Article,
The probability density and autocorrelation of short-term global and beam irradiance
Solar Energy, (1992)
Abstract
For individual hours, a characteristic bimodal pattern of short-term
global and beam irradiance is frequently observed, with modes at
high and low irradiances and with low probabilities near the hourly
averages. For such hours, averaging over the hour will imply smoothing
of quite significant variations within the hour. Models for the probability
density distributions of short-term (5 min or less) irradiances are
presented in this paper. These distributions are not unique functions
of the hourly averages, but depend heavily also on the irradiance
variability within the hour. This intrahour variability is found
to depend on the averaging time and also on the interhour variability
among three hourly averages, namely, the hour in question, the preceding
and the deceding hour. The distribution differences between 5 rain
averages and instantaneous values are, however, negligible for most
practical purposes. The lag one autocorrelation is evaluated as a
function of averaging time, and a first order autoregressive model
is presented. With hourly averages as the only input the probability
density and autoregressive model in combination produce time series
of short-term intrahour averages having realistic distributions and
autocorrelation structure.
