Hourly and daily clearness index and diffuse fraction at a tropical station, Ile‐Ife, Nigeria

Abstract

Dataset consisting of hourly global and diffuse solar radiation measured over the period February 1992 and December 2002 have been utilized to investigate the diurnal and seasonal variations of hourly and daily clearness index together with the diffuse fraction at a tropical station Ile-Ife (7.5°N, 4.57°E), Nigeria. Statistical analysis (the frequency and cumulative frequency distribution of the hourly and daily clearness index) and subsequent characterization of the sky conditions over the station based on these were also done, and their implications for solar energy utilization in the area discussed.Daytime ( Statistical analysis of hourly and daily clearness index showed that the local sky conditions at the station were almost devoid of clear skies and overcast skies (clear skies and overcast skies occurred for only about 3.5% and 4.8% of the time respectively). The sky conditions were rather predominantly cloudy (cloudy skies occurred for about 88% of the time) all the year round.

Spatio-Temporal Characteristics in the Clearness Index Derived from Global Solar Radiation Observations in Korea

“…There have been many studies characterizing sky conditions and long-term trends of solar radiation using KT for a particular location [1,2,4,8,9]. The sky conditions at tropical stations in Nigeria were analyzed by the diurnal and seasonal variations of hourly and daily KT [2,8].…”

“…The sky conditions at tropical stations in Nigeria were analyzed by the diurnal and seasonal variations of hourly and daily KT [2,8]. Based on the 40 years (1961-2000) of daily solar radiation and monthly sunshine duration from China, the significant decreasing trends in KT and global/direct radiation but increasing trend in diffuse radiation were evaluated [1].…”

“…Global solar radiation refers to the total amount of solar energy reaching the Earth's surface, which varies dramatically over temporal and spatial scales due to varying atmospheric conditions above the surface. These variations can be attributed to the absorption and scattering of radiation by clouds, water vapor, aerosols, and other gases in the atmosphere, but are difficult to predict, primarily because of non-linear interactions in this complex system [1][2][3].…”

Identification of clear days from solar irradiance observations using a new method based on the wavelet transform

“…data free of noise and cloudy periods, to obtain valid performance results [12]. The commonly used tool to characterize sky conditions (classification of day types) over a particular site is the clearness index k t [2,3,14,16,17,18,22,23,23,26,30]. On a clear day, the atmosphere causes a reduction of the extraterrestrial solar input by about 30 per cent and to nearly 90 percent in a very turbid (cloudy) day.…”

Analysis of Global Solar Irradiance over Climatic Zones in Nigeria for Solar Energy Applications

“…The diurnal and seasonal patterns of both the hourly and daily clearness and the cloudiness index were manifested in the datasets [4,5]. Using dataset of monthly global solar irradiance for a period covering at least 20 years, the optical sky conditions over some selected stations in the major vegetation zones of Nigeria have also been investigated [6].…”