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Dynamic Effects of Wind Loading on Photovoltaic Systems Mohammad Moravej1, Arindam Chowdhury2, Peter Irwin2, Ioannis Zisis2, Girma Bitsuamlak3 Department of Civil and Env. Eng., Research Assistant, Florida International University, Miami, Florida, USA 2 Department of Civil and Env. Eng., Faculty of Engineering, Florida International University, Miami, Florida, USA 3 Department of Civil and Env. Eng., Associate Professor, Western University, Ontario, CA
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ABSTRACT: Reliable wind loading information on Photovoltaic (PV) systems is essential for a safe and optimized design procedure. Having a robust and straight design method helps toward the large scale deployment of solar technologies. Currently limited research has been published on the comparison of full-scale and small-scale model studies on PV systems. In the current study, full-scale testing of a single PV system mounted on flat roof building was conducted with the 6-fan Wall of Wind (WOW) hurricane simulator. Model scale testing of similar configurations was conducted in a boundary layer wind tunnel (BLWT) at a scale of 1:12. The purpose of the research was to investigate the key differences between small-scale and full-scale testing of PV systems. The study shows the importance of capturing dynamic effects of small structures such as PV systems. The commonly used 1 Hz criteria as an indicator of the dynamic sensitivity, while useful for larger structures, is not such a useful guideline for smaller structures such as PV systems. KEY WORDS: Photovoltaic systems;Dynamic response; Full-scale testing; Wind tunnel; Mechanical Admittance; Aerodynamics. 1
INTRODUCTION
With the advancement in photovoltaic (PV) technology as a feasible renewable energy solution, an increasing demand for PV installation is expected for residential or commercial buildings. Due to the shape and the installation method of the PV panels and the racking structure, especially when installed on a roof top, they are prone to extreme wind loads and thus become vulnerable to damage. Having reliable information on wind loading on PV systems is essential to be able to design the systems in an efficient and economical way. Most research on evaluating wind effects on roof mounted solar panels systems has been carried either in boundary layer wind tunnel tests ( [1], [2], [3] [4], [5] , [6], [7]) or by computational fluid mechanic simulations ( [8], [9], [10]) which do not account for wind induced vibrations. Very few full scale studies have been performed ( [11], [12], [13], [14], [15]) to evaluate wind dynamic effects on PV systems. Consequently as a result of limited reported studies, there is little information in the current building codes and standards to address wind induced vibrations of such systems. Currently the American Society of Civil Engineers (ASCE) 7 standard indicates that provided a structure’s natural frequency is above 1 Hz, it will not experience significant wind induced vibration and the associated dynamic amplification of wind loads. However, this provision was originally developed with buildings in mind. When applied to smaller structures, such as PV systems on the roof top, it can be very misleading. To address wind induced vibrations of PV systems, a multi-scale wind load measurement study was conducted at Florida International University (FIU) using a single PV system located on the roof of a building model. The PV panel orientation was adjusted to different tilt angles that would generally encompass common residential installations in North America. The goal of the study was to compare the net aerodynamic forces acting on an individual roof-mounted PV panel measured at full scale using the 6-fan Wall of Wind (WoW) hurricane simulator at FIU with those measured in a boundary layer wind tunnel (BLWT) of RWDI USA LLC at a scale of 1:12. A complementary full-scale and scaled model testing approach, such as the one discussed in this paper, may draw upon the strengths of each test method. Full-scale testing affords engineers with opportunities to (1) investigate possible Reynolds Number (Re) effects to assess the applicability of experimental results to the real design cases [16], (2) determine load transfer information on actual PV support systems, (3) study wind induced vibrations of PV systems when subjected to severe winds, and (4) evaluate possible failure modes due to high winds. Conversely, small-scale testing in the BLWT is an established and reputable method for wind load estimations accepted by most code-writing authorities, and provides a convenient platform for testing many different model configurations and wind directions in shorter amounts of time and at significantly lower costs when compared to equivalent full-scale testing. As a result of a comparison between full- and small-scale test results, the main observation was that the dynamic effects were one of the main sources of discrepancy between wind tunnel and WoW measurements. A post-test analytical approach was used to incorporate the dynamic effects to correct the measurements obtained on rigid model tested in wind tunnel to bring them in line with full scale results.
