Influence of spacing parameters on the wind loading of solar array

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Journal of Fluids and Structures

Journal of Fluids and Structures ] (]]]]) ]]]–]]]

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journal homepage: www.elsevier.com/locate/jfs

Influence of spacing parameters on the wind loadingof solar array

Workamaw P. Warsido a, Girma T. Bitsuamlak b,n, Johann Barata a,Arindam Gan Chowdhury a

a Department of Civil and Environmental Engineering, Florida International University, 10555 W Flagler St, Miami, FL 33174, USAb WindEEE Research Institute/Department of Civil and Environmental Engineering, University of Western Ontario in London, ON, CanadaN6A 5B9

a r t i c l e i n f o

Article history:Received 2 May 2012Accepted 17 March 2014

Keywords:Wind loadSolar arrayGround mountedRoof mountedSheltering

x.doi.org/10.1016/j.jfluidstructs.2014.03.00546/& 2014 Elsevier Ltd. All rights reserved.

esponding author Tel.: þ1 519 661 2111x88ail address: [email protected] (G.T. Bitsuaml

e cite this article as: Warsido, W.P.,uids and Structures (2014), http://d

a b s t r a c t

Boundary layer wind tunnel tests were conducted to investigate the effects of differentspacing parameters on the wind loading of ground and roof mounted solar arrays. On theground mounted array the effect of lateral and longitudinal spacing between panels wasinvestigated. On the roof mounted array the effect of array perimeter gap from thebuilding edge was investigated. Based on the results obtained, the magnitude of force andmoment coefficients on the ground mounted array decreased across panel rows as a resultof sheltering effect from the neighboring upwind panels. The largest reduction of windload coefficients was observed on the second row panels but the amount of reductiondropped quickly afterwards, becoming minimal after the fourth row. It was also observedthat panels located in the outer array column could be subjected to relatively higher windloads compared to panels located in the inner array columns. Increasing the lateralspacing between array columns was observed to have minimal effect on the force andmoment coefficients. However, the wind load coefficients increased as the longitudinalspacing between panel rows was increased. The wind load coefficients on the roofmounted array decreased with increasing perimeter gap from the building edge.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Solar energy is enjoying an increased attention as one of the alternative energy sources to satisfy the fast growing global energydemand. Over the last couple of decades several studies have been conducted to make solar energy systems financially viable.One area of focus has been optimizing the solar panels and their support systems for wind loading. Since 1980s wind tunnel testshave beenwidely used to study wind effects on solar panels. Although several wind tunnel studies have been conducted, the resultsfrom very limited research or commercial works are publicly available. Radu et al. (1986) conducted wind tunnel tests on array ofsolar panels mounted on the top of a five-story residential building and noticed the reduction on mean force coefficients as a resultof sheltering effects from the building and the first row solar collectors. Peterka et al. (1987) conducted series of wind tunnel testson array of heliostats and investigated the wind load reductions obtained from the sheltering effects of neighboring upwind panelsand from utilizing protective fences. Wood et al. (2001) conducted wind tunnel tests on array of solar panels mounted on an

028; fax: þ1 519 661 3339.ak).

et al., Influence of spacing parameters on the wind loading of solar array. Journalx.doi.org/10.1016/j.jfluidstructs.2014.03.005i

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industrial flat roof building and studied the effect of solar panel position on the wind loading of the roof cladding as well as on thesolar panel support structure. Kopp et al. (2002) conducted wind tunnel tests on solar arrays and observed the significance of strongvortex shedding and turbulence of the wind flow on aerodynamically induced torque loads. Hosoya and Peterka (2008) conductedwind tunnel tests on parabolic trough solar collectors and provided force, moment and local pressure coefficients. Aly andBitsuamlak (2014) conducted wind tunnel investigation of solar panel mounted on sloped residential roofs. Besides aerodynamicwind tunnel tests, full scale tests were also conducted on solar panels by some researchers (e.g., Erwin et al., 2011; Geurts andSteenbergen, 2009). Moreover, computational fluid dynamic (CFD) simulation has also been implemented to study wind effects onsolar panels (e.g., Bitsuamlak et al., 2010; Bronkhorst et al., 2010; Shademan and Hangan, 2009; Wu et al., 2010) and nonlinearaerodynamic forces on a thin flat plate (Huang et al., 2014). This paper presents the results of wind tunnel tests conducted toinvestigate the effects of different spacing parameters on the wind loading of ground and roof mounted solar arrays. On the groundmounted array the variation of wind loads on solar panels with lateral and longitudinal spacings between panels was investigatedwhile on the roof mounted array the variation of wind loads with array perimeter gap from the building edge was studied. Effectsof the different spacing parameters could vary across the panel rows as well as with wind direction. Hence, the variations withwind direction will be shown on selected sample panels and the variation across panel rows will be shown on all panels fora selected wind direction. The paper also discusses sheltering effects from neighboring panels in different wind directions as well asat different locations across the solar array. In addition, wind load comparison between isolated and array systems is also presented.

2. Experimental procedure

2.1. Wind tunnel set-up

The wind tunnel tests were conducted at RWDI's boundary layer wind tunnel facility in Florida, USA. The tunnel hasa cross-section of 2.13 m � 2.44 m (7 ft�8 ft) and the test model was placed at 13.3 m (43.5 ft) down-stream of the tunnel

0 10 20 30 40 50 60 70 80 90 100

0

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0.0 0.2 0.4 0.6 0.8 1.0 1.2

TI(%)

Z(m

)

V/V30m

V(measured) V(Power Law,alpha= 0.15) TI(measured) TI(Target)

0.001

0.01

0.1

1

0.001 0.01 0.1 1 10

f*S V

(f)/

2

f*Lv/V

Measured von Karman

Fig. 1. (a) Normalized mean wind speed (V/V30m) and turbulence intensity (TI) profiles and (b) longitudinal turbulence spectrum at 5.64 m (buildingheight) of the simulated open exposure.

