Bypass diodes are generally employed to avoid hotspots. As
cells go into reverse bias, the substring can be effectively
shut down to avoid overheating. However, if cells experience
routine shade, soiling or cell cracks, then the cells are
consistently pushed into reverse bias which can accelerate
diode failure. While IEC 61215 testing does address diode
performance, the relatively short time frame of testing limits
insights into the long-term viability of these components.
Many solar panels are generally expected to last decades in
the field, and that can be a lot to ask of a diode constantly
under stress. A field study conducted in 2012, found that for
panels over 10 years old, approximately 20% of the diodes
had completely failed.
8
When the cells of the SunPower
Performance panel are shaded
or cracked, the current will take
the path of least resistance and
be shared by the neighbouring
cells (figure 11). The current in
any one string is a portion of the
operating current of a full cell,
greatly reducing the threshold
for power dissipation (I * V). As a
further mitigating measure, the
structure of the Performance
panel puts the affected cell in
direct physical contact to its
neighbouring cells which helps
to dissipate hotspot heat. With a
lower hotspot temperature in
the cell, energy loss is
minimised. In a conventional panel, cells are only connected
by ribbons, which have little heat transfer capability. Under
severe cell cracking or worst-case shading conditions of
conventional cells, SunPower Performance panels operate at
40-50 °C lower temperature (figure 12).
9
While SunPower Performance panels are passively safe,
bypass diodes are embedded in their junction boxes to
increase energy yield under partial shading should they be
required.
Under severe cell cracking or worst-case
shading conditions, Performance panels
operate at 40-50°C lower temperature.
9
Figure 12: SunPower Performance panels reduce the risk of
temperature-related failures through crack mitigation and
unique circuitry.
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