Abstract :
Global radiation measured on fixed-tilt, south-facing planes (40 and vertical) and a 2-axis tracker at NREL’s Solar Radiation
Research Lab. in Golden, CO is compared to predictions from ten transposition models, in combination with either optimal or suboptimal
input data of horizontal irradiance. Suboptimal inputs are typically used in everyday engineering calculations, for which the necessary
data are usually unavailable for the site under scrutiny, and must be estimated in some way. The performance of the transposition
models is first evaluated for ideal conditions when optimal data are used. In this specific case, it is found that the Gueymard and Perez
models provide the best estimates of global tilted irradiance under clear skies in particular.
The performance of four direct/diffuse separation models is also evaluated. Their predictions of direct and diffuse radiation appear
biased in most cases, with a model-dependent magnitude. Finally, the performance of the resulting combinations of separation and transposition
models is analyzed in a variety of situations. When only global irradiance is known, the accuracy of the tilted irradiance predictions
degrades significantly, and is mainly conditioned by the local performance of the direct/diffuse separation method. For the
south-facing vertical surface, inaccuracies in the ground reflection calculations becomes another key factor and significantly increase
the prediction error. The Reindl transposition algorithm appears to perform best in this case. When using suboptimal input data for
the prediction of plane-of-array irradiance on a moderately tilted plane (40 S) or a 2-axis tracking plane, the Hay, Reindl and Skartveit
models are less penalized than others and tend to perform better. It is concluded that further research should be conducted to improve
the overall process of predicting irradiance on tilted planes in realistic situations where no local high-quality irradiance or albedo measurements
are available.
2008 Elsevier Ltd. All rights reserved.
