STUDY OF FULL SCALE ROOFTOP SOLAR PANELS ERIN KELLY...

STUDY OF FULL SCALE ROOFTOP SOLAR PANELS

SUBJECT TO WIND LOADS

by

ERIN KELLY ANDOLSEK

B.S., University of Colorado Denver, 2010

A thesis submitted to the

Faculty of the Graduate School of the

University of Colorado in partial fulfillment

of the requirements for the degree of

Master of Science

Civil Engineering

2013

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This thesis for the Master of Science degree by

Erin Kelly Andolsek

has been approved for the

Civil Engineering Program

by

Frederick R. Rutz, Chair

Bruce Janson

Peter Marxhausen

November 29, 2013

iii

Erin Kelly Andolsek (M.S., Civil Engineering)

Study of Full Scale Rooftop Solar Panels Subject to Wind Loads

Thesis directed by Assistant Professor Frederick R. Rutz

ABSTRACT

Solar panels have become common rooftop features that must be designed to withstand

common environmental loads including wind. Current building codes and design

standards lack the information required to properly account for wind loading on solar

panels. The results of research on two full scale faux solar panels placed near the center

of a flat roof on the University of Colorado Denver campus are presented herein. The

primary objective of this research project is to provide data with which to compare wind

tunnel test results and values from standards for validation of both the analytical methods

and wind tunnel models. Faux solar panel frames were designed and constructed in such

a manner that actual force could be measured through the use of strain transducers. Wind

velocity and direction measurements were used to produce corresponding net resultant

forces acting on the face of the panel. A ratio of the net resultant force and the

measured actual force provided a means to derive the Coefficient of Force, CF.

The form and content of this abstract are approved. I recommend its publication.

Approved: Frederick R. Rutz

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DEDICATION

I dedicate this work to my parents Patrick and Joan Dowds, who have

always encouraged me to challenge myself and have supported me unwaveringly in my

ambitions.

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ACKNOWLEDGMENTS

This has been a long journey that I have had the pleasure of completing with the

help of several individuals. My thesis advisor, past professor and colleage, Dr. Frederick

R. Rutz, is due many thanks for his years of guidance and support. Your love of

engineering, attention to detail and dedication to not only this project, but to my

education, has been fundamental in getting me to where I am today. You have truly been

an inspiration. Countless thanks and praises to my fellow graduate student, co-researcher

and good friend, Jennifer Harris. Without your knowledge, help and comic relief along

the way this would have been a lot less fun. There is no one with whom I would have

rather carried sandbags up those dreadful ladders. I would like to gratefully acknowledge

Dr. Kevin Rens and Dr. Jimmy Kim of the Civil Engineering Departments for their

generous contributions to repairing the data logger so that it could be used for this

research. To Tom Thuis, Jac Corless, Denny Dunn and Eric Losty of the Electronic

Calibration and Repair Lab at UCD I offer my gratitude for all of your help with the

‘sparks and magic’ portions of this project including but not limited to fabricating the

steel members used in the panel frames, teaching us how to properly solder wire,

calibrating strain transducers and offering your knowledge on the components of the

testing equipment. You have been vital to the completion of this research. My sincerest

gratitude goes to the Auraria Higher Education Campus Facilities Department for

allowing us to utilize the roof of the Events Center Building and for adjusting the location

of pavers and to Pete Hagan for his coordination of such events. To Michael Harris I

would like to express my deepest appreciation for your willingness to assist Jenn and

myself with assembling the panel frames and for your long hours of hard work and late

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nights in the lab. Without your knowledge, tools and skills I would not be confident in

the quality of construction of the frames. I would like to acknowledge Andy Andolsek

and Rudy Herrera for assisting me with a good amount of heavy lifting that took place on

the roof. Finally I would like to take this opportunity to thank the several individuals

involved in the Wind Engineering community who I have had the pleasure of meeting

through the attendance of conferences and who have offered their suggestions and

invaluable knowledge regarding this research project including Dorothy Reed, David

Banks, Ted Stathopolous and Gregory Kopp among many others.

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TABLE OF CONTENTS

CHAPTER

I. OVERVIEW .................................................................................................................... 1

Introduction ................................................................................................................................. 1

Objective ...................................................................................................................................... 2

Procedure ..................................................................................................................................... 2

Outline ......................................................................................................................................... 3

II. THE IMPORTANCE OF ENERGY .............................................................................. 4

Introduction ................................................................................................................................. 4

Modern Solar Panels .................................................................................................................... 4

Conclusions ................................................................................................................................. 5

III. THE STUDY OF WIND BEHAVIOR ......................................................................... 7

Introduction ................................................................................................................................. 7

Wind Characteristics.................................................................................................................... 8

Wind Engineering and Current Codes and Standards ................................................................. 9

Wind Tunnel Testing on Solar Panels ....................................................................................... 18

The Future of Standardized Design ........................................................................................... 21

Conclusions ............................................................................................................................... 25

IV. PROJECT OVERVIEW AND PANEL LOCATION ................................................ 26

Introduction ............................................................................................................................... 26

Faux Solar Panel Test Frame Design ........................................................................................ 28

Faux Solar Panel Test Frame Construction ............................................................................... 30

Faux Solar Panel Frame Installation and Setup ......................................................................... 37

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V. EQUIPMENT............................................................................................................... 44

Introduction ............................................................................................................................... 44

Wind Measurements .................................................................................................................. 44

Thermocouple ............................................................................................................................ 45

Strain Transducers ..................................................................................................................... 46

Campbell Scientific Datalogger and Accessories ...................................................................... 51

Software ..................................................................................................................................... 54

VI. THEORY .................................................................................................................... 56

Introduction ............................................................................................................................... 56

Wind Behavior ........................................................................................................................... 57

Pressure Measured from Strain .................................................................................................. 62

Pressure Measured from Wind Velocity ................................................................................... 64

Coefficient of Force ................................................................................................................... 64

VII. RESULTS .................................................................................................................. 66

Introduction ............................................................................................................................... 66

Results ....................................................................................................................................... 66

Discussion .................................................................................................................................. 88

VIII. SUMMARY AND CONCLUSIONS ...................................................................... 94

Summary .................................................................................................................................... 94

Conclusions ............................................................................................................................... 95

Possible Sources of Error .......................................................................................................... 98

Recommendations for Further Research ................................................................................... 99

REFERENCES ............................................................................................................... 101

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APPENDIX

A. DATALOGGER PROGRAM.................................................................................... 104

B. HAND CALCULATIONS ......................................................................................... 108

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LIST OF FIGURES

Figure

1. Wind Interaction with Building. .................................................................................. 12

2. External Pressure Coefficients. .................................................................................... 14

3. Net Pressure Coefficients for Monoslope Free Roofs. ................................................ 16

4. Variables in Solar Panel Wind Load Determination.................................................... 18

5. Part one of Figure 29.9-1 from SEAOC Publication. .................................................. 23

6. Figure 29.9-1 from SEAOC Publication. ..................................................................... 24

7. Initial Panel Placement. ............................................................................................... 27

8. Project Panel Location. ................................................................................................ 27

9. Aerial View of Events Center Building and Surroundings. ......................................... 28

10. Special Wind Region in Colorado. ............................................................................. 29

11. Special Wind Region in Colorado. ............................................................................ 30

11. Panel A Detailed Section. ........................................................................................... 31

