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Development of a high-efficiency PV driven displacement pump system for application in rural areas

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Photovoltaic (PV) powered pumping systems offer the opportunity to improve rural living conditions by providing reliable and low cost water pumping for drinking or irrigation. Most conventional pumping systems are centrifugal pump systems with submerged ac motors. These have overall system efficiencies of only 3 to 4%, indicating that there is room for improvement. Some improvements in centrifugal pump efficiency may be possible, but new pumping concepts need to be designed to achieve even higher efficiencies. Prototypes of 2 innovative displacement pump systems have been developed: a rotary displacement pump with dc motor and a piston pump with matching valve. The rotary displacement pump with an ac motor had already shown promise in earlier trials to be a higher efficiency pump compared with traditional centrifugal pumping systems. By using a dc submersible motor, the pump could offer further significantly higher efficiencies without the high sophistication (and higher costs) of an inverter to drive the motor. The project team tested a suitable dc motor and developed an innovative dc/ac converter that could overcome the high starting torque needed for the pump, with the help of a capacitor that stores electric energy before the pump starts up. The piston pump has a high mechanical efficiency (85-90%) compared with centrifugal pumps (60-65%). By modifying the piston pump torque characteristics with the implementation of an adjustable valve in the piston to reduce the starting torque, the PV generator characteristics could be ideally matched with the pump requirements. Using experience with wind driven piston pumps, a pump with a valve optimally matched to the PV generator characteristics was developed and tested. The rotary displacement pump prototype achieved a maximum efficiency of more than 53%. This is the highest efficiency of a motor pump unit ever measured at the French testing facilities. The hydraulic efficiency of the pump showed that it is ideally suited to high heads and low flow rates, which are the main requirements for water needs and borehole characteristics in developing countries. For the piston pump, a special piston valve was incorporated that had previously been developed for use with wind pumps to improve pumping efficiency. This piston pump was tested under laboratory and field conditions to investigate the influence of different pump parameters on the characteristics of the pumping system. Subsystem efficiencies above 40% were achieved for insolation values above 400 W m{-2}, and subsystem efficiencies of about 55% are expected in future if further optimization work is performed.

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