Copyright © 2012, The Mattingley Publishing Co., Inc., Oakland, California, USA • All rights to reproduction reserved by the publisher.

Reproducing wind-induced vibration of solar PV modules

in the lab—without a dedicated wind tunnel

By HERBERT H. SCHUENEMANPresident

and JORGE CAMPOS Test Engineer II

Westpak IncorporatedSan Jose, California

As published in

TEST Engineering & Management

December/January 2012–13

3757 Grand Avenue, Suite 205

Oakland, CA 94610-1545

Phone (510) 839-0909 • FAX (510) 839-2950

www.testmagazine.biz

As published in TEST Engineering & Management magazine, December/January 2012–13 • www.testmagazine.biz

One of the greatest challenges facing the photovoltaic (PV) module business is overall reliability when exposed to the out-door environment for up to 25 years. The single biggest contributor to poor reliability has been identified as micro-cracks in the silicon base structure that will propagate as a function of excitation. Such crack propagation is known to worsen with thermal stresses and also with wind-induced vibra-tion. This latter topic has been very little studied in the overall reliability enhancement efforts to date.

One of the reasons that wind-induced vibration has not been well studied is the perceived need for a wind tunnel associated with the laboratory conducting the studies. Wind tunnels in general are rare and very expensive.

In 2010–2011, Westpak undertook a study to show that vibration test equipment could be used to duplicate the same excita- tion on solar modules as that experienced during outdoor wind exposure. When a solar module is analyzed as a spring-mass system, it becomes apparent that the system will respond to any excitation at its natural frequency, whether that excitation is a result of wind, mechanical forces, or any other excitation source.

In addition, testing solar modules in a vibration laboratory has another significant advantage—namely, the ability to increase the excitation level to a point where damage occurs (margin testing) and to study the exact nature of this damage in order to help rectify the problem in the future.

Measurement techniquesPrevious studies of wind excitation on

photovoltaic modules focused on dis-placement as the measured variable. While potentially very accurate, most of these techniques utilize a laser device that is relatively fragile and not adaptable to an outdoor measurement environment.

Westpak decided to focus on the possibil- ity of using off-the-shelf acceleration moni-toring devices, such as would be utilized to measure the road vibration environment

within an 18-wheel truck. These devices, commonly referred to as ride recorders, are self-contained, readily available, and within the budget of most testing laboratories. However, it was unknown if ride recorders would be sensitive enough to record the

acceleration response of a PV solar module responding to wind excitation in a typical field environment. Specifically, we were interested in a device that could record low- level vibration in response to wind velocities of 10 to 15 miles per hour (mph), as well as those responses to 50- to 60-mph wind.

To determine if this application was feas- ible, Westpak acquired two different types of ride recorders and fastened them to the backside of two different crystalline silicon (c-Si) photovoltaic modules mounted at a 30-degree angle to horizontal (a common field mounting angle). One of the ride re- corders was self-contained with “internal

By HERBERT H. SCHUENEMAN President

and JORGE CAMPOS Test Engineer II

Westpak Incorporated San Jose, California

accelerometers only,” while the other could utilize remote accelerometers. In the latter case, the recorder was fastened to the corner of the unit, while the transducer measuring the peak acceleration was located near the center of the module.

TEST #1: The modules were then excited in the laboratory using high-velocity wind generated by two high-pressure blowers. The wind velocity, measured at the surface of the modules, was 10 mph to 80 mph, in 10 mph increments. Two modules were tested; one a more typical 170-watt design measuring about 62 x 32 inches, and a larger 270-watt unit measuring about 75 x 51 inches.

In both cases, the ride recorders were able to successfully record the vibration response of the modules to wind velocities in the neighborhood of 20 mph. Acceleration levels of 0.1 g’s were sufficient to initiate the acceleration-sensitive triggering devices within the ride recorders and record the event.

Field recording of acceleration and wind-speed data

With the ride recorders attached to the solar modules, the modules were shipped to the National Renewable Energy Labor-atories (NREL) facility outside Denver, Color-ado (Figure 1). This facility was chosen be- cause it had pre-existing mounting facilities for solar photovoltaic modules, and the area was also under constant wind-speed surveillance; thus acceleration and wind speed could be correlated by comparing the date stamps for each set of data. The site was also famous for fierce winter winds.

The modules were allowed to collect wind-induced vibration data over a period of six months covering the winter of calendar year 2010–2011. The data was downloaded from the ride recorders after a three-month interval in order to guarantee that data was being successfully recorded. Hundreds of significant events were recorded, each

Reproducing wind-induced vibration of solar PV modules in the lab— without a dedicated wind tunnel

Herbert H. Schueneman is president of Westpak, Inc., a testing laboratory with locations in San Jose and San Diego, California. Schuen-eman co-founded Westpak in 1986, which now offers 75,000 square feet

of laboratory space and is staffed with 50 team members. A Certified Profes-sional, Schueneman is a member of the Packaging Hall of Fame.

The micro-crack propogation effects of wind-induced vibration in c-Si photovoltaic modules have not been well studied in the test lab because of the perceived need for wind tunnels in such testing. …this study shows that vibration test equipment can reproduce outdoor wind exposure excitation experienced by solar modules.

“

”

Jorge Campos is a test engineer at Westpak, Incor-porated, San Jose, California, where his major focus is on United Nations Performance Orien-ted Packaging (UN POP) certifications. Campos holds both

a BS and an MS degree in industrial and systems engineering from San Jose State University.

