For either PV-direct or battery-based solar water pumping, the following design practices can ensure a top-notch system that maximizes your investment.
Site Right. For optimal performance, the array must be free of shading during the day and must be facing as close to true south as possible. For most residential water-pumping systems, the modules are set at a fixed tilt. But for applications that require a lot of water pumping during the summer, like irrigation, a sun-tracking PV module mount can maximize power production and pumping.
Trackers keep the modules perpendicular to the sun as it moves through the sky, providing more power to the pump over a longer period of the day. In the summertime, a tracked system with a clear horizon-to-horizon solar window can provide 20% to 40% more water than a fixed array.
Minimize Voltage Loss. To limit wiring voltage loss and minimize the length and the gauge of wire needed, locate the PV array as close to the pump or battery bank as possible. The greater the distance from the PV modules to the pump, the greater the voltage drop. Too much voltage loss can significantly reduce the amount of water pumped, and in some cases may be so great that the pump will not operate effectively.
Voltage loss should not exceed the accepted standard of 2%. For long distances, wire size must be increased, at additional expense. With the price of copper at an all-time high, it’s usually cost-effective to locate the array as close to the pump or battery bank as possible. Your system designer can provide the wire-sizing calculation for you, or you can use software provided by the pump manufacturer.
Free Flow. Pipe friction is another important factor in optimizing pump production. The greater the distance between the pump’s outtake and the delivery point, the greater the flow restriction from pipe friction. Much like voltage drop can be mitigated with larger wire size, flow restriction can be reduced by increasing the pipe size or reducing the distance. For household use, 1.25-inch- or 1.5-inch-diameter pipe is typical. However, if the distance exceeds 500 feet or the flow rate is expected to be unusually high, consider increasing the pipe diameter to 2 inches to keep friction losses down.
Pump Protection. To regulate flow, pressurized systems use a pressure switch. If a holding tank is used, a float or level switch turns off the pump when the water rises to a certain level. Some pumps require a “reverse-acting” pressure switch. Most pressure switches operate by closing the contacts when the pressure has risen to a preset point. Conversely, in a reverse-acting pressure switch, the contacts open when the pressure falls to a certain level.
Just as important is providing a shutoff mechanism for the pump. Damage can occur to the pump if it pumps against “shutoff” head when the holding tank is full and there’s no overflow pipe or float switch in place. And to avoid damaging the pump if the water source runs dry, use dry-run protection (a water-level sensor) at the source. Some pumps have this built in, so check a given pump’s specifications.
Under Control. All submersible water-pumping systems should have a controller between the power source and the pump. In PV-direct systems, the control can be as simple as using a linear current booster (LCB). An LCB optimizes array voltage and current, maximizing the volume of water pumped and helping the pump start sooner in low-light conditions. More advanced controllers include pump motor speed-control and low-water cutoff circuits. Some even provide information related to the system status, including how many watts the PV modules are producing, and can be helpful for system troubleshooting. Most pump manufacturers offer one or more controller options designed to be used with their pump model.
Arresting Lightning. Proper system grounding will usually protect the pump and control. But some manufacturer warranties also require a lightning arrestor be installed. As with any PV system, the array and its modules should be properly grounded. While a direct lightning strike will likely damage the controller, the pump may survive if the system is well-grounded.