Page 123 - Applied Photovoltaics

are high. They consume little power when the load or battery is using only PV-
generated power. However, with the advent of low-resistance semiconductor
switching components, that is no longer a serious problem and the losses can be even
lower than for shunt regulators (Schmid & Schmidt, 2003). As previously discussed
(see Chapter 5), short circuiting can cause hotspots if no bypass diodes have been
used.

Series regulators control the array current when a certain preset voltage is reached.
The control element is placed in series between the array and the battery with a
corresponding voltage drop across the terminals. This can simply open-circuit the
array in the on/off configuration or apply a constant voltage as VR is approached by
acting as a variable resistor in the linear configuration. In the latter, the control
element dissipates power at all times (Fig. 6.6).
The pulse width modulation (PWM) controller applies repetitive pulses of current
with a variable duty cycle. It can be either a series or shunt arrangement
The sub-array switching topology is a refinement of the on/off controller. Instead of
switching off the whole array, sub-arrays are switched in and out as required (Fig.
6.6). They operate by disconnecting one array section at a time, as charging currents
increase towards midday. These are then reconnected as charging currents fall later in
the day. They are suitable for use in larger systems, with a number of solar array
sections.

Self-regulating systems operate without a regulator, with the array connected
directly to the battery, and rely on the natural self-regulating characteristics of the
photovoltaic panels. The slope of the current-voltage characteristic curve for a solar
cell or module progressively increases when shifting from the maximum power point
towards the open circuit condition. This automatic reduction in generating current,
with increasing voltage above the maximum power point, appears to be well suited
for providing charge regulation to a battery, provided the temperature remains
constant. However, due to the large temperature sensitivity of the voltage of a solar
cell, the day-to-day temperature variations and wind velocity inconsistencies can
make it quite difficult to design a reliable self-regulating system, particularly one
suitable for a range of locations. This approach is suitable only where the climate has
small seasonal temperature variations and the battery is large relative to the array size.

The other complicating factor regarding the design of self-regulating systems is that
different cell technologies are characterised by different effective values of series
resistance. The result of this is that the slope of the current-voltage curve between the
maximum power point and the open circuit point can vary quite significantly between
technologies. This clearly introduces additional complications when trying to design
such a system accurately.
The general approach adopted by manufacturers is to remove approximately 10% of
the solar cells from the standard modules to reduce the probability of over-charging
the batteries. This is because, with a self-regulating system, there is no longer a
voltage drop across the voltage regulator, and the excess voltage able to be delivered
by the solar panels to ensure batteries can be fully charged under the hottest
conditions, can no longer be included without risking over-charging the batteries on
cooler days. Consequently, self-regulating systems are substantially cheaper, not only

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