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When sizing a PV pumping system, the basic constraints are the availability of solar energy during the year, and the satisfaction of the user's water needs. The problem to be solved is the optimisation of the size of the photovoltaic generator and the pumps, taking the head and the electric PV-Pump matching into account, as well as the chosen System configuration.

 

We give an idea of the procedure for a first, rough estimation:

We start with the pump sizing.

We first determine the Hydraulic Energy needs over one day, assuming that Flow and Head are rather constant over the year (otherwise the day-by-day rough simulation provided in the presizing tool is unavoidable).

As a thumb rule, we can assume that on rather good days, the pump will run at its equivalent-full operation during about 6 hours; i.e; provide a flowrate [m³/h] of about the daily water yield [m³] / 6[h].

Assuming now a pump efficiency (usually about 50% for positive displacement, or 35-40% for centrifugal pumps), we can deduce the Pump Electrical nominal power suited for these clear day conditions.

We should of course increase this value, accounting for the possible bad weather conditions over the year. Here the day-by-day simulation is necessary if we want to determine accurately the nominal power required to get a predefined LOL.

At this stage, we should emphasize that the system layout has a great influence on the pump nominal power; namely if the threshold losses are important, as it is the case with "Direct Coupling" simple systems.

PV-array sizing

Again as a thumb rule, we can choose the nominal PV STC power as about 20%-30% over the pump nominal power. Oversizing the PV-array will result in unused energy by clear weather. Undersizing it will operate the pump at lower powers, where its efficiency may drop or the thresholds dramatically affect the yield by cloudy, or morning-evening conditions.

When using direct coupling configurations, the array voltage is also essential. Detailed optimizations can only be given by a set of detailed simulations, using real devices.

Tank sizing

The tank size is simply determined by the required autonomy, using the daily consumption defined by the user, and assuming no water production.

Other secondary characteristics of the pumping system should be determined in a second step: wire diameters between PV array and pump, pipe sizing, etc. These are involved in the Detailed Simulation Process.

Moreover, the sizing may be subjected to criteria which may take on different weights depending on the use:

-        Reliability of the supply, and the consequences of no-delivery periods (may be overcome by a back-up generator),

-        Investment and maintenance costs, which should take into consideration the cost of the PV generator, pump(s), regulation, and maintenance of the system. With battery buffered systems, also the initial cost of the batteries, as well as that of their maintenance and replacement.

-        Durability: Quality of the pumps and regulators, ease of maintenance and replacement, special wearing conditions like sands or impurities in the water, etc.