Page 260 - Applied Photovoltaics

V I R K ĭ N (12.2)
m m a
The electromagnetic torque (IJ e ) developed by such a motor is proportional to
the armature current (and hence I m ) and also the flux density ĭ (which is a
constant). This gives us
IJ K ĭ c I (12.3)
e m
where K' is a constant related to the physical dimensions of the motor and the
number of windings in the armature (Hambley, 2002).
When the motor drives a load (such as a water pump), the speed of the motor
will continue to alter until steady state is reached; that is, when

IJ IJ l (12.4)
e
where IJ l is the torque required to drive the load at that particular speed.
For any commercial pump, the torque versus speed (IJ l versus N)
characteristics should be available from the supplier. For each value of N, IJ l is
thus obtainable and, using Eqns. (12.3) and (12.4), I m can be calculated.
Subsequent use of I m and N in Eqn. (12.2) gives the corresponding value of
V m . Therefore, for each N of the pump, the motor voltage and current required
from the solar panels are determined. However, the actual voltage generated
by the solar panels needs to be about 2% higher than that calculated, to allow
for resistive losses in the wiring.
7. Appropriate sizing of the photovoltaic system will enable overall system
specifications to be met, while simultaneously maximising overall system
efficiency. For this, both the voltage and current at maximum power point
need to be optimised. Unfortunately, little choice exists with regard to the
voltages available with standard commercial modules. They are normally
designed for 12 V systems (including considerable excess voltage capacity to
allow for battery charging, regulation, blocking diode etc.), and can be
connected in series to increase system voltage to multiples of 12 V. This
restriction can be overcome by the use of a DC-to-DC converter. In
comparison, a reasonable choice in short circuit currents exists, owing to the
range of solar cell sizes and technologies used by different manufacturers. An
approach for optimising the photovoltaic configuration by matching the
requirements of the water pumping subsystem to the output of the
photovoltaics is provided in Appendix H.

REFE RE NCES
Updated World Wide Web links can be found at www.pv.unsw.edu.au/apv_book_refs.

Arab, A. H., Chenlo, F. & Benghanem, M. (2004), ‘Loss-of-load probability of
photovoltaic water pumping systems’, Solar Energy, 76(6), pp. 713–723.

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