Pumps are rated in flow (m3/hr) and head (m) and are connected to an electrical motor that is rated in kilowatts (kW). The motor draws power from the source (solar PV generator in this case) and drives the pump to deliver water.
Solar pumps can be broadly sorted into three categories:
- the motor power type – solar DC vs AC pumps;
- the pump end design: positive displacement vs centrifugal pumps;
- the installation set-up: submersible vs surface pumps.
The pump set should be matched to the water source, the power source, and the application.
Selection of the appropriate pump size is based on the duty point, which is the required flow output and head. Manufacturers’ pump performance curves or computer software are used to determine this. An explanation of the selection process is provided in Annex A and Annex B.
A key factor in pump selection is ensuring that the pump duty point is at the best efficiency point (BEP). A common practice is to select the duty point to the right of the BEP on the pump performance curve, so that as the pump wears out the duty point shifts towards the BEP, thereby achieving efficient operation over the life of the system.
Solar DC pumps vs solar AC pumps
Both DC and AC pumps are available for use. The distinguishing feature between DC and AC pumps is in the motor.
A DC pump system is the simplest SPWS configuration and consists of a PV array directly connected to a pump assembly with a DC motor via a DC controller. DC pumps have longer lifespans and are more efficient compared to an equivalent size of AC (up to 90 per cent versus 50–70 per cent for AC) as no power conversion is necessary. These pumps are, however, limited in head and flow and are generally used for lower head, lower volume (i.e. smaller) applications of up to 4 kW power demand. The pump design can be positive displacement or centrifugal type. The motors can be either brushed or brushless (both have permanent magnets).
The brushed motors have brushes that deliver current to the motor windings through commutator contacts, while brushless motors have none of these commutators. Brushed motors have the advantage of being less costly to buy with simple installation as they can be wired directly to DC power through a simple switch without the need of complicated electronics. Yet they are less efficient (75–80 per cent), electrically noisy, and have a maintenance cost due to wearing out of the brushes and commutators.
Brushless motors on the other hand, have a higher efficiency (85–90 per cent), longer lifespan, and are less costly to maintain as they do not require replacement of brushes. They do cost more than brushed motors and have an additional cost of an encoder and a driver to control.
In an AC pump system, the PV array is connected to a pump assembly with an AC motor. The motor is typically a brushless 3-phase induction (asynchronous) motor. These motors have a robust design with standard or enhanced insulation providing long, reliable service, minimum maintenance, and ability to withstand the voltage stresses encountered with most inverter drives. AC motors cannot operate with DC power and require a DC–AC inverter to convert incoming DC supply to power the AC pump. The pump design is commonly centrifugal due to their high flow capabilities.
An AC pump system is used for higher capacity applications that cannot be handled by a DC system.
Positive displacement vs centrifugal pumps
Examples of positive displacement pumps are helical rotor, diaphragm, or piston types. A helical design is used in submersible pumps. It features a rotor cased inside a rubber stator that spins with the motor, creating a vacuum that allows water into the cavity and effectively squeezes water out as it rotates. They deliver water with every rotation, with water output increasing with rotational speed, meaning their efficiency and lift capacity remains high even at low rotational speeds. Consequently they are appropriate for the varying solar radiation levels of solar-direct water pumping. This means higher volumes of water can be pumped per day in variable solar conditions. These pumps all fall within the DC range of solar pumps and therefore have a limited capacity, suitable for high heads and low-flow applications. Heads of up to 450 m are achievable while maintaining high efficiency.
Centrifugal pumps feature one or more impellers inside a chamber (referred to as a pump stage). Water enters the eye of the impeller and as the impeller spins the water is subjected to centrifugal force that pressurizes the water from one stage to another. To achieve high lift, multiple stages are stacked together (multi-stage pumps) with the pressure of the water increasing as it is pushed from one stage to the next. This is the reason high-pressure centrifugal pumps are tall, (submersible pumps can have up to 100 stages with heights of up to 6 m!).
Centrifugal pumps require a minimum speed to start and deliver water. They can achieve flows of up to 250 m3/hr with efficiency reducing at high heads and low flows. For this reason, positive displacement pumps are used for most systems that require high lift at low volumes. The efficiency of centrifugal pumps deteriorates as the speed varies, whereas positive displacement pumps can operate efficiently over a wide speed range. Positive displacement pumps can also operate at fairly constant flow over a wide pressure range.
Surface pumps vs submersible pumps
Submersible pumps are installed completely submerged in water. They are predominantly used for deep-well pumping. Submersible pumps are coupled to water-cooled or oil-cooled motors and must never be operated without water otherwise they will burn out due to dry running. They are available in both centrifugal and positive displacement designs.
A surface pump is installed outside the water source – it cannot be submerged. A surface pump has an air-cooled motor which should be installed in a well- ventilated location protected from the weather. It is prone to failure if it is submerged or splashed with water. Commonly, surface pumps are installed inside a pumphouse. They are designed to draw water from a depth of 3–7 m above the water-source surface level. If this vertical height (referred to as suction lift) is exceeded, the pump experiences cavitation (as the water enters the inlet of the impeller, low pressure causes it to vaporize, forming bubbles which collapse and erode the impeller as they collapse), eventually leading to pump failure. Surface pumps can be of centrifugal or positive displacement (diaphragm or piston) design.
Surface pumps tend to be more efficient at high-flow pumping and are less expensive than submersible pumps, but more complex to install and operate.
Manufacturer data sheets will state the type and name of pump.
Quality and performance considerations
Pumps are recommended to meet EN 809 and EN 60034-1 or equivalent standards, stainless steel with a minimum grade of AISI 304 or higher.
Centrifugal pumps may be constructed of other materials, such as cast iron and plastic, the choice of which will depend on various factors, such as the quality of water to be pumped. For example, water that has a lot of silt is better pumped with a pump of plastic internal construction, whereas corrosive water or hot water is better pumped with pumps of higher grades of steel. Importantly, the material used for pump construction should be corrosion resistant, permanently lubricated, and maintenance free, as well as able to handle the water temperature.
The pump motor is the piece of equipment most prone to failure and should be constructed with corrosion-resistant material, all stainless- steel exterior construction, stainless-steel shaft, ceramic bearings, NEMA mounting dimensions, hermetically sealed windings, water lubrication, and pressure-equalizing diaphragm, and able to withstand a certain maximum temperature.
The pump set must be of modular design to allow for replacement of individual parts (pump end, pump motor, and electronics) if failure occurs. The pump must have dry-run protection to protect it in the event of low water levels.