Assessment of Wind Energy Potential for the Generation of Power in Coastal and Sahel Savannah Locations in Nigeria

This study examines the wind power potential for different pressure heights in some Coastal and Sahel savannah stations in Nigeria. A 5-year hourly mean wind speed data at different heights and also at the hour of 00:00 and 12:00 were obtained from ERA Interim reanalysed data set. The data were statistically analyzed using Weibull distribution function. It was found out that Ikeja and Katsina stations have the maximum wind power density of 174.69 W/m and 196.88 W/m for both Coastal and Sahel savannah regions respectively at 100 m height and also at 12:00 hour of the day when there is occurrence of intensive wind speed as observed in the stations considered. Therefore, the analysis demonstrates the economic feasibility of using wind energy as a means of electricity supply at Ikeja in Coastal region and Katsina in Sahel savannah region of Nigeria. Thus, the wind power potential in these areas should be adequately harnessed which could be used to compliment the present power supply and reduce carbon dioxide emissions which contribute to global warming and lower long-term overdependence on fossil fuels in those areas and in Nigeria at large.


Introduction
Energy has a major impact on every aspect of our socioeconomic life. It plays a vital role in the economic, social and political development of our nation [1]. Despite the abundance of energy resources in Nigeria, the country is still in short supply of electrical power. Over 60% of the Nigerian population does not have power supply, with 40% not on the nation's grid [2]. The Nigerian grid supply of electricity is on average six hours per day rationed among inhabitants of the cities [3]. Almost all rural dwellers in Nigeria have little or no access to electricity. The majority of the electricity supply in Nigeria is generated by Kainji dam which produces about 3.2 x 10 8 W and 9.6 x 10 8 W at its peak [4]. This is due to underperforming hydro dams in the country. Another factor is the high cost of large oil and gas reserve, which compete favourably among the largest reserve in the world, Nigeria is battling with the severe energy crisis. There is therefore the need to harness renewable energy potential (such as wind, solar e.t.c) for reliable power supply in this country. Also the concern about global warming and continued apprehensions about nuclear power around the world should drive us into strong demand for wind generation according to [5]).
Several researchers have studied the prospects of wind energy resources around the world and in Nigeria especially the variation of wind energy potentials for power generation ( [6][7][8][9][10][11][12]). Each one of these researchers considered different sites and presented analyses to justify their results. It has been reported that due to the frictional drag of the ground, vegetation and building of the nation, large differences in wind distribution within the same locality exist. This is confirmed by the fact that wind resources are site specific and despite reports summarizing for the country, a site-bysite assessment is necessary in order to have proper wind classification for the nation. This paper will assess the wind power potential for different heights in some Coastal and Sahel savannah stations in Nigeria for wind energy generation.

Materials and Method
The hourly mean wind speed data for 00:00 and 12:00 hours at different heights ranging from 2006-2010 period (5 years) were used in the study and were obtained from ERA-Interim reanalysed data set for all the selected stations as shown in Figure 1 over Nigeria. Table 1 presents information about the locations.

Data Analysis
Weibull distribution has been found to be most adequate of the various statistical distributions that could be employed for describing and analyzing wind resource data. The wind power density (W/m 2 ) which is the quantitative measure of the wind energy available at any location was estimated from the equation (1) in terms of Weibull parameter ( [13]).
where: p (v) = wind power density (W/m 2 ), ρ = the air density at the site (kg/m 3 ), = gamma function. The Weibull parameters (c and k) were estimated by using the Empirical method with mean wind speed and its variance as shown in equation (1) and (2) according to ( [14,15] where: c = the Weibull scale parameter (m/s), k = the dimensionless Weibull shape parameter σ=standard deviation and m v = mean wind speed (m/s).
The c and k Weibull parameters account for intensity of wind speed and the stability of the atmosphere In a given area respectively.    Figure 6 shows the monthly variation of WPD at 12:00 hrs. for 100 m height over all the selected stations in Coastal and Sahel savannah regions. In the Coastal stations which comprises of Port-Harcourt, Ikeja and Calabar, fall in the same region and also characterized with the same climatic factors such as temperature, sunshine etc which could be a determinant to the wind energy generated in the stations [17]. The gradual increase in trend from January to April in all the three stations could be linked to the general rise in solar radiation associated to dry season, while the sudden rise experienced in Ikeja from May to August with a peak of 174.69W/m 2 in August within the rainy period could be attributed to thermal convection resulting from low temperature causing the upper air momentum to be transferred to lower layers ( [17]). Though, It may also be as a result of sea breeze ( [19]). In Sahel savannah region which is made up of Maiduguri, Katsina and Kano, it was noticed that Katsina has the maximum WPD in the month of December with 196.88 W/m 2 due to high temperature associated with the month and the minimum in Kano with 17.89 W/m 2 in September.

Results and Discussion
Wind energy potential can be harnessed properly in Coastal region and Sahel savannah regions as supported by ( [20]), because anywhere and each time there are differences in atmospheric (air) pressure, there will be a wind. The winds may be even stronger where the difference in the air pressure is greater. Figures (7-9) show the distribution of the WPD at 12:00 hours for 60 m 30 m, and 10 m heights respectively over all the selected stations in Coastal and Sahel savannah regions. The observation here is the same as that of Figure 6 discussed above. But it was noticed that at each height, wind energy potential was decreasing compared to the observation in Figure 6. Figure 10 shows the spatial distribution of wind power density for 00:00 and 12:00 hours at different heights over the Coastal and Sahel savannah regions.
It was observed that in each of the synoptic hours considered, Sahel savannah region due to its high temperature, has the highest wind energy potential in all the isobaric heights, then followed by Coastal region. Therefore, wind energy can be harnessed maximally in the Sahel savannah region as supported by ( [21]) where he said the northern part of the country is the best region to tap wind energy.
In addition, it was observed that wind power density is maximum at 100 m height in all the hours considered when compared to other heights especially 10 m height with little wind power distribution across the year. This is in support of the observation made by ( [12]) that the more the height as movement is made towards the mountain level, the more the wind energy that will be generated hence bringing forth a high wind speed and wind power density.

Conclusions
In this study, the assessment of wind power potential in some Coastal and Sahel savannah stations in Nigeria for wind energy generation was carried out. Five years hourly mean wind speed data at different pressure heights from ERA-Interim reanalysed data set were assessed and subjected to Weibull distribution function. Based on the analysis the following conclusion can be made: Wind speed was high in Ikeja and Katsina stations for both Coastal and Sahel savannah regions respectively especially at 100 m height. The turbine could be best installed at 100 m height which could be the best height to generate maximum wind energy potential and at 12:00 hour of the day. In conclusion, Ikeja and Katsina areas from the Coastal and Sahel savannah regions of Nigeria respectively are well endowed with abundant wind energy resources which can be used to generate electricity. Therefore, these areas are very well suitable for wind turbine application thus, the wind power potential in these areas should be adequately harnessed in order to increase the power supply and reduce the carbon dioxide emissions which contribute to global warming and lower long term overdependence on fossil fuels in those areas and in Nigeria at large.