14th International Conference on Wind Engineering – Porto Alegre, Brazil – June 21-26, 2015
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2 2.1
EXPERIMENTAL METHODOLY Full Scale Setup
Full-scale testing of a roof-mounted PV system was conducted using the 6-fan WoW at FIU. The 6-fan WoW is capable of generating wind conditions representative of a Category 1 hurricane according to the Saffir-Simpson scale [14]. For the scope of full-scale testing described in this study, a combination of passive flow management devices and active quasi-periodic fan throttle control inputs created turbulent flow conditions with mean velocity and turbulence intensity characteristics comparable to atmospheric flows over suburban terrain. Figure 1 shows the normalized wind speed profile obtained. A more detailed description of the flow management devices used at WoW and the generated flow characteristics may be found in [14]. A simplified flat roof model was constructed which is shown in Figure 2. The PV panel used was a Sun SV-T-190 module with dimensions of 157.1 cm × 95.1 cm × 4.1 cm (length × width × thickness). Also a minimum spacing of approximately 7.6 cm was provided between the PV module and the roof surface which is recommended as minimum clearance for roof mounted PVs to maintain the operating efficiency. Testing was conducted with a 0° wind angle of attack (AOA) when the PV panel was located at Position 1 and with 0° and 45° AOA when the PV panel was located at Position 2. The tilt angles tested were 0°, -15°, -45°, 15°, and 45° as shown in Figure 3. Net aerodynamic forces were determined by measuring the reaction forces between the PV racking system connections and the roof. This was accomplished by installing the PV system on four multi-axial load cells concealed underneath the roof surface. Each test was conducted for 3 min duration, and the force data were collected at a 100 Hz sampling frequency.
Figure 1 – Mean wind speed profile
Figure 2 – Full scale test setup at FIU Wall Of Wind
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2.2
Small Scale Setup
For a comparative study, a wind tunnel test was conducted in the BLWT of RWDI USA LLC to measure wind loads on geometrically scaled models of the roof mounted PV configurations described in Section 2.1 with length, velocity, and time scales of 1:12, 1:3, and 1:4, respectively. The simulation of mean wind speed and turbulence intensity profiles were accomplished through a combination of turbulence-generating spires installed at the upwind end of the tunnel and a long working section having floor roughness elements (Figure 4). The open wind profile developed with this combination produced a 0.15 power-law profile and a turbulence intensity of 20% at mean roof height. For each test case, pressure data was sampled for a duration of 90 sec with a sampling frequency of 512 Hz per channel. A transfer function was applied to correct for the pressure tubing effects. θ = 0˚ 45˚ Wind AOA
Test Position 2
θ
z Test Position 1 0˚ Wind AOA
θ = -15˚, -45˚
x θ
y
θ =15˚, 45˚
x
(a)
(b)
Figure 3 Flat roof building model: (a) plan view showing test positions, (b) elevation view showing PV tilt angles.
