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Materials Matter
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MITIGATING STRATEGIES FOR HOT SPOTS IN CRYSTALLINE SILICON SOLAR PANELS
Hot Spots
Hot spots are areas of elevated temperature affecting only part of the
solar panel. They are the result of a localized decrease in efficiency,
which results in lower power output and an acceleration of the
materials degradation in the affected area (Fig. 1). Solar panels
generate significant power and hot spots can occur when some of that
power is dissipated in a localized area. Hot spots are rarely stable and
will usually intensify until total failure of the panel performance in
terms of electricity production and/or safety.
In a crystalline silicon solar panel, the silicon cells are typically
connected in series, so that each 6-inch cell produces a current of
about 8 amps (A), and each voltage at about 0.6 volts (V) is added
through the string to build up the power (Power = Voltage*Current).
This leads to panels with a direct electrical output of about 8A and
typically 30 to 35V.
Hot spots in crystalline silicon solar panels represent an additional
stress for the materials used in module construction. This can be
temporary or permanent, depending on the type and cause of the
hot spot. The selection of materials with higher thermal stability
can help reduce the risk of performance and safety issues associated
with hot spots.
Module Design and Mitigating
Strategies
The causes of hot spots and susceptibility of solar panels have been
investigated in the past and test standards have been developed to
address this issue (Ref. 2).
Figure 2. Typical electrical connection of a crystalline silicon panel with
bypass diodes for hot spot protection
• If a cell cannot generate the peak current produced around it, it
will act as a power dissipation device (like a resistor). This is called
reverse bias.
• A resistor dissipates power by heating up, so a cell in reverse bias
will exhibit a hot spot.
Figure 1. Hot spots are evident in this infrared picture showing elevated
temperature where the cells are damaged along the frame
• Cells may not be able to produce the current because of shadows
(either cast or from soiling), or because of cell damage or inherent
cell lower performance (poor cell matching).
• Bypass diodes segregate cells in groups of 20 cells. This is supposed
to limit the reverse voltage across the cells to less than 10V.
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Materials Matter
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MITIGATING STRATEGIES FOR HOT SPOTS IN CRYSTALLINE SILICON SOLAR PANELS
The Causes of Hot Spots
There are multiple causes of hot spots, and they can be functional or
operational. The functional reasons can be divided into two areas.
• Cell mismatch occurs when cells of varying current production are
connected in series. This condition is the result of wide bins in the
cell sorting process. In general, this should only mildly affect the
temperature of the underperforming cells.
• Cell damage can occur during the production process because the
silicon cell will be subjected to many stresses in lamination and
handling. The cell is about 180μm thick and is very brittle.
The cell will be transported, soldered, handled as part of a string
and then laminated. Each stage subjects the cells to mechanical
and/or thermal stresses. Thereafter, module transport and
installation can generate further stresses on the cells which can
cause them to break. In particular, it was highlighted by the
Institute for Solar Energy Research Hamelin, Germany (ISFH)
that transportation of the panels parallel to the ground will
generate particular damage to the cells (Ref. 3).
The operational reasons for hot spots are related to the solar park
design and operation and can include:
• Situations where an engineering, procurement and construction
(EPC) company may want to accept shading conditions in
the winter (as it may only represent 10 percent of the annual
production in specific areas) to increase electricity production
in the summer. This means that the panel will suffer systematic
shading of the bottom row of cells every morning and evening for
several months. This is where it is particularly important to install
panels with the strings of cells parallel to the ground (landscape
orientation) to allow the bypass diodes to work and enable the
generation of 10 percent electricity, even in winter.
+20 °C
Figure 3. Partial shading of a panel caused by a "rooftop feature" (e.g. chimney)
• For solar installations on roofs, the topography of the roof can
sometimes present a challenge. Again, the installation designers
may decide that it is acceptable for a cell to be completely shaded,
thereby putting a lot of stress on the panel. This condition
may not be sufficient for the bypass diode to operate, resulting
in an increase in temperature which will accelerate the panel
degradation. Similarly, tree or tall vegetation growth around
a solar installation should be controlled to avoid partial
shading conditions.
• Panels can be soiled due to dust, dirt and other contaminants
during their lifetime. It can be useful to design the parks to
mitigate these sources of soiling. The operations and maintenance
(O&M) company should also identify situations requiring
cleaning, which means regular visits to the park. The frequency
of cleaning will be heavily dependent on the climate conditions
and ground surrounding the park. For example, a solar park that is
cleaned in the south of Italy will usually gain 2 percent for only a
few days and stabilize thereafter for many months. In this case, the
cost of cleaning may not be worth the effort. A solar park in dusty
desert conditions, however, can lose up to 30 percent power output
if not cleaned regularly. Today the Middle East has developed dry
cleaning methods to de-dust the solar panels, due to the scarcity of
water in the region.
