Improving the effectiveness of solar pumping systems by using modular centrifugal pumps with variable rotational speed

The possibility of improving the performance of deep well solar pumping systems by using centrifugal pumps with variable rotational speed and modular number of working stages (MNWS) was investigated and compared with traditional systems equipped with pumps having a fixed number of stages. Starting from commercially available pumps with a given head – mass flow characteristic, a visual simulation tool for the evaluation of the modified MNWS pump performance and costs was developed. In principle, it would be possible to use the desired number of modular stages, thus achieving the highest efficiency of the system for all conditions of radiation. In practice, in order to reduce the MNWS pump costs to an affordable level, only one shaft breakpoint is suggested (and then 2 modular blocks of stages), whose optimised position is determined by the simulation program on the basis of the location area on the earth’s surface, insulation curve during the daylight, required head and shaft speed. Referring to a 30 m2 panels surface (corresponding to a photovoltaic system of about 3000 W peak power) and to a well depth of 100 m and considering a commercial 46 stages submersible pump, it was found that an optimised breakpoint at the 31st impeller, produced an increase close to 9 % of the yearly pumped water yield with respect to a conventional, non-modular pump. Assuming an average international cost per cubic meter of lift water and supposing that the cost of a modified modular pump is about 1.5 times higher than that of a standard pump, the payback period is reduced of about one year when the water price is between 0.5 and 0.72 €/m3.