Figure 4 – Spires and roughness elements in BLWT
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ANALYSIS METHODS AND RESULTS
In the WoW testing, the net wind loads acting on the PV panel were determined from the measured force time histories. Net aerodynamic forces were calculated as follows: (1) 𝐹𝑥 = ∑4𝑖=1 𝑢𝑖 (2) 𝐹𝑧 = 𝑊 − ∑4𝑖=1 𝑤𝑖 The terms ui and wi are respectively defined as the x and z reaction forces measured by each load cell where x denotes the direction parallel to the roof and z is perpendicular to the roof (Fig. 1b). The term W denotes the combined weight of the PV panel and the mounting hardware. Equivalent peak forces were estimated for a 1-hr storm duration using the method developed by Sadek and Simiu [15] with a 95% probability of non-exceedance. The measured aerodynamic loading was normalized into a non-dimensional force coefficient using the 3-sec gust dynamic pressure, q, according to the following expression: 𝐹 (3) 𝐶𝐹 = 𝑞𝐴 14th International Conference on Wind Engineering – Porto Alegre, Brazil – June 21-26, 2015
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Where the term F is the force acting along a given axis, and A is the projected area of the PV panel normal to the applied force. On the other hand, net aerodynamic forces acting on the small-scale PV panel were calculated directly from the simultaneous BLWT pressure time histories measured on upper and lower surfaces of the solar panel by using the pressure integration method. A summary of the obtained results for the estimated peak force coefficients in the normal surface direction is shown in Tables 1. The normal force is calculated as the resultant of horizontal (F x) and vertical (Fz) forces acting on the panel surface. Only results for the flat roof case and Position 1 are presented here for brevity. Table 1 – Peak Normal Force Coefficients angle
CFn, Wall of Wind
CFn, Wind Tunnel
0
1.32
0.62
-15
1.63
0.91
-45
2.53
3.52
15
1.44
0.97
45
-2.07
-2.47
From this table a difference between the results of wind tunnel and WoW is observed. For the angles 45º and -45º the percentage differences between the full-scale and small-scale results were relatively lower than those for the other shallow tilt angle cases (where the panels were parallel or near-parallel to the roof deck). The full scale observations showed that there were notable vibrations in the panels when tested at shallow tilt angles. Resonant responses were observed with frequencies around 13 Hz, close to the natural frequency of the PV systems. These fluctuations were not simulated in rigid small scale model testing and were found to be the main source of discrepancy between the wind tunnel and the WoW results. Studying the power spectrum helps to scrutinize the difference in a clear way. Figures 5-7, show the power spectra of normal surface force coefficients at 0, +15 and 15 degree tilt angle respectively.
Figure 5 – CFn Spectra comparison at 0 degree
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Figure 6 - CFn Spectra comparison at 15 degrees
Figure 7 - CFn Spectra comparison at -15 degrees The spikes in the spectra of WoW measured force coefficients show the effect of resonant responses which was the main reason of mismatch between the results of WoW versus wind tunnel. The dynamic effects captured at full scale can be mathematically expressed in terms of a mechanical admittance function. 1⁄2 (4) |𝐻(𝑓)| = [
1 2 𝑓 2 𝑓 2 {1−( ) } +4𝜁 2 ( ) 𝑓0 𝑓0
]
, X(t)max=Xstatic|𝐻(𝑓)|
Where f0, f and ζ are natural frequency of the structure, frequency of the applied force, and the damping ratio respectively. H(f) can be calculated for each desired vibration mode and to incorporate the effect of higher modes, a resultant function should be used which is calculated by Equation 5: (5) 𝐻2 𝑅𝑒𝑠𝑢𝑙𝑡𝑎𝑛𝑡 = 𝐻21 ∗ 𝐻2 2 ∗ 𝐻2 3 ∗ … 𝐻2 𝑛 14th International Conference on Wind Engineering – Porto Alegre, Brazil – June 21-26, 2015
