data of different sites of our country to find out the wind energy potentials to be properly utilized at particular heights during the best possible seasons of the year when the available wind speed is adequate enough to produce electricity from it.
Wind energy is the kinetic energy of the moving air mass. The power, P, in watts, possessed by wind blowing with a velocity of V, in meters per second, is
directly proportional to the rotor swept area and to the cube of the wind speed, and is given by;
P = (Â½). r. A. v3(in Watts) (1)
where, A is the area perpendicular to the direction of
flow, meter2, r, density of air, kgm-3, is approximately1.2 kg/m3, v, wind velocity, meters per second. Only a part of the total available power calculated by equation 1 can be extracted and is given by;
P = (Â½). r. A.v3. Cp (in Watts) (2)
Cp, the power coefficient  is the ratio of power extracted by a wind turbine to power available in wind at that location. A theoretical maximum of 59.3% of available power can be extracted; i.e. Cp is0.593. Practically a typical maximum of 40% is achievable. Now the power per unit area of wind cross-section is given by
P = (Â½). r. v3. Cp (in Watts) (3)
The collected raw data has been processed to compare the wind speed among the sites over one year. The raw data which were in knot, are transferred into m/s using following conversion formula:
1 knot = 0.514 m/s (4)
The wind speeds are measured at height of 2m above ground level. But the speed changes with height. So this data have been converted to get wind speed at those different heights where the installation of wind turbine is usually feasible. The rate of increase of velocity with height depends upon the roughness of the terrain. The variation of average wind speed can be determined from the following power law expression,
Vz/Vref = (h/href)Î± (5)
where, Vz and Vref are the average speeds at height 'h' in meter and at the reference height of h m ref =10
above the ground respectively and Î± varies from 0.1 to 0.4 depending on the nature of the terrain.
WIND DATA ANALYSIS
Very recent (jul-08 to jun-09) data of wind speed for eight different locations of Bangladesh from Metrological Department of Bangladesh have been collected. Among these eight locations Isordi is located at north western part, Hatia & Shandip are located at southern part, Shitakunda & Shrimangal are located at north eastern part, Mongla is located at southern part and Feni is located at east southern part of Bangladesh.
The wind speeds in different months of the year in knots are given in table-1 at a height of 2m above ground. Variations of wind speed with the seasons in m/s of the considered location are shown in fig-1 to fig-8. It shows that the wind speed from October to January is comparatively very low. From April to August wind flow in these sites is quite reasonable. So during this period wind can be a convenient source of energy. Fig-9 shows the monthly wind speed of all the locations in a common scale. It is very clear from fig-9 that the wind speed is very low during October to January. From fig-10 which is the radar view of the wind speed of different sites for a duration of twelve months from July 2008 to June 2009.
Table-1: Wind speed of different locations in knots
Fig-1: Wind speed of Srimangal
Fig-2: Wind Speed of Hatia
Fig-3: Wind Speed of Sandwip
Fig-4: Wind Speed of Mongla
Fig-5: Wind speed of Ishordi
Fig-6: Wind Speed of Shitakunda
Fig-7: Wind Speed of Rangamati
Fig-8: Wind Speed of Feni
3.1 Proposed Time Duration for Installing Wind Power Plants
To generate enough energy from the wind power plants at least 4 m/s of wind speed is needed which is not available in Bangladesh through the whole year. Table-1 shows the different wind speeds of eight different sites for duration of twelve months from June 2008 to July 2009. In fig-9 the average wind speed of different sites have been plotted in clustered column. From the figure it is seen that wind speed of almost all sites from October to January is very low which is not enough to generate energy from the wind plants. This is clearly seen in fig-10 which is the radar form of wind speed data. In the figure from October to January the peak towards the wind speed is very low compared to the other months. So during these months the energy will be low too. That is why we eliminated these four months and again plotted the wind data in clustered column and radar form in figure 11 and 12. We propose to install wind power plants during June to September and February to May as wind speed is enough during this period for generation of energy.
Fig9: Different sites with unsatisfying average wind speed in clustered column
Fig11: Different sites with unsatisfying average wind speed in radar format
Fig10: Different sites with satisfying average wind speed in clustered column
Fig12: Different sites with satisfying average wind speed in radar format
3.2 Proposed Heights for Installing Wind Power Plants
The satisfying wind speeds which have been in plotted in figure 10 and figure11for different sites are for a height of 2m above sea level except Feni which is for height of 10m. Obviously wind speed at 2m is not satisfying enough to generate energy. That is why wind speed at higher point is calculated by equation (5). The range is selected from 25 to 39 meters. For conversion we have selected terrain effect as 0.28 for general case except site Feni. For Feni the terrain effect has been selected 0.40 as wind speed at Feni is very low. The wind speed with varying height from 25 to 39 meters has been given in Table 2. This wind speed data has been again plotted in clustered column [fig13] and radar format [fig14]. From the table it is seen that almost all districts have speed more than 4 m/s except Feni.
In Shrimongal and Hatia the wind speed is quite satisfactory enough for generation of energy at height 33 to 37 meters. For shrimongal the minimum height is 37 meters and for shitakunda the height is 33 meters. Here the terrain value is 0.28. At this height the wind speed is enough fir generation as well as economical for the installment of power plant. In case of Shitakunda, Sandwip, Rangamati and Mongla the wind speed is not satisfactory. In that case we have to increase the value of terrain to 0.40. For Feni the wind is very low though the terrain effect has been considered as 0.40. So it will not feasible to install a wind power plant in Feni unless the height of the site has been increased which will be very much expensive.
Table-2: Wind speed of different locations with different heights in m/s
Fig13: Wind speed of different locations with different heights in m/s in clustered column
Fig14: Wind speed of different locations with different heights in m/s in radar format
3.3 Proposed Wind Power Plants Considering both Time Duration and Height
From the above analysis we may reach to our final proposal considering both time and height. From table-2 we will select the height for every site. For Srimongal, Ishordi we can select minimum height as 35 meters for generation of energy. For Hatia the height is 35 meters. For Shitakunda, Sandwip, Rangamati the height should be more than 37 meters and terrain value may increase depending upon the value. Proposed wind speed for installment of wind power plants considering both time duration and height are plotted in figure 15 and figure 16.
Table-3: Wind speed of different locations with different heights in m/s
Fig15: Wind speed of different locations considering heights and time in m/s in clustered column
Fig16: Wind speed of different locations considering heights and time in m/s in radar format
Our study is concentrated on the prospect of wind energy in our country to utilize properly so that we can get maximum possible efficiency with considerably low cost. The analysis shows that if we select the time duration between February to July for running wind power plant then we can get more or less satisfied amount of wind speed to harness electricity from that. Again if we can raise the height of the wind turbine then for some sites even though the speed is lesser compare to others we can still have acceptable amount of power by sacrificing cost. The other time of the year when the wind speed is not adequate enough we can think of wind-solar or wind-tidal (in some special sites) hybrid system to ensure that the power plant is not out of order during that particular time period. Solar energy or tidal energy can be used as an alternative source of energy to run the plant through the year.