Please cite this article as: Warsido, W.P., et al., Influence of spacing parameters on the wind loading of solar array. Journalof Fluids and Structures (2014), http://dx.doi.org/10.1016/j.jfluidstructs.2014.03.005i




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entrance. The lower part of the atmospheric boundary layer was simulated in the wind tunnel to generate an open upwindexposure. The mean wind speed and turbulence intensity profiles of the open exposure simulated are shown in Fig. 1(a).The meanwind speed profile fits well to a target profile obtained with a power law exponent of 0.15 (�1/6.5). Moreover, theturbulence intensity profile also fits well to a target profile of 1/ln(z/zo) suggested based on the large-scale depressionsystem measurements near the ground (Holmes, 2007). A full scale value of 0.02 m was taken for the roughness coefficientzo as per the recommendation of ASCE-7 (2010) for exposure category C. Fig. 1(b) shows the normalized longitudinalturbulence spectrum which also shows a good match with von Karman spectrum.

Each solar panel model was produced from a 3.175 mm (1/8 in) thick, 0.3 m by 0.045 m (1 ft�0.15 ft) plexiglass plate torepresent a 9.14 m�1.34 m (30 ft�4.4 ft) panel at 1:30 scale (see Fig. 2(a)). The solar panel was mounted on a pair of 251inclined trapezoidal supports having 81.28 cm (32 in) short legs (see Fig. 2(b)) produced at 1:30 scale panel scale. Eventhough this is a relatively large scale compared to commonly used scales for testing buildings in the wind tunnel it wasrequired for practical simulation of the solar panel. Such relatively large scales have been used in the past by differentresearchers (e.g., Kopp et al., 2005; Suaris and Irwin, 2010; Visscher and Kopp, 2007) to model small structures in the windtunnel. Detail study on scale effects has been also conducted by Aly and Bitsuamlak (2013). For such large scale wind tunneltests, one of their proposed solution is to simulate a portion of the atmospheric boundary layer at a relatively large scale.Hence, the open upwind profile developed for the present study simulated only the lower portion of the atmosphericboundary layer at large scale.

Both the ground and roof mounted solar arrays tested in the present study constitute multiple rows and columns.In order to facilitate testing the different configurations of the ground mounted array, the dummy (un-instrumented) panelsin one column were glued to a single acrylic sheet at the required longitudinal spacing. However, the instrumented panelswere not glued to a single acrylic sheet instead narrow plexiglass bars were used to get the required longitudinal spacing.In this manner, the different configurations were tested effectively without disconnecting the instrumented panels from thedata acquisition system. For the roof mounted test, a two story flat roof rectangular building having 5.64 m (18.5 ft) heightand 27.43 m (90 ft)�35.66 m (117 ft) plan dimensions was produced at 1:30 scale. The maximum blockage obtained from

Fig. 2. (a) 3D drawing showing full scale dimensions of the panel (in m) and location of pressure taps; (b) orientation of the coordinate axes and positivedirections of normal force and overturning moment shown on side view of the panel.






the roof mounted test is 4.2% which is less than 5% hence no correction was required for blockage effect (Holmes, 2007).The building model was constructed in four different blocks to facilitate testing of different configurations. One block wasdedicated to mounting the instrumented panels and was always kept at the center of the turn table while the remainingthree blocks with the dummy panels were rearranged to create the required test configurations. Photographs of sample testconfigurations of the ground and roof mounted arrays are shown in Figs. 3 and 4 respectively. For each test configuration,pressure readings were taken on the top and bottom surfaces of 10 panel rows on the same column with 48 pressure tapsinstalled on each panel (24 on each surface). It was attempted to exactly overlap the top and bottom surface pressure taps ateach of the pressure measurement locations, however, with the thickness of panel used in the present study it was notpossible to do so. Instead, the top and bottom surface taps at each location were separated by a distance equivalent to thepressure tube diameter which is 0.053 in (1.34 mm). Considering the magnitude of this distance, the top and bottom surfacepressure taps can be considered to overlap at each measurement location for net pressure calculation. This has been a verycommon practice in the wind tunnel testing of similar structures such as canopies. It is to be noted that the adopted tapdistribution could be on the lower side particularly for My assessment. It might be useful, provided the size of the testingfacility allows, to use a higher density of taps using large scale test models. The wind tunnel tests were conducted at a windspeed of 15 m/s (50 ft/s), measured at 1.524 m (60 in) height above the tunnel floor, in multiple wind directions rangingfrom 01 to 1801 at 101 interval. For each wind direction, the pressure data was collected for 90 s duration at a samplingfrequency of 512 Hz. The resonance effect from vibration of the PVC tubes used to connect the pressure taps with thepressure sensing system was accounted for by treating the measured data with an inverse transfer function specificallyderived for the tubes used in the experiment (Irwin et al., 1979).

Instrumented panels Dummy panels

Lateral spacing

Longitudinal spacing

Fig. 3. Photograph of a sample ground mounted array test configuration.

Instrumented panels

Array perimeter gap from the building edge

Dummy panels

Fig. 4. Photograph of a sample roof mounted array test configuration.





Fig. 5. Location of solar panels used in the present study on a typical ground mounted array field.


