Page 320 - Applied Photovoltaics

Modules specifically tailored to give the exact desired voltage at maximum power
point can of course be designed and constructed by using a different number of series-
connected cells. This, however, will cost a premium price, making it uneconomical to
use specially-designed modules unless the quantities required are enormous.

Considering the standard modules at 45°C:
x one single module (36 cells) should give V mp = 15.5 V
x two in series (72 cells) should give V mp = 31.0 V
x three in series (108 cells) should give V mp = 46.5 V
x four in series (144 cells) should give V mp = 62.0 V.
As can be seen from Fig. H.3, the maximum power point voltage changes very little
with variations in light intensity.
We therefore choose an appropriate number of series-connected modules to give us a
maximum power point voltage (at 45°C) as close as possible to the voltage at which
the subsystem attains maximum operating efficiency (allowing for a 2% voltage drop
along the length of the wiring).
The final part of the design procedure is the determination of the current-generating
capacity required for the solar panels. An iterative approach is simplest and well
suited to solving through the use of computers. An initial choice of array size (current
rating) is made by following the guidelines of steps (4) and (5) in Section H.2, above,
where it is suggested that we want the subsystem to operate at its maximum
efficiency when the light intensity (L mp ) is given by
L . 0 80 I (H.13)
mp sa
2
Therefore, we want a rated maximum power point current (I mp ) at 1 kW/m insolation
of
100 I
I m (H.14)
mp
. 0 80 I sa
where I m is the motor current at maximum subsystem efficiency.
Once the required I mp value is determined, we must appropriately upgrade the rating
to ensure the solar panels will in fact produce the current I mp . The contributing
reasons for necessitating this upgrade in rating include:
x dust on the surface (Halcrow & Partners, 1981; Hammond et al., 1997)
x insolation levels possibly lower than anticipated
x tolerances in solar panel outputs
x degradation of solar panels.
These factors can be compensated for by appropriately over-sizing the solar array
design. Typically, we should allow for 6% loss owing to dust, depending on location,
10% for degradation of the solar panels (unless manufacturers give guarantees to the
contrary) and 10% for combined tolerances in solar panel outputs and insolation
levels. A combined reduction of 26% from rated value translates to an over-sizing
requirement of 35% (1/0.74). We call this a derating factor DR, which in this case is
0.74. In other words, we select solar panels with a rated maximum power point

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