Page 258 - Applied Photovoltaics

6. Select module mounting method—fixed or manual tracking.
For a system using batteries, step 5 simply involves the use of ‘stand-alone system’
design principles, outlined in earlier chapters.
However, prior to following these guidelines, it is useful to ascertain whether a
directly-coupled system (no batteries, no inverter and no power conditioning
circuitry) is feasible for the particular application. If so, such a system may be
advisable, even though its use provides reduced flexibility in component choice and
system configuration, while maximum power point tracking circuitry is becoming
more freely available. However, there are occasions when directly-coupled systems
are unsuitable. These include:
1. When pumping heads are too large to be able to use a centrifugal pump with
reasonable efficiency.
2. When suitable DC motors are not available, such as with some large systems.
3. When the pumping rate in bright sunshine exceeds the water source
replenishment rates.
4. When it is essential batteries be used for energy storage (i.e. where
‘availability’ of pumped water must be very high and tank storage is
unsuitable, e.g. portable units).
5. Locations characterised by excessive cloudy weather, making the poor part-
load efficiencies of a directly-coupled system unacceptable.
Many water pumping applications are not characterised by any of the above and are
accordingly suited to a directly-coupled system. However, the use of maximum
power point tracking circuitry is increasingly common for pump applications,
including in many commercially supplied pump systems (von Aichberger, 2003).

12.3 DESIGN OF A DIRECTLY-COUPLED SYSTEM
A directly-coupled system is one where a low starting torque pump (such as a
centrifugal pump) can be driven by a DC motor that receives its power directly from
the solar panels. No batteries, inverters, or power conditioning circuitry are used,
other than perhaps safety cut-out relays activated by level, flow or pressure sensing
transducers. When the sun shines sufficiently brightly, the system operates and water
is pumped either for storage or direct use.

An approach for designing directly-coupled PV-powered water pumping systems is
provided in Appendix H. Important considerations are as follows:
1. The volume of water to be pumped and over what period. The volume to be
pumped may vary significantly throughout the year and in fact may be
entirely non-critical for some months of the year, as for some irrigation
applications. This will have important implications regarding array tilt angles.
For instance:
x If the demand profile throughout the year is reasonably constant (such as
for a domestic water supply), a tilt angle in the vicinity of latitude + 20°
will be necessary to give the most uniform insolation levels throughout
the year falling on the solar panels.

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