Page 32 - Applied Photovoltaics

A cloudiness index, corresponding to the fraction of sky blocked by clouds, has been
correlated with global radiation (Lorenzo, 1989) and cloud cover (sky cover) data,
and is discussed by Iqbal (1983), who considers this form of data to be less reliable
than sunshine hour correlations.

Also available for some locations are nephanalysis charts, which portray cloud data
by standardised symbols and conventions, showing cloud types, amounts and sizes,
the spaces between them and various types of cloud lines and bands. The degrees of
coverage are determined by ground-level observations, in conjunction with the
satellite picture. Cloud types are usually identified on nephanalyses as stratiform,
cumuliform, cirriform and cumulonimbus, with each type able to be characterised in
terms of its effects on incident insolation.

Satellite-derived insolation estimates
NASA (2004a) makes freely available satellite-derived estimates of global insolation
for the world, on a grid of cells, each 1° latitude u 1° longitude. The data are
considered to be the average over the area of the cell. The data are not intended to
replace ground measurement data but to fill gaps where ground measurements are
missing and to complement ground measurements in other areas. The data quality
may at least be accurate enough for preliminary feasibility studies.
Various models are applied to estimate diffuse and direct components and global
radiation on tilted surfaces, with the applied methods being documented clearly
(NASA, 2004b).

1.8.3 Global and diffuse components
Diffuse insolation is produced by complex interactions with the atmosphere, which
absorbs and scatters, and the earth’s surface, which absorbs and reflects.
Measurements of diffuse insolation, which require pyranometers fitted with shadow
bands to block direct sunlight, are available for far fewer sites than are measurements
of global insolation. Hence, methods have been developed to estimate the diffuse
fraction from the global value.

Clearness index
Liu and Jordan (1960) estimated the diffuse fraction of sunlight from the monthly
average clearness index, K , defined by:
T
K R R (1.16)
T o
the ratio of monthly averages of daily diffuse and extraterrestrial global radiation.
The procedure to estimate R , the monthly average daily diffuse radiation on a
d
horizontal surface, from published or measured values of R is simply:
1. Calculate R for each month using Eqn. (1.7), then
0
K R R (1.17)
d d o
where R is the desired result. The latter expression yields the diffuse
d

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