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The spectrum containing the resonant part is related to the one which doesn’t include the dynamic effect by the Equation 6: 𝑺𝑪𝑭 (6) 𝒏 (𝑩+𝑹) 𝑺𝑪𝑭 = 𝟐 𝒏 (𝑩) 𝑯 𝑹𝒆𝒔𝒖𝒍𝒕𝒂𝒏𝒕 Applying the admittance function to the wind tunnel spectrum, the modified spectrum is obtained as depicted in Figures 8 to 11. A substantial improvement is observable after implementing the mechanical admittance which makes a better match between the two spectra. The modified wind tunnel force coefficient spectra, including the resonant responses, is used to calculate the modified peaks adopting a simplified method through using Equations 7 and 8. (7)
𝑖
̅ + √ 𝑔2 𝐵 + ∑ 𝑔2 𝑅 𝑖 𝐶̂𝐹𝑛 = 𝐶𝐹𝑛 𝐵 𝑅𝑖 1
𝑔𝑅𝑖 = √2𝑙𝑜𝑔𝑒 (𝜐𝑅𝑖 𝑇) +
0.577
(8)
√2𝑙𝑜𝑔𝑒 (𝜐𝑅𝑖 𝑇)
B and R are the background and resonant part of the spectra, 𝜐𝑅𝑖 can be conservatively taken as the natural frequencies of structure, and 𝑔𝐵 (given as 𝑔𝑄 in ASCE-7) is given as 3.4 in ASCE 7. The peaks calculated by this approach are presented in Table 2. Results show that the corrected BLWT peaks better match the full-scale results when the dynamic effects were incorporated using the post-test analytical method. Since for +45o and -45o there was a better match and no vibrations was noted so no dynamic behavior correction was done, hence not included in the table. For the +15o case a higher peak value of 2.16 was calculated which is well explained by looking at the spectra. Comparing the wind tunnel and WoW spectra it can be seen that there is a difference between the low frequency parts of the spectra which suggests that a better method of accounting for low frequency fluctuations in the WoW is required in future. Theoretical methods for compensating for low frequency fluctuations have been developed by Mooneghi et al. in another paper at this conference. Table 2 – Comparison table for modified peaks of zero and shallow tilt angle cases PV angle(deg) 0 -15 15
WoW Data CFn, max
1.32 1.63 1.44
Figure 8 – Flat Roof, tilt angle=0, position 1
BLWT
BLWT, Modified
CFn, max
CFn, max
0.62 0.91 0.97
1.47 1.60 2.16
Figure 9 - Flat Roof, tilt angle=15, position 1
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Figure 10 - Flat Roof, tilt angle=-15, position 1 4
Figure 11 - Flat Roof, tilt angle=45, position 1
CONCLUSION
The differences between the full-scale and scaled model results for PV systems were higher for low tilt angles (PV panel being parallel or near-parallel to the roof deck). For shallow tilt angles, significant vibrations were noted during system-level full-scale testing which were not captured when using rigid scaled models. Analysis of the force coefficient spectra revealed significant dynamic effects for the full-scale testing which was the main cause of discrepancy between the WoW and BLWT results for zero and shallow tilt angles. Based on the natural frequency and damping of the actual PV systems, mechanical admittance functions were developed and used to modify the wind tunnel spectrum to accommodate dynamic effects. Promising improvement was achieved while comparing the full-scale and modified wind tunnel results that incorporated effects of PV vibrations. The study indicates the importance of conducting full scale testing to evaluate vibrations under extreme wind conditions to which these panels can be subjected during windstorms. The post-test analytical method also can serve as a tool to incorporate dynamic effects in results obtained economically using wind tunnel rigid scale models. Since the missing low frequency turbulence in WoW spectrum (Figure 5-11) is believed to be another source of discordance between results, further improvement is expected by using the Partial Turbulence Simulation [PTS] method [19] which has been developed for the 12-fan Wall of Wind facility and is the subject of another paper at this conference. [20] Testing on full-scale PV systems indicated that significant wind-induced vibration may occur in systems with much higher natural frequency (~13Hz) than the ≤1 Hz criterion indicted in the ASCE 7 for categorizing dynamically wind sensitive structures. Thus the current ASCE 7 criterion related to dynamic amplification, originally developed with buildings in mind, is not really applicable to judge the dynamic sensitivity of smaller structures, such as PV systems. Additional research is underway using the more powerful 12-fan Wall of Wind to develop a more appropriate criterion