Article,
Fluctuations in instantaneous clearness index: Analysis and statistics
Solar Energy, (2007)
Abstract
Solar radiation is characterized by short fluctuations introduced
by passing clouds. An analysis of these fluctuations with regard
to solar energy applications should focus on the instantaneous clearness
index. Its probability distribution for a given mean clearness index
is, as a first approximation, independent from the season and partly
also from the site. This is verified for four annual datasets from
three different sites. An analysis of fluctuations in solar radiation
must focus on their amplitude, persistence, and frequency of occurrence
rather than their location in time. The Fourier analysis cannot satisfactorily
provide this information since time series of the instantaneous clearness
index exhibit no periodicity. Instead, a localized spectral analysis
based on wavelet bases rather than on periodic-ones has been applied.
This analysis allows the decomposition of the fluctuating clearness
index signal into a set of orthonormal subsignals. Each of them represents
one specific scale of persistence of the fluctuation. The annual
mean square values of all subsignals have been analysed, permitting
the allocation of the signalâs power content to the different scales
of persistence of a fluctuation. These annual mean values agree well
for the different datasets, indicating the existence of statistically
significant mean square values of the fluctuations as a function
of their persistence. The analysis offers a valuable tool for the
estimation of power flow fluctuations introduced by direct solar
energy systems. With further elaboration it may be applied by power
system operators for network planning in distribution grids with
a high density of embedded generation.