2.2. Array test configurations

2.2.1. Ground mounted arrayIn a typical ground mounted array, such as the one shown in Fig. 5, the most critically loaded panels are located near

the corner regions. Hence, the present study considers the northwest and southwest corner regions of the groundmounted array which will also cover the northeast and southeast corner regions by symmetry. As it can be seen fromFig. 5, 10 rows of panels extending over three columns were investigated. The northwest corner panels were tested for0–901 wind Angle of Attack (AoA) and the southwest corner panels were tested for 90–1801 wind AoA. It is to be notedthat in all subsequent figures the row number counting in the range of 0–901 wind AoA starts from the front row panelsof the northwest corner region while in the range of 100–1801 wind AoA it starts from the front row panels of thesouthwest corner region. Since relatively a large number of taps were installed on each panel it was only possible toinstrument 10 panels at a time. By systematically testing the configurations shown in Fig. 6(a, c and e) separately andassembling the test results together it was possible to cover the northwest corner region. Similarly, by testing theconfigurations shown in Fig. 6(b, d and f) separately and assembling the test results together, it was possible to cover thesouthwest corner region. In all test configurations, the instrumented array columnwas kept at the center of the turntableas shown in Fig. 6. The ground mounted array was tested for three full-scale lateral spacings (0, 0.91 m (36 in) and 1.83 m(72 in)) and three full-scale longitudinal spacings (0.61 m (24 in), 1.22 m (48 in) and 1.83 m (72 in)) between panels(see Fig. 3). An informative nomenclature is adopted based on the instrumented column number as well as lateral andlongitudinal spacings to refer to the different test cases. The column number where the instrumented panels werelocated is designated by the prefix ‘C’ followed by the column number; the lateral spacing between columns isdesignated by the prefix ‘Y’ followed by the full-scale value of spacing in inches; the longitudinal spacing between rowsis designated by the prefix ‘X’ followed by the full-scale value of spacing in inches. For instance, column three of the arraywith lateral and longitudinal spacings of 36 in and 48 in respectively is referred as ‘C3Y36�48’. Summary of the testcases for the ground mounted arrays is given in Table 1.

2.2.2. Roof mounted arraySimilar to the ground mounted case, the northwest and southwest corner regions of a typical roof mounted array

were investigated, which by symmetry also covered the northeast and southeast corner regions respectively. For thesame reason as in the ground mounted case the northwest corner panels were tested for 0–901 wind AoA while thesouthwest corner panels were tested for 90–1801 wind AoA. Moreover, in all subsequent figures the row numbercounting in the range of 0–901 wind AoA starts from the front row panels of the northwest corner region while in therange of 100–1801 wind AoA it starts from the front row panels of the southwest corner region. For the roof mountedtest, 10 rows of panels from the last two columns of the array were selected from the two representative corner regions.The instrumented panels were the same panels used in the ground mounted study where it was possible to instrumentonly 10 panels at a time. By systematically testing the configurations shown in Fig. 7(a and c) separately and assemblingthe test results together, however, it was possible to cover the northwest corner regions. Similarly, by testing theconfigurations shown in Fig. 7(b and d) separately and assembling the test results together, it was possible to cover thesouthwest corner region. As it can also be observed from Fig. 7, the three column array was arranged in such a way thatthe instrumented column was always kept at the center of the turn table. In one configuration the instrumented columnwas placed between two columns of dummy panels while in the other configuration both columns of dummy panels





Fig. 6. Wind tunnel configurations used to test the first column (a and b), second column (c and d) and third column (e and f) of the ground mounted array.


were moved to one side and the first column of the array was instrumented. In the roof mounted array the panels werearranged with a constant full-scale longitudinal spacing of 1.22 m (48 in) but without any lateral spacing. The array wastested for three different full-scale array perimeter gaps (0, 0.91 m (36 in) and 1.83 m (72 in)) from the building edge (seeFig. 4). Similar to the ground mounted case an informative nomenclature is adopted based on the instrumented columnnumber and the array perimeter gap from the building edge to refer the different test cases. The column number wherethe instrumented panels were located is designated by the prefix ‘C’ followed by the column number and the arrayperimeter gap from the building edge is designated by the prefix ‘D’ followed by full-scale value of the gap in inches. Forinstance, a test case where the array perimeter gap was 36 in and the instrumented column was located between twodummy columns (i.e. column 2) is referred as ‘C2D36’. Summary of the test cases for the roof mounted array is givenin Table 2.





Table 1Summary of test cases for ground mounted array.

Test name Inst. column # Lateral spacing (Y) Longitudinal spacing (X) Tested wind AoA

C3Y0�24 3 0 24 in (0.61 m) 0o–180o

C3Y0�48 3 0 48 in (1.22 m) 0–180o

C3Y0�72 3 0 72 in (1.83 m) 0–180o

C3Y36�48 3 36 in (0.91 m) 48 in (1.22 m) 0–180o

C3Y72�48 3 72 in (1.83 m) 48 in (1.22 m) 0–180o

C2Y0�24 2 0 24 in (0.61 m) 0–180o

C2Y0�48 2 0 48 in (1.22 m) 0–180o

C2Y0�72 2 0 72 in (1.83 m) 0–180o

C2Y36�48 2 36 in (0.91 m) 48 in (1.22 m) 0–180o

C2Y72�48 2 72 in (1.83 m) 48 in (1.22 m) 0–180o

C1Y0�24 1 0 24 in (0.61 m) 0–180o

C1Y0�48 1 0 48 in (1.22 m) 0–180o

C1Y0�72 1 0 72 in (1.83 m) 0–180o

C1Y36�48 1 36 in (0.91 m) 48 in (1.22 m) 0–180o

C1Y72�48 1 72 in (1.83 m) 48 in (1.22 m) 0–180o


3. Discussion of results

3.1. Data analysis

The pressure data collected from the wind tunnel tests were normalized by a reference dynamic pressure to get non-dimensional pressure coefficients as follows:

CP ¼P�Po12 ρaV

2H

; ð1Þ

where CP is the non-dimensional pressure coefficient; P is the pressure measured on the panel surface; Po is the mean staticpressure at a reference height; ρa is the density of air and VH is the mean wind speed at a reference height which was takento be the mean height of panels from the wind tunnel floor. The reference height for the ground mounted case is equal to themean height of the trapezoidal panel supports (i.e. 1.09 m (43 in) from the ground at full-scale). However, for the roofmounted solar array case it is considered as the sum of mean height of the trapezoidal panel supports and the buildingheight (i.e. 6.73 m (265 in) from the ground at full-scale). Time history of the net pressure coefficients was obtained bycombining the time histories of the top and bottom surface pressure coefficients as follows:

CPNðtÞ ¼ CPT ðtÞ�CPBðtÞ; ð2Þwhere the subscripts N, T and B denote the net, top surface and bottom surface pressure coefficients respectively. At each taplocation, the time history of the net force was computed by multiplying the time history of the net pressure coefficient withthe corresponding tributary area and the reference dynamic pressure. The time history of the total net force on the panelwas obtained by superimposing the forces from all tap locations:

FnðtÞ ¼ ∑m

i ¼ 1

12 ρaV

2HCPNðtÞAi; ð3Þ

where Fn is the total net force which is normal to the panel surface, Ai is the tributary area corresponding to tap i, and m isthe total number of taps. Moreover, the force at each tap location creates an overturning moment about the geometriccenter of the panel. Similarly, the time history of the total overturning moment, My(t), was computed by superimposing theoverturning moments from the individual forces. Positive directions of the normal force and overturning moment areshown in Fig. 2(b). Hence, based on the sign convention adopted the critical wind loads for the structural design of the solarpanel are the largest negative normal force and positive overturning moment. The normal force and overturning momentwere converted into non-dimensional coefficients through the following normalization:

CFnðtÞ ¼FnðtÞ

12 ρaV

2HBL

; ð4Þ

CMyðtÞ ¼MyðtÞ

12 ρaV

2HB

2L; ð5Þ

where CFn and CMy are the normal force and overturning moment coefficients respectively; L and B are length and width ofthe panel respectively (see Fig. 2(a)). Most of the discussions in the following sections are based on these normal force andoverturning moment coefficients. In addition, mean and root-mean-square (rms) values of the coefficients were computedfrom the time history data. However, rather than using measured peak values from the time history a more stable statisticalpeak estimation approach, which was suggested by Sadek and Simiu (2002), was adopted to compute peak values of the





Table 2Summary of test cases for roof mounted array.

Test name Inst. column # Perimeter gap (D) Tested wind AoA

C1D0 1 0 0–180o

C1D36 1 36 in (0.91 m) 0–180o

C1D72 1 72 in (1.83 m) 0–180o

C2D0 2 0 0–180o

C2D36 2 36 in (0.91 m) 0–180o

C2D72 2 72 in (1.83 m) 0–180o

Fig. 7. Wind tunnel configurations used to test the first column (a and b) and second column (c and d) of the roof mounted array.


coefficients. The method involves fitting a combination of gamma and normal distributions to the time history of wind-induced responses and uses the standard translation processes approach suggested by Grigoriu (1995). The method wasimplemented after checking goodness of the probability distribution fit to the time history of responses from the probabilityplot correlation coefficient (PPCC) (Filliben, 1975) as well as from the probability and cumulative distribution plots.

3.2. Variation of wind load across array rows

Fig. 8 shows the variation of peak normal force and overturning moment coefficients on the 10 panel rows in differentwind directions for a sample ground mounted test case C3Y0�48. As it can be seen from the figure, the largest negativeforce coefficients are observed on the first row panel for northerly wind close to 01 AoA. The largest positive forcecoefficients are observed on the first row from the southerly wind close to 1801 AoA. The largest overturning momentcoefficients are also observed on the first row panel for southerly wind. The force and moment coefficients show reductionacross panel rows. The highest reduction occurs on the second row and it becomes minimal after the fourth row. Thisindicates that most of the energy in the oncoming wind flow is dissipated by the first three panel rows resulting in a





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C Fn(p

eak)

Azimuth(degrees)

Row-1 Row-2 Row-3 Row-4 Row-5 Row-6 Row-7 Row-8 Row-9 Row-10

Northwest corner Southwest corner

-0.2

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0.5

0.6

0 20 40 60 80 100 120 140 160 180

C My(p

eak)

Azimuth(degrees)



Fig. 8. Variation of peak (a) normal force and (b) overturning moment coefficients with wind direction on each of the 10 panels for a sample groundmounted test case C3Y0�48.


significant reduction of the wind speed which is directly related to the wind loads on solar panels. Minimal variation of thewind load after the fourth row panel also shows that the turbulent wind flow will be well mixed after the first four panelrows. It can also be observed that the wind load reduction on the second row panel is higher for southwesterly windbetween 1001 and 1801 AoA than for northwesterly wind between 01 and 901 AoA. Hence, higher sheltering effect from thefirst row panels is observed when they are inclined along the direction of wind flow than when they are inclined against thedirection of wind flow. This is because panels inclined along the direction of wind flow help to divert the wind to flow abovethe downstream panels unlike panels inclined against the direction of wind flow. Fig. 9 shows the variation of peak normalforce coefficients on the 10 panel rows for sample roof mounted test cases C2D0 and C2D36 in different wind directions.As it can be observed from Fig. 9(a), the force coefficients for test case C2D0 show reduction up to the fourth row particularlyfor northwesterly wind between 01 and 901 AoA. The force coefficients for test case C2D36 show a more complex variationas it can be observed from Fig. 9(b). For the northerly wind close to 01 AoA the force coefficients show reduction up to thefourth row but for diagonal wind from northwest (i.e. close to 451 AoA) slight increase of force coefficients is observed. For asoutherly wind close to 1801 AoA the largest peak force coefficient is positive on first row panel but it is negative on thesecond row panel due to wake effects from the first row panel. From the third row up to the sixth row magnitude of thenegative force coefficient shows slight reduction and at the seventh row it changes back to positive and shows slightincrease up to the tenth row. Sign change of the force coefficient at the seventh row can be attributed to the potentialreattachment of wind flow separated from the windward edge of the roof.





-3.0

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-1.0

0.0

1.0

2.0

3.0

0 20 40 60 80 100 120 140 160 180

C Fn(p

eak)

Azimuth(degrees)



-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 20 40 60 80 100 120 140 160 180

C Fn(p

eak)

Azimuth(degrees)



Fig. 9. Variation of peak normal force coefficients with wind direction on each of the 10 panels for sample roof mounted test cases (a) C2D0 and (b) C2D36.


