Solar Water Pumping Module 2

Introduction

This module contains information about the components of a solar water pumping system. A conventional solar water pumping system comprises four identifiable major component systems:

• Photovoltaic (PV) Array
• System Control Unit and Maximum Power Point Tracker (MPPT)
• Pump Motor
• Pump

The System control unit and Maximum Power point tracker are usually integrated into one unit commonly referred to as the pump controller and maximiser. Similarly in some pump units the motor and pump come as an integrated unit. In most cases the motor used for driving the pump is a direct current (DC) motor, although some units may utilize an AC motor.

The relationship of the components is as follows:

Components of a Solar Water Pumping System – PV Modules

The principle of a photovoltaic module

The photovoltaic effect is the basis for a PV module , and is described below.

When activated by sunlight, a PV cell generates an electrical charge across its faces – if an electrical circuit is connected to the PV cell (e.g. a globe or a water pump motor), then the electrical charge converts to a flow of electrons, or a flow of electrical current. This current is forced through the circuit by an electrical voltage (or electrical “pressure”), and work is done – in this case water can be pumped.

Of course, the energy provided by one small PV cell is not sufficient to do much useful work, but if many cells are connected together then the energy total is much larger and the required level of work (pumping) can be done.

It is important to remember that the photovoltaic effect is dependent on sunlight and not heat to produce energy.

A PV module consists of individual PV cells which are connected in series and parallel in such a way that the required output voltage and current are achieved. Similarly by connecting PV modules together and mounting them on a frame they can then be used to deliver to a system the required electrical voltage and current (and thus power).When the modules are set up this way, with interconnecting wiring and suitable mountings, the collection is known as an ARRAY. The array is the energy collection/conversion device, converting the light energy of the sun to electricity dependent on the photovoltaic effect.

Types and Characteristics of Photovoltaic Modules

There have been many different varieties of PV cells developed, but for simplicity only Monocrystalline (Mono), Polycrystalline (Poly) and Amorphous Silicon (AmSi.), the most widely used types for water pumping applications, will be discussed.

Mono and Poly PV's are manufactured using silicon wafers (crystals), whilst the last (AmSi) is a silicon based material applied to a backing surface. The efficiency of a PV cell is its ability to convert sunlight to electrical current. The efficiencies of these cell types range from around 8% to 16% for normal commercially available product. However, research laboratories developing PV Cell technologies are heading towards PV cell efficiencies of 30%.

The output of a PV module is affected by the amount of light falling on the module surface and the temperature of the module.
The maximum output of a module is achieved in very bright sun conditions and at low ambient temperatures. Typically for crystalline cell technologies the hotter the module gets the lower its output will be. Equally the less bright (e.g. on a dull cloudy day) the sunlight falling on a module will also cause a reduction in the output.

The efficiency of the solar module relates to the area of active cell area exposed to the sunlight. A low efficiency cell will require a larger area than a high efficiency cell to produce the same output in the same conditions. The selection of the modules for an array for a pumping system is not only based on the cost of the individual module, but more importantly on the predicted total water pumping output for the system using a complete array of modules along with frame and mounting hardware.

By connecting modules together electrically, a specific amount of energy collection can be achieved, meaning that a specific amount of work can be done. The electrical output of a PV cell has the following features:

• the voltage produced in varying levels of sunlight is quite constant,
• the current it can deliver is directly dependent on the size of the cell and the light levels.

A module is rated in Watts. This value is the electrical power output for the module at specific reference conditions. The Electrical power (watts) is the product of current (amps) and voltage (volts). The power (in watts) delivered by a PV array will vary with the sunlight intensity (i.e. how bright the light is) – usually maximum around midday on a cloud free day, and minimum at sunrise and sunset. This is very suitable for pumping water in summer conditions – long sunlight hours match the heavier demand for water – stock, crops etc.

The sunlight intensity on the panel is also affected by the angle of the sun light falling on a PV module. If the surface of the module is facing towards the sun the output will be greater than if it is turned away from the sun.

Crystalline modules generally produce around 150 watts for each square metre in sunlight. Amorphous silicon modules around half the value, 80 watts for each square metre. For further information on PV Cells refer to the Information Portal.

Installation and Characteristics of Photovoltaic Arrays

Several different structures can be used for installing the PV array for a solar water pump. These include:

• a ground mount type,
• a pole mount type,
• or even a mechanical tracking (following the sun) type.

