Environmental Engineering Reference
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Figure 4.15 Extended Equivalent Circuit of a Solar Cell (One-diode Model)
diode factor m of an ideal diode is equal to 1; however, a diode factor between
1 and 5 allows a better description of the solar cell characteristics.
A current source connected in parallel to the diode completes the simple
equivalent circuit of an irradiated solar cell. The current source generates the
photocurrent I ph , which depends on the irradiance E and the coefficient c 0 :
(4.33)
Kirchhoff's first law provides the current-voltage characteristics of the simple
solar cell equivalent circuit illustrated in Figures 4.13, and Figure 4.14 shows
the characteristic curves at different irradiances):
(4.34)
Extended equivalent circuit (one-diode model)
The simple equivalent circuit is sufficient for most applications. The differences
between calculated and measured characteristics of real solar cells are only a
few per cent. However, only extended equivalent circuits describe the electrical
solar cell behaviour exactly, especially when a wide range of operating
conditions is to be investigated. Charge carriers in a realistic solar cell
experience a voltage drop on their way through the semiconductor junction to
the external contacts. A series resistance R S expresses this voltage drop. An
additional parallel resistance R P describes the leakage currents at the cell edges.
Figure 4.15 shows the modified equivalent circuit including both resistances.
The series resistance R S
of real cells is in the range of several milliohms
(m
. Figures 4.16 and
4.17 illustrate the influence of both resistances in terms of the I-V
characteristics.
), the parallel resistance R P is usually higher than 10
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