Page 125 - Applied Photovoltaics

Another problem with a self-regulating system is that the photovoltaic-generating
capacity has to be well matched to the load requirements. For instance, during the
night the load must partially discharge the batteries so that, on the following morning
when the weather is cooler and hence the photovoltaic voltage is higher, the batteries
can accept the charge generated. Later in the day, once the solar panels operate closer
to the anticipated design temperature, if the batteries are close to full state-of-charge,
the same problem will not result as the self-regulation will automatically cause the
generating current to fall. This charging scenario has important implications for
system maintenance and down-time. Failure to disconnect the batteries from the
photovoltaic arrays during periods of no load will result in severe over-charging of
the batteries, as they commence each day already at full state-of-charge. Examples of
this in the field have led to rapid destruction of the batteries resulting from severe
over-heating, over-charging and rapid loss of electrolyte.
Self-regulating systems are best suited to batteries such as nickel-cadmium that can
tolerate substantial amounts of over-charging. Lead-acid batteries, on the other hand,
should rarely be used in such systems without particular care and monitoring.
Maximum power point trackers seek to transform the array voltage at its
instantaneous maximum power point for the pertaining insolation and temperature, to
the appropriate voltage required for the charging regime. This allows the array to
continually operate at its maximum efficiency except when charging is reduced or
suspended to protect the battery. The tracking circuitry is essentially a DC-to-DC
converter, commonly using a pulse width modulation topology. Care should be taken
when systems are specified to check that the additional expense and complexity is
justified by the energy gains. The three main advantages are reduced sensitivity to
voltage drops across wires between the array and the battery, reduced sensitivity to
the number of cells per module, thereby permitting the use of modules with fewer
large-area cells, and the opportunity to use more complex charging current profiles
(Schmid & Schmidt, 2003).
Protection against excessive discharge basically requires disconnection of the load
at the LVD point and reconnection at the LVR point after sufficient recharge. In a
high reliability system, with large array and battery relative to the load, the battery
tends to have shallow cycles and the low voltage disconnect protects the battery only
under abnormal conditions. In low reliability systems, though, the disconnect
frequently protects the battery in normal operation. It has been recommended (Usher
& Ross, 1998) that loads be disconnected at 40% depth of charge, even for batteries
with higher rated maximum discharges. Some controllers allow the low voltage
disconnect to be overridden by the user but this is not recommended (UPM, 2003).
Freeze protection is important in many climates. The freezing temperature of
electrolyte depends on its density, which depends on state-of-charge. The load
disconnect setpoint should be raised in colder conditions to prevent freezing (Usher &
Ross, 1998; Spiers, 2003).
6.7.3 Inverters

Inverters are needed in PV-based power systems when power is required as
alternating current (AC), rather than the direct current (DC) produced by the PV
array. Inverters use switching devices to convert DC to AC power, at the same time
stepping up the voltage, typically from 12, 24 or 48 V dc to 110 or 240 V ac for small

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