Observation and Simulation of Available Solar Energy at N’Djamena, Chad

The objective of this work is to evaluate the available solar potential at N’Djamena (12 ̊08N, 15 ̊04E) from 2017 to 2018. To achieve this goal, we used various datasets and model including: the in situ shortwave radiation (by pyranometer) measurement and sunshine duration (by Campbell-Stokes heliograph) obtained from N’Djamena station, observations from MODIS (aerosol optical depth (AOD) and precipitable water) satellite sensors, and simulations from Streamer radiative code. The results show the presence of a good available solar potential with an annual global potential of 4.71 kWh/m/d. At the intra-seasonal time scale, there are two maximums for the global solar potential. The first maximum is registered in the month of March (spring) with value of 5.7 kWh/m/d and the second in October (autumn) with value of 5.18 kWh/m/d. However, the minimum of global potential is recorded in winter (from December to February) with values around 3.86 kWh/m/d. Then, the measured global irradiation allowed validating the Streamer radiative transfer code with a score of more than 98%. Subsequently, this model was used to simulate direct normal and diffuse irradiation for several types of days (clear, dusty and cloudy days). An examination of the dust influence on solar radiation based on selected cases (AOD = 2.05) indicates a mean decrease of 3.33 and 3.17 kWh/m/d, respectively, for the total and direct normal potential. This corresponds to an increase of the diffuse potential of 0.52 kWh/m/d. Finally, an increase of 5.82 cm of precipitable water per day tends to decrease the overall potential of 0.73 kWh/m/d and the direct normal potential of 1.74 kWh/m/d. For this cloudy day, the potential has increased more than 0.89 kWh/m/d. How to cite this paper: Goni, S., Adannou, H.A., Diop, D., Kriga, A., Khayal, M.Y., Nebon, B., Beye, A.C., Niang, A.S.A. and Drame, M.S. (2019) Observation and Simulation of Available Solar Energy at N’Djamena, Chad. Smart Grid and Renewable Energy, 10, 165-178. https://doi.org/10.4236/sgre.2019.106011 Received: June 2, 2019 Accepted: June 23, 2019 Published: June 26, 2019 Copyright © 2019 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/


Introduction
Africa is often considered as and referred to the "Sun continent" or the continent where the Sun's influence is the greatest [1]. The theoretical reserves of Africa's solar energy are estimated at 60,000,000 TWh/year, which accounts for almost 40% of the global total, thus definitely making Africa the most sun-rich continent in the world [2].
The global solar irradiation of Africa is very favorable to the solar energy exploitation. Some parts of the continent are among the sunniest in the world in terms of intensity of solar radiation [3] [4] or duration of insolation [5]. For instance, the center of the eastern Sahara is the driest region of the globe [6]. Likewise, the region between Libya, Egypt, Sudan and Chad, is the sunniest part of the Earth with over 4300 hours per year [7], which is equal to 97% of the possible total [8]. This region also has the highest mean annual values of solar radiation (the maximum recorded being over 220 kcal/cm 2 ) [9]. However, the solar potential is not homogeneous across region and countries in Africa. Indeed, this radiation varies according to the climatic zone (altitude, longitude and latitude) and the atmospheric components (rainfall, aerosol, precipitable water).
The focus of this work is on N'Djamena (12˚08N, 15˚04E) administrative capital of Chad from 2017 to 2018. The climate is characterized by warm and dry continental type. From north to south of Chad, we have a succession of Saharan climate in the north, Sahelian in the center and Sudanese in the South [10] [11] [12]. Indeed, Chad is one of the world's sunniest countries which receives more than 3030.91 ± 176.33 hours of sunning corresponding to 8.9 hours daily [12].
Similarly, there is a strong gradient between the north and south of the country.
For example, daily insolation is 10 ± 0.41 hours in the north, 8.85 ± 1.1 hours in the center and 7.75 ± 1.8 hours in the south [12]. Besides, there is a marked seasonality of the duration of sunshine with maxima in the dry season and minima in the rainy season. The lowest values of sunshine duration are in August (less than 7.5 hours per day). On the contrary, maximums are recorded from November to February with values greater than 9.5 hours per day. The aim of this work is to evaluate the available solar potential (direct normal, diffuse and global) over N'Djamena in Chad. For the first time a pyranometer has been used to characterize the global potential and Streamer model which serves to estimate the direct normal and diffuse potential. The paper is organized as follows. "Data and Methodology" presents the study area (N'Djamena station), in situ measurement (by pyranometer and heliograph), the space (MODIS sensor) data (aerosol optical depth (AOD) and precipitable water) and Streamer radiative transfer code. The results are presented in "Results and discussions".