12. Panel B Detailed Section. .......................................................................................... 32

13. Panel C Construction Detail........................................................................................ 32

14. Tension Tie Connection Detail. .................................................................................. 35

15. Panel Cross Section View in Weak Axis. .................................................................. 35

16. Panel Connection Detail. ........................................................................................... 36

17. Completed Panel B. ................................................................................................... 36

18. Angle Connection Shop Drawing Detail. .................................................................. 38

19. Panel A Steel Tube Leg Shop Drawings.................................................................... 39

20. Panel B Steel Tube Leg Shop Drawings. ................................................................... 40

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21. Panel Layout. ............................................................................................................. 42

22. Anemometer Tree. ..................................................................................................... 43

23. RM Young 3101 Anemometer and RM Young 3301 Wind Sentry Vane. ................. 45

24. Strain Transducer Assembly. ..................................................................................... 46

25. Strain Transducer A Calibration Curve. .................................................................... 47

26. Strain Transducer B Calibration Curve. ..................................................................... 48

27. Strain Transducer C Calibration Curve. ..................................................................... 48

28. Strain Transducer E Calibration Curve. ..................................................................... 49

29. Strain Transducer F Calibration Curve. ..................................................................... 49

30. Strain Transducer Placement Diagram. ...................................................................... 51

31. Campbell Scientific Measurement and Control Datalogger. ..................................... 52

32. Solar Panel Providing Power to Datalogger. ............................................................. 53

33. Campbell Scientific SDM-INT8 8-Channel Interval Timer. ..................................... 54

34. Wind Behavior at Panel Location. ............................................................................. 58

35. Streamer Experiment at Roof Edge. .......................................................................... 60

36. Streamer Experiment 20 feet From Roof Edge. ......................................................... 60

37. Streamer Experiment 40 feet from Roof Edge. ......................................................... 61



40. Schematic Diagram of Faux Solar Panel Theory. ...................................................... 63

41. CF vs. Wind Direction. ................................................................................................ 68

42. Wind Velocities of Each Anemometer. ..................................................................... 68

43. Wind Velocity, Strain and CF Values Data from 10/4/13 02:51AM. ........................ 69


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55. Wind Velocity, Strain and CF Values Data from 10/5/13 12:13PM. ......................... 82

56. Wind Velocity, Strain and CF Values Data from 10/5/13 12:13PM. ......................... 83

57. Wind Velocity, Strain and CF Values Data from 10/11/13 11:39AM. ...................... 84

58. Wind Velocity, Strain and CF Values Data from 10/11/13 11:39PM. ....................... 85



61. Schematic Time History of Wind and Strain Curves................................................. 90

62. Net Pressure Coefficients for Monoslope Free Roofs. ............................................. 97

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LIST OF TABLES

Table

1. Summary of CF values for Panel B from Figure 43. .................................................... 69

2. Summary of CF values for Panel A from Figure 44. ................................................... 70








10. Summary of CF values for Panel A from Figure 52. ................................................. 78

11. Summary of CF values for Panel B from Figure 53. .................................................. 80








19. Summary of Net Peak CF Values for Panel B. .......................................................... 92

20. Summary of Net Peak CF Values for Panel A. .......................................................... 92

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CHAPTER I

OVERVIEW

Introduction

Structural engineers are tasked with the great responsibility of ensuring life safety.

“Engineers shall hold paramount the safety, health and welfare of the public and shall

strive to comply with the principles of sustainable development in the performance of

their professional duties” (Code of Ethics 2013). Engineers must evaluate several

probability based design load combinations including environmental phenomena such as

snow, wind and earthquakes. The general study of wind has been ongoing for centuries,

however the term “Wind Engineering” became a common expression only as recently as

the early 1970s (Cochran 2010). Over time, wind engineering has become a significant

and essential branch of the structural engineering profession.

With roof-mounted solar panels becoming an increasingly popular means of

generating energy comes the obligation of providing a sound design to resist the

somewhat unpredictable, yet probable, wind loads that will affect them. Common

engineering codes and standards frequently used by engineers, such as ASCE7, are silent

on the subject of wind loads on rooftop solar panels. Thus, the engineer is left to use his

or her best judgement, in combination with what information is provided within the codes

and standards, when developing a method to determine what wind loads the solar panel,

its various connections and the supporting roof structure should be designed to withstand.

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Objective

Although efforts to determine wind loads on solar panels have been ongoing for

sometime, few full-scale experiments are reported (Harris 2013). The purpose of this

research is to provide baseline data, in the form of a coefficient, for comparison to wind

tunnel study results. When wind tunnel studies were relatively new technology,

significant amounts of full-scale research were performed to validate the results (Cochran

2010). Similarly, solar panels are fairly new rooftop features for which wind tunnel test

results need to be validated using full-scale models.

Procedure

The procedure of obtaining data for comparison included gathering real time

measurements of wind velocity and direction and corresponding strain measurements.

Using the wind velocity in combination with the barometric pressure it is possible to

derive the wind-induced forces on the faux solar panel. Using the strain measurements in

combination with geometrical equations it is possible to derive the net force acting on the

face of the faux solar panel. Computing the ratio of these two calculated values renders a

coefficient that is of great use. This coefficient, deemed the Coefficient of Force, or CF, is

a number that can be compared to the Coefficient of Pressure, or Cp, as calculated using

ASCE 7. Comparing these two numbers provides significant insight into the difference

between measured pressure and pressure calculated with wind tunnel values that are

written into current standards.

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Outline

This thesis contains 8 chapters. The first chapter is dedicated to describing the

background information and main objective of this research. Chapter 2 is a literature

review of available sources of information regarding wind engineering and solar panels.

Several topics including the history of wind engineering, an overview of wind tunnel

studies and the history of solar panels are explored. In Chapter 3 the study of wind

behavior and current standard design procedures are presented. An overview of the

project set up and panel construction including the fuax solar panel design, location and

installation is presented in Chapter 4. Chapter 5 covers the instrumentation and

equipment used to conduct the research for this thesis. In Chapter 6 the theory behind

this research is discussed. The results of the research are presented in Chapter 7,

followed by discussion. Chapter 8 includes a summary of the project and a conclusion of

the results as well as possible sources of error and suggestions for future research.

There are two appendices that are included in this thesis. Appendix A provides

the program that was used to collect data for this study. The calculations used for

determining design wind pressure acting on the panels, designing the steel members of

the panel frames, and the derivation of key equations are presented in Appendix B.

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CHAPTER II

THE IMPORTANCE OF ENERGY

Introduction

Harnessing the power of the sun is not a new technology. Generations before us

recognized the importance of the sun and its ability to bring forth light, heat and life.

Early civilizations dating back to the 7th

Century B.C. used the magnifying glass to create

fire (History of Solar 2012). In the 6th

Century A.D. sun rooms were common in

buildings and “sun rights” were initiated so that individual access to sun light was

available (History of Solar 2012). The first solar collector was built in 1767 by Horace

de Suassure and years later in 1954 the first silicon photovoltaic cell capable of running

everyday electrical devices by converting the sun’s energy, running at 4% efficiency, was

created at Bell Telephone Laboratories (History of Solar 2012). In the last 50 years solar

technology has advanced significantly. Today photovoltaic cells are used to power

satellites, airplanes, automobiles and both residential and commercial buildings.

