Page 316 - Applied Photovoltaics

where X i and Y i represent the fraction of sunny and cloudy days, respectively.
For a directly-coupled system, we make the assumption that no pumping
takes place during cloudy weather because of poor sub-system performance in
the presence of greatly reduced operating currents. We therefore use the
sunny weather component of the global radiation in Eqn. (H.3) (X × N × 6.76
× I s ) as being the useful global insolation (R u ) incident on the solar panels.

Figure H.2. Profile of light intensity throughout a typical sunny day, as a function
of the peak daily intensity I. The histograms of width N represent an
approximation to the daily light intensity profile, with N being determined by the
time of the year.

3. Average the calculated values of I si throughout the year (or period for which
the design is being undertaken), to give I sa . For this, each I s value should be
weighted according to the number of days in the particular ith month (M i ) and
the percentage of those that are expected to be sunny. For instance, for i
representing the month or particular period of time
¦ i M i I si X
I i (H.4)
sa
¦ i M i X
i
We then convert each sunny day (for each month), as given by Fig. H.2, into
an appropriate equivalent number of hours of sunshine (E) as given by

. 6 76 N I
E i si (H.5)
i
I
sa

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