Page 259 - Applied Photovoltaics

x If the amount of water to be pumped is to be reasonably constant
throughout the year, but with a definite bias towards summer months
(such as for drinking water), a tilt angle in the vicinity of latitude + 10°
will probably be desirable.
x If the annual amount of water pumped is to be maximised (such as with a
large storage reservoir), a tilt angle in the range latitude to latitude – 10°
should be used.
x If the water pumped during summer months is to be maximised (such as
for some irrigation applications), a tilt angle in the vicinity of latitude –
20° will be preferable, to ensure the solar panels point more directly at the
midday summer sun.

In general, more uniform pumping throughout the year will be provided by
increasing the tilt angle.
2. The pumping head and its seasonal variations must be known and, where
possible, information regarding water source replenishment rates should be
obtained.
3. The inclusion and economics of water storage should be considered in
conjunction with consumer needs.
4. Any available insolation data should be obtained and used in conjunction with
the guidelines given in Appendix B and Chapter 1. Fig. H.1 indicates the
procedure for determining the light intensity incident on the solar panels at
angle ȕ at noon.
5. Select a pump to suit starting torque requirements, the range of operating
heads, any physical dimension constraints imposed by the application, and
one that will pump the required volume of water when operating at its
maximum efficiency point. It is essential the torque-speed characteristics of
the selected pump be known, to facilitate system matching.
6. Select a motor with a torque-speed characteristic compatible with that of the
pump. It is important that the motor operate near maximum efficiency when
producing the necessary torque, to drive the pump at its design speed. Recall
that

V I R K ĭ N (12.1)
m a a
where V m is the motor voltage, I a is the armature current, R a is the resistance
of the armature, K is the motor constant, ĭ is the flux density and N is the
speed of rotation.
In Eqn. 12.1, the voltage applied to the motor terminals (V m ) has two
components—I a R a is the resistive voltage drop across the armature windings
and KĭN is the back emf generated, which is hence proportional to the speed
of rotation N and the flux density ĭ.
If we now consider a permanent magnet DC motor, then ĭ remains
approximately constant, independent of the voltage applied or current
consumed, and I a becomes the total motor current I m , since no current is
required for field windings. Thus, we can rewrite Eqn. (12.1) as

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