The capacity of the battery will depend mainly on the need for autonomy without solar generation. Periods of bad weather in the region are mainly limited to October when there are often rainy spells that can last for 3 or 4 days. During the rest of the year, it is rare for the sky to be covered for more than two consecutive days. The duration of 3 days without Sun is therefore a fairly correct value that will considerably limit the risk of breakdown of the electrical equipment.
For the pumping system, it was decided to use a value of 4 days to ensure water supply without interruption throughout the year (in this case the storage is in the tank and not in a battery, which is much cheaper!).
The daily consumption of the main system is 191.2 Ah at 24 V (= 4590 Wh/24). With an initial coefficient of 0.7 and 3 days of autonomy without solar generation, the battery capacity needs to be
191 2 y 3
Cn = 9 0 7 3 = 820 Ah (5.33)
With this capacity requirement, and bearing in mind that the premises are occupied, it is clear that the best solution is an open lead battery (with liquid electrolyte). The maintenance can easily be carried out regularly and there is no need to have recourse to a sealed battery. The technology chosen will be a model with tubular electrodes, which can deliver 500 discharge cycles at 80%.
There is a model of 800 Ah nominal capacity that would be compatible with the loss coefficient of 0.72.
Since the ambient temperature is between 15 and 25 °C most of the time, the temperature loss of capacity can be estimated to be 10% in winter. Still using the loss coefficient of 0.72, this gives an order authorised depth discharge of 80% (0.72 = 0.8 x 0.9). This is correct because one can then have 500 cycles available.
We will therefore use an open lead battery composed of twelve 800 Ah-2 V elements with tubular electrodes. The size of each 2 V element is 200 x 150 x 650 mm. If they are aligned in two rows of six elements, the battery bank will fill a space of 400 x 900 mm on the ground with a height of around 800 mm (for this type of arrangement, see Figure 5.50, and the accompanying explanations). Its charge current efficiency is at least 80%.