Fig. 10 shows the variation of peak overturning moment coefficients on the 10 panel rows for roof mounted test casesC2D0 and C2D36 in different wind directions. As it can be observed from the figure, the moment coefficients for both testcases show complex variations across panel rows. For both test cases the largest moment coefficient is observed on the firstrow panel and significant reduction of the coefficient is observed on the second row panel for northerly wind close to 01 AoAand for southerly wind close to 1801 AoA. The variation of moment coefficients after the first row is far more complex and itdoes not show consistent trend in different wind directions. The clear decreasing trend in the wind load coefficients that isobserved across the ground mounted array is not observed for the roof mounted array. This shows the impact of buildinggenerated flow speed-up, flow separation and turbulence on the wind loading of the roof mounted array which is observedto be a function of the array perimeter gap from the building edge.

Some of the notable differences considering the force coefficients on the first row panels of the ground and roof mountedarrays for southwesterly wind between 1001 and 1801 AoA given in Figs. 8(a) and 9 respectively are discussed as follows. Theforce coefficients on the ground mounted panel are positive while the corresponding coefficients on the roof mounted panelare positive for test case C2D36 and negative for test case C2D0. Similarly, considering the overturning moment coefficientson the first row panels of the ground and roof mounted arrays for southwesterly wind between 1001 and 1801 wind AoA(see Figs. 8(b) and 10 respectively) the following has been observed. The moment coefficients on the ground mounted panelare positive while the corresponding coefficients on the roof mounted panel are positive for test case C2D36 but negative fortest case C2D0. Theses variations in wind load coefficients between the ground and roof mounted test cases can be furtherexplained by looking at the pressure distribution on the ground and roof mounted panels.





-0.8

-0.6

-0.4

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0.0

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0.4

0 20 40 60 80 100 120 140 160 180

C My(p

eak)

Azimuth(degrees)



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0.10

0.15

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0.25

0.30

0.35

0 20 40 60 80 100 120 140 160 180

C My(p

eak)

Azimuth(degrees)



Fig. 10. Variation of peak overturning moment coefficients with wind direction on each of the 10 panels for sample roof mounted test cases (a) C2D0 and(b) C2D36.


Fig. 11 shows the mean top surface, bottom surface and net pressure coefficients on the first row panels of the groundand roof mounted arrays at 1801 wind AoA. As it can be observed, the top surface pressure is positive on the groundmounted panel but negative on the roof mounted panel for both test cases. In the case of ground mounted panel, there is noupwind structure which disturbs the wind flow in front of the solar panel. Hence, the wind interacts with the solar paneldirectly and induces positive pressure on the top surface of the 251 inclined panel. However, in the case of roof mountedpanel the wind flow will be deflected by the leading edge of the roof thus a negative pressure zone is created in theseparation bubble developed close to the windward edge of the roof. Hence, the mean top surface pressure is negative onthe first row of the roof mounted panels for both test cases. Since the largest suction occurs close to the windward edge ofthe roof, magnitude of the negative top surface pressure for test case C2D0 is larger than for test case C2D36. The meanbottom surface pressure is negative for both the ground mounted and roof mounted test cases. The net mean pressure onthe ground mounted panel is positive since the top surface pressure is positive and the bottom surface is negative. The netmean pressure for the roof mounted panel test case C2D36 is positive since the magnitude of the negative bottom surfacepressure is larger than the top surface pressure. The net mean pressure for the roof mounted panel test case C2D0 is negativesince magnitude of the negative top surface pressure is larger than the bottom surface. For all the three test cases,magnitude of the net pressure decreases from the windward edge to the leeward edge of the panel. Hence, with this type ofpressure distribution, the net positive pressure which produces positive normal forces and results in positive overturning





C3Y0X48

C2D0

C2D36

C3Y0X48

C2D0

C2D36

C3Y0X48

C2D0

C2D36

WIN

D a

t 180

oW

IND

at 1

80o

WIN

D a

t 180

o

Fig. 11. Mean (a) top surface, (b) bottom surface and (c) net pressure coefficients at 1801 wind AoA for ground mounted test case C3Y0�48 and roofmounted test cases C2D0 and C2D36, (a) mean top surface pressure, (b) mean bottom surface pressure, (c) mean net pressure.


moment, and the net negative pressure produces negative normal forces and results in negative overturning moment. Thus,for the roof mounted test case C2D0 the largest peak normal force and overturning moment coefficients on the first rowpanels are negative in southwesterly wind directions. However, for the roof mounted test case C2D36 the largest peaknormal force and overturning moment coefficients on the first row panels are positive in southwesterly wind directions.Although such a complex flow mechanism can be difficult to explain only from the pressure data, it can be investigated byusing flow visualization techniques such as particle image velocimetry (PIV) method (see Pratt and Kopp, 2012) ortomographic PIV based turbulent flow structure investigation (Deria et al., 2013) or CFD simulations.

3.3. Variation of wind load across array columns

Peak normal force and overturning moment coefficients on the first row panels of the first, second and third columns ofthe ground mounted array are compared in Fig. 12(a and b). The coefficients are obtained from sample test cases C1Y0�48,C2Y0�48 and C3Y0�48 which correspond to the first, second and third columns respectively. Similarly, the peak normalforce and overturning moment coefficients on the first row panels of the first and second columns of the roof mounted arrayare compared in Fig. 12(c–f). Fig. 12(c and d) shows the comparison of wind load coefficients between test cases C2D36 andC1D36 for which the array perimeter gap from the building edge is 0.91 m (36 in). Fig. 12(e and f) shows the comparison ofwind load coefficients between test cases C2D0 and C1D0 for which the panels were mounted flush with the building edgewithout leaving any gap. As it can be observed from Fig. 12(a–d) the roof mounted test case with 0.91 m (36 in) array





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Fig. 12. Variation of peak normal force (a, c and e) and overturning moment coefficients (b, d and f) on the first row panels of the different columns in theground and roof mounted arrays with wind direction.


