Page 246 - Applied Photovoltaics

the load impedance to match the optimum impedance of the array. The circuitry of
course must consume very little power to justify its inclusion and, in most systems,
will typically consume 4–7% of total power (Matlin, 1979). It is also expensive,
usually costing more than the electric motor (Halcrow & Partners, 1981), while
historically often providing reliability problems.
As the light intensity falls, the current generated by solar panels falls proportionately
while the voltage at the maximum power point remains approximately constant.
However, for a motor/pump, as the current falls, the voltage also falls. Consequently,
without power conditioning circuitry, as the light intensity falls, the solar array
operates at a current and voltage progressively further and further from its maximum
power point.
For instance, with centrifugal pumps, the torque is approximately proportional to the
speed squared, while the torque produced by the motor will be directly related to the
current flowing in the motor windings. Consequently, as the current from the solar
array falls, the torque produced by the motor falls, the speed of the pump therefore
decreases, the back emf produced by the motor correspondingly falls, and hence the
voltage required by the motor falls. In this situation for a DC motor, the required form
of power conditioning is for a DC-to-DC converter, to effectively convert the excess
voltage able to be produced by the solar panels into additional current.
For displacement pumps, the torque required for pumping is, in general, primarily
dependent on the pumping head, pipe and pump friction, and pump pipe diameters,
but depends little on the speed of pump operation (neglecting high break-away
torques). In this instance, a certain threshold current is required by the motor to
provide the torque necessary to maintain operation of the pump. The speed of
pumping is then determined primarily by the driving voltage available, as the
pumping rate will increase until the back emf produced in the motor matches the
applied voltage from the solar panels. Consequently, the motor/pump load line
appears as a horizontal line when superimposed on the current-voltage characteristic
of the solar panel. This is an unacceptable mechanism of operation since the falling of
the array current below the required level will result in no pumping at all while
current generating potential above the critical level will be wasted. Again, a DC-to-
DC voltage converter is required and, in fact, is essential when a DC motor is driving
a typical displacement pump.
In addition, the high starting (breakaway) torques require high starting currents,
which in general cannot be supplied by the solar panels. When starting, the speed is
zero and there is no back emf produced. Consequently, a DC-to-DC converter can
again be beneficially used to produce the high starting currents by effectively
converting the excess array voltage into current. Fig. 11.16 shows a circuit that, in
principle, facilitates such a conversion, although the control circuitry, which can be
the most demanding and unreliable component, is not shown. An alternative approach
commonly used for providing high starting currents is through the use of a ‘starting
capacitor’, which stores sufficient charge to provide a large current burst to start the
motor/pump.

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