Page 235 - Applied Photovoltaics

Long driving shafts were therefore required between motor and pump, which in turn
led to other complications. For this reason, submersible AC motors were often used,
despite their lower efficiencies and requirements for inverters. Now, however,
submersible DC motors have become more readily available. In these, electronic
commutation is used to remove the need for brushes.
Another alternative has been the use of self-priming centrifugal pumps (side pumps),
where a chamber containing water at the side of the pump keeps the pump effectively
submerged and hence primed.

The major trade-off involved with the design and use of centrifugal pumps is the
requirement for high efficiency versus the need for an impeller with long life and
good tolerance of aggressive impurities in the water (Halcrow & Partners, 1981).
High efficiency can be obtained with small clearances and narrow passages, but this
is undesirable for pump reliability and the ability to pump liquids contaminated with
particles. In addition, high efficiency can be obtained with a high speed impeller,
which again acts to shorten the life of the pump. In summary, pumps need to be
designed and selected for specific applications and environments.
Apart from the usual use of photovoltaic water pumps for the supply of potable water,
there is also a significant Australian market for centrifugal pumps for desalination of
land affected by elevated water tables.

11.3.2 Displacement or volumetric pumps
Displacement or volumetric pumps are the other class of pumps often used for water
pumping applications, particularly for lower pump rates from deep wells or bores.
Included within this class are piston pumps, diaphragm pumps, rotary-screw type
pumps and progressive cavity pumps, such as the popular helical rotor (Fig. 11.7)
(Revard, 1995).

222