Technical Analysis and Comparative Study of three Wind Turbines for a 50MW wind farm in Laayoune City Morocco

. Wind energy is in the spotlight, it is nowadays the fastest growing source of energy in the world. The objective of this work is to examine wind power potential of Laayoune site using wind speed, wind direction, and other meteorological data collected during one year. This type of complete examination provides information of wind characteristics of potential sites and helps in choosing appropriate wind turbine. In the first part, we have been present a comparison of three methods for estimating parameters of Weibull distribution function wind speed data namely Standard Deviation , Least Squares and Mabchour’s. The comparison has been done using Excel sheet and ALWIN software. In second part, we have been compare three various wind turbines: WES 18-100KW, XANT M-21 100KW and Northem Power NPS 21-100K for the production of a 50MW electric power from wind energy farm installed in Laayoune city. The criteria of turbine choice is based on the price per kilowatt hour and the higher annual production. The simulation results using HOMER and ALWIN software showed that the wind turbine: XANT M-21 100KW has the higher annual production with low cost of installation.


Introduction
In recent years the world has seen a huge increase in the consumption of electricity generation based on energy resources. There is a great need to use these renewable resources such as solar energy, wind energy, hydro energy, biomass… [1][2]. Currently, among the renewable energy sources, wind energy is discovered, this energy is clean and non-polluting, it is the most promising source in recent decades [3][4]. In addition, the development of wind turbines proposes a great investment in the field of technological research. These systems that produce electrical energy according to the principle of converting the wind kinetic energy that converts into mechanical energy and then into electrical energy with the help of wind turbines and electrical generators [5]. The general objective of this work is the choice of wind turbine technology to install in Laayoun city. The select Turbine is based on electricity production and technology cost. So it is necessary to choose an Equipment that gives a higher annual production but with a low cost. In this work, we will be interested in Processing meteorological data of Laayoune site and wind speed Data collected during twelve months to assess its potential, its direction and frequency. In order to know the properties of this site, we have used the most important exploited Weibull Distribution statistical [6]. In addition to determine the Weibull factors, we have used the following methods Standard Deviation, Least Squares and Mabchour's method the validation of these methods are executed using the Excel sheet and ALWIN software. In the * Corresponding author: najouamrabet@gmail.com simulation part, three medium-sized wind turbines: WES 18-100KW, XANT M-21 100KW and Northem Power NPS 21-100KW were compared by sheet and ALWIN software. To choose the higher annual energy production in that comparison we have used three different methods: Standard Deviation, Least Squares and Mabchour's method in order to confirm the most effectual method is evaluated with ALWIN software. Finally to choose the Wind turbine installed in the Laayoune city it is necessary to know the cost of KWh produced for each Wind turbine. The calculations are made by using Homer software. This paper is organized as follows: section 1 explains the study feasibility of dimension of wind farm by sheet excel and Alwin software, and at section 2, we present the results of dimensioning a wind farm of Laayoune , in section 3, we choose the wind turbines and calculate their annual energy production by Excel sheet, Homer, Alwin software. We compared in section 4 the results of the production of wind farm on Laayoune also the results the cost of KWh produced for each turbine.

Presentation of Site
Laayoune is the most important city in Western Sahara. It is located on the Atlantic coast, 500 km south of Agadir and 400 km west of Tindouf, on the road to Dakhla. Geographical coordinates: (Area: 21 km²; Contact information: located at 27° 09′ 13″nord, 13° 12′ 12″ouest; Climate: is characterized by its arid and temperate aspect).

Weibull distribution
The Weibull distribution is a continuous probability distribution used for statistical modelling of wind speeds [8][9]: And the probability function is given by: ( ) is the Weibull probability density function. It represents the probability of observation of a wind speed , in m/s. C is the scale factor of the Weibull law, in m/s. It is related to the average wind speed by the form factor, k is the form factor of the Weibull law, and it is generally between 1 and 3.

Weibull parameters Determination
Several methods are used to determine the Weibull parameters C and k from statistical wind data. Among the most used are [10,11]:

Least squares method
With the help of this method the parameters are estimated with regression line equation by cumulative density function. From Equation 2, the cumulative density function of Weibull distribution function with two parameters can be written as: The cumulative Weibull distribution function is transformed to a linear function like below: 1 ln ln ln ln 1 ( )

k v k c Fv
     (5) Equation (4) can be written as = +b.
By taking the logarithms =ln ( ) and =ln [-ln (1-pi)], we obtain after a line = + whose coefficients are adjusted by linear regression. And we will have the parameters c and k, k= a and c=exp (-b/a).

