Page 236 - Applied Photovoltaics

Figure 11.7. Helical rotor type of displacement pump (after Revard, 1995).

Fig. 11.8 gives the performance curves of a typical positive displacement pump. The
pumping rate with these pumps is directly related to the speed of operation, with a
fairly constant torque required. The resulting flat torque-speed characteristic makes it
almost impossible to drive these pumps directly from a photovoltaic source (Halcrow
& Partners, 1981). This is because the torque developed by a motor depends directly
on the current in the armature. The requirement for this to remain approximately
constant (to suit a constant torque pump) therefore requires an approximately constant
current. This type of load is not well matched to the output of solar cells, where the
current generated is directly proportional to the light intensity. For instance, if the
operating torque corresponds to a current from the solar modules that closely matches
their maximum power under bright sunshine, then a small reduction in light intensity
will result in insufficient current being generated to maintain the pumping speed. The
pump/motor will accordingly slow down in pumping rate so as to require less current.
However, because of the flat torque-speed characteristic, the pump will actually cease
entirely whenever the current generated drops below the critical level. To prevent this
happening for large parts of the day, a critical current would have to be selected that
was well below the maximum current generated by the solar panels throughout the
day. This of course means sacrificing much of the power-generating capabilities of
the solar panels, hence producing a system with low overall efficiency.

223