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Materials Matter
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MITIGATING STRATEGIES FOR HOT SPOTS IN CRYSTALLINE SILICON SOLAR PANELS
Impact of Hot Spots on the Electrical
Protection of the Panel
The consequences of hot spots can range from dramatic fires to
accelerated aging of the materials and in most cases, we will see the
more diffuse temperature increase leading to an accelerated aging of
the backsheet/encapsulation material set (Fig. 4).
A hot spot resulting from or leading to a short circuit between the
front and the back of the cell will result in very localized high
intensity heating. This type of hot spot can result in melting of
the backsheet and can lead to fires.
Deep Crack
(Polyester)
Hot Spots Lead to Bubbling
Micro Crack
(Polyester)
Bubbling Leads to Cracking
Delamination & Crack
(Polyester)
Deep Crack
(PVDF)
Delamination
(PVDF)
Figure 5. High intensity hot spots in PVDF-based backsheets
Figure 4. Hot spots will accelerate the aging of materials. Examples of panels
with 4 years of exposure in Spain made with polyester and polyvinylidene
fluoride (PVDF)-based backsheets show cracking and delamination
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Materials Matter
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MITIGATING STRATEGIES FOR HOT SPOTS IN CRYSTALLINE SILICON SOLAR PANELS
Mitigating the Impact of Hot Spots
While hot spots may not be directly caused by poor material selection
and may be more directly related to processing problems (from cell
manufacturing to panel installation and maintenance),
certain materials have been found to be more sensitive to hot
spot conditions. While the hot spot condition can be temporary
(such as for partial shading or soiling), it is important to select
materials which can withstand the temporary occurrence of hot spots.
There are two variables which are of interest in the backsheet to
mitigate the likelihood of permanent damage.
1. The softening of the backsheet inner layer may compromise the
adhesion of the insulation layer, thereby affecting the electrical
insulation of the panel.
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2. The smaller the thermal coefficient of expansion, the fewer stresses
there are at the edges of the hot spots. Large expansion coefficients
may increase the likelihood of backsheet delamination.
The backsheet is the electrically insulating component on the back
of the solar panel. It is usually made of several layers with a core of
polyester (PET). The polyester middle layer or core layer provides the
main electrical insulation function. The other layers fulfill protection
and/or adhesive functions. DuPont™ Tedlar® polyvinyl fluoride (PVF)
film, used in a tri-layer backsheet composed of Tedlar®/PET/Tedlar®
(referenced as TPT in figures 7 and 8) is well recognized to provide
the best combination of electrical insulation and durability against
moisture, UV and temperature degradation. DuPont™ Tedlar® PVF
film is also used in a corresponding single-sided structure (Tedlar®/
PET/Tie layer, known as TPE). The tie layer, which is in contact
with the ethylene vinyl acetate (EVA) encapsulant, is also called the
inner layer.
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In situations of high hot spot intensity caused by shunts or
electrical arcs, no material can be expected to withstand the very high
temperatures reached. Some cells may be more prone to
edge shunts. Arcs can occur within the panel when the soldering bus
wires are placed too close to one another inside the panel and the
water saturation of the EVA reaches equilibrium with the ambient
moisture levels. These situations can only be remedied at the cell
selection and the panel design level and should be addressed by the
panel manufacturer.
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Figure 7. Different backsheet structures can resist melting of the inner layer
and retain adhesion and electrical insulation for longer periods of time
Figure 6. Example of catastrophic hot spot failure
Figure 8. Thermal coefficient of expansion can help mitigate backsheet
delamination on diffuse hot spots
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Materials Matter
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MITIGATING STRATEGIES FOR HOT SPOTS IN CRYSTALLINE SILICON SOLAR PANELS
Dependence of Hot Spot Conditions
on Installation Type
DuPont™ Tedlar® PVF film-based backsheets used in a TPT
construction demonstrate the best thermal characteristics to mitigate
the impact of diffuse hot spots for three reasons:
Cells and panels can operate at elevated temperatures for reasons
other than a malfunction in the panel. Rooftop systems can create an
environment where the temperature of the panel can be more than
40 °C above the ambient air temperature (Fig. 9).
