Estimation of Average of Global Solar Radiation Depending on Sunshine Duration Hours for Iraqi Metrological Stations

In this study, the global solar radiation for the locations of fourteen Iraqi metrological stations was studied and calculated. This was performed because most of the Iraqi stations lack solar radiation measuring devices. The equation postulated by Angström (1924) and modified by Prescott (1940) was utilized for the estimation of the solar radiation for the fourteen Iraqi metrological stations depending on sunshine duration measurements of these stations. Empirical constants of AngstromPrescott equation that are adopted by the Food and Agriculture Organization (FAO) were used for obtaining the results. The utilized data reported in this study were taken from the Republic of Iraq Meteorological Office (RIMO). The calculations and diagrams were carried out and the results were obtained by making advantage of Excel's program capabilities.


Introduction
In addition to energy security concerns, the environmental concerns about fossil fuels led to significant interest in using of renewable energy (RE). The sun affecting the Earth is providing renewable energies that are mostly inexhaustible. Renewable energy sources include solar, wind, ocean waves, and thermal sources, noting that the incoming energy from the sun in a visible radiation form of energy to the surface of the earth is known as solar radiation [1]. Solar energy is friendly to the environment and available almost everywhere on Earth, unlike fossil fuels that are available only in limited locations of the world. In addition to the above advantages, solar energy is available and is never subject to price changes related to the oil market. Solar radiation may be harnessed either as a solar or photoelectric radiation. [2]. The differences in the received solar radiation at a given place are caused bythe sequence of the day of the year, latitude, sea level (altitude) and climatic factors such as clouds, humidity and dust [3].In principle, most of these variables are well understood. Difficulty arises for not describing physical weather conditions in both spatially and temporally dimensions. Other difficulties are related to clouds as they are highly variable in both spatial and temporal dimensions and in spatial dimension. Clouds are impossible to be described even for a specific short time in a specific location [4]. Daily mean solar radiation data are necessary for the study and the design of solar energy control devices [5]. Sunshine duration is one of the most important parameters of meteorology measurement because it plays an active role to determine the data of solar radiation. Also, through its relationship with global solar radiation, relative humidity and other climatic variables, sun duration represents the most efficient parameter [6]. A huge source of energy is represented by solar energy, with about 170 trillion kw of being usually received to the Earth. Global solar radiation H with its components (i.e. diffuse H d and direct H r ) are active parameters [7]. Angstrom-Prescott equation is utilized to calculate global solar radiation (H) on a horizontal surface by using the sun duration data [8]. The modified form of Angstrom equation is a simple model utilized for estimating the global radiation. In 1924, Angstrom was able to develop the first relationship between the sun duration and global solar radiation, the irradiation using a linear model. After that, Prescott developed the equation in a favorable form when he replaced the global solar radiation on a clear sky by extraterrestrial solar radiation, as follows: Where H is the global solar radiation on horizontal surface in a day (MJ.m -2 .d -1 ), n is the daily bright light duration (Hours per month), N is the maximum possible daily bright duration (Hours per month), [9]  The following equation was used to calculate the monthly average daily extraterrestrial radiation on horizontal surfaces (MJ.m -2 .d -1 ): Where o E is the emendation of receiving solar radiation due to the changing distance in the path between the Sun and the Earth. It is given by the equation: Where φ is the latitude, d is the Julian day number and δ is the declination angle given by the equation: Where w is the sunset hour angle given by the equation:

Weather in Iraq
Iraq as a Middle Eastern country is one of the countries which are situated on a yellow belt of Earth that can receive the maximum light during the day and in different months in the year. Iraq climate is described as hot weather in summer and cold in winter. A typical meteorological data set is not available in Iraq, mainly due to the lack of sufficient raw data [11].

The RIMO data
The utilized RIMO data of the studied sunshine duration of the fourteen Iraqi metrological stations are illustrated by using an Excel program in Figure-       The results also show that the astronomical day length N and the extraterrestrial solar radiation H o values decrease with the increase in the distance from the equator, as shown in Figures-(5 and 6). The results obtained from this research generally indicate that the analysis of the geographical location is the most influential factor on global solar radiation values. Therefore, the values of solar radiation decreased with the increase in the distance from the equator, whereas the solar radiation values typically decreased with the increase in the latitude, as shown in Figure-7.
The calculated values of the monthly average daily of H o and N were applied in equation 1 to determine the monthly average daily values of global solar radiation H., as illustrated in Table-2. Figure-8 shows the change in H values with stations distribution from south to north of Iraq.

Conclusions
This study showed that it is possible to calculate the values of extraterrestrial radiation (Ho) and global solar radiation (H) by means of the geographical location (latitude) and sunshine duration data, making the application of Angstrom equation possible. The values of the regression constants A and B of the FAO The values of the constants were dependent on the location of the station within the Iraqi land. In this study, the application of Angstrom equation provided a successful estimation of global solar radiation (H) for all the fourteen stations. This was deduced by the fact that the calculated values decreased gradually with the increase in the distance from the equator, which is consistent with what was expected. Figures-(6, 7), which are related to the changes in the monthly mean values of extraterrestrial solar radiation (Ho) and global solar radiation, show that the maximum value of Ho (about 43) and H (about 28) for the fourteen stations were recorded during the summer, especially in June and July, while the minimum values of Ho (about 15.1) and H (about 8) were observed during the winter, especially in January and December.