Page 257 - Applied Photovoltaics

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.

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