Page 237 - Applied Photovoltaics

Figure 11.8. Performance curves of a typical positive displacement pump.
(Speeds shown are pump speeds. For motor, multiply by 8.5. Efficiencies include
belt transmission between motor and pump.) (Used with permission of Halcrow &
Partners, 1981).

The other problem limiting the use of these pumps in direct connection to solar panels
is the high starting (breakaway) torque associated with binding of the seals.
Furthermore, the low number of strokes per unit time of volumetric pumps
necessitates the use of an appropriately-geared transmission to match to the speed of
the motor (Halcrow & Partners, 1981), which adds further complexity to the system.
Despite these limitations, for large heads (>20 m) the efficiencies obtainable exceed
those of single-stage centrifugal pumps, particularly under part load conditions.
Having mentioned this benefit, it is important to understand the implications of using
these pumps under part load. Under part load, these pumps require the same current
but reduced voltage, because of the flat torque-speed characteristic. This is not
compatible with photovoltaic output and hence power conditioning circuitry would be
required before this benefit could be utilised.
Another benefit is that they are less sensitive to head variations (seasonal and during
pumping) and are self-priming, which alleviates the need for submersible motors and
long driving shafts between motors and pumps. Also, their efficiencies tend to suffer
less from partial load conditions than do centrifugal types (Sharma et al., 1995).

The performance of volumetric pumps is quite poor for small heads, owing to the
large component of friction. The exception to this is perhaps the free-diaphragm
pump, which has low internal friction and hence may be well suited to small heads.
Fig. 11.9 gives the typical performance curves of such a pump, and it has been hoped
that these may be able to be operated directly by an actuator, although to date, no
great success in this area has been achieved. Diaphragm pumps are available for

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