As the well is 30 m deep, the pump must be of the submerged type (surface pumps are only generally suitable for wells of less than 6 m depth). Its PV system will be independent of the main system.
The pumping requirements are not very demanding: there are many suitable pumps available on the market. In fact there are PV pumps capable of pumping tens of cubic metres per day from a depth of 200 m (see Section 5.4.4).
The farm’s requirements are more modest: the average daily volume required is 900 l/day from March to September and 550 l/day for the rest of the year, with the water being pumped from a depth of 30 m.
To meet these characteristics, and for the sake of simplicity, we would choose a DC pump to work directly off the Sun without a battery. The 4 m3 tank will act as a storage buffer.
The model chosen is a small volumetric pump (height 30 cm, diameter 9.5 cm), suitable for boreholes of 100 mm diameter. It is a three chamber diaphragm model, and the 24 V DC motor has a permanent magnet with thermal protection.
According to the manufacturer’s specifications, with only 100 Wp installed, at 24 V DC, the volume pumped should reach 1.2 m3 for solar radiation of
4.5 kWh/m2/day and a depth of 30 m (from the pump to the tank feed). It is adequate for our daily requirement of 900 l. Between March and September, the horizontal total radiation is 5 kWh/m2/day (Figure 5.58). As the modules will be slightly tilted to avoid standing rainwater and as they are likely to get slightly dirty, it is good to have this margin. During the remainder of the year, the solar radiation will be between 3 and 5 kWh/m2/day. Bearing in mind the length of the days, it is probable that in winter the instantaneous radiation will rarely reach 1000 W/m2. It will therefore be worthwhile to install uphill from the pump a booster that will enable it to start even in weaker sunshine, so as not to risk creating a water shortage. A booster also has the advantage of protecting the pump against accidental overloading or overvoltages.
The pumping system is thus made up of the following:
• a 100 Wp-24 V PV module (of the same type as those of the main system);
• a 24 V submerged volumetric pump 9.5 cm diameter;
• an electronic starter booster;
• a half-inch diameter water pipe;
• a waterproof feed cable;
• a4m3 tank mounted on the ground;
• an animal trough of around 500 l.
Figure 5.59 shows the positioning of these different elements.