On the importance of relative humidity for prediction of diffuse solar radiation in warm and humid climate of Iran

ABSTRACT

In numerous applications of the solar energy, diffuse solar radiation plays crucial role while their availability in many countries is scarce. Many models have been developed to estimate the diffuse solar radiation that each of them is a function of different metrological parameters. The objective of this study is obtaining a model to estimate the diffuse solar radiation in warm and humid climate in Iran and specify the importance of metrological parameters especially relative humidity. Many empirical models in three categories are selected. By using the long-term data for horizontal global solar radiation and sunshine duration and by employing the regression analysis, nine new models have been created. Then, by entering two parameters, the monthly mean daily ambient temperature and relative humidity in the empirical models, nine other new models have been developed. Performances of all the models are evaluated by means of different statistical indicators. Obtained results showed adding of the ambient temperature and especially relative humidity parameters cause increasing a few accuracy even though in warm and humid climate of Iran. The result shown in the two-degree model, in terms of the clearness index and relative sunshine duration, is the best model.

KEYWORDS:

• Diffuse solar radiation
• statistical indicators
• warm and humid climate
• relative humidity

Nomenclature

$G_{sc}$	=	solar constant (equal to 1367 W/m 2)
$\bar{H}$	=	monthly mean daily global radiation on a horizontal surface (MJ/m 2)
${\bar{H}}_d$	=	monthly mean daily diffuse radiation on a horizontal surface (MJ/m 2)
${\bar{H}}_o$	=	monthly mean daily extraterrestrial on a horizontal surface (MJ/m 2)
${\bar{H}}_{di,c}$	=	ith calculated values of ${\bar{H}}_d$ (MJ/m2)
${\bar{H}}_{di,ave}$	=	ith averaged values of ${\bar{H}}_d$ (MJ/m2)
${\bar{H}}_{di,m}$	=	ith averaged values of ${\bar{H}}_d$ (MJ/m2)
${\bar{H}}_{dc,avg}$	=	average of the calculated values of ${\bar{H}}_d$ (MJ/m2)
${\bar{H}}_{dm,avg}$	=	mean of the averaged values of ${\bar{H}}_d$ (MJ/m2)
${\bar{K}}_T=(\bar{H}/{\bar{H}}_o)$	=	monthly mean clearness index
$K_T$	=	daily clearness index
${\bar{K}}_d=({\bar{H}}_d/\bar{H})$	=	monthly mean diffuse fraction or cloudiness index
$n_{day}$	=	number of days
$\bar{n}$	=	monthly mean daily sunshine hours (h)
$\bar{N}$	=	monthly mean daily maximum possible sunshine hours (h)
${\bar{R}}_h$	=	monthly mean daily relative humidity
$R^2$	=	coefficient of determination
${\bar{T}}_{avg}$	=	monthly mean daily ambient temperature

Greek Symbols

$\varphi$	=	Latitude (°)
$\delta$	=	Solar declination (°)
${\omega }_s$	=	sunrise hour angle (°)

Acronyms

RMSE	=	Root mean squared error (MJ/m2)
MAPE	=	mean absolute percentage error (%)
MABE	=	mean absolute bias error (MJ/m2)

Additional information

Funding

The authors are grateful to the Energy Research Institute of the University of Kashan for supporting this research by Grant No .358125/2 to carry out this work.