Estimating solar radiation on tilted surfaces with various orientations: a study case in Karaj (Iran) |
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Authors: | Gh A Kamali I Moradi A Khalili |
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Institution: | (1) Atmospheric Science and Meteorological Research Center, Tehran, Iran;(2) Department of Physical Geography, Kharazmi University, Tehran, Iran;(3) Department of Irrigation, Faculty of Agriculture, University of Tehran, Karaj, Iran;(4) Now at the Renewable Energy Organization of Iran (SUNA), Tehran, Iran |
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Abstract: | Summary The aim of this study is the evaluation of models that estimate daily global solar radiation on tilted surfaces from that
measured on horizontal surfaces. Global solar radiation incident on a tilted plane consists of three components: beam radiation,
diffuse radiation and reflected radiation from the ground.
The Klein (1977) method, modified by Andersen (1980), was used for estimating direct solar radiation incident on tilted surfaces
and an isotropic model was used for estimating reflected solar radiation incident on a tilted plane. In contrast models for
the diffuse radiation component show major differences, which justifies a validation study which has been done.
Eight models for derivation of daily slope diffuse irradiance from daily horizontal diffuse irradiance were tested against
recorded slope irradiances at Karaj (35°55′ N; 50°56′ E), Iran. The following models were included: Badescu (2002), Tian et
al. (2001), Reindl et al. (1990), Skartveit and Olseth (1986), Koronakis (1986), Steven and Unsworth (1980), Hay (1979) and
Liu and Jordan (1962).
All the models use the same method for calculating beam radiation as well as ground reflected radiation. However, only diffuse
component of radiation was compared. Statistical indices showed that Reindl’s model gives the most accurate prediction for
the south-facing surface and Koronakis’s model performs best for the west-facing surface. The Relative Root Mean Square Errors
(%RMSE), except for Steven and Unsworth’s model that has unacceptable results, for whole data range from 1.02 to 10.42%. In
general, Reindl’s model produces the best agreement with the measured tilted data. |
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