perimeter gap and the ground mounted test case shows the same pattern of variation of wind load coefficients with winddirection. For northwesterly wind between 201 and 801 AoA the magnitude of force and moment coefficients on the outer(first) column panels are larger than the coefficients on the inner (second and third) column panels. For southwesterly windbetween 1001 and 1801 AoA the wind load coefficients from the outer and inner columns of the array are more or lesssimilar. This shows that bluff body obstruction to wind flow across the array columns is relatively higher when the panelsare inclined against the direction of wind flow than when they are inclined in the direction of wind flow. In the roofmounted test case with zero array perimeter gap a different pattern of variation of wind load coefficients with winddirection is observed. As it can be observed from Fig. 12(e and f) for northwesterly wind between 01 and 901 AoA themagnitude of force and moment coefficients on the outer column panel are larger than the corresponding coefficients on theinner column panel. However, for southwesterly wind between 1101 and 1801 AoA the magnitude of force and momentcoefficients on the inner column panel are larger than the corresponding coefficients of the outer column panel. It can alsobe observed that the critical wind directions on the roof mounted array panels could vary with the location of the panels inthe array. For instance, the largest negative force coefficients on the outer column panel occur when the wind comesdiagonally from northwest close to 451 AoA while the corresponding coefficients on the inner column panel located in thesame region occur for northerly wind close to 01 AoA. The increase in the wind load coefficients of outer column panels fordiagonal wind directions can be attributed to the effect of conical vortices developed on the roof when the wind comes fromdiagonal directions. As it was shown by several researchers conical vortices can induce very high suctions in the corner






regions of low-rise building roofs (e.g. Banks and Meroney, 2001; Marwood and Wood, 1997). Hence, roof mounted panelsparticularly those located in the corner region can be subjected to significantly higher wind loads for diagonal winddirections.

3.4. Effect of lateral spacing between panels

Fig. 13 shows the variation of peak force and moment coefficients with lateral spacing between panels in different winddirections. The sample panels used for comparison are taken from the first row of the third (inner) and first (outer) columnsof the ground mounted array. In both the inner and outer column panels, the force and moment coefficients obtained from0.91 m (36 in) and 1.83 m (72 in) lateral spacings are close for all wind directions. However, the peak force and momentcoefficients obtained without any lateral spacing show a different pattern from the corresponding coefficients obtained withthe two non-zero spacings. As it can be seen from Fig. 13(a and c), for northwesterly wind between 401 and 701 AoA the peakforce and moment coefficients on the inner column panel obtained without any lateral spacing are slightly less than thecoefficients obtained with the two non-zero spacings. As it can be seen from Fig. 13(b and d), for northwesterly windbetween 01 and 901 AoA the magnitude of peak force and moment coefficients on the outer column panel obtained withoutlateral spacing are larger than the coefficients obtained with the two non-zero spacings. It can also be noticed that thevariation of wind load coefficients with lateral spacing is minimal for southwest wind directions between 1001 and 1801 AoAcompared to northwest wind directions between 01 and 901 AoA. Considering 401 wind AoA the variations of mean, peakand rms values of the force and moment coefficients across the inner and outer column panels are shown in Fig. 14. On theinner column panels the variation of force and moment coefficients with lateral spacing is minimal except the reduction ofwind load coefficients observed on the first three rows at zero lateral spacing. On the outer column panels the magnitude ofpeak, mean and rms values of the wind load coefficients obtained without lateral spacing are larger than the correspondingcoefficients obtained with the two non-zero lateral spacings.

3.5. Effect of longitudinal spacing between panels

Fig. 15 shows the variation of peak force and moment coefficients with longitudinal spacing between panels in differentwind directions. The sample panels used for comparison were taken from the second row of the third (inner) and first(outer) columns of the ground mounted array. As it can be observed from the figure, the peak force and moment coefficientsgenerally increase with increasing longitudinal spacing between panels. This increase of force and moment coefficients with

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Fig. 13. Variation of peak normal force and overturning moment coefficients on the first row panels of the third (a and c) and first (b and d) columns in theground mounted array with lateral spacing in different wind directions.





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Fig. 14. Variation of (a) peak CFn, (b) peak CMy, (c) mean CFn, (d) mean CMy, (e) rms CFn and (f) rms CMy with lateral spacing across the first and third columnsof the ground mounted array at 401 wind AoA.


increasing longitudinal spacing can be attributed to the reduction of sheltering effect from neighboring upwind panels.It can also be observed that, the magnitude of force coefficients on the outer column panel obtained with 1.22 m (48 in)spacing are slightly larger than the coefficients obtained with 1.83 m (72 in) longitudinal spacing for northwesterly windbetween 01 and 501 AoA. Moreover, the force and moment coefficients obtained with 1.22 m (48 in) and 1.83 m (72 in)longitudinal spacings are relatively close on the outer column panel than they are on the inner column panel. As it can beobserved from Fig. 15(c and d), the peak overturning moment coefficients obtained with 0.61 m (24 in) spacing are negativefor most wind directions even though positive moments are expected just like the other two spacings (1.22 m (48 in) and1.83 m (72 in)). However, when the panels are placed close enough to be highly sheltered by the upwind panel themagnitude of pressure on the windward edge of the panel could be lower than the pressure on the leeward edge which willinduce a negative overturning moment.

Fig. 16 shows the variation of mean, peak and rms values of the coefficients across panel rows on the third column fornortherly wind close to 01 AoA. As it can be seen from the figure the effect of longitudinal spacing is pronounced on thesecond row panels. From the third to the tenth rows all the peak, mean and rms force coefficients obtained with the threedifferent longitudinal spacings closely match while the moment coefficients obtained with 0.61 m (24 in) longitudinalspacing are slightly less than the corresponding coefficients obtained with the other two spacings. Another interestingphenomenon observed is the slight increase in the force and moment coefficients of the first row panel with increasing





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Fig. 15. Variation of peak normal force and overturning moment coefficients on the second row panels of the third (a and c) and first (b and d) columns ofthe ground mounted array with longitudinal spacing in different wind directions.


longitudinal spacing. This shows that the modification of wake flow behind first row panels by the downstream panels canhave an impact on the wind loading of first row panels.

