Accumulated frequency estimation for daily clearness index

Abstract

In this paper, solar radiation description using the analysis of the daily clearness index accumulated frequency for Gran Canaria island (Canary Islands — Spain) is studied. The Canary Islands are placed at the north west of Africa. To reach this aim, some different climate models have been used. Most of the models are designed to work for specific geographic situations, so it was necessary to compare the results obtained with the local canary conditions.Monthly accumulated frequency graphics have been obtained using two different models for K T (average monthly clearness index), K max and K min. Both models have been compared taking into account its accuracy to the local canary data. The models studied are the exponential model developed by Bendt (1981) and a polynomial model based upon normalized parameters developed by (2000). The Bendt model was designed to work with some localities in North America, but it fits pretty well to Gran Canaria data if K max and K min are estimated appropriately. To estimate both parameters (K max and K min) using K T , we used some expressions developed for different climatic conditions in different parts of the earth. Moreover, an empirical expression to forecast K min in the island, was developed. The daily analysis of the available data in six different measurement stations distributed in different points all around the island, takes us to consider two different zones in the island. The behaviour of the data from the north part was quite different to the behaviour of the stations from the south. The north part of the island is strongly affected by the Alisios winds during the aestival season what causes the appearance of clouds coming from north. Finally, we obtained that the Bendt model gets the best results using an expression developed by K. Babu to estimate K max and an empirical expression developed for Gran Canaria, for estimating K min.