Copyright © 2012, The Mattingley Publishing Co., Inc., Oakland, California, USA • All rights to reproduction reserved by the publisher.

with time stamps. The data was cross-referenced with wind-speed data recorded at that site by NREL and validated. Figure 2 is a side view of the PV modules and the arrows represent the direction of the wind flow measurements relevant to the study.

Vibration response was recorded at wind speeds as low as 25 mph to speed exceeding 60 mph; however the most severe wind currents—those of 50–60 mph—were the focus of this study.

Following completion of the six months’ outdoor exposure, the modules where re- turned to the San Jose facility of Westpak Inc. Initial review of the data showed that the modules had been exposed to wind velocities in the neighborhood of 60 MPH (Figures 3 and 4).

Laboratory test setupTEST #2: With the modules back in the

laboratory, the first order of business was to make sure that we could reproduce the same excitation response on the module as had been experienced in the field environment. To accomplish this, we mounted the modules to the table of a vibration test machine in the same orientation as they had been mounted in the field—that is, 30 degrees to horizontal (Figure 5). The input transducer used to drive the vibration test machine was then located on the solar module in the same location as the ride recorder had recorded the field data. The field vibration spectrum was then loaded into the vibration controller and the machine was instructed to reproduce that spectrum at that location.

The results showed that we could easily reproduce the field-induced vibration spec-trum on the module as anticipated. In addition, recording the transfer function on the vibration table surface was easily accomplished.

The observer will note that this type of test set-up is not considered to be good labora- tory practice in most environments. This is exactly why we were anxious to measure the transfer function on the vibration table surface in order to use this information for future tests.

TEST #3: For the final test, the input accelerometer used to drive the vibration table was returned to the table surface, and this time the controller was fed the inverse transfer function from test #2. In theory, when the table was driven with that inverse transfer function recorded from test #2, the response of the module at the critical

FIG. 1—Recorder mounting locations.

FIG. 3—Recorded on 1-16-2011 at 10:47 PM, 50–60 mph wind event (time domain)—Module A.

FIG. 4—FFT of the time domain shown in Figure 3.

FIG. 5—Spring/mass model of solar panel test (L); laboratory test setup (R).

location should be identical to that obtained in the field during wind excitation. To verify that this was the case, Westpak conducted a series of tests on both modules where the mounted specimen was subjected to the inverse transfer function recorded in test #2, while the result was monitored at the reference location on the module. See Figure 6.

Test resultsThe results showed that

the module response was virtually identical to that de-termined during the outdoor test exposure. Test results are shown in Figure 7, Figure 8, Figure 9, and Figure 10.

Value of the approach

To demonstrate the value of this approach, both modules were functionally checked and found to be in near perfect condition. Both modules passed the max power output test (Pmax), as well as examination using electro luminescence (EL1).

Both modules were then subjected to approximately

FIG. 2—Vibration response was recorded at wind speeds as low as 25 mph to speed exceeding 60 mph; however, the most severe wind currents were those of 50–60 mph—the focus of this study.

➤

As published in TEST Engineering & Management magazine, December/January 2012–13 • www.testmagazine.biz

FIG. 7—Module A: Wind-induced vibration spectrum. FIG. 8—Module A: Laboratory vibration response spectrum.

FIG. 6—Schematic of the relationship of the input and response on a spring/mass system and how the drive was reversed from test #2 to test #3.

Reproducing wind-induced vibration in the lab (continued)

five minutes of excitation equal to a wind velocity of approximately 60 mph. Note that these tests were conducted separately, as the responses of the modules were differ-ent, and therefore required separate testing.

The results show that one module suffered very little damage when exposed to this duration of high wind velocity equivalent excitation. Both the Pmax and the EL1 evaluations changed very little.

The other module, however, showed a degradation of approximately 10 percent in its Pmax and the EL1 evaluation showed sig- nificant crack propagation within the mod- ule structure itself. These cracks resulted in "hot spots" which were observed using the electro luminescence camera. This module would be considered damaged beyond repair and would represent a significant reliability risk in the field.

ConclusionsThe purpose of this

study was to determine if wind excitation of c-Si photovoltaic modules in the field could be repro- duced in the laboratory using traditional vibration test procedures. The re- sults clearly show that this is very possible and will indeed result in a test technique that should be very useful in improving the reliability of these de-vices.

Note that the actua l drive spec-

Copyright © 2012, The Mattingley Publishing Co., Inc., Oakland, California, USA • All rights to reproduction reserved by the publisher.

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FIG. 10—Module B: Laboratory vibration response spectrum.

trum necessary to reproduce a certain wind velocity excitation on a module will vary from module to module, depending on its stiffness, orientation, and other factors. However, the data also shows the value of wind excitation reproduced in the laboratory using high-pressure blowers mounted in a fashion where the distance between the blower and the module can be varied and the orientation of the wind can be adjusted as necessary. Once the field vibration excitation and the wind speed are correlated, the data can be easily incorporated into laboratory vibration techniques that are very favorable to improving overall product reliability. T

FIG. 9—Module B: Wind-induced vibration spectrum.

To continue this discussion with Herb and/or Jorge, go to www.testmagazine.biz/info.php/12dj145

For more information about Westpak's testing services, go to www.testmagazine.biz/info.php/12dj162