Page 250 - Applied Photovoltaics

2. Voltage regulation is necessary to protect the batteries from over-charging or
excessive discharging.
3. They have fairly limited lives.
4. They are costly.
5. They reduce the reliability of a system that should potentially be extremely
high.
6. Their columbic efficiency (charge out divided by charge in) is normally only
about 85%.
7. They necessitate a blocking diode being placed at the array output to prevent
the batteries from discharging through the array at night.
8. They require considerable over-sizing of the solar arrays, with respect to
voltage, to allow them to reach a state of full charge at roughly 14 V (while at
the worst extremes of temperature), to compensate for the voltage drop across
the blocking diode, to allow for a power loss (voltage drop) in the regulator
and to make some allowance for small losses in voltage that accompany
variations in light intensity.
9. They can cause heavy metal pollution if not properly recycled, and recycling
is not a viable option in many parts of the world.
To illustrate the effect of (8), for a 12 V system, a typical solar panel for battery
charging will have 36 cells connected in series, each with an open circuit voltage
2
(V oc ) of about 600 mV under standard test conditions (1 kW/m , 25°C), giving a solar
panel V oc of 21.6 V, and a voltage at the maximum power point (V mp ) of 18.0 V. With
a 30° temperature rise, V mp falls to 15.8 V. Allowing for variations in light intensity
and a 1 V drop across the regulator reduces V mp to about 13.8 V, which is considered
necessary to bring the battery to a full state of charge (14–14.5 V). To illustrate the
enormous loss incorporated into such a setup, 26 cells series-connected rather than 36
(with the inclusion of batteries) should be sufficient to provide 12 V output in a
directly-coupled system. Such a solar panel would then have a V oc of 15.6 V under
standard test conditions and a V mp of 13.3 V. Allowing for a more typical 20°C rise in
temperature to 45°C reduces V mp to 12.2 V. No allowance has to be made for
blocking diode losses, voltage regulator losses, and variations in light intensity. The
reason the latter can be neglected is because under reduced solar insolation
conditions, excess voltage will always exist for a directly-coupled system because of
the relatively greater reductions in current.
If batteries are required, they need to be capable of regular deep discharges and
should be housed in a weather-resistant box, which is preferably non-metallic, if non-
sealed batteries are selected. When interconnected, each parallel string of batteries
should be provided with a fuse as close as possible to the battery terminals.
The advantages and disadvantages of lead-acid batteries as opposed to nickel-
cadmium batteries are discussed in Chapter 6. These are valid when considering
batteries for storage. However, when batteries are to be used simply for power
conditioning, nickel-cadmium batteries should be seriously considered. The
additional cost becomes less significant owing to the small quantity required, while
the ability to be deep cycled, overcharged and left for long periods fully discharged,

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