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Pump technologies

There are two classes of pump technologies:

Centrifugal Pumps

The water is moved with a rapid-rotating impeller. The pump should rotate at a sufficient speed for reaching the head required by the external system. The efficiency is mainly related to the flowrate. It shows an increasing curve (from zero efficiency at zero flowrate) until a maximum, which usually doesn't depend much on the Head. After this maximum the efficiency decrease is more marked for lower heads.

The flowrate has a quadratic behaviour as a function of the power, with a power threshold depending on the Head; this corresponds to the minimum speed before reaching the external head.

Centrifugal pumps are suited for systems with rather 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, the (uncompressible!) water is shut in an impervious moving volume, either with valves or with moving pieces with special geometric shape. Therefore some water is pumped as soon as the pump is rotating, and the flowrate is directly proportional to the pump speed. Power threshold is due to electrical losses in the motor before reaching a sufficient force for overcoming the torque.

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

There are several technologies:

  • Piston pumps, where an alternating piston in a cylinder draws up the water from the inlet or pushes it out of the chamber to the outlet, using non-return valves. |
  • Membrane pumps act in a similar way, except than the piston's imperviousness is replaced by a moving membrane. |
  • Progressive cavity pumps use a special vis-shaped rotor in a cylinder, which imprisons a volume of water in the input chamber and pushes it along the tube to the output. |
  • Rotating displacement pumps are made of rotor resembling to 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 rather constant for different flowrates.

Surface and deep well

"Normal" surface pumps are made of a motor and a pump aggregate, which are not necessarily integrated in a single case, giving the opportunity of coupling different motor types with different pump devices. The pump should be placed not too far from the water source (and at a maximum of about 5 mWater height for avoiding cavitation problems). There are no problems of accessibility for maintenance. But in many cases the fact of being placed above the water level requires a priming procedure, and some precautions for avoiding air entries.

For deep wells, submersible pumps have to be placed at the bottom of the well. These should have of course a cylindric shape adapted to the well diameter, and the electrical part should be perfectly waterproof all over the life time. The technical constraints are more severe, and their quality should be much higher as the maintenance is not easy. Therefore the price of such pumps is usually much higher than for surface pumps.

Moreover, it is technically very difficult (or impossible) to put several pumps in the same well.

Nevertheless, there are now very sophisticated well immersed solar pumps on the market, some of them even including the power converter, and accepting a very large range of input voltages. These dramatically facilitate the system design.