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1.3 Energy Production by Equivalent Cell Circuitry
ОглавлениеThe circuit of a solar cell as a current generator can be represented as shown in Figure 1.19. If the current generated by absorption of photons is IS, diode current is ID and leakage current is IL, then, the current I obtained at the output terminals is given as follows [88-90],
Figure 1.19 Schematic shows the electrical circuit of solar cell with current contribution due to PV effect (IS), diode current (ID), leakage current (IL), open-circuit voltage (VOC) when no current flows across the load, l eakage conductance (G) and output current (I). It also shows a diode through which ID flows, and series RS offered due to resistance offered to movement of charge carriers.
In the circuit diagram shown below, a series resistance (RS) has been shown which depends on the thickness of the junction resisting the flow of current across the barrier, types of impurities and contact resistance. The leakage conductance (G) directly relates to the leakage current in series under normal conditions of operations. The conversion efficiency of a solar cell is least affected in a variation of value of G, while a small variation of RS has a pronounced effect on it. In case of open circuit when no current flows across the load, the voltage is given as follows,
The diode current ID is expressed by formula for the DC flow and is given as,
where, q is the charge on an electron, T is temperature, k is Boltzmann constant, A is the identity factor of diode depending on recombination effects inside the diode and ISS is the saturation current of diode. As a result, current generated across the load is given as follows from equations (1.29) & (1.30) in equation (1.28),
(1.31)
The typical current-voltage characteristics of a solar cell module are shown in Figure 1.20. The generated current is the highest (ISC) under short-circuit conditions, whereas it is the least in case of open-circuit and voltage is the maximum (VOC). The produced electric power in both these conditions is zero. Under all the conditions other than these two conditions, the produced power increases as a function of the voltage. Hence, different parameters involved in the characterization of a solar cell module are the short-circuit current (ISC), open-circuit voltage (VOC), current and voltages (Im and Vm) produced at the maximum power. The filling factor (FF) is the ratio of the maximum power (Pm) to the product of the open-circuit voltage (VOC) multiplied by the short-circuit current (ISC).
Figure 1.20 Schematic shows the electrical circuit of solar cell with current contribution due to PV effect (IS), diode current (ID), leakage current (IL), open-circuit voltage (VOC) when no current flows across the load, leakage conductance (G) and output current (I). It also shows a diode through which ID flows, and series RS offered due to resistance offered to movement of charge carriers.
Solar panels will display the maximum efficiency when angle of incidence of sunlight is always perpendicular in a direction to the surface of the panel. The panels must be oriented in a direction specified by azimuthal angle (γ) which is the deviation in reference to the optimal direction to the north in the Southern Hemisphere or with respect to optimum direction in south in the Southern Hemisphere [88-90]. Azimuth angle can be defined as the angular distance determined from north to east along the horizontal line which is the point of intersection of sphere with the axis.
