TECHNICAL INFORMATION

Types of Solar Pumps

There are really only two basic types of solar pumps; positive displacement and centrifugal and these can both be subdivided into surface mount and submersible categories. There are really only two basic types of solar pumps; positive displacement and centrifugal and these can both be subdivided into surface mount and submersible categories.

Your water source will dictate whether you need to use a surface mount or submersible pump while your daily volume requirement and total dynamic head (TDH) will determine if you need a positive displacement pump or centrifugal pump. In general, positive displacement pumps (diaphragm, piston, helical rotor, etc.) are best used where the TDH is high and the daily volume requirement is low. Positive displacement pumps usually require less power to operate than a centrifugal pump and they will produce some water even in cloudy conditions where a centrifugal pump might not. Centrifugal pumps are good in situations where the TDH is low and the daily volume requirement is high. One of the disadvantages of a centrifugal pump is that it has to operate at a high enough rpm to push the water all the way out of the well. If it is cloudy and the solar array is not producing enough power, the pump/motor may be turning but not fast enough to do this. Using a tracker is highly recommended with a centrifugal pump since it increases the solar array's power output over a longer period of time which increases your daily volume of water delivered.

What is Total Dynamic Head (TDH)

Head is measured in feet/meters or psi and it is a term for the distance that a pump has to push the water from your source to your tank. It may seem odd to be able to list head in terms of pressure or distance, but keep in mind that a column of water 2.31' (70 cm) tall produces 1 psi (atm/bar 0.06)at the bottom of that column so there is a correlation between distance and pressure. The greater the vertical distance is in between the water source and the tank, the harder the pump has to work to get the water there. Static head is the simple measurement from the surface of the water (when the pump is not running) to the tank. Dynamic head takes into account the amount the water level in the well drops when the pump is running plus the friction loss in the discharge pipe run. Total Dynamic Head is the sum of the static head and dynamic head.

Should batteries be used in solar pumping system?

It is not recommended to use batteries in solar pumping system, except in the case of having a very low volume producing water source (well/spring) and the user must be pumping as much water as possible each day. Otherwise, there are several reasons to avoid batteries in a pumping system:

• Using batteries in a solar pumping system lowers the efficiency as compared to going solar direct since it usually lowers the operating voltage of the pump which lowers the operating speed and flow rate of the pump. 

• Adding batteries to a pumping system costs more money for the batteries themselves plus a required charge controller, battery enclosure and additional disconnects and wiring.

• Batteries require care and maintenance that isn't always possible in a remote location.

• Batteries performance level and expected lifetime are closely connected with the temperatures that they are subjected to. If batteries are undercharged and subjected to freezing temperatures they could be damaged. If batteries are overcharged and subjected to high temperatures their lifetime will be shortened dramatically. Care must be taken to shield batteries in a remote location from temperature extremes.

Accordingly, it is recommended to install a solar pumping system capable of fulfilling the entire daily need during the daylight hours without having to resort to using batteries. It is further recommended to install a water storage tank that can hold a minimum of 3 days worth of water for livestock watering system or gravity fed domestic application.