Understanding Pump Specifications:
In the specification sheet, notice that voltage varies from 8 to 24 V DC and the maximum head (zero flow) is around 3 meters. The flow rate is zero at maximum head. There is no flow at maximum head; this just shows what the pump could do if it hummed away, maximum power in, and just pushed a water column up as far as it could. The water column would reach 3 meters. No water would spill out the top.
12 Volt is nominal voltage. Power is equal to current times voltage (P=IV). On page 1 of the same brochure, voltage for all models of the pump varies from 6 V to 24 V DC, which is the great advantage of the PV direct pump: output is adjusted electronically based on a varying solar input. Solar panel output varies in both current and voltage. To achieve maximum power point (MPP) tracking, the pump adjusts both torque and speed. The pump can run fast, high RPM with a high flow, at low head: this works with a lower voltage and high current. Torque required is small at lower head. Even at a fairly low voltage, the pump could absorb full power; the power is all going to high flow. The converse is true as well: to get the maximum head, which requires maximum torque, the voltage would have to be at a maximum value, closer to 24 V, and the current would be low, just enough to turn the pump fast enough to prevent back flow.
Solar PV direct circulation pumps are built to circulate water from a solar collector on the roof to a tank that is usually in the building below. The pump needs to have a closed loop; which means water pressure in to the pump, coming from the roof, is almost the same as water pressure out of the pump. If water circulates from the roof back down to the tank, then the pump is only overcoming friction in the pipe, not the head to the roof. There has to be water in the return line and no air gaps, otherwise this little pump would not have the power to push water all the way to the roof. Once the whole line is full of water, though, then all the pump has to do is overcome friction, not gravity. This is the secret to being able to use such a small pump to circulate water to a high roof: there is no head to overcome because the system is closed loop.
Electronic controllers historically cause the most trouble in a solar water heater systems. This solar PV direct pump eliminates the need for controllers or probes, eliminating the largest source of solar water heater system failures.