measurement and comparison of solar radiation estimation models ...

horizontal surface by the DMI. The yearly and monthly optimum tilt angles were
determined by converting available data. It has been concluded that the yearly total
optimal tilt angle was 39.40° for the year 2000. The smallest optimal tilt angle was 6.7°
in June and the largest was 65.2° in November.
Ulgen and Hepbasli (2004) reviewed solar radiation models for Turkey in
general and some of its provinces. 41 models used to estimate the monthly average daily
global solar radiation on a horizontal surface were categorized in four groups namely;
lineer models, polynomial or quadratic models, angular models, and modified
Angstrom-type models. They concluded that most of solar radiation models developed
for Turkey over a 19 year period was in the polynomial forms and the models gave
reasonably well results for Turkey or elsewhere with similar climatic conditions.
Aras et al. (2006) developed empirical models to predict the monthly-average
daily global solar radiation over twelve provinces in the Central Anatolia Region (CAR)
of Turkey and to compare calculated values obtained from developed models with data
measured by the Turkish State Meteorological Service (DMI) in the period from
January 1990 to December 1996 based on the various statistical methods. The general
linear, quadratic, and cubic polynomial models were derived for the region. Values for
the maximum monthly average daily measured global solar radiation ranged from 24
(for Cankiri, Turkey) and 27.10 (for Nevsehir, Turkey) to 25.85 MJ/m2. Maximum
average daily measured sunshine duration values were in the range of 9.6 (for Cankiri,
Turkey) and 11.90 (for Nevsehir and Sivas, Turkey) to 11.12 h.
Ulgen and Hepbasli (2004) compared some existing models used for estimating
the monthly-average daily global solar radiation for Istanbul, Ankara and Izmir of
Turkey. They also developed some empirical models for these cities. The MBE, RSME,
MPE, and t-statistic methods were used to evaluate the performance of the models.
They concluded that the 2 new empirical models were found to be reasonably good for
all the test methods.
A new correlation for the estimation of monthly average daily global solar
radiation was developed by Akinoglu and Ecevit (1990) and they compared with the
correlations of Rietveld, Benson et al., Ogelman et al. The overall results show that the
quadratic form gives better performance in terms of global applicability. The new
quadratic model should be preferred for the monthly average global solar radiation
estimation when the data for bright sunshine hours are available.
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