Category Photovoltaics is the process of converting

ARRAY WIRING AND MOUNTING

11.7.1 Array wiring

Reference should be made to the stand-alone system design guidelines mentioned in Chapter 6 (Standards Australia, 2002). Array cables should be heavy duty, with all connections in water-tight junction boxes with strain relief connectors (Ball & Risser, 1988). The gauge of wire should be selected so as to keep resistive losses to less than 2.5%. For reliability, splicing of the leads from the motor to the array output cable should utilise crimp-on connectors with resin-filled heat shrink tubing or equivalent, to ensure long lasting, dry connections. All wiring should be attached to support structures with, for example, nylon cable ties. PVC conduit should be used for the array output wiring to the motor/pump, regulator or batteries...

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BATTERIES

Batteries may be used in water pumping systems for two independent purposes— storage of energy and as a power conditioning mechanism.

As a storage medium, batteries are greatly inferior, in most instances, to pumped stored water. However, with some critical applications, or where continuous pumping is necessary, batteries may be required, in which case three days’ storage is commonly used.

The role of batteries for power conditioning was briefly considered in the last section. They allow power to be transferred to the load under optimal conditions, whereby both pump and motor can potentially be at their respective peak efficiencies whenever operating. This has four significant advantages relative to a directly-coupled system:

1...

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POWER CONDITIONING CIRCUITRY

The role of power conditioning circuitry (Ross, 2003; Schmid & Schmidt, 2003) is to provide the motor/pump with the most suitable voltage-current combination, while ensuring the solar panels operate at their maximum power points. In effect, it alters the load impedance to match the optimum impedance of the array. The circuitry of course must consume very little power to justify its inclusion and, in most systems, will typically consume 4-7% of total power (Matlin, 1979). It is also expensive, usually costing more than the electric motor (Halcrow & Partners, 1981), while historically often providing reliability problems.

As the light intensity falls, the current generated by solar panels falls proportionately while the voltage at the maximum power point remains approximately constant...

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Integrated pump/motor machines

Integrated pump/motor machines, where the pump and motor are matched and interconnected within the same housing by the manufacturer, have become popular. Such configurations act to simplify systems and provide high efficiencies when operating at or near their design point. However, careful attention should be paid to performance losses and mismatch that results from using these machines away from their design point, such as with a different head or flow rate.

It is feasible to integrate permanent magnet brushless motors with pumps to such an extent that the motor also serves as the impeller, within an hermetically-sealed, submersible unit (Divona et al., 2001).

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Magnetic circuit

Imperfections in the magnetic circuit will always contribute losses although, if properly designed, these should be quite small. Low efficiency motors generally have losses in the magnetic circuit as the dominant cause of poor performance. Permanent magnet motors generally need to be carefully designed to ensure operation at the maximum of their BH product.

Motor heating

Motor heating can be a serious loss mechanism and can lead to lower reliabilities and shortened lifetimes. As the temperature increases, the resistance of windings increases, thereby increasing the resistive losses, which in turn act to further increase the motor temperature. It is therefore necessary to keep motors cool, both to achieve high performance and to increase reliability and lifetimes.

Submersible motors are eas...

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Motor losses Support bearings

Friction in the support bearings has a load-dependent term and a load-independent term, which jointly contribute significantly to the losses in high efficiency motors.

To achieve high reliabilities and low maintenance, lubrication with grease is essential. Although expensive, a high quality lubrication grease that has a temperature – independent kinematic viscosity is recommended, and has been demonstrated to reduce frictional losses by up to 60%.

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AC motors

A wide range of AC motors are commercially available, owing to the wide range of applications for which they have been used for many years. However, with most of these, the emphasis has been on low cost rather than operating efficiency. In particular, small motors of about 1 kW or less suffer from very low efficiencies, making them not well suited to photovoltaic-powered systems. In addition, they require costly inverters at their inputs, which can exacerbate reliability problems. Furthermore, to provide high starting currents, additional power conditioning circuitry is generally required. In general, however, AC motors are very reliable and relatively inexpensive, being typically half the cost of an equivalent size DC motor.

The two basic types of AC motor available are asynchronous induc...

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DC motors

Fig. 11.13 shows the cross-section of a two-pole DC machine. Crosses represent current flowing into the page and dots represent current flowing out of the page.

There are four basic types of DC motors:

1. The series DC motor, as represented by Fig. 11.14a, has the field windings connected in series with the armature windings. This configuration has a severe limitation when being driven directly by photovoltaic panels because a
drop in motor current accompanying a fall in light intensity on the solar panels affects both the field and armature windings. On the other hand, they tend to be able to pump more water than shunt DC motors on sunny days (Cultura, 2004).

2. The permanent magnet DC motor, as illustrated in Fig. 11.14b, overcomes the above limitations of the series connected DC motor...

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