LOAD CHARACTERISTICS AND DIRECT-COUPLED SYSTEMS

A PV cell will operate at the voltage and current at which its characteristics match that of the load to which it is connected. Examples of load characteristics are a resistive load, a battery, a series motor, and the power grid. The exiunples in this section are of direct – connected systems, in the following section, note is made of power conditioning equip­ment (maximum power point trackers) that can be used where there is serious mismatch between generator and load characteristics.

Figure 23.4.1 shows the characteristics of three resistive loads and three I-V curves for a solar cell module of 65 W rated capacity. The I-V curves are typical of those for three times of day for modules with fixed orientation. Hie dunce resistive loads shown each intersect the module I-V curves at the maximum power points. At any time, the combination of the module and the load will operate at the intersection of the charac­teristic curves of the two components. The optimum load resistance, that is. the slope of the resistive load curve, will vary throughout n day. A fixed resistive load is thus not an optimum load for a PV generator. Khouzam ct al. (1991) studied the problem of deter­mining the optimum fixed resistance for clear days. An electronic load that effectively changes the load resistance with solar radiation to follow the maximum power point is called a maximum power point tracker, or MPPT (see Section 23-5).

The charging and disehaigmg characteristics of a single cell of ц typical 250-Ah leud-acid storage battery are shown in Figure 23.4.2 (see Section 8.10). The internal

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resistance of a battery depends on the rate of charging, and the battery characteristic thus has a finite slope. Also, the voltage required to charge a battery (and the voltage produced by a battery on discharge) is a function of the state of charge F so that as the state of charge nears 100% the voltage rises rapidly with charging current. As shown tn Figure

23.4.3

Подпись: 2.2 2.3 2.4 Voltage (V) Figure 23.4.2 Chaining and discharging characteristics оГ a lead-acid storage battery. Подпись: &

battery-charging characteristics can more nearly match the locus of maximum

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Voltage (V)

Figure 23.4.3 Current-voltage characteristics for a PV array at three radiation levels and a storage battery.

power points than that of a resistive load. In this figure tire ceil temperature is 259C. As the celt temperature increases, the curve of the PV maximum power points will shift to the left, providing an even belter match (see Figure 23.2.5).

Photovoltaic generators may be used to drive machines such as electric pumps, re­frigerators, and other devices. Direct-current motors are of several types, each having different characteristics. The speed at which motors ran is a function of voltage and torque, which in tum depend on the characteristics of the load on the motor. Thus an analysis of the operation of PV generators supplying energy to motors must be a simul­taneous solution of the characteristics of all of the components. Townsend (1989) and Eckstein (1990) have compared series, shunt, and permanent-magnet DC motors, includ­ing as loads ventilating fans, centrifugal pumps, and positive-displacement pumps, figure

23.4.4 shows the characteristic of two different DC motors (connected to a water-pumping system) and PV generator characteristics at three radiation levels. The system with the separately excited motor begins delivering water at a lower incident radiation than the system with the series motor. Both systems operate far from the maximum power point at both low and high radiation levels. Thus this load is not well matched to PV cell characteristics, direct connection is not an efficient arrangement, and power-conditioning equipment would probably be justified.5

Electrical loads that are combinations of components may be used. A battery may be wired so as to charge when there is current from the PV generator that exceeds load current and discharge when the generator does not produce enough. A common circuit to accomplish this i$ shown in Figure 23.4.5. In such a parallel circuit the voltage across the PV panel, the battery, and the load Is the same. At operating conditions the PV array

Section 23.5.

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figure 23.4,4 Characteristics of an army of ceil modules <n five radiation levels coupktl to л water pumping system using two different DC motors. Adapted from Townsend (19S91.

 

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current is the sum of the toad current mid the battery cmresU us shown in Figure 23.4.6. The operating voltage in this situation is found by moving the operating voltage line shown in the figure until /WilJ. – ftalW}. + /Ьчй1,

The effective use of PV generators in many applications will depend on system arrangements more complex than those of the direct-coupled arrangements inferred here. Devices for controlling voltage (with some sacrifice of power) can be used for tracking maximum power points. Some of these control devices and strategies are noted in the following section.

Updated: August 20, 2015 — 2:40 pm