Page 233 - Applied Photovoltaics

11.3.1 Centrifugal pumps
Centrifugal pumps have a rotating impeller that throws the water radially against a
casing, as indicated in Fig. 11.5. The kinetic energy added to the water by the
impeller is then converted to potential energy in the form of pressure or ‘head’ in the
diffuser or volute section. As the water exits the pump through the diffuser, the cross-
section increases, causing the velocity (kinetic energy) to reduce and, by conservation
of energy, the potential energy to increase (United Nations Economic & Social
Commission for Asia & the Pacific, 1991; Dufour & Nelson, 1993; Sahdev, 2004).
They are normally used in low head/low pressure, high volume applications,
particularly if direct connection to the solar panels is required. They are well suited to
high pumping rates and, because of their compactness, wherever small diameter bores
or wells exist. They are characterised by the torque being proportional to the square
of the speed (angular velocity of the impeller). Fig. 11.6 shows a performance curve
for a typical centrifugal pump. As can be seen, these pumps have relatively high
efficiencies, although typically lower than for helical rotors, but rapidly lose pumping
performance as their speed reduces and in fact do not pump at all unless quite
substantial spin speeds are achieved. This is, of course, a concern for a photovoltaic-
powered system when light intensity is low. However, maximum performance is
achieved at high spin speeds, making them easy to match to motors, which tend to
develop maximum torque (maximum efficiency) at similar speeds.

Figure 11.5 Centifugal pump operation (Used with permission of The Chemical
Engineers' Resource Page, Sahdev, 2004).
For conventional centrifugal pump designs, high efficiencies are only obtained for
low pumping pressures and hence relatively small pumping heads of less than about
25 m. To overcome this limitation, either multistage or regenerative centrifugal
pumps can be used. With the latter, water that leaves the pump under pressure is

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