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Wind energy potential in Tamil Nadu India: Prediction and mapping using GIS
D. Jayakumar, Prashanthi Devi. M, S. Suriyanarayanan,S. Balasubramanian Dept of Environmental Sciences, Bharathiar University, Coimbatore641046. Tamil Nadu. India C. R. Ranganathan Dept of Mathematics, Tamil Nadu Agricultural University Coimbatore 641003 Tamil Nadu. India Email: sbalasubra2001@rediffmail.com Phone No: 91 0422 425459, Fax No. 091422422387 Wind energy is one of the important free renewable clean and nonpolluting sources, which vigorously pursued in many countries. Of the several alternative energy sources wind is perhaps the most suitable and cost effective. Compared with solar energy sources wind is more sensitive to variations with topography and weather patterns. These properties make wind resources assessment  the characterization of wind as an energy resource  a very important part of wind energy applications (Pacific North West Laboratory, 1979). Thus unlike other renewable resources, the wind resource varies with time of day and season of year and even some extent from year to year. Wind energy has inherent variances and hence it has been expressed by distribution methods like Weibull, Bivariate Normal and Gamma distributions (Christofferson and Gillette (1987), Ranganathan et al., 1991). With this in view, the present study has been initiated to attempt to estimate the available wind power potential using Gamma distribution in different parts of Tamil Nadu and mapped using GIS. Study Area and Data The study area Tamil Nadu lies within the latitude 8° 5'  13° 35'N and longitude 76º 15' 80º 20' E. Tamil Nadu is situated in the southern end of Indian peninsula, which is bounded by Karnataka & Andhra Pradesh in the north, Bay of Bengal in the east and Indian ocean in the south and Kerala in the West. The total area of this state is 1,30,058 Km^{2}. The basic data for this study consists of mean monthly wind speed data of 18 meteorological stations randomly distributed in Tamil Nadu for 11 years (19701980). The readings were taken at a height of 10meters above the ground level. The locations of the stations, the altitude and the descriptive statistics of monthly wind speed were analysed. Wind power estimation and Methodology The observed mean monthly wind speed has inherent variability and can be best described by a probability distribution method. Gamma distribution is one of the most useful continuous distributions often used to model natural events like precipitation and other climatological data of series (WMO, 1996) In this paper, we have used Gamma distribution to estimate the mean wind power of 18 stations in Tamil Nadu. The mathematical analysis of wind power calculations and fitting procedure of Gamma distribution to observe data are as follows: The probability density function of the twoparameter Gamma distribution is defined by
(1.0)
where G(b) is the Gamma function defined by
(2.0)
The Gamma distribution has a single peak at x = (b  1)/a, (b < 1) and it takes a variety of shapes, depending on the values of b, ranging from reverse J shaped for b < 1 to single peaked for b > 1. The constant b is called shape parameter and a is called scale parameter. The cumulative Gamma distribution function is defined by
(3.0)
The value of the above function can't be given in a closed form and it can be evaluated numerically using tables of incomplete Gamma function, which is available in several handbooks (eg. Pearson, 1957). The mean wind speed, says wbar, in any locality is given by the mean of the Gamma distribution and can be shown as
(4.0)
The wind power at any location is given by where r is the air density. So the available mean wind power at any location is given by
(5.0)
To estimate this value, we have to find the value of E (w^{3}). This is done as follows. If G (v) is the cumulative distribution function of w^{3}, then
G (v) = P {w^{3} £ v} = P {w £ v^{1/3}} = (6.0)
Hence if g (u) is the density function of u, then, The above equation gives an expression for available wind power at any location. As an example, for Kodaikanal station, the estimates of values of a and b are22.8 and 7.27 respectively. Hence the average wind speed is given by E (w) = 22.8/7.27 = 3.14 and the mean power, assuming the value of air density to be 1.25 is In a similar manner, the mean wind speed and wind power were estimated for the remaining stations. The results of the twoparameter Gamma distribution were compared with twoparameter Weibull distribution estimates of both Method I & Method II (Ranganathan et al., 1991). These values were found to be on par with each other. Mapping and GIS GIS mapping method was used to map the wind power of the 18 different locations randomly distributed in Tamil Nadu. GIS package MapInfo was used to digitize the necessary layers of Tamil Nadu and to geocode the 18 wind power stations for this study. Using the predictors obtained from Gamma distribution, thematic maps were prepared for seasonality on monsoon scale and wind speed/power. All the 18 stations in Tamil Nadu were selected to prepare wind power maps for different monsoon seasons. The data were classified into four ranges namely High, Moderate, Low and Very low for each season and mapped using GIS package MAPINFO. An overall map of Gamma distribution for wind power was first prepared. Three stations namely Coimbatore, Kanyakumari and Tuticorin were found to have high wind power with values ranging from 51 to 82.4 (W/m2 ). In the moderate ranges four stations namely Pamban, Karaikal, Nagapattinum & Pondicherry