Page 21 - North American Clean Energy July August 2015

a wind storm with a 90 mph gust, an array Summary
Colleen O’Brien, P.E. is a principal

of 13 ft wide (4 m) rack tilted at 20° will be Geographic location, surrounding terrain, PV system geometry and the engineer at DNV GL, where she
subjected to a signiicant amount of wind dynamic response of the mounting structure govern the magnitude of wind manages the Commercial Solar Due
energy with excitation frequencies between loads on PV systems. he dynamic response of many PV mounting systems
Diligence group.
1 and 3 Hz in the array interior. If the natu- is often overlooked and can result in a signiicant increase in wind loading

ral frequency of the mounting system is 2 relative to the static loads that are typically estimated by PV system designers.
Hz, the DAF can be above 5 if the damping he risk of a structural failure in a PV system is best assessed by a statistical Dr. David Banks is a leading expert
is low (under 1%).
analysis (fragility curve) that combines statistical data for material strengths on aerodynamics and wind tunnel
Clearly, ignoring this efect will signii- and historical wind data for the location. Statistical data that quantiies the testing and leads Solar Services and

cantly increase the likelihood of a failure. impact of real-world wind loading on PV modules is currently unavailable but Special Projects at CPP.
For this reason, a rule of thumb of keeping is needed to fully assess the risk of PV system damage due to wind loading.
the natural frequencies above 4 Hz is some- DNV GL | www.dnvgl.com
times used for such systems. Note that this

frequency threshold will increase for small-
er structures or when higher wind speeds
are likely (such as in hurricane zones).
Dynamic wind loading is well understood

for tall buildings, and as a result many struc-
tural engineering irms are familiar with
methods of integrating wind tunnel test data

with structural properties to predict dynamic
loading. Solutions can involve increasing the
damping and/or the natural frequency.
Dynamic wind loads are most evident

for winds approaching from the high side
of the array (i.e. the north side in a south-
facing ixed-tilt PV array), so that changes
in array alignment to avoid the worst winds

at a site can reduce this efect as well.

Risk Assessment
he last step in this process is to incorpo-

rate the likelihood of a component (such as
a pier) breaking under a certain wind load.
Code calculations take a pass/fail approach
to this issue, which has a built-in assump-

tion, but all components will have a range
of strengths. Statistics about the material
strength can be combined with statistics
about the wind loading (a combination of

everything discussed above) to produce a
fragility curve, which shows the probability
of a given amount of damage over the life of
the structure (see Figure 1).

he y-axis on the fragility curve can be
converted to a cost of repair. he cost of
moving between the curves on the plot is
the cost of increasing the strength of the

members or structure.
To create a fragility curve, it is necessary
to understand how the component is af-
fected by wind loads. Unfortunately, this

is poorly understood for one of the key
components: the PV modules themselves.
Existing mechanical loading test protocols

for PV modules do not relect the actual
wind loading environment experienced by
the modules; simply put, the load tests are
static, whereas the real wind loads vary

rapidly in time, and are not uniform across
the panel. As a result, modules may fail the
tests when they are in fact suitable, or be
installed and warrantied only to experi-

ence unexpected failures and degradation.
With an industry trend toward reducing
frame rigidity, and glass and cell thickness,
it may be important to screen PV modules

for long-term resistance to the type of wind
loading it will be subjected to in the real
world. Unfortunately, such a test protocol
does not yet exist.

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