Page 161 - A Comprehensive Guide to Solar Energy Systems

162 A CoMPrehensIVe GuIde To soLAr enerGy sysTeMs

Considering solar cell as an energy source, the characteristics are used in the form
shown in Fig. 8.7. The output power depends on the load, as indicated in Fig. 8.8.
The most important points on the I–V characteristic are
• open-circuit voltage V oC ,
• short-circuit current I sC ,
• maximum output power characterized by the voltage V mp and current I mp . For
characteristic evaluation, the fill factor (FF) of the solar cell has been introduced,

I
FF = V mp mp . (8.19)
I
FF=VmpImpVoCIsC. V OC SC
The cell efficiency η is the next very important parameter that is defined as the ratio of
the maximum output power (the photovoltaically generated electrical output of the cell in
a case of the optimum load) to the radiation power falling on the cell area:
I
I FF
η = V mp mp = V OC SC . (8.20)
η=VmpImpPin=VoCIsCFFPin. P in P in
All these parameters depend on both input radiation power and spectral distribution,
which must be precisely defined. usually, parameters P m , V oC , I sC , FF, and η are given for
−2
so-called standard testing conditions (sTCs), that is, the radiation power 1000 W m of
spectrum AM 1.5, and cell temperature 25°C. (The AM 1.5 spectrum is defined in the Inter-
national standard IeC 60904/3 and it is based on the spectral distribution of solar irradi-
ance tilted at 42 degree to the horizontal.)
Important features of the I–V characteristic can be derived from eq. (8.18). The short-
circuit current I sC can be obtained from eq. (8.18) by putting V = 0. For a very low series
resistance R s , the short-circuit current I sC ≈ A ill J PV ; it depends only very weakly on tempera-
ture (a very slight increase due to decrease of the recombination rate). I sC decreases with
increasing series resistance R s . In an ideal case, R s = 0 and the FF ≈ 0.89. With increasing R s ,
both maximum power I mp V mp and FF decrease and, consequently, efficiency decreases. The
influence of series resistance R s on cell characteristics is schematically shown in Fig. 8.9A.

FIGURE 8.7 I–V characteristic of a PV cell.