Page 144 - Advances in Renewable Energies and Power Technologies

1. Introduction 117

PV Current Battery Load
Sense
PV Cells

PV Voltage
Sense
MPPT Control
FIGURE 4.2
PV energy system with maximum power point tracking (MPPT) system.

in Esram and Chapman and Salas et al. [1,2], and it will be introduced and discussed
in the following sections.

1.1 DIRECT-COUPLED METHOD
In the direct-coupled method [3e5], the PV array is connected directly to the loads
without power modifier. To match the MPPs of the solar array as closely as possible,
it is important to choose the solar array characteristics according to the characteris-
tics of the load. The direct-coupled method cannot automatically track the MPPs of
the solar array when the insulation, temperature, or, load changed.

1.2 CONSTANT-VOLTAGE MPPT
It is clear from the PeVcurve of Fig. 4.1 that the ratio of the array’s maximum
power voltage, V mp , to its open-circuit voltage, V oc , is approximately constant.
So, the PV array can be forced to work as a ratio of its open-circuit voltage. The
literature reports success with 73%e80% from V oc [6e9]. The implementation
of this technique can be obtained by temporarily isolating the solar cells from
the MPPT, and a V oc measurement is taken. Next, the MPPT calculates the correct
operating point and adjusts the array’s voltage until the calculated V mp is reached.
This technique is very simple to be implemented butithas twomajor problems.
The first one is the difficulty to choose the optimal value of the constantdthe ratio
between V mp and V oc dthe other one is the momentary interruption of PV power to
determine the open-circuit voltage. The later difficulty can be overcome by using
pilot cells.