Considerable emphasis in this book is placed on the design of photovoltaic water pumping systems, firstly because water pumping is a major application for photovoltaics and secondly because the design of each system is considerably more complicated than most applications, owing to the large range of water source types, consumer requirements and system configurations. Where batteries are required for storage, design procedures are relatively straightforward and follow the design principles outlined in earlier chapters for stand-alone systems. However, direct interfacing between photovoltaic panels and the water pump motor introduces significant mismatch problems as the light intensity varies. This leads to large variations in overall system efficiency throughout each day and between ‘sunny’ and ‘cloudy’ weather, making it inappropriate to assume that the power delivered to the load is directly related to the light energy incident on the solar panels. It is therefore necessary to process insolation data differently for directly-coupled systems. The basic design principles are covered in this chapter, with worked examples given in Appendix H.
12.2 BASIC STEPS IN SYSTEM DESIGN
Designing a photovoltaic water pumping system has two very important aspects:
1. Selection of the most suitable system component types—crucial in providing a low maintenance, long life system of high reliability.
2. Matching of system components—a difficult area requiring considerable know-how and expertise, and ultimately responsible for efficient operation of the system.
To demonstrate the importance of the latter, the World Bank analysed one of the most efficient water pumping systems from their testing program at the time (referred to in the last chapter) and found the components to be poorly matched. Improved matching was demonstrated to give an 18% improvement in operating efficiency, on top of a 30% increase obtained through the introduction of manual tracking (Halcrow & Partners, 1981). It appears that this level of mismatch or worse is quite common with many system designs.
One of the most important questions to be asked before designing a particular system is: “What level of reliability is necessary and to what extent can maintenance be carried out?”
The answer to this will indicate a bias towards either a direct-coupled system with simplicity, reliability, low maintenance and long life, or a system that sacrifices these attributes, to an extent, to gain greater efficiency. The features included in the latter, which contribute to the increased complexity, higher maintenance, poorer reliability and shorter life expectancy, include power conditioning circuitry, inverters, and perhaps batteries.
Of course, other constraints influence the type of system selected, and each system needs to be designed on its own merits. No one system will be ideal for all applications and, of all photovoltaic applications, water pumping probably introduces the greatest variability of system design with regard to configuration and component selection. Several computer simulation and design tools and methodologies are available to assist designers (e. g. Mayer et al., 1992; Sharma et al., 1995; Protogeropoulos & Pearce, 2000; Arab, 2004). However, use of some of these requires a high level of water pumping knowledge and good data on site conditions and component performances. Thomas (1987) describes a sizing system based on nomographs to assist potential buyers. There are also practical guides to solar water pumping, such as that by Dankoff (1997).
The general approach to designing a system can be summarised as follows:
1. Determine the volume of water to be pumped each day, and at what head.
2. Calculate the pump rate from the number of sunlight hours.
3. Select the pump type.
4. From the torque-speed characteristic of the pump, select a motor with a compatible torque-speed characteristic.
5. Select appropriate solar modules.
6. Select module mounting method—fixed or manual tracking.
For a system using batteries, step 5 simply involves the use of ‘stand-alone system’ design principles, outlined in earlier chapters.
However, prior to following these guidelines, it is useful to ascertain whether a directly-coupled system (no batteries, no inverter and no power conditioning circuitry) is feasible for the particular application. If so, such a system may be advisable, even though its use provides reduced flexibility in component choice and system configuration, while maximum power point tracking circuitry is becoming more freely available. However, there are occasions when directly-coupled systems are unsuitable. These include:
1. When pumping heads are too large to be able to use a centrifugal pump with reasonable efficiency.
2. When suitable DC motors are not available, such as with some large systems.
3. When the pumping rate in bright sunshine exceeds the water source replenishment rates.
4. When it is essential batteries be used for energy storage (i. e. where ‘availability’ of pumped water must be very high and tank storage is unsuitable, e. g. portable units).
5. Locations characterised by excessive cloudy weather, making the poor partload efficiencies of a directly-coupled system unacceptable.
Many water pumping applications are not characterised by any of the above and are accordingly suited to a directly-coupled system. However, the use of maximum power point tracking circuitry is increasingly common for pump applications, including in many commercially supplied pump systems (von Aichberger, 2003).