Identifying periods of clear sky direct normal irradiance

When modeling the effect of the cloud transients in the Direct Normal Insolation (DNI), it is particularly relevant to identify those moments in which there are no clouds between the observer and the sun. In this paper, we present a simple algorithm for offline detection of situations where the sun path to the observer is not obstructed by any cloud. The algorithm is based on the characterization of the relations between the measured and the clear sky curves. The clear sky identification module consists of three evaluation and detection metrics: hourly mean, slope, and line length criterion. All of them rely on the assessment of the measured data against the clear sky generated data. The conjunction of the fulfillment of the three criteria leads to the clear sky hour identification. We validate our algorithm by comparing our results with those obtained from a recently published clear sky detection algorithm that uses high temporal resolution Global Horizontal Irradiation (GHI) as the input. We obtain a 98% agreement when having more than 50 min identified as clear.