Page 249 - Applied Photovoltaics

However, all systems using batteries should include voltage regulation circuitry to
prevent the battery from over-charging (see Chapter 6). In addition, it may be
necessary to include control circuitry to switch the load on and off. If, for instance,
the pumping rate exceeds the replenishment rate for the water source, then the load
must be switched on and off accordingly.
In this mode of operation, the response of motor/pump combinations to transient
operation should be considered when choosing and designing the system. Short-term
electromechanical storage (e.g. flywheel and synchronous machine) has been
proposed as a means of allowing centrifugal pumps to operate intermittently, at full
load and hence high efficiency (Landau et al., 1992).
When AC motors are used, a DC-to-AC inverter is essential. Such an inverter may be
variable frequency, to add greater flexibility to the control, and may include an MPPT
and various other forms of power conditioning circuitry. Unfortunately, many
inverters can only operate from a stable voltage source and hence require batteries or
DC-to-DC converters at their inputs, thereby adding to the system cost. Also, many
inverters produce square waves (or at least poor sine waves) and can hence cause
problems with motor heating. Centrifugal pumps are commonly driven by three-phase
AC motors from pulse-width modulating inverters, an arrangement that allows
relatively easy maximum power point tracking of the photovoltaics (Schmid &
Schmidt, 2003).

11.6 BATTERIES

Batteries may be used in water pumping systems for two independent purposes—
storage of energy and as a power conditioning mechanism.

As a storage medium, batteries are greatly inferior, in most instances, to pumped
stored water. However, with some critical applications, or where continuous pumping
is necessary, batteries may be required, in which case three days’ storage is
commonly used.
The role of batteries for power conditioning was briefly considered in the last section.
They allow power to be transferred to the load under optimal conditions, whereby
both pump and motor can potentially be at their respective peak efficiencies whenever
operating. This has four significant advantages relative to a directly-coupled system:
1. It ensures the solar array continues to operate at the maximum power point
when the light intensity is reduced, hence ensuring the maximum possible
electrical energy is generated.
2. It avoids the situation whereby motors/pumps operate at reduced load for
considerable portions of each day. Centrifugal pumps in particular suffer from
greatly reduced pumping efficiencies when operated at reduced load.
3. Batteries can provide the high currents required by some loads when starting.

4. Some inverters for AC motors will only operate from batteries.
The disadvantages of using batteries, however, are quite substantial:
1. Most batteries require regular maintenance.

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