Estimation of global solar radiation by means of sunshine duration

Abstract

This paper analyses the relationship between global solar irradiation and sunshine duration with different estimation models for the island of Gran Canaria (Spain). These parameters were taken from six measurement stations around the Island, and selected for their reliability and the long period of time they covered. All data used in this paper were handed over by the Canary Islands Technological Institute (I.T.C.). As a first approach, it was decided to study the Angstrom lineal model. In order to improve the knowledge on solar resources, a Typical Meteorological Year (TMY) was created from all daily data. TMY shows differences between southern and northern locations, where Trade Winds generate clouds during the summer months. TMY resumes a data bank much longer than a year in duration, generating the characteristics for a year series of each location, for both irradiation and sunshine duration. To create the TMY, weighted means have been used to smooth high or low values. At first, Angstrom lineal model has been used to estimate solar global irradiation from sunshine duration values, using TMY. But the lineal model didn’t reproduce satisfactory results when used to obtain global solar radiation from all daily sunshine duration data. For this reason, different models based in both parameters were used. The parameters estimation of this model was achieved both from TMY daily and monthly series and from all daily data for every location. Because of the weather stability all over the year in the Island, most of the daily data are concentrated in a close range, occasioning a deviation in the lineal equations. To avoid this deviation it was proposed to consider a limit condition data, taking into account values out of the main cloud of data. Additionally, different models were proposed (quadratic, cubic, logarithmic and exponential) to make a regression from all daily data. The best results were obtained with the exponential model proposed in this paper.The lineal equation obtained from the limit condition data reproduces good results working with daily and monthly TMY series. The root mean squares (%rms) observed is around 6% in each location. Otherwise, with all daily data, the exponential model obtained the best results. Furthermore, this proposed model reproduces similar %rms as the lineal one to estimate TMY series. In the North location of the Island, the results were not so good, probably due to the high cloudiness of this area compared to the South. In the close range of the TMY series, both the lineal and the exponential model estimate satisfactory global solar irradiation from sunshine duration, whereas using all daily data, the exponential model reproduces the best results. A general conclusion is that the exponential model proposed in this paper is the most adequate to estimate global solar irradiation from sunshine duration.