Photovoltaic pumping systems are used nowadays as a valuable alternative to supply water to communities living in remote rural areas. Owing to the seasonal variation and the stochastic behavior of solar radiation, at certain times the supply of water may not be able to meet demand. A study has been made of the relationship between water pumping capacity, reservoir size and water demand, for a given water deficit. As a result, curves of equal water deficit (iso-deficit lines) can be obtained for various combinations of PV pumping capacity and reservoir size. A methodology to generate those curves is described, using as its main tool the characteristic curve of the system, that is, the relationship between water flow and collected solar radiation. The characteristic curve represents the combined behavior of the water pumping system and the well. The influence of the minimum collected solar radiation level, necessary to start the system's operation (the critical radiation level I_C), is also analyzed. Results show that PV pumping systems with different characteristic curves, but with the same critical levels, yield the same set of iso-deficit lines. This drastically reduces the number of necessary solutions to those corresponding to a few values of I_C. Iso-deficit lines, calculated for the locality of Recife (PE), Brazil, are used to illustrate the sizing procedure PV water supply systems.
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