to address vibration of small structures. REFERENCES [1] K. Strobel and D. Banks, "Effects of vortex shedding in arrays of long inclined flat plates and ramifications for groundmounted photovoltaic arrays," Journal of Wind Engineering and Industrial Aerodynamics, vol. 133, p. 146–149, 2014. [2] G. Kopp, D. Surry and K. Chen, "Wind loads on a solar array," Wind and Structures, vol. 5, no. 5, pp. 393-406, 2002. [3] G. A. Kopp, S. Farquhar and M. J. Morrison, "Aerodynamic mechanisms for wind loads on tilted, roof-mounted, solar arrays," Journal of Wind Engineering and Industrial Aerodynamics, vol. 111, p. 40–52, 2012. [4] K. Chung, K. Chang and Y. Liu, "Reduction of wind uplift of a solar collector model," Journal of Wind Engineering and Insdustrial Aerodynamics, vol. 96, pp. 1294-1306, 2008. [5] K. Chung, K. Chang and J. Chou, "Wind Loading of Solar Collector Models," in Seventh Asia-Pacific Conference on Wind Engineering, Taipei,Taiwan, 2009. [6] R. N. Meroney and D. E. Neff, "Wind effects on roof-mounted solar photovol-taic arrays: CFD and wind-tunnel evaluation," in the Fifth In-ternational Symposium on Computational Wind Engineering (CWE 2010), Chapel Hill, North Carolina, 2010. [7] S. Barkaszi and C. O’Brien, "Wind Load Calculations for PV Arrays.," Solar American Board for Codes and Standards Report, 2010. [8] G. T. Bitsuamlak, A. K. Dagnew and J. Erwin, "Evaluation of wind loads on solar panel modules using CFD," in 5th International Symposium on Computational Wind Engineering (CWE2010), 2010. [9] A. Bronkhorst, J. Franke, C. Geurts, C. Bentum and F. Grepinet, "Wind tunnel and CFD modelling of wind pressures on solar energy systems on flat roofs," in 5th International Symposium on Computa-tional Wind Engineering, 2010. [10] M. Shademan and H. Hangan, "Wind Loading on Solar Panels at Different In-clination Angles," in 11th Americas Conference on Wind Engineering, 2009. 14th International Conference on Wind Engineering – Porto Alegre, Brazil – June 21-26, 2015
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[11] J. Erwin, G. Bitsuamlak, A. Gan Chowdhury and S. Barkaszi, "Full Scale and Wind Tunnel Testing of a Photovoltaic Panel Mounted on Residential Roofs," in Advances in Hurricane Engineering, Miami, FL, 2012. [12] P. Hauranta, C. Ménézoa and P. Dupeyratc, "The PHOTOTHERM project: full scale experimentation and modelling of a photovoltaic - thermal (PV-T) hybrid system for domestic hot water applications," Energy Procedia, vol. 48, p. 581 – 587, 2014. [13] C. Geurts and P. Blackmorec, "Wind loads on stand-off photovoltaic systems on pitched roofs," Journal of Wind Engineering and Industrial Aerodynamics, vol. 123, p. 239–249, 2013. [14] A. Naeiji, F. Raji, I. Zisis, A. Gan Chowdhury and P. Irwin, "Wind-Induced Pressures and Forces on Solar Panels Mounted on Flat, Gable or Hip Roof Residential Buildings," in 14th International Conference on Wind Engineering, Porto Alegre, Brazil, 2015. [15] A. Naeiji, F. Raji and I. Zisis, "Large-scale Wind Testing of Photovoltaic Panels Mounted on Residential Roofs," in Structures Congress, ASCE, 2015. [16] R. Kargarmoakhar, A. Gan Chowdhury and P. Irwin, "Reynolds Number Effects on Twin Box Girder Long Span Bridge Aerodynamics," J. Wind & Structures, vol. 20, no. 2, pp. 327-347, 2015. [17] P. Huang, A. Gan Chowdhury, G. Bitsuamlak and R. Liu, "Development of Devices and Methods for Simulation of Hurricane Winds in a Full-Scale Testing Facility," Wind and Structures, vol. 12, no. 2, pp. 151-177, 2009. [18] F. Sadek and E. Simiu, "Peak non-gaussian wind effects for database-assisted low-rise building design.," ASCE Journal of Engineering Mechanics, vol. 128, no. 5, pp. 530-539, 2002. [19] M. Asghari Mooneghi, Experimental and Analytical Methodologies for Predicting Peak Loads on Building Envelopes and Roofing Systems, FIU Electronic Theses and Dissertations. Paper 1846., 2014. [20] M. Asghari Mooneghi, P. Irwin and A. Gan Chowdhury, "Partial Turbulence Simulation Method for Small Structures," in 14th. International Conference on Wind Engineering, Porto Alegre, Brazil, 2015.
14th International Conference on Wind Engineering – Porto Alegre, Brazil – June 21-26, 2015