Modern Solar Panels

The basic function of a solar panel is to convert sunlight to energy, a relatively

simple concept that appeals to the masses due to the fact that sunlight is readily available

and free of charge. A solar cell is composed of several layers, the most important of

which are two semiconductor layers. When photons from sunlight are absorbed by the

solar cell an electron is freed. The electron is naturally attracted across the boundary

electric field that is created where the two semiconductor layers meet, causing an

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imbalance in electric charge within the cell. In order to reinstate a balance of charge

within the semiconductor the electron must be expelled. The electric field only operates

in one direction, therefore the electron must travel through an external circuit, generating

electricity. The outermost layer of a photovoltaic cell is the glass surface, which is used

to protect the cell from the environment. A clear, antireflective coating is located just

below the glass surface. The purpose of this antireflective coating is to reduce the

amount of sunlight reflected by the glass. Without the coating approximately 30% of the

sunlight that comes into contact with the panel is reflected away from the cell, compared

to 5% when the coating is utilized. In order to maximize energy output the amount of

sunlight absorbed by the cell needs to be maximized.

Solar panels are mounted in a variety of locations including on the ground and on

the roofs of buildings. If shading is a factor, as it often is within an urban environment,

roof mounted solar panels will be exposed to more sunlight and thus produce more

energy. In the Northern hemisphere it is common practice to install solar panels so that

they face south, where the sun makes its daily path through the sky. It is becoming

increasingly more common to see multiple solar panels installed on the roofs of

commercial buildings and residential homes.

Conclusions

It is an undeniable fact that human beings have made an impact on this planet and

its ecosystem. The primary fossil fuels that are refined into different sources of energy

used on a daily basis around the world include petroleum, natural gas and coal. Burning

coal is the largest source of energy for the generation of electricity to supply power to the

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population. Burning coal has a negative impact on the Earth’s biosphere by emitting

large amounts of carbon dioxide, a greenhouse gas, which has been linked to

controversial topics like climate change and global warming. Coal is the largest

contributor to the human-made increase of carbon dioxide in the atmosphere. In the

United States in 2011, coal accounted for 20% of the available energy resources, while

renewable energy accounted for just 9% (DOE 2013). That same year, 91% of coal

burning processes went to generating energy for electric plants, whose primary business

is to sell electricity to the public (DOE 2013). Electric plants account for 40% of the total

energy consumption, more than transportation (DOE 2013). It is obvious that there is a

need for increased utilization of renewable energy resources, especially in the area of

electricity production.

In an effort to mitigate the extensive amount of energy used to burn coal and

supply power to the booming communities around the world, several alternative energy

sources have been studied and some have been implemented on a large scale. The

advancements in solar technology have made solar panels both more accessible and more

affordable for the average American business and homeowner. To increase the appeal of

using solar panels as an alternative power source, the Energy Policy Act of 2005 began

the Residential Renewable Energy Tax Credit program, which offers a 30% tax rebate on

qualified expenditures for a solar-electric system. As a result, the solar industry has

surpassed the engineering industry in terms of preparedness.

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CHAPTER III

THE STUDY OF WIND BEHAVIOR

Introduction

Efforts to expand the body of knowledge on the wind loading of structures has

been evident through the heightened amount of research that has taken place around the

world in the last 35 years (Holmes 2001). The design of every structure takes into

consideration both gravity and lateral design. In each individual situation wind loading

competes with seismic loading for controlling the lateral design, which is largely

dependant on the location of the structure. Although seismic events are often more feared

than wind events and the loading from earthquake induced movement is typically greater

in magnitude than that of the design wind load, it has been shown that the frequency of

shaking resulting from an earthquake can often be comparable to the buffeting caused by

wind, proving these natural disasters can produce equally devastating outcomes (ASCE

2005). Although wind storms and earthquakes have created roughly the same amount of

damage over the years, wind storms are much more common and widespread than

earthquakes (Holmes 2001).

Between the years of 1980 and 2010 a total of 640 natural disasters were reported

in the United States (Prevention Web 2013). Of those occurences, 24 were earthquake

related and 392 were storm related (Prevention Web 2013). Of all of the types of natural

disasters reported over that same time period the greatest number of people killed and the

highest economic damages were due to storms (Prevention Web 2013). Note that for the

purposes of this statistical study storms and floods are classified as separate disasters,

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however floods are recognized as the overflow of bodies of water caused by wind events,

such as hurricanes (Prevention Web 2013). Storms are classified under the meteorlogical

disaster subgroup and types of storms include thunderstorms, severe storms, tornadoes,

and orographic storms, which are associated with high winds (EM-DAT 2013). The

United States has the largest occurence of tornadoes in the world (ASCE 2005). This

data suggests that although seismic events and wind related events have produced

roughly the same amount of damage, wind related events occur more frequently, affect

more people over widespread locations, and in some cases are more severe. Designing

for wind loading is a very necessary component of the design of any structure. It has

been acknowledged that wind is a somewhat unpredictable component of building design.

Therefore, researchers and engineers generalize wind pressures to fall within a reasonable

envelope of design parameters.

Wind Characteristics

Wind has been depicted as a mysterious act of nature, and seems to occur

completely at random; in reality, wind is driven by the solar heating of the earth’s

atmosphere which leads to pressure differentials and ultimately wind flow (ASCE 2012).

Wind is a dynamic force, a three-dimensional and time-variant phenomenon, which

emulates the characteristics and movement patterns of a fluid. In fact, wind profiles are

derived theoretically from principles of fluid mechanics. Several independent factors

influence wind flow including the surrounding environment, terrain and topography,

elevation and directionality. Mean wind speeds measured over a specific time interval

are of some importance. A wind gust is defined as “the noticeable increase in wind speed

9

relative to the mean speed over a short duration” (ASCE 2012). The peak gusts in the

mean wind speed are incorporated in design wind values in codes and standards.

Wind engineering is mostly concerned with the region of these enormous

atmospheric motions that collides with the surface of the earth. The ground surface and

all that is attached to it creates friction, called surface drag, within this local circulation.

Surface drag has a significant effect on the wind near the surface of the earth, called the

Atmospheric Boundary Layer (ASCE 2012). One impact that surface drag produces is

the slowing of the mean wind flow near the gound, which is why surface roughness is an

important factor when considering wind design. The influence of surface drag decreases

as elevation increases which indicates that the mean wind flow is a function of height

(ASCE 2012). Turbulence is another product of protrusions and terrain interfering with

surface drag (ASCE 2012).

Wind Engineering and Current Codes and Standards

"Wind engineering is best defined as the rational treatment of interactions

between wind in the atmospheric boundary layer and man and his works on the surface of

Earth” (Banks 2011). Building codes and standards are regulations that are enforced by

local building departements with the intention of ensuring uniformity, quality and safety

among building design and construction. One of the most commonly used engineering

standards in the United States is the American Society of Civil Engineers (ASCE)

standard number seven titled Minimum Design Loads for Buildings and Other Structures

(ASCE 7). Typically a designer begins his or her wind engineering analysis by

determining the appropriate wind speed and resulting wind pressures from ASCE 7 to be

10

used in the deisgn of the main wind force resisting system and the components and

cladding. In the case of solar panels, the design wind pressure is used to calculate the

resulting downward and uplift forces acting on the panel so that it can be properly

attached to roof support structure. Note that mechanically attached solar panels will not

increase the total wind load acting on the roof surface, rather the structural support

member will need to be designed so as to sufficiently resolve the panel’s forces (Banks

2011).