Standard deviation method
The standard deviation method can be used for the K and C parameters determination based on the standard deviation calculation of the wind speeds variation and their average. According to this method [12], we have: The average wind speed can be expressed as: Equation (6) can be simplified as: By substituting a Gamma Function: (7) and let 1 1 y k  then we have: The standard deviation of wind speed v is given by Using: Put 2 1 y k  , then the following equation can be obtained: Hence, we get:

Mabchour's method
Mabchour's method (MMab). The proposed method by Mabchour (1999), was selected in the Assessment of wind energy potential k and c as [13]: ρ stands for the density of the air (kg/m3 ), v is the average wind speed in (m/s) and s the wind collector surface (m2). With i: class of wind speed [5,7,9]

Wind turbine energy
Where: E is the energy produced in kWh, is the time period ( ) is the power in kW given by the wind turbine generator at speed .with i: class of wind speed

Presentation of results
We processed and analysed the data of station in Laayoune of 369 days for an assessment of the wind potential using the menu under "Annual Review". We found the frequency distribution of wind speed as follows [8, 9, and 13].

Least squares method Result
Least squares method can be used to determine the parameters k, c by numerical of the equations (4, 5).
Weibull parameters for the Laayoune site by the least squares method :( k = 2.95 = 6.93).

Standard deviation method Result
Standard deviation method used to determine the parameters k, c by numerical of the equations (14, 15).

Mabchour's method
Mabchour's method can be used to determine the parameters K, C by numerical of the equations (16, 17). Table 4. Calculation of the Weibull parameters using the Mabchour's method for the Laayoune.
Weibull parameters for the Laayoune site by Standard deviation method (k = 2.419 = 6.89).

Wind potential of the of Laayoune
Wind potential of Laayoune site of 369 days for an assessment of the wind potential using the menu under "Annual Review".   The main characteristics of these wind turbines are presented in the table below.  The main characteristics of these wind turbines are presented in the table below.

Calculation of the production by Excel
The tables 6, 7 and 8 represent the annual production for the XANT M-21 100KW turbine calculated by Excel.
We can see that: The annual production measured by method 1 is: 441.39 MWh. The annual production by method 2 is: 449.96 MWh. The annual production by method 3 is: 436.57 MWh. It can be seen that the annual production by method 3 Is almost identical with the measured annual production.

Calculation of the production by ALWIN
The graph below represents the annual production for the WES 18 -100KW turbine calculated by ALWIN.   For the other medium-sized wind turbines: WES 18-100KW and Northem Power NPS 21-100KW it is the same principle. So we obtain the following results: It can be seen that the annual production by Method 3 is almost identical with the measures annual production.

Calculation of the cost per KWh produced by each technology
According to figures 5, 6 and 7 estimated with the help of HOMER software we find:

Conclusion
This project presents a study relating estimation of the wind potential on the site of Laayoune and on the other hand the estimation of the annual energy production. The analysis of the results shows that the site of Laayoune has a significant wind potential and is more favourable to the exploitation of this type of energy for electricity production. To install a 50MW wind farm in Laayoune, we can use on the one hand medium wind turbines: 500 turbines type WES-18 [100KW] whose annual production of the wind farm is about 117.10GWh (according to the results of Weibull ALWIN). Then we can install 500 XANT M-21 100KW turbines with an annual production of 139.65GMWh, and finally we can choose 500 Northem Power NPS 21-100K turbines with an annual production of 136.7GWh.
The choice of the installed technology is based on the costs of the technology. Therefore, it is necessary to choose a technology that gives a higher annual production but with a low cost. According to the results obtained by the different methods, we can conclude that the annual energy production obtained by the XANT M-21 100KW turbine is generally good compared to the other turbines.
In addition to that, the electrical production of the XANT is superior with a very low total cost. Therefore, for the Laayoune site, the XANT M-21-100KW turbine is the most efficient, the most profitable and the best suited to the climatic variables of the site.