Technical Application Papers No.10 Photovoltaic plants - ABB

2.8.2 Temperature of the modules
Contrary to the previous case, when the temperature of
the modules increases, the produced current remains
practically unchanged, whereas the voltage decreases
and with it there is a reduction in the performances of
the panels in terms of produced electric power (Figure
2.12).
Figure 2.12
3
2
1
0
The variation in the no-load voltage V oc of a PV module
with respect to the standard conditions V oc,stc , as a
function of the operating temperature of the cells T cell ,
is expressed by the following formula (Guidelines CEI
82-25, II ed.):
where:
β is the variation coefficient of the voltage according to
temperature and depends on the typology of PV module
(usually -2.2 mV/°C/cell for crystalline silicon modules and
about -1.5 ÷ -1.8 mV/°C/cell for thin film modules);
N s is the number of cells in series in the module.
Therefore, to avoid an excessive reduction in the performances,
it is opportune to keep under control the service
temperature trying to give the panels good ventilation
to limit the temperature variation on them. In this way
it is possible to reduce the loss of energy owing to the
temperature (in comparison with the temperature of 25°C
under standard conditions) to a value around 7% 7 .
7 The reduction in efficiency when the temperature increases can be estimated as 0.4
to 0.6 for each °C.
E = 1000 W/m 2
0.2 0.4 0.6
20
40
60
80
100
Voltage
V oc (T) = V oc,stc - N S . β . (25-T cel ) [2.13]
2.8.3 Shading
Taking into consideration the area occupied by the modules
of a PV plant, part of them (one or more cells) may
be shaded by trees, fallen leaves, chimneys, clouds or
by PV panels installed nearby.
In case of shading, a PV cell consisting in a junction
P-N stops producing energy and becomes a passive
load. This cell behaves as a diode which blocks the
current produced by the other cells connected in series
thus jeopardizing the whole production of the module.
Moreover the diode is subject to the voltage of the other
cells which may cause the perforation of the junction
due to localized overheating (hot spot) and damages to
the module.
In order to avoid that one or more shaded cells thwart the
production of a whole string, some diodes which by-pass
the shaded or damaged part of module are inserted at
the module level. Thus the functioning of the module is
guaranteed even if with reduced efficiency. In theory it
would be necessary to insert a by-pass diode in parallel
to each single cell, but this would be too onerous for the
ratio costs/benefits. Therefore 2÷4 by-pass diodes are
usually installed for each module (Figure 2.13).
Figure 2.13
+
Solar radiation
By-pass diode
Shadow
I I
Photovoltaic plants
–
25
2 Energy production