A PV Panel consists of several photovoltaic cells in series and parallel connections. Series connections are responsible for increasing the voltage of the module whereas the parallel connection is responsible for increasing the current in the array. Typically, Solar Panel has converting efficiency of 8 to 15 % of the Incident Solar Irradiance into electrical energy.
I-V characteristics of a Solar panel is as shown in the fig 1. below.
At the open circuit voltage VOC and the short circuit current ISC, the power generated is zero.
The Maximum power (Pm) is generated at a point where the product Vm*Im is maximum and this point is called as Maximum Power Point. Maximum power point tracking technique is used to improve the efficiency of the solar panel.
There are different techniques used to track the maximum power point, few of the most popular techniques are
1) Perturb and Observe (hill climbing method)
2) Incremental Conductance method
3) Fractional short circuit current
4) Fractional open circuit voltage
5) Neural Network
6) DC-Link Capacitor Droop Control Technique
Perturb & Observe Method
The P&O algorithm is also called “hill-climbing”. Hill-climbing involves a perturbation on the duty cycle of the power converter and P&O a perturbation in the operating voltage of the DC link between the PV array and the power converter. In this method, the sign of the last perturbation and the sign of the last increment in the power are used to decide what the next perturbation should be. As shown in fig.2 on the left of the MPP incrementing the voltage increases the power whereas on the right decrementing the voltage increases the power. If there is an increment in the power, the perturbation should be kept in the same direction and if the power decreases, then the next perturbation should be in the opposite direction. Based on these facts, the algorithm is implemented. The process is repeated until the MPP is reached. This technique holds good when irradiance is constant and it can track MPP in the wrong direction for dynamic changes in atmospheric condition.
Fig.2 PV panel characteristic with MPP operating points
Incremental Conductance Method
In this method, the MPP can be tracked by comparing the instantaneous conductance I/V to incremental conductance dI/dV
At MPP the slope of the PV curve is 0 (zero).
dI/dVMPP = – I/V at MPP
dI/dVMPP > – I/V at left of MPP
dI/dVMPP < – I/V at right of MPP
Here we are sensing both the voltage and current simultaneously. Hence the error due to change in irradiance is eliminated. However, the complexity and the cost of implementation increases.
The techniques so far discussed are the most popular one and presently in use. The selection adapting any of these differ in many aspects such as required sensors, complexity, cost, range of effectiveness, convergence speed, correct tracking when irradiation and/or temperature change, hardware needed for the implementation or popularity, among others.