In the Southern Hemisphere the PV array orientation faces North, and is tilted to an angle similar to the latitude of the site. This tilting tends to “even out” the variations of PV array output caused by the apparent seasonal movement of the sun from summer to winter. Ideally you would change the tilt such that for maximum summer water pumping the array should be close to horizontal but this would give poor winter performance where the array should be more vertical. This manual changing of the array tilt angle is not done as the extra benefit of manually tilting is lost in the extra cost and inconvenience to the user.

However, PV Array mounting systems are available that use a mechanical tracking device (which attempts to face the array directly at the sun at all times) on a daily basis. Using such a system in some conditions, it is possible to pump up to 40% more water than a fixed array of the same power. The cost of the tracking device must be considered in the economic analysis of the whole PV pumping system. On a small system, it may be better to add another module or two to a fixed array to get more water pumped than to invest in the extra cost of the tracker.

An important factor in PV pump system design is the amount of the sun’s energy falling on an array's surface. A commonly used factor is the “peak sun hours” which is an indicator of the total energy from the sun falling on a surface (usually horizontal) which changes throughout the year due to the sun’s apparent movement and climatic conditions. The peak sun hours is representative of the sunlight intensity over a given period. The pattern of daily water pumped follows the peak sun hour levels as shown below (Figure 5), and the system design must allow for the operation requirements in different months of the year – obviously, winter sunlight levels are cumulatively lower than summer levels – this may not be a problem, but the solar pump buyer should check for “best month design” or similar comments.

Figure 5 The number of “peak sun hours” per day varies with the time of year	Power produced by an array by hours of the day ( tracked vs fixed)

Example Photovoltaic Arrays

A four module (pole) tracking array with a floating pump (on the water surface) AmSi array total power approximately 250 watts or 0.33 Horsepower.
A two module poly crystalline fixed array driving a surface pump – lifting over the bank of the dam  - peak power around 150 watts	A twenty four module monocrystalline ground mount array – around 2000 watts peak power

Components of a Solar Water Pumping System – Motor and Pumps

Types and Applications of Solar Pumping Systems

Typical high efficiency DC motors for solar water pumping systems applications	Surface or transfer pump application – centrifugal or similar pump directly coupled to a high efficiency motor
Left – typical surface drive unit – DC motor driving rotating head unit turning a sub surface mechanical rotation pump (helical screw or “progressive cavity” type)	Below – typical solar submersible pump – motor and pump and MPPT can be  in one unit – waterproof cables only to the surface PV array

Motors D.C. (for solar applications)

High efficiency (from 75% to over 90%). Can be brushless (less maintenance than brush motors, but more costly to repair when needed) or have brushes which need replacement each 4 to 6 years. Good selection of sizes to allow for most applications.

Pumps for Solar Applications

Must be high efficiency where possible (range 40 to 70%), especially for high volume and high head applications (high pressure pumping). Generally direct displacement types for high pressure pumping. Centrifugal pumps are OK for low head medium volume applications. Can be submersibles, floating, or surface applications, both “lift and push”

Pump / Motor Efficiency (the “system efficiency”)

Must be as high as possible, as the PV array must be sized to provide for the system losses as well as the pumping power. System efficiency is the product of the motor and pump efficiency. ( e.g. motor 80% , pump 50%, system efficiency = 0.8 x 0.5 = 40%). Use this value (40%) for an example exercise.

Components of a Solar Water Pumping System – Controllers & Maximisers

The system control unit and maximum power point trackers are usually integrated into one unit commonly known as the pump controller and maximiser.

The function of the maximiser (or maximum power point tracker – MPPT) is to ensure that as much power as is possible is directed to pumping effort, regardless of the light levels, and so the pump motor is receiving the correct voltage and current to pump water more effectively. In addition the maximiser ensures that the PV array is working at its most efficient operating point.

Functions of controllers include:

• starting and stopping the motor with signals from a flow switch, or pressure switch, to protect the system from damage due to loss of water or other problems.
• starting and stopping the motor with signals from a water level switch
• the advanced controller can also start and stop the system remotely through telecommunication options
• in some cases controllers can record (or log) data for analysis or troubleshooting.
  

This is the end of Module 2.