Solar PV Theory.pdf

12 v lead acid battery i sc e oc a discharged battery

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12 V lead acid battery I SC E OC (A) Discharged battery (B) Fully charged battery (C ) Overcharged battery
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Storing Energy from Solar Panels into Batteries Discussion 2 4 Solar Power Figure 33 . Typical E-I curve of a 36-cell PV module used to charge 12 V lead-acid batteries. Battery connected to a PV module in the dark So far, we have discussed battery charging with a PV module when it is illuminated. But what happens at night when the PV module is in the dark? Representing a 36-cell PV module using the simplified equivalent diagram of a PV cell shown in Figure 2 3 suggests that no current (or a very low current) would flow in the system as the current source in each PV cell no longer produces current and the battery open-circuit voltage E OC is only able to apply a weak forward bias to the diode in each PV cell. For instance, when the battery open-circuit voltage is 12.4 V, the forward-bias voltage across each PV cell is only 0.34 V as shown in Figure 34 . Actually, however, this is not exactly what happens as some current does flow from the battery to the PV module, thereby discharging the battery. A more complete equivalent diagram of the PV cell is thus required to explain what happens. 0 5 10 15 20 E (V) 25 50 75 100 I (mA) PV cell equivalent diagram R P = 79.4 R P calculation Δ I = 3.5 mA @ 10.0 V R P for PV module (36 cells) = E / Δ I = 2857 R P for 1 cell = 2857 ÷ 36 = 79.4 R S calculation Δ E = 1.5 V @ 70.0 mA R S for PV module (36 cells) = Δ E / I = 21.4 R S for 1 cell = 21.4 ÷ 36 = 0.6 E = 0.98 V I = 103 mA E = 14.5 V I = 98 mA E = 15.6 V I = 95 mA E = 16.4 V I = 80 mA E = 17.7 V I = 20 mA 0 R S = 0.6 V
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Storing Energy from Solar Panels into Batteries Discussion Solar Power 2 5 Figure 34 . Lead-acid battery connected to a PV module (in the dark) represented using the simplified equivalent diagram of a PV cell. Equivalent diagram of a PV cell Comparing the simplified equivalent diagram of a PV cell shown in Figure 2 3 with the actual equivalent diagram of a PV cell shown in Figure 35 reveals that the latter includes a resistor R P in parallel and a resistor R S in series with the current source and diode. Figure 35 . Actual equivalent diagram of a PV cell. The value of resistors R P and R S can be evaluated from the E-I curve of a PV cell as shown in Figure 36 . The presence of the parallel resistor explains why the PV cell current decreases a little with voltage in the constant-current region of the E-I curve – the lower the value of R P , the higher the decrease of cell current with R S R P PV module at night 0.34 V 0.34 V 0.34 V 36 PV cells Battery open-circuit voltage = 12.4 V 12 V lead-acid battery
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Storing Energy from Solar Panels into Batteries Discussion 2 6 Solar Power voltage. The presence of the series resistor explains why the PV cell voltage decreases a little with current in the constant-voltage region of the E-I curve – the higher the value of R S , the higher the decrease of cell voltage with current. Figure 36 . Evaluation of resistors R P and R S from the characteristic E-I curve of a PV cell.
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