Page 234 - Applied Photovoltaics

channelled back through cavities in the casing into an adjacent chamber, where it is
pumped to a greater pressure, hence suiting increased pumping heads.
Efficiencies of these pumps, however, tend to be a little lower, owing to leakage of
water from the high pressure chamber to the low pressure chamber. In addition, the
clearances between impellers and casing need to be substantially less to give good
performance, which creates reliability problems. Another modification to suit
centrifugal pumps to larger heads is to include a water injector (jet pump). However it
is more common to use multistage centrifugal pumps for larger heads. These have
been used successfully to pump water up to heights of 600 m but not with solar power
(von Aichberger, 2003). Unfortunately, centrifugal pumps can only operate at
maximum efficiency at a single operating point, even when a maximum power point
tracker is used, so efficiency is reduced at all but an optimal insolation level (Sharma
et al., 1995).

Figure 11.6. Typical efficiency characteristics of a centrifugal pump at variable speed
3
(60 m head, 10 m /h flow rate, 3 kW power). (Used with permission of Thomson,
Landau, M., Sachau, J. & Raatz, A. (1992), ‘Photovoltaic pumping system for
intermittent operation’, Proc. 11th EC Photovoltaic Solar Energy Conference,
Montreux, Switzerland, pp. 1391–1394, Fig.1, Published by Taylor & Francis 1993)

Other advantages of centrifugal pumps include their simplicity (with a minimum of
moving parts) and corresponding reliability, low cost, robustness, tolerance to
pumping particulates and low starting torque. On the other hand, another potential
limitation of centrifugal pumps is their inability to be self-priming, although
technology exists for overcoming this impediment (von Aichberger, 2003).
Consequently, they are frequently used as submersible pumps, preferably in
conjunction with a submersible motor. For many years this has been a problem, since
the preferred DC motors were not submersible owing to the presence of the brushes.

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