Estimating the Effect of Meteorological Parameters on the Decrease of Earth Received Radiation Emphasizing the Relative Humidity Changes to Set Solar Sites

Solar radiation, a renewable energy, is the most effective, economical and the safest source of energy which has the potential to be the major source of energy in near future. As a result optimum use of solar energy needs precise siting of solar site. The most accurate way of measuring solar radiation is using pyranometer which is not so popular around the world because of its high charge and lack of facilities. Therefore, nowadays the researchers use climatic and environmental parameters to estimate solar radiation (Belcher and DeGaetano, 2007: 10). Although meteorological parameters, namely relative humidity, cloudiness percentage, temperature and hours of sunshine affect the amount of earth received radiation, research has proved that hours of sunshine is the most effective factor on receiving radiation; it has been proved in the model of linear regression of Angstrom in the way of FAO- penman-Monteith method. Approximately 1445">   of Iran area, having 240-250 sunny days yearly is capable of producing a lot of solar energy. Angstrom model (1924) was made based on the relationship between received radiation and sunshine hours (Angstrom, 1924: 122). It was revised in 1940 (Prescott, 1940: 115). Sabziparvar (2008: 1002) believed that out of various studies, aiming at presenting a simple equation to estimate solar radiation in central arid region of Iran, Sabagh method has the least number of errors. This study aimed at finding the effect of climatic parameters, especially relative humidity on the amount of total received radiation between Minab and Bandarabas stations.
Material and method
In the present research daily data of synoptic meteorology stations of Minab and Bandarabas during the temporal period of 2006-2009 were studied in order to estimate not only the amount of received radiation but also the effect of relative humidity on radiation decrease in real standard mode. The aforesaid stations were chosen due to their similar features like the height difference less than 20m altitude, 4◦ latitude while different distance from the source of moist; Oman sea and Persian gulf. The amounts of solar radiation was estimated, using Bird and Hulstrom model, by considering both locative features (altitude, latitude and longitude) and climatic features (relative humidity, temperature, pressure, sunshine hours, sun height, atmosphere albedo, particle absorbing, earth albedo, atmosphere mass, ozone absorption and Rayleigh distribution). Moreover in order to estimate the effect of relative humidity on total received radiation decrease the values of climatic variables were supposed stable in standard mode. Finally daily and monthly values were presented as tables and graphs.
Discussion and Conclusion
The findings of this study showed that the maximum amount of total received radiation of Minab station was in June, 14.48 MJ/m^-2/ d^-1, while for Bandarabas station was in May, 13.97 MJ/m^-2/ d^-1. In fact this difference during a month is due to the difference in the amount of relative humidity. In other words, high radiation of Bandarabas station in May is the result of both its less relative humidity than warm months and high solar height. As summer comes radiation decreases, for the amount of moist in atmosphere increases. The lowest amount of total received radiation of Bandarabas is in cold months in which slight angle of sun are the major reason for low received radiation. Besides total received radiation of Minab during winter is less than Bandarabas which is the result of more moist in June. Minab station received maximum amount of solar radiation coinciding with highest height of sun. In addition, sunshine hour decrease leads to minimum total received radiation during cold month. The most considerable point is total received radiation of Minab which is less than Bandarabas, for relative humidity of Minab is more than Bandarabas. Studying the effect of relative humidity on standard atmosphere showed that the amounts of total received radiation of the stations are not stable: it varies both in January form maximum amount, 6.4 MJ/ m^-2/ d^-1, with relative humidity of 10% to minimum amount, 5.6 MJ/ m^-2/ d^-1 with relative humidity of 90%, and in June from maximum amount, 18.4 MJ/m^-2/ d^-1 with relative humidity of 10% to minimum amount, 15.8 MJ/ m^-2/ d^-1, with relative humidity of 90%. As a matter of fact the amount of relative humidity in average mode has more effect on total received radiation compared with its low or high mode. Moreover it is necessary to say that the amount of total received radiation in different hours is 4 MJ/ m^-2/ d^-1, minimum, and 16 MJ/m^-2/ d^-1, maximum, considering the effect of relative humidity. Totally, it seems that Minab than BandarAbas station has more potential for energy production to Set Solar Sites because Minab station recive more radiation.