Page 321 - Applied Photovoltaics

current 1.35 times greater than that calculated as being necessary. To convert this to a
short circuit current rating, simply divide by 0.95.
We are now in a position to refer back to Fig. H.3 and consider the vertical axis. If I sc
is the rated short circuit current, as specified by the manufacturer, then we must:

1. Use Fig. H.2 to make adjustment for the light intensity being different from
2 2
1 kW/m (= 100 mW/cm ), which means multiply by (0.99 × I / 100), for the
top curve, where I is given by Eqn. (H.1).
2. Multiply by the derating factor DR (which is suggested to be 0.74).
Consequently, for the top curve, we get a short circuit current of:

0.99 × 0.74 × I / 100 × I sc
or

(0.0073 × I) × I sc
2
where I is in mW/cm .
Having now selected the array size, the vertical and horizontal axes of Fig. H.3 can be
specified, and the load line (with efficiency weightings) superimposed. This
facilitates, in conjunction with Fig. H.2, the calculation of pumped volumes of water
and system average efficiencies throughout both sunny and cloudy days and for the
whole design period. If this procedure is being carried out by a computer, it becomes
a relatively simple task to try different values for the current to iteratively tune into
the system design for maximum overall efficiency and preferred pumping regime.
It should be noted that a degree of conservatism has automatically been built into the
design back in steps (4) and (5) when we defined the number of pumping hours (E) in
terms of the number of equivalent hours of sunshine with light intensity (I sa ), but then
subsequently used the light intensity of 0.80I sa as the one for which the maximum
subsystem efficiency would correspond. This conservatism was necessary for two
reasons. Firstly, it allows for reduced subsystem performance (efficiency) when
operating under light intensities greater or less than the design value of 0.80I sa .
Secondly, it compensates for subsystem performance below that expected, which
could result from:
x discrepancies between manufacturers’ curves and those measured in practice
x degradation of subsystem components
x unexpected changes in static head etc.
The net result of the conservatism is that the design, on paper, should result in 10–
20% more water being pumped than is required in the specification. This figure may
be increased or decreased as required, although any modifications should probably be
done in consultation with the consumer.

308