Environmental Engineering Reference
In-Depth Information
proximately 100 cm
2
. However, particularly for grid-connected installations also
large-surface solar modules of capacities of up to 300 W are available. Cell sur-
faces of 225 cm
2
on a single wafer are offered on the market today.
Parallel connection
Series connection
Combined parallel
and series
connection
6
6
6
4
4
4
2
2
2
+
-
+
-
0.6
0.6
1.2
1.8
0.6
1.2
1.8
1.2
1.8
Fig. 6.15
Change of the characteristic current-voltage curve when combining various
photovoltaic cells to a module exemplarily for cells of 2 A short-circuit current and 0.6 V
open-circuit voltage (according to various sources)
Voltage in V
Voltage in V
Voltage in V
If individual cells of an operating module are shaded or if their original power
rating is impaired by defects, they no longer function as generators within the
interconnected cells but act as a load. According to the type of connection, they
will either be operated in reverse-biasing (wrong voltage direction) or at voltages
exceeding their open-circuit voltage (wrong current direction). Under unfavour-
able conditions they may heat up stronger than the neighbouring cells ("hot spot"
effect).
Independent of the described detrimental effects, also partial shading of serial
connected cells causes considerable losses. As a first order approximation, the
current in a string of serial connected cells is determined by the current of its
weakest cell. Thus losses due to partial shading are considerably higher compared
to the ratio of the shaded area to the total area. For strings or cells connected in
parallel, losses are only proportional to the shaded area.
Particular consideration of the shading effects for building-integrated photo-
voltaic cells is necessary, as, for instance, façade elements, frames, and windows
very often cause partial shading. This is why designs need to be carefully analysed
to prevent excessive losses.
Available protection measures for a connection of several modules are outlined
in Fig. 6.16; in principle, they also apply to the connection of several cells com-
posing one module. Bypass diodes (freewheeling diodes), arranged in parallel to
the cell strings, prevent overheating of possibly shaded solar cells. Incorporated
blocking diodes prevent equalising current over cell strings in the wrong direc-
tion, if the voltages are lower compared to those of the neighbouring strings due
to partial shading or property changes (i.e. damaged cells).
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