Presentation of Instruments and Data
Several data are used in this study of solar potential in N'Djamena from 2017 to 2018. First, solar irradiation measurements are taken by pyranometer (vantage Pro2 weather station) installed at the National Research and Development Center in N'Djamena in 2017. This is the first time that such types of in situ measurements have been carried out in Chad. Likewise, daily insolation measurements (at sunrise and at sunset) are carried out using a Campbell-Stokes heliograph come from this center. And finally, the atmospheric data (aerosol optical depth at 550 nm and precipitable water) which serve as input to Streamer model and intercomparison tools are derived from the MODIS sensor from space. MODIS is a sensor transported by the TERRA satellites since December 1999 and Aqua in April 2002. TERRA sweeps the earth surface from the North to the South around the equator in the morning around 10:30 am while Aqua occurs in the evening, around 10:30 am in an orbit oriented South-North of the Equator [13]. MODIS has 36 spectral bands that enable it to provide measurements on the atmosphere, the Earth and the ocean, 7 of which are used to study aerosols (466, 553, 644, 855, 1243, 1632 and 2119 nm). In addition, it uses different algorithms to invert aerosol properties on Earth [14] and on seas [15] where measurements are made with a spatial resolution ranging from 1 to 250 km and temporal from 1 to 2 days. For our study, MODIS-Terra and MODIS-Aqua Deep-Blue inversions at 550 nm (available on NASA's Giovanni site (https://giovanni.gsfc.nasa.gov/giovanni/) are used indeed, the Deep Blue algorithm takes into account cloud masks, the aerosol model and the reflection of shiny surfaces [13] [16]. This makes it possible to eliminate contaminations due to the reflection of the shiny surfaces and improve the qualified observations in level 2 in areas like the desert of the Sahara, the arid, semi-arid and urban regions where reflectivity is very significant [17].

The Streamer Code for Radiative Transfer
The streamer code developed by [18] and [19] is a simple and fast model for as-Smart Grid and Renewable Energy or without isotropic reflection through its bidirectional reflectance distribution function [20]. For our study, the rural model is used considering the nature of dominating particles in the Sahel, especially in Chad. Moreover, for the simulation of surface level solar flows, the model must be thoroughly and accurately completed [21].
In this study, the Streamer code inputs data for surfaces fluxes estimation for each day are: the aerosol optical depth, the precipitable water, the aerosol model, the surface albedo, the ozone, and the site geographic coordinates.

Global Solar Potential on the Horizontal Plane
Sizing of photovoltaic systems requires precise knowledge of available solar potential on a plane, usually on the horizontal plane.
From the global radiation (G) measured, the available solar potential (E g ) is calculated using the following equation: Ls, time of sunrise (h); Cs, time of sunset (h).

Selection of Dates
In order to assess and simulate the solar potential (global, diffuse and direct normal) in N'Djamena, a classification was performed based on three types of days organized as follow: 1) clear days (a day without aerosol), 2) dusty days (maximum of aerosol) and 3) cloudy days (maximum of clouds). This classification is based on aerosol optical depth and water precipitable provided by MODIS sensors. Similarly to the previous study by [22], the upper threshold of aerosol optical depth is fixed to 0.1 per day. Table 1 shows the daily averaged total AOD (at 550 nm), precipitable water (WP) and global solar potential (H) Smart Grid and Renewable Energy for a total of 6 selected days (2 days per day type) for N'Djamena station.

Seasonal Effects of AOD, Precipitable Water and Insolation on Global Solar Potential
To evaluate the impact of the atmospheric parameters and the insolation on the monthly potential solar, we are performed qualitative comparisons. Figure 2 shows the seasonal evolution of the global solar potential in comparison with insolation ( Figure 2  Overall, we note that insolation duration (Figure 2(a)) follows the same trends that the solar potential except in winter (December to February). Indeed, the increasing trends are visible in these two curves from March to August. Furthermore, we note a decreasing trend of these two parameters from August to November. During the rainy season (especially in August), the insolation and the global solar radiation are minimums which provide cloud cover. The minimum of solar potential and sunstroke recorded during the rainy season (especially in august) is mainly due to cloud cover. However, insolation peaks recorded in winter (from November to February) are not correlated with solar potential. During this season, we note the lowest values of the solar potential whereas these are the longest days in terms of sunshine.  In summary, it can be said that insolation, aerosols and clouds are parameters that influence the incident solar radiation. The combination of the effects of these three parameters makes it possible to better evaluate the solar potential from February to November. However, the lowest values of solar potential recorded in winter (from December to February) may be due to the height of the sun.

Diurnal Validation of Streamer Code in N'Djamena
The  In summary we can say that this model is validated by the observations with  correlation coefficients of more than 98% for the global potential. Then we will be able to trust this model to simulate the direct normal and diffuse potential for which we do not have observations.

Simulation of Direct and Diffuse Radiation for a Few Days
In this part we use the streamer model to simulate the diffuse and direct normal irradiation. For this, only dusty and cloudy days are used to evaluate the impact of aerosols and clouds on these radiations. For each day, two simulations were performed: a first simulation (with aerosol or with cloud) and a second (without aerosol or without cloud).

Conclusion
This study involved evaluating the available solar potential by observations and