It is important to understand that the design wind speeds provided in ASCE7 are

probabilistic in nature. When data points are accumulated over a long period of time a

pattern eventually emerges. This pattern is analyzed by statisticians, meteorologists and

wind engineers and is known as a probability distribution for the ASCE 7 standard. This

means that the wind speed that a building is designed for has a 7% probability of being

exceeded over a period of 50 years (ASCE 2010).

Wind is composed of moving air, which is a gas; because both gases and liquids

are classified as fluids, it is not surprising that the movement of wind emulates the lfow

of a liquid. For this reason, the main equation for determining the design wind pressure

has evolved from the well known Bernoulli principle. Bernoulli’s theorem states that an

increase in the speed of a fluid occurs proportionately with an increase in its pressure

(Finnemore et al. 2002). A simplified equation demonstrating the theorem is presented

below.

= (1)

Where p is the pressure, ρ is the density of the fluid, and V is the velocity of the fluid.

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The same principle can be applied to the incompressible flow of wind. Equation

27.3-1 from ASCE7, which determines the velocity pressure of wind with respect to the

height above the ground surface, is presented below.

= 0.00256 (2)

Where qz is the pressure, Kz is the velocity pressure coefficient, Kzt is the topographic

factor, Kd is the wind directionality factor, and V is the basic wind speed. The 0.00256

term is simply a conversion to mass density of air at standard atmospheric pressure and

temperature. The velocity pressure coefficient accounts for the height above ground level

and the exposure at the building site, which is known to affect the surface drag and in

effect the mean wind flow. The topographic factor accounts for wind speed-up effects in

relation to the surrounding topography. The wind directionality factor takes into

consideration the angle at which the wind flow will collide with the bluff body.

Therefore, Equation 2 is essentially equivalent to Equation 1, and is a valid method of

calculating wind pressure.

Figure 1 below demonstrates the behavior of wind flow as it approaches and

consequentially is interrupted by a building with a parapet and a flat roof. The

streamlines reach the face of the building and in effect must be redirected. To simplify

design, it is assumed that the streamlines above the midpoint of the surface continue their

path upwards along the wall, and the streamlines below the midpoint are relayed

downwards. Once the streamlines reach the leading edge, in this case a parapet, a

separation point is formed. The shear layer is then generated from this separation point at

a slope of 2:1 towards the building (SEAOC 2012). The shear layer separates

streamlined flow above from turbulent recirculation below. At a distance of between

12

approximately one and two building heights from the edge, the shear layer reattaches to

the roof surface and streamlined flow is reestablished.

Figure 1. Wind Interaction with Building.

This figure represents the behavior of wind flow as it comes into contact with a building

surface.

Although efforts to establish a method to determine wind loads on solar panels

have been ongoing for a number of years, a standard approach has not been adopted by

any building codes or standards. While the ASCE 7 document provides in depth

information on design wind speeds and wind pressures for buildings, components and

cladding and rooftop structures, there is no guidance on design values to be used in

conjunction with rooftop mounted solar panels. Similarly, other documents, including

the International Building Code (IBC) and the International Residential Code (IRC), are

silent on the subject. The result is that practicing engineers use the materials and

information that is available to them combined with their best judgement to design the

structural components of a roof mounted solar panel system to withstand estimated wind

pressures.

13

In the U.S., there are currently two approved methods for determining wind loads

on solar panels. The first method is to use tables provided in ASCE 7 and the second

method is conducting a wind tunnel test (Banks 2011). It should be noted that for the

following two procedures presented for determining wind loads on solar panels, the

velocity pressure is calculated at mean roof height with the same Kz, Kzt, and Kd factors,

as well as the same importance factor, as would be used in the design of the building

itself. This is true because it is generally not common practice to design components

placed on a building to higher standard then the building itself, however it is necessary

for those components to be able to withstand the same design wind occurrence that may

be imposed on the building.

Figure 30.4-1 in ASCE7-10, shown in Figure 2, is often used to approximate the

external pressure coefficient for flush mounted solar arrays. This figure is actually

intended to determine the wind pressure acting on the components and cladding on the

roof of a partially enclosed building with a gable roof of varying slopes, however this

method will yield conservative results (Banks 2011). The equation to be used in

conjunction with the aforementioned figure is as follows.

= − (3)

Where qh is the velocity pressure evaluated at mean roof height (see Equation 2), GCp is

the external pressure coefficient as determined from the appropriate figure, and GCpi is

the appropriate interal pressure coefficient determined from Table 26.11-1. The internal

pressure coefficient is constant for each enclosure classification, but the external pressure

coefficient varies with the effective wind area.

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Figure 2. External Pressure Coefficients.

Figure 30.4-1 from ASCE 7-10 used to determine external pressure coefficients on

components and cladding of enclosed buildings (ASCE 7-10 2010, used with permission

from ASCE).

15

For tilted panels, the data for monoslope free roofs in Figure 27.4.4 in ASCE7-10,

shown in Figure 3, is often used to determine wind loads on tilted solar panels. The

equation used in conjuction with this figure to determine the net design pressure is

presented below.

= (4)

Where p is the net pressure, qh is the velocity pressure evaluated at mean roof height (see

Equation 2), G is the gust-effect factor, and CN is the net pressure coefficient as

determined from the appropriate figure. The gust effect factor, G, is determined in

accordance with Section 26.9 and is permitted to be taken as 0.85 for a rigid structure,

however it is recommended that G be increased to 1.0 when using this method to

determine wind loads on solar panels (Banks 2011).

16

Figure 3. Net Pressure Coefficients for Monoslope Free Roofs.

Figure 27.4-4 from ASCE7-10 used to determine wind pressure on monoslope free roofs

(ASCE 7-10 2010, used with permission from ASCE).

17

Figure 4 shows the many variables that can have an effect on the complex wind

flow around a solar panel. With the exception of the roof corner and edge regions, these

methods have proven to be reasonable (Banks 2011). Cornering winds can produce

wind speeds nearly 20% greater than the mean wind flow, therefore for the design of

panels placed within two building heights of a roof corner it is suggested that GCN be

multiplied by Kcorner = 1.5 (Bank 2011). It is advised that an uncertainty factor of 1.4 be

utilized in the design of panels to be placed within two building heights of a roof corner

or edge due to uncertainty related to the interaction of the solar array with flow patterns

in these regions (Banks 2011). It is also advised that tilted panels never be placed within

two panel heights plus the parapet height of a roof edge due to the high wind speeds

associated with this region (Banks 2011). This suggestion was deliberately overlooked in

research conducted at the University of Colorado Denver by Jennifer Harris in order to

gather information on the interaction of tilted panels with edge-induced wind velocities

(Harris 2013). It is true that in some cases the parapet can offer some sheltering effects,

but that is generally specific to the region immediately behind the parapet (Banks 2011).

“However, two or three parapet heights from the roof corners, the magnitude and extent

of the wind acceleration a short distance above the roof is increased by the parapet, and

can result in wind loads that are 50% greater than in the absence of a parapet, particularly

for unprotected tilted panels” (Banks 2011). While the research presented in Study of

Wind Loads Applied to Rooftop Solar Panels was not performed on panels placed within

two or three parapet heights from roof corners, it was conducted on panels placed within

two or three parapet heights from the edge of the roof (Harris 2013). The placement of

the panels did result in significant wind velocities (Harris 2013).

