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A. Virtuani
connectors, etc.). Losses due to mismatched cells (i.e. cells with different electrical properties), can be, as well, a critical loss mechanism, but can be minimized by
proper sorting of the cells.
On the other hand, the integration of cells into modules leads as well to optical
gains (partly offsetting CTM losses), mostly thanks to an improved optical coupling
(glass/encapsulant/cell) that reduces the relatively high reflectivity of the cell at the
air/cell interface. Additional optical gains can be related to internal reflectance and
coupling promoted by the reflection of the light over the metallization and the rear
cover, particularly if a white backsheet is used [2, 3]. If the angle of incidence of
the light is not normal part of the light reflected by the cells, the interconnects or the
backsheet in fact will undergo multiple reflections in the encapulant/glass structure
and eventually impinge over the active surface of an adjacent cell.
9.1.4 By-Pass Diodes
The electrical layout of a solar module generally contains by-pass diodes, which are
used to minimize the effects of partial shading on cells. Such partial shading can
seriously affect the output of a module (small shadows, big losses!), and can even
cause damages to the materials composing the module.
Partial shading can easily occur due to the presence of trees, leaves or bird droppings. It can be particularly severe in the built environment, where it can originate
from the presence of chimneys, antennas, neighbouring buildings, etc.
If a single cell (or a group of cells) is shaded, it will become reverse-biased, not
generating power any longer, but becoming a power sink that dissipates the power
generated by the other cells of the string. This will result for the shaded cell (or the
shaded portion of a cell) in local overheating (in so-called “hot-spots”). The same
effect will take place, if one of the cells is partially or fully damaged. Temperatures
≥100 °C, depending on weather conditions and the number of cells in a string, can be
reached and, in turn, lead to destructive effects (e.g. encapsulant browning, backsheet
and glass cracking, loss of adhesion, etc.), which in the long run can compromise the
performance and electrical insulation of the module, as will be described in the next
chapter. Additionally, a large reverse bias applied to a solar cell (which will depend
on the number of cells connected to the shaded cell in the string) can lead to junction
breakdown completely destroying the cell.
The effects of a hot-spot can be prevented through the use of a by-pass diode
connected in parallel with an opposite polarity to that of the solar cell, as Fig. 9.5
illustrates. Under normal operating conditions, each solar cell is forward biased. The
by-pass diode will then be reverse biased and carries no current. When a solar cell
becomes reverse biased, due to shading or cell damage, the by-pass diode will then
allow the current from the illuminated solar cells to flow in the external circuit, bypassing the shaded/damaged cell and preventing hot-spot formation. The maximum
reverse bias across the shaded/damaged cell is limited to a single diode drop (i.e.
~−0.6 V), irrespective of the number of cells connected in series.
A. Virtuani
connectors, etc.). Losses due to mismatched cells (i.e. cells with different electrical properties), can be, as well, a critical loss mechanism, but can be minimized by
proper sorting of the cells.
On the other hand, the integration of cells into modules leads as well to optical
gains (partly offsetting CTM losses), mostly thanks to an improved optical coupling
(glass/encapsulant/cell) that reduces the relatively high reflectivity of the cell at the
air/cell interface. Additional optical gains can be related to internal reflectance and
coupling promoted by the reflection of the light over the metallization and the rear
cover, particularly if a white backsheet is used [2, 3]. If the angle of incidence of
the light is not normal part of the light reflected by the cells, the interconnects or the
backsheet in fact will undergo multiple reflections in the encapulant/glass structure
and eventually impinge over the active surface of an adjacent cell.
9.1.4 By-Pass Diodes
The electrical layout of a solar module generally contains by-pass diodes, which are
used to minimize the effects of partial shading on cells. Such partial shading can
seriously affect the output of a module (small shadows, big losses!), and can even
cause damages to the materials composing the module.
Partial shading can easily occur due to the presence of trees, leaves or bird droppings. It can be particularly severe in the built environment, where it can originate
from the presence of chimneys, antennas, neighbouring buildings, etc.
If a single cell (or a group of cells) is shaded, it will become reverse-biased, not
generating power any longer, but becoming a power sink that dissipates the power
generated by the other cells of the string. This will result for the shaded cell (or the
shaded portion of a cell) in local overheating (in so-called “hot-spots”). The same
effect will take place, if one of the cells is partially or fully damaged. Temperatures
≥100 °C, depending on weather conditions and the number of cells in a string, can be
reached and, in turn, lead to destructive effects (e.g. encapsulant browning, backsheet
and glass cracking, loss of adhesion, etc.), which in the long run can compromise the
performance and electrical insulation of the module, as will be described in the next
chapter. Additionally, a large reverse bias applied to a solar cell (which will depend
on the number of cells connected to the shaded cell in the string) can lead to junction
breakdown completely destroying the cell.
The effects of a hot-spot can be prevented through the use of a by-pass diode
connected in parallel with an opposite polarity to that of the solar cell, as Fig. 9.5
illustrates. Under normal operating conditions, each solar cell is forward biased. The
by-pass diode will then be reverse biased and carries no current. When a solar cell
becomes reverse biased, due to shading or cell damage, the by-pass diode will then
allow the current from the illuminated solar cells to flow in the external circuit, bypassing the shaded/damaged cell and preventing hot-spot formation. The maximum
reverse bias across the shaded/damaged cell is limited to a single diode drop (i.e.
~−0.6 V), irrespective of the number of cells connected in series.