1. The highest melting temperature of the inner layer provides
stable mechanical properties of the electrical insulation at higher
temperatures.
The occurrence of a hot spot on a rooftop system can further
accelerate the degradation and affect the electrical safety protection.
An additional challenge is that the effect of a hot spot on the
backsheet will not be apparent in most cases (e.g. on pitched
rooftops). System owners may have a serious electrical safety
protection issue without knowing. In any case, higher operating
temperatures due to poor air ventilation of the panels require the use
of materials that have higher thermal stability.
+15 ˚C
Air
2. The lowest thermal expansion coefficient reduces the stresses at the
edges of the hot spot and the damage to the electrical insulation.
3. Ensures the highest combined UV, humidity and thermal resistance.
+15 ˚C
Ground
Flat Rooftop
+10 ˚C
BAPV
BIPV
Figure 9. Levels of temperature intensity for various types of solar installations
Source: Creep in Photovoltaic Panels: Examining the Stability of Polymeric
Materials and Components (2010) 35th IEEE Photovoltaic Specialists
Conference (PVSC ’10) Honolulu. David C. Miller, Michael Krempe,
Stephen Glick and Sarah Kurtz. Viridian Solar – January 2014 (5)
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Materials Matter
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MITIGATING STRATEGIES FOR HOT SPOTS IN CRYSTALLINE SILICON SOLAR PANELS
Conclusions
Hot spot conditions are never desirable in a solar park and are
usually associated with power loss (Ref. 5). Any power loss means
lower return on investment and a higher levelized cost of electricity
(LCOE) for the system owner. While some hot spots can be
remedied, others are the sign of irreversible damage to the panel.
Hot spots stress the panel materials and can eventually degrade them
to the extent that not only the power of the panel is substandard,
but more importantly, the safety of the panel and potentially the
installation is compromised. It is important to identify hot spot
situations and remedy the ones that can be prevented, by removing
partial shading situations or implementing a cleaning cycle.
If the panel materials have been selected carefully, mild hot spot
situations resulting from partial shading or soiling should not lead to
instantaneous damage of the panel. This will give the O&M team the
time they need to detect these by infrared (IR) thermal inspection,
for example, and to remedy the situation before significant damage
can occur.
For more information on
mitigating risks linked to
solar panel hot spots, please
contact your regional DuPont
Photovoltaic Solutions
representative.
EUROPE, MIDDLE EAST & AFRICA
Dr. Lucie Garreau-Iles
[email protected]
+41 (0)22 7176622
References:
(1). “PV module failures observed in the field – solder bond and bypass diodes failures,” Kazuhiko Kato, 25th EU PV Solar
Energy Conference 2010.
(2) “Hot spot susceptibility and testing of PV modules,” E. Molenbroek, D.W. Waddington, K.A. Emery. IEEE Conference,
1991, pp. 547-552.
(3). “Crack statistics of crystalline silicon photovoltaic modules,” M. Kontges, 26th EU PV Solar Energy Conference 2011.
(4). “Backsheet and module durability and performance and comparison of accelerated testing to long term fielded modules,”
W.J. Gambogi, 28th EU PV Solar Energy Conference 2013.
(5). “Reliability of IR-imaging of PV-plants under operating conditions,” C.I. Buerhop, Solar Energy Materials & Solar Cells
107(2012), 154-164.
(6). “Creep in Photovoltaic Modules: Examining the Stability of Polymeric Materials and Components” (2010) 35th IEEE
Photovoltaic Specialists Conference (PVSC ’10) Honolulu, David C. Miller, Michael Krempe, Stephen Glick and Sarah Kurtz,
Viridian Solar – January 2014.
NORTH AMERICA
Dr. Alexander Bradley
[email protected]
+1 302 999 4734
INDIA
Rahul Khatri
[email protected]
+91 8800677768
ASIA PACIFIC
Oakland Fu
[email protected]
+86 21 28921289
(7). “Evaluation of high-temperature exposure of photovoltaic modules,” S. Kurtz, K. Whitfield, G. TamizhMani, M. Koehl, D.
Miller, J. Joyce, J. Wohlgemuth, N. Bosco, M. Kempe and T. Zgonena, Progress in Photovoltaics: Research and Applications
Volume 19, Issue 8, pages 954–965, December 2011.
Copyright © 2015 DuPont. All rights reserved. The DuPont Oval Logo, DuPont , Tedlar and Materials Matter
are trademarks or registered trademarks of E.I. du Pont de Nemours and Company or its affiliates. (03/15)
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