3.6. Effect of array perimeter gap from the building edge

Fig. 17 shows the variation of peak force and moment coefficients of the first row panels from the first (outer) and second(inner) columns of the roof mounted array with array perimeter gap from the building edge in different wind directions.As it can be observed from the figure, force and moment coefficients decrease with increasing perimeter gap from thebuilding edge. Moreover, when the panels are mounted flush with the building edge (zero array perimeter gap)the magnitude of force and moment coefficients are significantly larger than the coefficients obtained with 0.914 m(36 in) and 1.83 m (72 in) gaps left from the building edge. Moreover, the force and moment coefficients obtained with zeroarray perimeter gap show different patterns of variation with wind direction from the corresponding coefficients obtainedwith 0.914 m (36 in) and 1.83 m (72 in) array perimeter gaps. As it can be seen from Fig. 17(a), for 0.914 m (36 in) and 1.83 m(72 in) gaps the largest negative force coefficients on the inner column panel are obtained for northerly wind close to 01 AoAwhile for zero perimeter gap the largest coefficients are observed for northerly wind close to 01 AoA and for southerly windclose to 1801AoA. The moment coefficients obtained with 0.914 m (36 in) and 1.83 m (72 in) gaps are all positive and thelargest coefficients are obtained for northerly wind close to 01 AoA and for southerly wind close to 1801AoA. However, forzero perimeter gap the moment coefficients are positive for northwesterly wind between 01 and 901 AoA and negative forsouthwesterly wind between 1001 and 1801 AoA and the largest coefficients are obtained for southerly wind close to 1801.Taking sample wind directions, 101 wind AoA for inner column panel and 451 wind AoA for outer column panel, variations ofmean, peak and rms values of the force and moment coefficients across the panel rows are shown in Fig. 18. As it can beobserved from the figure the effect of array perimeter gap is the highest on the first row panel. On the inner column panels,the effect becomes minimal after the second row for 0.914 m (36 in) and 1.83 m (72 in) gaps while for zero perimeter gapthe effect can be observed up to the fourth row. From the fourth to the tenth rows of the outer column panels the largestpeak, mean and rms values of the force and moment coefficients are obtained with 1.83 m (72 in) perimeter gap while theleast peak, mean and rms values of the force and moment coefficients are obtained with 0.914 m (36 in) perimeter gap. Thevariation of force and moment coefficients with array perimeter gap is caused by the turbulence created from the flowseparation occurring at the building edge as it was discussed before. The influence of this building generated flow separationand turbulence is the highest on the first row panels particularly when they are mounted flush with the building edge and itdecreases with increasing gap from the building edge.





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Fig. 16. Variation of (a) peak CFn, (b) peak CMy, (c) mean CFn, (d) mean CMy, (e) rms CFn and (f) rms CMy with longitudinal spacing across the third column ofthe ground mounted array at 01 wind AoA.


3.7. Comparison with an isolated solar panel

A single (isolated) ground mounted solar panel, which has the same size as the panels used in the array, was tested in thewind tunnel to understand the variation of wind loading between isolated and array solar panel systems. Fig. 19 shows thecomparison of peak normal force and overturning moment coefficients obtained from an isolated solar panel and from thefirst row panels of the first (outer) and second (inner) columns of the ground and roof mounted arrays. Fig. 19(a)shows the comparison of force coefficients of the isolated panel with the corresponding coefficients of the outer columnpanels of the roof and ground mounted arrays. As it can be observed, the isolated panel has the largest force coefficients inall wind directions except when the wind comes diagonally from northwest between 401 and 601 AoA where the outercolumn roof mounted panel for test case C1D0 has the largest force coefficients. It can also be seen that the outer columnroof mounted panel for test case C1D36 has the least force coefficients in all wind directions. Moreover, for northwesterlywind between 101 and 701 AoA, the outer column roof mounted panel for test case C1D0 has larger force coefficients thanthe outer column ground mounted array panel. However, for southwesterly wind between 1001 and 1801 AoA the outercolumn ground mounted array panel has larger force coefficients than the corresponding roof mounted panel for test caseC1D0. Fig. 19(b) shows the comparison of peak force coefficients of the isolated panel with the inner column panels of theground and roof mounted arrays. As it can be observed, the isolated solar panel has the largest force coefficients in all wind





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Fig. 17. Variation of peak normal force and overturning moment coefficients on the first row panels of the second (a and b) and first (c and d) columns ofthe roof mounted array with array perimeter gap from building edge in different wind directions.


