How Solar Water Pumping Systems Work

A solar pump does the same hydraulic job as any other pump. What changes is the energy source, and with it the whole design logic: instead of a fixed supply you have a varying one, strongest exactly when demand is highest. Design around that and solar pumping is the cheapest water most South African farms will ever pump. Design against it and you end up with a battery bank you did not need.

The four components

The array

Photovoltaic panels wired in series and parallel to produce the voltage and current the controller expects. Series strings raise voltage; parallel strings raise current. Panels are rated in watt-peak at standard test conditions, which is a laboratory figure – in the field you plan on roughly 75 to 85 per cent of rated output because of heat, dust and cable losses.

The controller or drive

This is the intelligent part. It takes the varying DC from the array, tracks the maximum power point as irradiance changes through the day, and delivers the right voltage and frequency to the pump motor. On AC systems it is effectively a solar-input variable speed drive; on DC systems it is a dedicated pump controller. Either way it also carries the protection: dry-run cut-out, over-voltage, reverse polarity and tank-full shutdown.

The pump

Two families. DC helical rotor pumps move modest flow at high head very efficiently and start turning at low light, which makes them excellent on deep, low-yield boreholes. AC centrifugal submersibles driven by a solar drive cost less per litre at higher flows and use standard, widely serviceable pump ends. Most farm-scale installations in South Africa now use the AC route.

Storage

A reservoir. This is the component people try to skip, and it is the one that makes the whole system work.

Why water storage beats battery storage

The obvious way to make a solar pump run at night is a battery bank. It is almost always the wrong answer. Storing energy costs far more than storing the water that energy would have moved.

The design rule follows directly: size the reservoir for two to three days of demand, run the pump only in daylight, and let gravity or a small pressure pump handle the delivery. A tank on a stand or a hill is a battery that never degrades.

Sizing the array

Array sizing works backwards from the water you need.

1. Daily water requirement in cubic metres, at peak season.
2. Total dynamic head – pumping water level plus discharge height plus friction, exactly as for any pump. See calculating pump head.
3. Hydraulic energy = volume in cubic metres times head in metres times 0.002725 to get kilowatt-hours.
4. Divide by system efficiency – typically 0.40 to 0.50 for a complete solar pumping system from panel to water.
5. Divide by peak sun hours for the site in the worst design month to get the required array size in kilowatts-peak.

Worked example

A cattle operation near Kuruman needs 18 cubic metres a day. The borehole pumping water level is 58 m and the reservoir inlet is 6 m above the wellhead, with about 4 m of friction – total dynamic head 68 m.

• Hydraulic energy: 18 x 68 x 0.002725 = 3.34 kWh per day
• System efficiency 0.45: 3.34 / 0.45 = 7.4 kWh of solar energy per day
• Northern Cape design figure of 5.5 peak sun hours: 7.4 / 5.5 = 1.35 kWp
• Array: 1.5 kWp, say four 385 W panels
• Pump: 1.1 kW AC submersible, ten stages, on a solar drive
• Storage: 50 000 litre reservoir, just under three days of demand

Note that the array is comfortably larger than the pump motor rating. That is deliberate and normal – oversizing the array by 20 to 40 per cent extends the productive pumping window into the early morning and late afternoon, which buys more daily water than a bigger pump would.

Hybrid and load-shedding applications

Solar pumping is not only an off-grid technology. On farms and estates that already have a grid connection, a solar drive with grid backup lets the pump run on sunshine by default and fall back to Eskom or a generator only when the array cannot carry it. During load shedding the borehole keeps filling the tank regardless of the schedule, which for a lodge or a dairy is the entire point.

What goes wrong

• Shading. A single shaded panel in a series string drags the whole string down. Keep the array clear of trees, tanks and the wellhead structure, and account for winter sun angles.
• Dust. On Highveld and Karoo sites, soiling can cost 10 to 20 per cent of output by the end of a dry season. Rinse the array quarterly.
• Undersized reservoir. A three-day cloudy spell with one day of storage means a dry trough. Storage is the cheapest insurance in the system.
• Cable losses. DC cable runs from array to controller must be sized for voltage drop just like a submersible drop cable.
• Theft. Panel clamps, anti-theft bolts, and fencing are part of the specification in most of the country, not an afterthought.

Running costs and payback

A solar pumping system has essentially no running cost beyond cleaning and an occasional controller or pump service. Against a grid-powered borehole running four hours a day, payback on a typical farm installation lands between three and five years at current tariffs, and shorter where the alternative was diesel. Against extending a grid line to a remote borehole, solar is usually cheaper on day one – a kilometre of new line costs more than most complete solar pump sets.

For the pumping fundamentals behind all of this, see how borehole pumps work, and for the sizing arithmetic how to size a borehole pump.

Wiring and mounting the array

Panels in series add voltage; strings in parallel add current. The controller’s datasheet gives you a maximum input voltage and an operating window, and the array must sit inside that window at both temperature extremes – open-circuit voltage rises on a cold winter morning, which is when undersized controllers fail.

Fixed-tilt mounting is the norm on pumping systems because tracking adds moving parts to an installation that is often unattended for months. Set the tilt to favour the season you most need water.

Mount the array high enough that livestock and long grass cannot reach it, on a frame rated for the site wind loading, and use anti-theft fasteners as standard. Keep the DC cable run from array to controller short and sized generously – voltage drop here is lost water, not just lost efficiency.

Keeping it working

• Rinse the panels quarterly in dry regions, monthly during dust storms or harvest. Soiling is the largest recoverable loss on most installations.
• Check the array output against the controller display seasonally; a string that has dropped out is easy to miss when the tank still fills.
• Inspect connectors and cable ties annually – UV degrades both.
• Verify the dry-run and tank-full cut-outs work. The pump depends on them.
• Record daily delivered volume. A gradual decline is either soiling, a failing string or a falling water level, and the three are easy to distinguish once you have a record.