18

Figure 4. Variables in Solar Panel Wind Load Determination.

Wind Tunnel Testing on Solar Panels

Scientists and early thinkers have been contemplating the science of wind for

many years, however the use of wind tunnel testing is relatively new technology

(Cochran 2010). Several historical events related to wind induced failures, including the

collapse of the Tacoma Narrows Bridge in 1940, led engineers to the conclusion that

further experimentation must be done and more stringent precautions must be taken.

When wind tunnels started to become an accepted means of studying wind effects on

buildings, the prestigious Twin Towers of the World Trade Center in lower Manhattan in

New York City were among the first buildings to receive testing in the wind tunnel at

Colorado State University (Cochran 2010). After several years of both validating wind

tunnel test results and expanding the abilities of wind tunnels to effectively model real

world conditions, wind tunnel testing became and is currently the only permissible

19

procedure with which to override written code on wind loads (Cochran 2010). In fact,

much of the results obtained from wind tunnel tests have provided the technical basis for

the pressure coefficients used in establishing current codes and standards on determining

wind loads on buildings and other structures (Cochran 2010). Wind tunnel testing has

long been regarded as the most accurate and reliable method while maintaining cost-

effectiveness. “There have been a large number of projects tested by more than one wind

tunnel laboratory where results were very close, typically within about 10%” (Griffis

2006). Wind tunnels have many applications including the study of pedestrian-wind

conditions, dispersion of air pollutants, forensic studies and wind effects on structures.

Wind tunnels are large tubular structures through which air flow is forced by way

of powerful fans. Physical modeling is of the utmost importance with wind tunnel

testing. A precise model of the subject under testing is crucial, along with a detailed

proximity model of the surrounding terrain and upstream topography. Until recently, the

preferred method of replicating the test subject was machined Plexiglass pressure models

(Cochran, 2005). Technology has made it possible to generate complex shapes with

integrated pressure tap paths, making the use of laser-induced stereolithography pressure

models the current favorable approach (Cochran 2005). The flow in the wind tunnel is

most naturally scaled in the range 1:400 to 1:600, and the scale of the model test subject

should be approximately equivalent (Davenport 2007). “In all cases, it is the mean wind

speed profile and the turbulence characteristics over the structure that are most important

to match with those expected in full scale” (Davenport 2007). The object under testing is

mounted to a turntable so wind from multiple directions can be studied. Simply put, the

scaled model of the test subject is instrumented with pressure taps, in some cases up to

20

1,000 transducers can be applied (Cochran 2005). The pressure taps convert the wind-

induced pressure at any given location on the model into an electrical signal, which can

be stored and analyzed subsequently (Cochran 2006). What is significant is that pressure

time-series can be collected over the entire building simultaneously and then developed

into design curves that appropriately envelope the peak pressures.

As the second currently approved method for determing wind loads on solar

panels, wind tunnel testing is increasing in popularity in the commercial wind

engineering community. Many wind tunnel tests have been performed in recent years on

various solar arrays and layouts for numerous solar energy companies; unfortunately, due

to the high cost of wind tunnel testing and the desire of these companies to keep the

findings private, much of the results are proprietary information that is unavailable to the

general public. Therefore it is difficult for practicing engineers to compare their

calculations with actual test results.

Wind tunnel testing on modeled solar panel arrays can help a designer understand

the impact that the array size, shape and placement has on the influence of the wind that

current code methods simply do not address. Through wind tunnel testing it has been

shown that the wind loads are reduced as the array gets larger and that the location on the

roof influences the wind load (Banks 2011). Wind tunnel tests performed for SunLink at

the Boundary Layer Wind Tunnel Laboratory at the University of Western Ontario were

consistent in showing that the maximum pressures measured over a large surface area of

panels was much less than the maximum pressures measured over smaller surface area of

panels (Tilley 2012). In addion, wind tunnel tests have demonstrated that some panel

sheltering occurs. Panels along the edge of an array typically see two to three times the

21

wind load that interior panels experience (Banks 2011).

The findings of several wind tunnel studies performed on solar panels mounted on

flat roofs are presented in Wind Loads on Solar Collectors: A Review (Stathopolous et al.

2012). The comparative results, however, portray very dissimilar conclusions mostly due

to the different configurations of the tested panels. Therefore no reasonable conclusion

was possible in this case. In the case of solar panels mounted on pitched roofs, two

separate studies, one of which involved wind tunnel testing and the other full scale

testing, are presented. The results suggest that the maximum net pressure coefficients

measured from the full scale studies are consistently higher than that of the wind tunnel

testing (Stathopolous 2012). While the specific findings are not presented, the final

verdict claims that “it appears doubtful that many of the systems being deployed can

demonstrate sufficient structural capacity needed to meet code-level requirements”

(Tilley 2012).

The Future of Standardized Design

“In the absence of detailed guidance from ASCE 7 for wind loads on photovoltaic

arrays on flat roof low-rise buildings, designers often attempt to use a hybrid approach of

the ASCE 7 components and cladding tables for enclosed buildings and main force

resisting system tables for open structures, or they use the wind tunnel procedure of

ASCE 7. The hybrid approach can lead to unconservative results” (SEAOC 2012). This

statement is obviously a contradiction of the aforementioned approved methods, evidence

of an extreme need for some form of clarity and specific requirements. Recently there

have been developments in the wind engineering community and a new standard has

22

been developed for the use of determining wind loads on roof-mounted solar panels. The

document Wind Design for Low-Profile Solar Photovoltaic Arrays on Flat Roofs was

published in 2012 by the Structural Engineers Association of California (SEAOC) and is

the culmination of the research of the vast majority of wind engineering experts around

the country and the world. The SEAOC document, shown in Figures 5 and 6, serves as a

proposal for inclusion in the next edition of ASCE 7 and is formatted accordingly. It

provides the general guidelines, definitions, familiar looking equations and coefficients,

figures, and tables that design engineers are accustomed. In addition, the newly

formulated procedure incorporates the location of the panels on the roof, the normalized

wind area and takes into account whether the panels are in a sheltered or edge area, all of

which are known to have a significant impact on the resulting wind pressure. The

equation of interest is presented below, noticeably including a new coefficient.

= (5)

Where p is the velocity pressure evaluated at mean roof height (see Equation 2) and

(GCrn) is the combined net pressure coefficient for solar panels as determined from

Figure 5.

23

Figure 5. Part one of Figure 29.9-1 from SEAOC Publication.

Part 1 of the figure in the document Wind Loads on Low Profile Solar Photovoltaic

Systems on Flat Roofs used to determine design GCP values as published by SEAOC,

August 2012 (SEAOC 2012, used with permission).

24

Figure 6. Figure 29.9-1 from SEAOC Publication.

Part 2 of the figure in the document Wind Loads on Low Profile Solar Photovoltaic

Systems on Flat Roofs used to determine design GCP values as published by SEAOC,

August 2012 (SEAOC 2012, used with permission).

