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Pump technologies

There are two classes of pump technologies:

Centrifugal Pumps

Water is moved by a rapidly rotating impeller. The pump must rotate at a sufficient speed to reach the head required by the external system. Efficiency is mainly related to flowrate. It shows an increasing curve (from zero efficiency at zero flowrate) up to a maximum, which usually does not depend strongly on head. After this maximum, efficiency decreases more sharply for lower heads.

Flowrate shows quadratic behavior as a function of power, with a power threshold depending on head; this corresponds to the minimum speed required to reach the external head.

Centrifugal pumps are suited for systems with relatively low heads and high flowrates.

For extending the head range, many pumps use a multistage technique, that is implementing several impellers in series on the same motor axis, each ensuring a part of the required head.

Positive displacement pumps

In a positive displacement pump, (incompressible) water is enclosed in an impervious moving volume, either with valves or with specially shaped moving parts. Therefore, water is pumped as soon as the pump rotates, and the flowrate is directly proportional to the pump speed. The power threshold is due to electrical losses in the motor before sufficient torque is reached to overcome resistance.

In many pumps, this starting torque is higher than the running torque (friction losses are higher when the pump is stationary), requiring a starting inrush current.

Several technologies exist:

  • Piston pumps: an alternating piston in a cylinder draws water from the inlet or pushes it from the chamber to the outlet using non-return valves.
  • Membrane pumps: similar to piston pumps, except that the piston is replaced by a moving membrane.
  • Progressive cavity pumps: use a specially shaped screw-like rotor in a cylinder, which traps water in the inlet chamber and pushes it along the tube to the outlet.
  • Rotating displacement pumps: made of a rotor resembling a paddle-wheel rotating in a cylinder with inlet and outlet openings.

Positive displacement pumps are well suited for high head systems. Their efficiency is usually relatively constant across different flowrates.

Surface and deep well

Standard surface pumps consist of a motor and a pump unit that are not necessarily integrated in a single case, allowing different motor types to be coupled with different pump devices. The pump should be placed not too far from the water source (at a maximum of approximately 5 m water height to avoid cavitation problems). Maintenance accessibility is not an issue. However, since the pump is placed above the water level, a priming procedure is often required, and precautions must be taken to prevent air entry.

For deep wells, submersible pumps must be placed at the bottom of the well. They must have a cylindrical shape adapted to the well diameter, and the electrical part must be perfectly waterproof throughout its lifetime. The technical constraints are more stringent, and quality requirements are higher because maintenance is not easily accessible. Consequently, such pumps are usually much more expensive than surface pumps.

Furthermore, it is technically very difficult (or impossible) to install multiple pumps in the same well.

However, very sophisticated submersible solar pumps are now available on the market, some even including the power converter and accepting a very wide range of input voltages. These greatly simplify system design.