The Impact
of Typhoon on PV Systems (2/2)- Module Strength
3. Module
Strength
Module strength mainly depends on the
glass, frame, cell breakability and the interaction of all these factors.
3.1 Glass
Only few years ago, 4mm glass was
commonly used and it was much stronger than 3.2mm glass. The capability to
resist bending force is proportional to the cube of the thickness of the
component, so the 4mm glass is ‘stiffener’ than 3.2mm by (4/3.2)3 = 1.95
times. That means the bending resistance of 4.0 mm glass is almost twice as
much as 3.2 mm glass. Currently almost all module manufacturers use 3.2mm glass,
and even 1.6~2mm glass is already in mass production. Generally 3.2mm glass is
still sufficient to resist 5400Pa uniform pressure, but it is not strong enough
for local loads like people stepping on the module. (read more in ‘Can people step on the modules?’). Thinner glass with
1.6~2mm thickness is insufficient to resist 5400pa uniform load with
traditional design, the module need additional reinforcement in frame or fixing
measure, otherwise the huge deflection will result in serious cell cracks..
3.2 Frame
Frame looks sample, but there are a
lot of tricks inside. Below is a cross section of module frame, three important
factors are front width D, frame height H and bottom width W. When D is too
small, the glass is not held deep enough, so the glass is easy to be blown away,
especially under negative pressure. The photo below shows an example happened
in the recent typhoon, the glass is blown away but the frame is still there.
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Glass
is gone and the frame is still there
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The frame height H and width W
determine the capability of bending resistance. As mentioned earlier, the
bending resistance is proportional to the cubic of frame height, therefore H is
critical for module deflection under pressure. W is also helpful in the bending
resistance, but not so much as H.
There are more and more manufacturers
reduce the frame height to reduce the transportation cost, but if they do not
strengthen the module in other aspects, the conventional frame will be too weak
to resist high external pressure. The photo below shows broken and deflected
frames caused by typhoon.
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Frame
breakage and deformation
|
Another trick to reduce frame cost is
reducing the frame thickness. Frame thickness is not visible from module
appearance, so customers usually do not notice the frame thickness. This kind
of frame is easy to have dents caused by local impact, and the frame is easy to
be torn off because of stress concentration around the fixing hole (as shown in
the photos below).
 |
Torn
frame around fixing hole
|
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Torn
frame side wall
|
Another problem rarely noticed is the
frame corner. Most modules use L-key to fix frame corner, but it is also
getting thinner and thinner as cost reduces, and the consequence is the corner
strength is also getting lower. The photo below shows many open corners in the
modules damaged by typhoon.
 |
Module
corner opened
|
For modules with the same glass and
frame design, the deflection under pressure could be very similar, then the
cell ‘breakability’ plays an important role. The breakability mainly depends on
the wafer material, and also depends on the micro cracks and internal stress
induced by stringing process.
The photo below shows a system we
evaluated with EL inspection after typhoon, the upper left module was hit by
other object and the glass was completely broken. The rest of the modules had
perfect appearance, but the EL image showed serious cracks. Fortunately they
had EL inspection in the acceptance test, and the images showed no cracks at
beginning. That means the cracks are induced by the typhoon. The module was
certified with 5400Pa load resistance, and the wind load was not over 5400Pa,
why the cells break? Sometimes the soldering parameter is not appropriate, or
the wafer has more defects, the cells are prone to have cracks. The module
suppliers should prevent this kind of defect with internal control, like
in-line EL inspection and cell peel test.
 |
System
EL inspection after typhoon
|
As a summary, the strength of each
part of the PV construction is equally important. I have not seen any structure
supplier has a product specially designed to prevent typhoon issues, but I did saw
one module supplier has ‘Typhoon-resistant’ solar module in PV Taiwan Exhibition.
After checking the design of the Typhoon-resistant module, and with WINAICO’s consent,
I may comment on this product.
There are three main features in this
prodct: 1. HeatCap cell with high breakage resistance. 2. Outer corner key to
increase the corner strength. 3. Higher frame strength design.
HeatCap cells have special material
coated on the rear side of the cells to resist breakage. WINAICO provided the
test result from ITRI (Industrial Technology Research Institute) PV Lab, and
the result shows it outperforms traditional modules in mechanical load test
with 4000Pa (figure below). The coating successfully stopped the cracks from
rapid growth under large module deformation. It is specially helpful for mono
cells because the cracks in mono cells grow much faster than those in multi
cells.
 |
ML
test result from ITRI (provided by WINAICO)
|
The outer corner key is to prevent the
corner problem mentioned earlier. The figure below shows the corner key in a
Typhoon-resistant module. Another figure shows the comparison of inner corner
key and outer corner key. Normal inner corner key is completely inserted in the
frame, the width is about 2cm. The outer corner is thick and wide at the
corner, which means the width is 4cm at the corner, so the strength is surely
higher than normal corner key.
 |
Typhoon-resistant
Solar Module (provided by WINAICO)
|
 |
Comparison
of inner corner key (left) and outer corner key (right) (Provided by WINAICO)
|
Regarding the frame strength, the
bottom width W is added to 50mm to increase the bending resistance. These three
features all together really make the PV module capable of resisting strong
wind in Typhoon.
A small negative is that the cost is a
bit higher. From my understanding, the module price is few cents/Wp higher than
normal modules. But it is reasonable, high quality always come at a price. When
you ask for high strength products, you have to pay more, but the customers
also get more. You got to make the choice by yourself.
Note:
In order to explain the factors
resulting in damages, the root cause of the damage in the photos shown in this
article is simplified to be one single factor. The real root cause could be an
interaction of several factors, which needs more investigation at site to make
a conclusion.
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About the author:
Dr. Jay Lin, Chief Consultant of PV Guider Consultancy
Task Force Leader in SEMI International Standards
Technical Committee of CNS National Standards
12 years experience (since 2004) in PV technology research and module production.
As an independent consultant, he has audited more than 50 module factories for insurance companies, investors and EPCs.
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