25

Conclusions

There is much confusion and contradiction in combination with limited guidance

in the way of determining wind loads on solar panels. Although wind tunnels have

proven to be an indispensable aid to the practice of structural engineering, it is clear that

they too need validation with full scale testing. As is the case regarding building design,

“the next step in improving our knowledge of highrise building response is to convince

the developer or owner to instrument (with accelerometers, pressure transducers, and

strain gauges) their buildings for research purposes” (Cochran 2006). It is possible that

solar panel systems currently installed on roofs around the country are underdesigned due

to “the lack of validation with the full scale (as was done in the early years of wind-

tunnel modeling)” (Cochran 2006). In fact, the discovery that physical modeling of wind

effects requires a properly simulated boundary-layer flow was reinforced by comparison

of mean pressure measurements from a scale model in a wind tunnel with field

measurements on the full scale building (Cermak 2003). It seems that the experts agree:

full scale testing on wind loads on solar panels is necessary.

In order to help fill a gap in the literature and research, two full scale faux solar

panels were deployed on the roof of the Events Center building on the University of

Colorado Denver’s Auraria Campus in Denver, Colorado. Comprehensive results are

presented in the form of unitless coefficients, making it possible to directly compare them

with the results from past and future wind tunnel and numerical studies.

26

CHAPTER IV

PROJECT OVERVIEW AND PANEL CONSTRUCTION

Introduction

The concept for this experiment was proposed in the summer of 2012 (Dowds

2012) and proceeded to evolve into two separate research projects. The first part of this

study involved collecting data from two faux solar panels placed close to the edge of a

flat roof, intentionally not adhering to guidelines set forth in a recent design standard

(SEAOC 2012). The panels were designed so that the shear layer would intersect

approximately at the midpoint of the face of Panel B while Panel A was well below the

shear layer in the turbulent recirculation region. Higher wind speeds were expected to

occur in correlation with larger CF values at this location. The second portion of this

study pertained to collecting data from the same two faux solar panels placed on the same

flat roof approximately 80 feet, or two times the height of the building, from the roof

edge. It is between the roof leading edge and this location that the shear layer is expected

to reattach to the roof surface, resulting in lower wind speeds than measured during

previous research. The original intention was to fabricate and deploy a total of three faux

solar panels on the roof, but due to site conditions and panel size it was found that

utilizing a total of two panels was more appropriate and Panel C was removed from the

experiment. Figures 7 and 8 illustrate the original location of the panels and the location

of the panels farther from the edge of the roof, respectively.

27

Figure 7. Initial Panel Placement.

Figure 8. Project Panel Location.

28

Figure 9. Aerial View of Events Center Building and Surroundings.

Faux Solar Panel Test Frame Design

The faux solar panels were installed on the flat roof of the Events Center Building

on the Auraria Campus in downtown Denver, Colorado in the spring of 2013, as shown

in Figure 9. This building was chosen for its desireable aerodynamic qualities that

emulate other simple building models that have been tested in wind tunnel studies.

Figure 26.5-1a in ASCE7-10, shown in Figure 10, denotes the basic wind speeds for Risk

Cateogry II buildings and locations of Special Wind Regions (ASCE7 2010). A special

29

wind region exists all along the Front Range in Colorado and is bordered on the east by

Interstate 25. The Auraria Campus is located just east of this boundary. The prevailing

wind direction at the panel site is from the northwest direction, which is approximately

normal to the building. The elevation at the site is approximately 5,248 feet and the

height of the building is 38 feet. The Exposure Category, as defined in Section 26.7.3 of

ASCE7-10, was taken as B in accordance with the urban surroundings. The design wind

speed in the Denver Metro is 115 mph for Risk Category II Buildings per Figure 26.5-1a

in ASCE7-10. In the absence of guidance on rooftop solar panel design wind pressures,

Figure 27.4-4 from ASCE7-10 were used to approximate design values because the shape

of the solar panel test frame closely resembles the monoslope free roof diagram.

Figure 10. Special Wind Region in Colorado.

Figure 26.5-1A from ASCE7-10 used to determine Basic Wind Speeds for Occupancy

Category II Buildings (ASCE 7-10 2010, with permission from ASCE).

30

Figure 11. Special Wind Region in Colorado.

The design wind pressure was calculated using the equation given in ASCE7-10.

This computed wind pressure was applied over the entire face of the panel and the net

resultant force was determined. Geometry and statics provided a means to resolve the

uplift forces at each leg of each panel resulting from the wind loads acting on the face of

the panel. In addition, the tension that would be applied to tension tie was determined.

Using all of this data every componenet of the test frame was analyzed using the typical

structural steel and wood design calculations. All hand calculations can be found in

Appendix B.

Faux Solar Panel Test Frame Construction

The original panel design was based on the estimated location of the shear layer

as it detaches from the parapet at a slope of 2:1. The height of Panel A, shown in Figure

12, is roughly 2’-6” which is assumed to be a good distance below the shear layer when

the panel is located in its original position at 4’-0” from the edge of the roof. Panel B,

31

shown in Figure 13, is significantly taller than Panel A at just over 5’-1”. Panel B was

designed so that, when placed approximately 4’-0” from the edge of the roof, the shear

layer intersects with the midpoint of its surface. A third panel, Panel C shown in Figure

14, was planned to be part of this study as the panel located well above the shear layer.

After much deliberation it was decided that the size of Panel C made it rather difficult to

both fabricate and mobilize and it was removed from the experiment. Relatively high

wind speeds up to 40 mph and CF values averaging 4.6 were reported when the panels

were located near the edge of the roof (Harris 2013). At a distance of approximately 2h,

or 80 feet, from the edge of the roof it was expected that more streamlined wind

velocities and much lower CF values would be recorded.

Figure 11. Panel A Detailed Section.

32

Figure 12. Panel B Detailed Section.

Figure 13. Panel C Construction Detail.

33

The faux solar panels are composed of common materials that are readily

available in most hardware stores. The surface of each panel is constructed with two foot

wide by four foot long segments of 3/8 inch plywood. The vertical legs are 16 gauge one

inch square tube steel. Steel angles are used to fasten each vertical leg to the sheet of

plywood with ½ inch diameter bolts, shown in the connection detail of Figures 15 and 17.

In order to make it possible to move the panels as needed and to prevent damage to the

roof, it was decided that the panels should not be directly connected to the roof structure.

For this reason the legs are bolted to 2x6 wooden members at the base of the frame,

which are weighted down at each end to resist the uplift forces. Tension ties are installed

diagonally between each front and back leg. The tension ties provide the strain data that

is used to calculate the total resultant force acting on the face of the panel. Each tension

tie is composed of 7/16 inch diameter threaded rod and bolted to a strain transducer

through a ½ inch hole. The tension ties are pre-tensioned with tightened nuts on both

sides of the walls of the strain transducers to ensure proper performance. A ¼ diameter

eye bolt is installed through pre-drilled holes at the top and bottom of each vertical leg.

Originally another eye bolt was then slotted onto that eye bolt at each end of each leg and

a coupler was used to fasten the tension tie to the eye bolt. It was determined that the

coupler was becoming too loose and fatigued to maintain a reliable connection so another

solution was found. A ¼ inch diameter hole was drilled through a 7/16 inch diameter

coupler. The coupler is threaded directly onto the eye bolt and the tension tie is screwed

into the coupler, as shown in the connection detail of Figure 15. This provides a means

of direct contact between the panel legs and tension ties. The intention was that this

connection remained a pinned connection in order to direct all horizontal components of

34

force into the diagonal tension tie where it could be measured via the strain transducers.

In order to provide some stability in the short direction of the frame, cable was used to

create an X-brace between the two back legs of the panel, as shown in Figure 16.

