Evaluation of clear-sky conditions in high altitude sites

Abstract

The correct assessment of cloud conditions in any site is important for determining the characteristics of solar resource availability. The criterion proposed by Iqbal has been widely used for this purpose, through the classification of the daily clearness index Kt. However, evidence was found to suggest that this criterion may not be applicable to data measured in high altitude sites, because partially cloudy days are incorrectly classified as clear-sky days. This paper analyzes the cloudiness state of the sky by observing the values of the clearness index Kt and the clear-sky index Kc. These indices were obtained from reliable global solar irradiance data measured over a year in three places of Argentina located at different altitudes (25, 1200 and 3700m.a.s.l). ESRA and ARG-P models were used to estimate daily clear-sky global solar irradiation values Hc. They also reveal that the most probable value of Kc which corresponds to an actual clear-sky day of a particular location and time depends on the model used to estimate the clear-sky global solar radiation values. Thus, it was confirmed that, unlike the index Kc, the Iqbal's criterion does not correctly classify the cloud cover status from data measured in high altitude sites (>1000m.a.s.l.). •A clear-sky day is defined from an irradiance vs. time graph.•Clearness index Kt and clear-sky index Kc values are compared.•The ESRA and ARG-P clear-sky models are used to estimate the Kc values.•The Iqbal's criterion is inconsistent with the genuine cloud status in the high altitude site. © 2013 Elsevier Ltd.