were found at a range of 29.0 to 50.99 W/m^{2}. The low range consisted of six stations (Palayamkottai, Kodaikanal, Madurai, Tiruchirapalli, Cuddalore, Meenampakkam) at a range 12.0 to 28.99 W/m^{2}. The very low wind power was observed at Nungampakkam, Vellore, Mettur and Ooty (stations), which come in the range 2.0 to 11.99 W/m^{2} (Map 1). In the Premonsoon map of wind power, the values were classified into four ranges. In the high range from 39 to 64 W/m^{2}, three stations namely Tuticorin, Kanyakumari and Pondicherry were seen. In the moderate range of 19to 38 W/m^{2} four stations were observed. In the low range six stations ranging from 9 to 18 W/m^{2} were observed. The very low range of 0.8 to 8.0 W/m^{2} of the Premonsoon wind power consisted of five stations (Map 2). In the monsoon season, three stations (Kanyakumari, Coimbatore, Tuticorin) recorded the high wind power whereas five stations recorded low wind power, which are Ooty, Mettur, Vellore, Nungampakkam and Cuddalore (Map3). On comparison of the Premonsoon wind power map with the monsoon wind power map, it was observed that Tiruchirapalli of Premonsoon low range had shifted to monsoon high range and Pondicherry had shifted from the high range to the low range. In the post monsoon season 4 stations namely Pamban, Tuticorin, Kanyakumari and Pondicherry recorded the highest wind power and five stations namely Ooty, Mettur, Vellore, Meenampakkam and Adirampattinam recorded the lowest wind power (Map 4). On comparison of the percentage of the Gamma with the three monsoons, it was observed that the monsoon season recorded a high of 10.14% and the lowest recorded as 2.90% at post monsoon season. The highest wind power was recorded as 192.56% at Coimbatore stations in the monsoon season. Tuticorin and Kanyakumari remained in the high range throughout the year. On similar comparison with the other ranges, it was observed that there was a constant shift among the moderately low stations. In the very low range, Ooty, Vellore and Mettur remained constant throughout the year. Constant shift was observed in the other 12 stations during the change of season. Discussion and Conclusion The estimated parameters a and b and k and c of and the mean estimated wind speed U (m/s) and power P/(Wm^{2}) of the Gamma and Weibull distribution methods, respectively. The analysis of the results revealed that the mean wind speed varies from 1.21 to 4.60 in Ooty and Tuticorin, respectively. The respective wind power was also estimated. Using the wind power estimates for 18 stations, a thematic mean wind power map was prepared (Map 1). From the thematic map the wind power was ranged into four classes namely high (51  83 W/m^{2}), moderate (29  50.99 W/m^{2}), low (12  28.99 W/m^{2}) and very low (2  11.99 W/m^{2}). The mean wind energy varies from 2.46 to 82.39 W/m^{2} in Ooty and Coimbatore. From Map 1, it is inferred that the stations like, Coimbatore, Kanyakumari and Tuticorin, were found to have high wind power. Since, wind as a substantial stochastic component in both spatial and temporal variations. This fact leads to advantages and disadvantages for wind as a possible energy sources. Therefore, temporal wind power variation analysis was also performed. Using the wind speed and power, seasonal wind power thematic maps for Premonsoon, Monsoon, and Postmonsoon were prepared and presented as Maps 2 4. From the thematic maps it is inferred that Tuticorin, Kanyakumari and Pondicherry has recorded higher wind power in the Premonsoon season (Map 2). In the post monsoon season Pondicherry is an addition (Map 4). But Coimbatore, Tuticorin and Kanyakumari has recorded overall maximum wind power in monsoon season (Map 3). The results show that Coimbatore has the high wind power of 192.56 W/m^{2} in monsoon season, which is higher than the wind power of other 17 locations in all seasons. When we consider the wind power energy throughout the year, the result reveals that Tuticorin and Kanyakumari were found to have high wind power. Comparing with Weibull distribution methods of wind power estimation of Ranganathan et al., the present method (Gamma) is the best method for estimating wind power. In this study, an effort was made to identify new wind power potential data from different regions of Tamil Nadu using twoparameter Gamma distribution model. To view and assess the wind power potential of the state GIS technique was adopted to delineate wind power in different regions. From the analysis it was observed that Coimbatore, Tuticorin and Kanyakumari is the best potential area for setting wind power system. The estimated high wind power area is 4.53% of the total area of Tamil Nadu.
Table 1. The calculated % area of wind power potential in Tamil Nadu
The moderate and low wind power areas are 6.34% and 20.05%, respectively (Table 1). The wind power potential is high (6.16%) in post monsoon season followed by 4.56% in monsoon season of high potential area. Further it is inferred that stations like Pondicherry, Tiruchirappalli and Pamban have a good prospect on wind energy in pre and postmonsoon seasons respectively followed by Coimbatore in the monsoon season. It is clear from the thematic maps that high wind power potential area is maximum in the postmonsoon season and it is varied according to the season. From the study, it is concluded that spatial and temporal diversity, however, tends to increase the chance of wind availability, if the wind energy machines are interconnected as an array on the same utility grid with light and strong wind speed on either directions of the network. Reference
 