35

Figure 14. Tension Tie Connection Detail.

Figure 15. Panel Cross Section View in Weak Axis.

36

Figure 16. Panel Connection Detail.

Figure 17. Completed Panel B.

37

Faux Solar Panel Frame Installation and Setup

Prior to construction of the panel frames, all materials were ordered and collected

from local hardware stores. While some of the construction work was possible with little

experience and common tools, much of it was rather complicated and required the proper

equipment. An experienced contractor performed a majority of the assembly of the

panels. Shop drawings were provided for use in the production of the steel members of

the frames, which was carried out by the Electronics Calibration and Repair Lab at the

University of Colorado Denver. The steel componenets were cut to the proper lengths

and drilled for bolted connections as indicated in the shop drawings, shown in Figures 18,

19 and 20.

38

Figure 18. Angle Connection Shop Drawing Detail.

39

Figure 19. Panel A Steel Tube Leg Shop Drawings.

40

Figure 20. Panel B Steel Tube Leg Shop Drawings.

Once Panel A and Panel B were completed they were mobilized for placement on the

41

roof of the Events Center. Both panels were carried to the building and maneuvered up

three flights of stairs to the roof. In addition, approximately 1,020 pounds in sand bags

were transported to the roof to be used as a means of resisting the uplift on the panel legs.

The panels were situated on a 10 foot square area of concrete pavers in order to prevent

damage to the roof and evenly distribute the additional load to the precast concrete roof

structure. The panels are two feet apart and 80 feet from the edge of the roof. The sand

bags were stacked up on the ends of the 2x6s at the base of each frame. It became

apparent that the sand bags might have some influence on air flow around Panel A due its

shorter dimensions. In an effort to reduce the impact, the sand bags at the base of Panel

A were replaced with much less intrusive, but equally heavy, sections of wrought iron. A

layout of the entire system is shown in Figure 22.

42

Figure 21. Panel Layout.

An anemometer tree, as shown in Figure 23, was fashioned out of pipe sections

and located between the two panels. Three anemometers were used to measure the wind

speed at different elevations. The top anemometer was originally positioned so that it

would be located well above the shear layer. The middle anemometer was originally

intended to intersect the shear layer and the bottom anemometer was originally intended

to be located in the recirculation region. At a distance of 80 feet from the edge of the

roof, all of the anemometers were expected to be located in the streamlined flow that

occurs beyond the attachment point of the shear layer. Thus, the wind velocity readings

43

from each anemometer were anticipated to be very close. The location of the

anemometer tree in proximity to the solar panels can be seen in Figure 22.

Figure 22. Anemometer Tree.

44

CHAPTER V

EQUIPMENT

Introduction

Sophisticated technology was necessary in order to take several measurements at

short time intervals. Campbell Scientific products were utilized to measure and record

wind velocity and wind direction. Strain gauges were used to measure strain

differentiation in the panels.

Wind Measurements

Three RM Young 3101 Wind Sentry Anemometers were utilized to record the

wind velocity at three different elevations above the roof surface. Each anemometer has

a threshold of 1.1 mph and records wind speed by producing a sine wave that is directly

proportional to the wind velocity each time the cup wheel makes a full rotation

(Campbell Scientific 2007). One RM Young 3301 Wind Sentry Vane was used to

accurately measure the wind direction. The vane was installed at the same elevation as

the highest anemometer. The output of the vane sensor is a voltage that is directly

proportional to the azimuth of the wind direction (Campbell Scientific 2007). The wind

vane was oriented in such a manner that due south was at 0 degrees and the direction

normal to the face of the building, which is north, was set to 180 degrees. Figure 24

shows the anemometer and wind vane.

45

Figure 23. RM Young 3101 Anemometer and RM Young 3301 Wind Sentry Vane.

Figure courtesy of Cmapbell Scientific, Inc., Logan, Utah.

Thermocouple

A Campbell Scientific A3537 Type T Thermocouple wire was used to measure

the ambient air temperature at the panel location. The thermocouple consists of copper

wire and constantan wire (Campbell Scientific 2007). The thermocouple wire is two feet

long and was fastened to the outside of the metal box in which the datalogger is enclosed.

This was done so that the wide ranges of temperatures that the panels were subjected to

were recorded. The recorded temperature values were then compared to reported

temperature values for the same time period.

46

Strain Transducers

A total of five strain transducers were utilized in this study. Each strain transducer

is composed of 2 inch sections of 3 inch diameter steel pipe. A 350Ω strain guage was

adhered to the inside face of each steel pipe section as shown in Figure 25 below. Two ½

inch holes were drilled, 180 degress apart and 90 degrees from the strain gauge, through

the steel pipe section. The proper wiring was then soldered to the strain gauge and set

with epoxy to prevent it from being dislodged from the pipe section. Pieces of silicone

were placed over each strain gauge and taped down with electrical tape for protection.

Once the strain transducers were assembled they were calibrated in order to

produce calibration curves for use in correlating measured strain to subjected load. They

were calibrated with a MTS machine in the Structures Lab at the University of Colorado

Figure 24. Strain Transducer Assembly.

47

Denver. The corresponding strain was measured as the machine imposed load on the

transducer in increments of 100 pounds. The calibration curve for each strain transducer

can be seen in Figures 25 through 29.

Figure 25. Strain Transducer A Calibration Curve.

y = 0.8527x + 299.42

R² = 0.99952

0

100

200

300

400

500

600

700

800

-300 -200 -100 0 100 200 300 400 500 600

Loa

d (

lbs)

Strain (ue)

Strain Transducer A

48

y = 0.8722x + 291.29

R² = 0.99776

0

100

200

300

400

500

600

-300 -200 -100 0 100 200 300

Loa

d (

lbs)

Strain (ue)

Strain Transducer C

Figure 26. Strain Transducer B Calibration Curve.

Figure 27. Strain Transducer C Calibration Curve.

y = 0.8358x - 31.747

R² = 0.9879

0

100

200

300

400

500

600

700

800

0 100 200 300 400 500 600 700 800 900

Load

(lb

s)

Strain (ue)

Strain Transducer B

49

y = 0.8922x - 41.947

R² = 0.97348

0

100

200

300

400

500

600

0 100 200 300 400 500 600

Load

(lb

s)

Strain (ue)

Strain Transducer F

Figure 28. Strain Transducer E Calibration Curve.

Figure 29. Strain Transducer F Calibration Curve.

y = 0.8809x - 76.335

R² = 0.9998

0

100

200

300

400

500

600

0 100 200 300 400 500 600 700

Lo

ad

(lb

s)

Strain (ue)

Strain Transducer E

50

Four transducers were installed on the panels, one on each leg, and one transducer

was used to measure strain related to temperature only. Strain transducers A and B were

installed on Panel A and transducers C and E were installed on Panel B. Strain

transducer F was placed on the roof surface near the panel setup and recorded strain

related to thermal effects. The strain transducers are a very important part of the design

of the faux solar panels. They provide information that is vital to the extraction of the

Coefficient of Force. The tension tie, as described in Chapter IV, was threaded through

the holes on either side of the strain transducer and fastened with 7/16 inch diameter nuts.

When the faux solar panels were subjected to wind the frames flexed and the tension ties

on each leg were pulled, thus creating a tension force in the transducer and producing a

change in strain. This change in strain was measured with the strain guage and recorded.