directions. Moreover, the inner column ground mounted array panel has relatively larger force coefficients than the innercolumn roof mounted panel in all wind directions. Fig. 19(c) shows the comparison of peak moment coefficients of theisolated panel with the outer column panels of the roof and ground mounted arrays. As it can be observed from the figure,for southwesterly wind between 1001 and 1801 AoA the isolated panel has the largest moment coefficients compared toboth the ground and roof mounted array panels. For northwesterly wind between 01 and 90o AoA the isolated panel and theouter column roof mounted panel for test case C1D0 have equivalent moment coefficients. For southwesterly wind between100o and 180o AoA the outer column roof mounted panel for test case C1D0 and the outer column ground mounted panelhave moment coefficients which are in the same order of magnitude but with opposite signs. Fig. 19(d) shows thecomparison of the moment coefficients from the inner column panels of the roof and ground mounted arrays with theisolated panel. As it can be observed from the figure, the isolated solar panel has the largest moment coefficients in all winddirections. Moreover, the inner column ground mounted array panel has relatively larger moment coefficients than theinner column roof mounted panel in all wind directions. As it can be seen from Fig. 19(a and b), the minimum and maximumpeak force coefficients of the isolated panel have the same order of magnitudes and occur for northerly wind close to 0o AoAand from south close to 180o AoA respectively. In both the inner and outer column panels of the ground mounted arraypanels the maximum force coefficients which occur for southerly wind close to 180o AoA have larger magnitudes comparedto the minimum force coefficients which occur for northerly wind close to 0o AoA. The variation of force coefficients on theroof mounted array is observed to be a function of the array perimeter gap as well as the location of the panel in the array.For the outer column panel of the roof mounted array the minimum force coefficient occurs when the wind comesdiagonally from northwest close to 451 AoA and has larger magnitude compared to the maximum force coefficient whichoccurs for southerly wind close to 1801 AoA. The maximum force coefficient on the inner column roof mounted panel fortest case C2D36 occurs for southerly wind close to 1801 AoA and has larger magnitude compared to the minimum forcecoefficient which occurs for northerly wind close to 01 AoA. However, the inner column roof mounted panel for test caseC2D0 has negative force coefficients in all wind directions except for westerly wind close to 90o AoA. Moreover, theminimum force coefficients occur for northerly wind close to 0o AoA and for southerly wind close to 180o AoA. As it can beseen from Fig. 19(c and d), the moment coefficients of the isolated panel, ground mounted array panel and the roof mountedpanel for test cases C1D36 and C2D36 are positive for all wind directions and the maximum values occur for southerly windclose to 180o AoA. However, for the roof mounted test cases C1D0 and C2D0 the moment coefficients are negative forsouthwesterly wind between 100o and 180oAoA. Moreover, the minimum moment coefficients for southerly wind close to180o AoA have larger magnitudes compared to the maximum moment coefficients for northerly wind close to 0o AoA.To summarize the discussion presented in this section, the wind loads on isolated systems are generally larger than the wind





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Fig. 18. Variation of (a) peak CFn, (b) peak CMy, (c) mean CFn, (d) mean CMy, (e) rms CFn and (f) rms CMy with array perimeter gap from building edge acrossthe first and second columns of the ground mounted array at 451 and 101 wind AoA respectively.


loads on individual panels of the ground and roof mounted solar arrays. Although the panels used for comparison from theground mounted array are the first row panels, which are not subjected to sheltering effect from upwind panels, the forceand moment coefficients on the array panels are relatively lower than the coefficients on the isolated panel. Moreover, thekind of equivalence observed between the minimum and maximum force coefficients of the isolated panel is not observedin the individual array panels. Hence, the wind loads on first row panels can be affected by the downstream panels locatedin the wake of first row panels. Comparison of the first row panels from the roof and ground mounted array shows that theroof mounted array starts to show the same pattern of wind loading as the ground mounted array when the array perimetergap from the building edge is increased. Moreover, depending on the array perimeter gap from the building edge, the winddirection and location of the panel in the array, the wind loads on the roof mounted array panels could be higher or lowerthan the ground mounted array panels. For instance, when the array perimeter gap is zero and the wind comes fromnorthwesterly directions normal force coefficients on the outer column panels of the roof mounted array are larger than thecorresponding coefficients on the ground mounted array. However, when the array perimeter gap is 1 m (36 in) or when thewind comes from southwesterly directions, normal force coefficients on the outer column panels of the ground mountedarray are larger than the corresponding coefficients on the roof mounted panels. Moreover, if the same comparison is carriedout between the second column panels of the roof and ground mounted arrays it can be observed that the normal forcecoefficients on the ground mounted panel are larger for all cases tested in the present study.





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Fig. 19. Comparison peak normal force and overturning moment coefficients of an isolated ground mounted panel with the coefficients on the first rowpanels in the first column (a and c) and second column (b and d) of the ground and roof mounted arrays.


4. Conclusions

The influence of different spacing parameters on the wind loading of ground and roof mounted solar arrays was investigatedthrough wind tunnel tests. The ground mounted array was tested for a combination of different lateral and longitudinal spacingsbetween panels while on roof mounted array the perimeter gap from the building edge was varied, while keeping the lateral andlongitudinal spacings constant. The variations of normal force and overturning moment coefficients with the different spacingparameters were investigated across panel rows as well as along different columns of the ground and roof mounted arrays andseveral wind directions. From the results obtained it was observed that high reductions of force and moment coefficients due tosheltering effect occur on the second row panel and the amount of reduction drops quickly, becoming less significant beyond thefourth row. Comparison of wind loads across array columns showed that panels located in the outer array column could besubjected to relatively higher wind loads than panels located in the inner array columns. In particular, the outer column roofmounted panels located close to the roof corner regions could be subjected to significantly high wind loads when the wind comesfrom diagonal directions. The effect of lateral spacing between panels on the inner column panels was found to be minimal, exceptthe slight reduction of force and moment coefficients observed on the first three panel rows, for the case of zero lateral spacing.On the outer column ground mounted panels, the coefficients obtained with zero lateral spacing were larger than the coefficientsobtained with 0.91 m (36 in) and 1.83m (72 in), spacings and the variation was observed across all the 10 panel rows. Both theforce and overturning moment coefficients on the ground mounted panels increased with increasing longitudinal spacing betweenpanels. Moreover, when the panels were placed close enough to be strongly influenced by the wake flow from the neighboringupwind panel, unusual wind loading such as negative overturning moment, can be observed. The force andmoment coefficients onthe roof mounted array decreased with increasing array perimeter gap from the building edge. It was also observed that the flowseparation occurring at the building edge can have a significant impact on the first row panels, particularly when they are mountedflush with the building edge. Comparison of wind load coefficients of an isolated solar panel with the corresponding coefficientsfrom the first row panels of ground and roof mounted arrays revealed that isolated panels are generally subjected to higher windloads than individual panels in a solar array.

Acknowledgment

Financial support through Florida International University's International Hurricane Research Center and the CanadaResearch Chair to the second author is greatly acknowledged. Any opinions, findings, and conclusions or recommendationsexpressed in this material are those of the authors and do not necessarily reflect the views of the granting agencies.






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Influence of spacing parameters on the wind loading of solar array