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Chapter 4 Solar Energy

In this chapter, we will discuss a more accurate equivalent circuit for a PV cell. This circuit helps us to have better prediction and understanding of PV cell current-voltage characteristics as well as at different types of load connected to the PV panel, which is made up of an array of PV cells. Later in this chapter we will discuss the concept of maximum power point tracker (MPPT). | Chapter 4 Solar Energy II 4.1 Introduction In this chapter we will discuss a more accurate equivalent circuit for a PV cell. This circuit helps us to have better prediction and understanding of PV cell current-voltage characteristics as well as at different types of load connected to the PV panel which is made up of an array of PV cells. Later in this chapter we will discuss the concept of maximum power point tracker MPPT . 4.2 More Accurate Equivalent Circuit for a PV Cell If we connect two simple PV cell model as described in Fig. 3.7 in series and simulate the situation when one of the PV cell is shaded while the other is under strong sun rays the current source is essentially of zero current. However in the real PV panel with PV cells connected in series such as the BP SX10 with 36 PV cells connected in series measurable current is recorded when some of the cells are shaded. This implies that the simple PV cell is insufficient to model the PV panel I-V characteristics. We need some path to be added to the model to show the current flow within the PV cells. Fig. 4.1 shows a more accurate PV cell equivalent circuit using one parallel resistor Rp and one series resistor Rs to realize the extra current paths. Let us first derive a mathematical expression for this equivalent circuit. By KCL the current enters node 65 4. Solar Energy II 66 x equal the currents leave this node Isc Id Ip I 4.1 Substitute 3.23 into 4.1 and re-arrange the terms we obtain I Isc - Io eqVd kT - 1 -V 4.2 Rp And Vd and V are related by the following equation Vd I Rs V 4.3 We cannot find a solution of 4.2 with close-loop form. However we may first use an assumed value of Vd to evaluate I and use the value of I to calculate V from 4.3 . An example as follows shows how to work that out. Example Given the PV cell has short-circuit current I SC 4A and at 25 C its reverse saturation current is Io 6 X 10-1oA. Show the I-V curves under different resistance values a Rp TO Rs 0 b Rp TO Rs 0.06Q c Rp .