Analysis of statistical methods for estimating solar radiation

Aleksandar Janković ,Branislava Lalić

doi:10.5937/geopan1401001j

Abstract

Daily global radiation is not only required for a large number of ecological, physiological and agro-meteorological models but also is required for the design and evaluation of solar energy systems. Global radiation data is quite frequently not available for all locations. For such locations daily global radiation can be estimated statistically using continuous series of other measurable meteorological parameters. This paper considers four statistical methods for estimating the daily global radiation: Angstrom- Prescott, Coulsen, Hargreaves and Supit-van Kapel method. The empirical coefficients that characterize these methods were determined using the least squares method for the two locations near Belgrade in Serbia. The results showed that calculated empirical coefficients are similar to values that were previously determined in similar researches for neighboring countries. Further analysis verified accuracy and applicability of four empirical relations, where Ǻngstrom-Prescott and Supit-van Kappel methods proved to be by far the most reliable in the assessment of daily global radiation.

Highlights

For stations where the global radiation is not measured or is partly missing there are several techniques for estimating and modeling global radiation
Ǻngstrom proposed model for assesment of daily global radiation that was later altered and improved by Prescott. According to this method daily global radiation Gd is dependent on daily solar radiation that reaches the top of the atmosphere Ga and the relative sunshine duration Sr: Gd = Ga
They are approximate to values of empirical coefficients that Supit determined on the basis of long-time series of measured global radiation for adjacent land Croatia (Split, as = 0.11, bs = 0.36 and Zagreb, as = 0.10, bs = 0.32) (Supit, van Kappel, 1998)

Summary

Introduction

For stations where the global radiation is not measured or is partly missing there are several techniques for estimating and modeling global radiation. The first group is based on the astrophysical properties of the Earth, atmospheric physics and geometry of location for which global radiation needs to be estimated (Paulescu, et al, 2013) Such modeling is called prognostic or physical modeling, where it is common to model seperately shortwave and longwave solar radiation. The second group includes those models which are based on statistical data or data obtained from satellite observations This approach is called statistical modeling, where the most commonly used method is based on empirical relations and requires the development of equations for estimating solar radiation according to the commonly measured meteorological variables. This type of modeling is known as statistical modeling of global radiation. The most accessible and measured data are used as input data for estimating such as sunshine duration, air temperature, air temperature combined with cloudiness and air temperature combined with total daily rainfall (Trnka, et al, 2005)

Methods

Results

Conclusion