Solar (PV) Water-Pumping

 

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Photo Credit: NREL

Water pumping has a long history, so many methods have been developed to pump water with a minimum of effort. These have utilised a variety of power sources, namely human energy, animal power, hydro power, wind, solar and fossil fuels for small generators. The relative merits of these are laid out in Table 1 below.

Applications

• village water supply
• livestock watering
• irrigation

A solar pump for village water supply is shown schematically in Figure 1.With village water supply, a constant water demand throughout the year occurs, although there is need to store water for periods of low insolation (low solar radiation). Typically in Sahelian Africa the storage would be 3-5 days of water demand. In environments where rainy seasons occur, rainwater harvesting can offset the reduced output of the solar pump during this period. The majority of the 6000 or more solar pumping systems installed to date are for village water supply or livestock watering.

The suitability of major irrigation systems for use with solar pumps is shown in Table 2.

The technology

Systems are broadly configured into 5 types as described below:

Submerged multistage centrifugal motor pumpset - Figure 3

This type is probably the most common type of solar pump used for village water supply. The advantages of this configuration are that it is easy to install, often with lay-flat flexible pipework and the motor pumpset is submerged away from potential damage.

Either ac or dc motors can be incorporated into the pumpset although an inverter would be needed for ac systems. If a brushed dc motor is used then the equipment will need to be pulled up from the well (approximately every 2 years) to replace brushes. If brushless dc motors are incorporated then electronic commutation will be required. The most commonly employed system consists of an ac pump and inverter with a photovoltaic array of less than 1500Wp.

Submerged pump with surface mounted motor - Figure 4

This configuration was widely installed with turbine pumps in the Sahelian West Africa during the 1970s. It gives easy access to the motor for brush changing and other maintenance. The low efficiency from power losses in the shaft bearings and the high cost of installation has been disadvantages. In general this configuration is largely being replaced by the submersible motor and pumpset.

Reciprocating positive displacement pump - Figure 5

The reciprocating positive displacement pump (often known as the jack or nodding donkey) is very suitable for high head, low flow applications.

The output is proportional to the speed of the pump. At high heads the frictional forces are low compared to the hydrostatic forces often making positive displacement pumps more efficient than centrifugal pumps for this situation. Reciprocating positive displacement pumps create a cyclic load on the motor which, for efficient operation, needs to be balanced. Hence, the above ground components of the solar pump are often heavy and robust, and power controllers for impedance matching often used.

Floating motor pump sets - Figure 6

The versatility of the floating unit set, makes it ideal for irrigation pumping for canals and open wells. The pumpset is easily portable and there is a negligible chance of the pump running dry. Most of these types use a single stage submersed centrifugal pump. The most common type utilises a brushless (electronically commutated) dc motor. Often the solar array support incorporates a handle or 'wheel barrow' type trolley to enable transportation.

Surface suction pumpsets - Figure 7

This type of pumpset is not recommended except where an operator will always be in attendance. Although the use of primary chambers and nonreturn valves can prevent loss of prime, in practice self-start and priming problems are experienced. It is impractical to have suction heads of more than 8 metres.

Performance

The performance of some commercially available products is shown in Figure 8. It can be seen that solar pumps are available to pump from anywhere in the range of up to 200m head and with outputs of up to 250m³/day.

Solar pumping technology continues to improve. In the early 1980s the typical solar energy to hydraulic (pumped water) energy efficiency was around 2% with the photovoltaic array being 6-8% efficient and the motor pumpset typically 25% efficient. Today, an efficient solar pump has an average daily solar energy to hydraulic efficiency of more than 4%. Photovoltaic modules of the monocrystalline type now have efficiencies in excess of 12% and more efficient motor and pumpsets are available. A good sub-system (that is the motor, pump and any power conditioning) should have an average daily energy throughput efficiency of 30-40%.