Each strain transducer was wrapped with an insulating foil material to attempt to

maintain a balanced temperature and deter outside weather interference. Figure 31 shows

the strain transducer layout.

51

Figure 30. Strain Transducer Placement Diagram.

Campbell Scientific Datalogger and Accessories

A Campbell Scientific CR5000 Measurement and Control Datalogger, shown in

Figure 32, was used to record, store and collect data for this project. This particular

datalogger has several input channels and is capable of measuring a large amount of

sensors. The datalogger was kept in a metal box throughout the duration of this research

in order to keep it dry and safe from the elements. The datalogger was last calibrated in

March 2013.

52

Figure 31. Campbell Scientific Measurement and Control Datalogger.

Fiugre courtesy of Campbell Scientific, Inc., Logan, Utah.

An external battery was used to charge the datalogger. A Campbell Scientific

SP20 Solar Panel, shown in Figure 33, was used to provide power to datalogger’s battery.

The panel was oriented towards the south in order to receive maximum sun exposure.

53

Figure 32. Solar Panel Providing Power to Datalogger.

All recorded data was stored to a Campbell Scientific CFMC2G 2GB Compact

Flash card. Since measurements were taken at 0.1 second, a large amount of data was

recorded and exceeded the capacity of the datalogger storage system. The use of the PC

card allowed for data to be stored and downloaded more quickly and less often.

A Campbell Scientific SDM-INT8 8-Channel Interval Timer, shown in Figure 34,

was used to output individual data from each of the three anemometers. The interval

timer allows for individual programming of each of the eight channels and outputs data to

a datalogger (Campbell Scientific 2007).

54

Figure 33. Campbell Scientific SDM-INT8 8-Channel Interval Timer.

Figure courtesy of Campbell Scientfic, Inc., Logan, Utah.

Software

Campbell Scientific RTDAQ Version 1.1 Support Software for High Speed

Dataloggers was used in conjunction with the datalogger. RTDAQ Version 1.1 is

compatible with Microsoft Winds XP, Windows Vista and Windows 7. Short Cut and

CRBasic Editor are functions of the program that were used to create the program used to

record the measurements from the strain gauges, anemometers, thermocouple and wind

direction vane (Harris 2013). The program that was used in this research is available in

Appendix A. The output that the program produced included a date and time stamp,

55

strain recorded from the five strain transducers in microstrain, wind velocity from the

three anemometers in miles per hour, wind direction in degrees, and air temperature in

degrees Farenheit. The RTDAQ program also has a function called Card Convert. Card

Convert was used to convert the data to a format that was recognizable by Microsoft

Excel.

In order to connect a computer to the datalogger, the driver software Trendnet

must be installed on the computer prior to connecting the datalogger cord via the USB

port.

56

CHAPTER VI

THEORY

Introduction

There are few reports of full-scale solar panels used for the purpose of research in

the area of structural engineering design. The overwhelming majority of studies have

occurred in wind tunnels where pressure taps are the main source of information. Each

scaled model has several pressure taps built in which record and report the wind data at

that specific location. This results in a possible source of error in wind tunnel study

practice. Many miniature models of solar panels are virtually covered with pressure taps,

creating an unrealistic replica, which leads to the need for full-scale validation. Although

a viable alternative, due to the high cost of pressure taps, among other issues, the use of

such technology was ruled out early in this project. It should be noted that there were

two options if pressure taps were in fact to be used in this experiment. The first option

included installing a minimal amount of pressure taps in the high priority zones of the

faux solar panel surface, like the corners. In this case, data would be unrepresentative of

the actual wind load acting over the whole surface of the panel. The second option

incorporated pressure taps over the entire surface of the faux panel, thus compromising

the structural integrity of the frame, which is unacceptable. Therefore, it was decided to

forgo the pressure taps and proceed with a more primitive design concept. The ultimate

goal of this research is to provide a coefficient that is comparable to the Coefficient of

Pressure, CP, as provided in ASCE 7. This is a simple way of comparing apples to apples

in order to see the real life effects of wind on solar panels. To do this, an equation was

57

formulated to derive the net force acting on the surface of the panel from the measured

strain. Similarly, an equation to calculate the force acting on the panel due to the

dynamic pressure from the wind velocity was also established. The ratio of the force

measured by the strain transducers to the force from the velocity pressure results in the

coefficient termed CF, the Coefficient of Force. Because no direct pressure measurments

were recorded, this coefficient is not called the Coefficient of Pressure, CP, but it is

comparable to CP.

Wind Behavior

As previously reviewed in Chapter 3, wind flow around a bluff body is a rather

complex phenomenon. When wind collides with the face of the building, its trajectory is

redirected. The flow travels up the side of the building until it can continue back on its

original path. Once the flow reaches the top of the surface, a separation point is

established. It is from this separation point which the shear layer originates. The shear

layer creates a boundary between streamlined flow above and turbulent flow below. At a

distance between approximately h and 2h, where h is the building height, the shear layer

reattaches to the roof surface, as depicted in Figure 34.

58

Figure 34. Wind Behavior at Panel Location.

For this particular research, the panels were installed at a distance of 2h from the

face of the building. It was expected that the recorded wind velocity would be fairly

consistent among all three anemometers in the more streamlined flow beyond the shear

layer. An experiment was performed on the roof on a windy day in order to reinforce this

theory. A 20 foot long section of pipe was assembled. Four five foot long pieces of

orange construction tape were fastened to the pipe at three equally spaced intervals 5 feet

apart. The pole was projected into the wind and photographs were taken to document the

behavior of the wind flow at three different distances from the edge of the roof. As

demonstrated in Figures 35 through 39, it was found that the lowest streamer was rather

turbulent when the pole was close to the edge of the roof. The location of the lowest

streamer is identified with the arrow in each figure. As the distance between the edge of

59

the roof and the pole increased, the turbulence of the streamers decreased. Once the pole

was located directly in front of the panel set up, the flow pattern of all three streamers

appeared to be synchronized and streamlined thus confirming the assumption that the

panels are located beyond the reattachment point.

60

Figure 35. Streamer Experiment at Roof Edge.

Figure 36. Streamer Experiment 20 feet From Roof Edge.

61

Figure 37. Streamer Experiment 40 feet from Roof Edge.


62


Pressure Measured from Strain

In order to derive the net force that is produced when a gust of wind collides with

the underside of the faux solar panel surface, it is necessary to implement relatively

simple geometry and statics basics. A schematic drawing illustrating the force vectors

acting on the panel is shown in Figure 40. Strain is produced when wind causes the test

frame to flex and then measured via the strain transducer that is connected to the tension

tie running diagonally from the bottom of the taller back legs of the panel frame to the

top of the shorter front legs. The strain transducers were intentionally installed as close

to the bottom of the back legs of the panel frame as possible because the largest amount

of reactive force was expected to occur at that location. The equation that was used to

63

compute the net total pressure acting on the faux solar panel surface is provided below

and its derivation can be found in the calculations of Appendix B.

It was taken into consideration that thermal effects could have a significant impact

on the strain readings that were recorded. In order to eliminate this possibility, all of the

data was averaged over 3.0 seconds. Theoretically, it is not possible that the temperature

of the climate surrounding the strain transducers could change drastically over such a

short time period. Therfore, any change in strain that was recorded over that same time

period could not have occurred due to thermal activities, leaving only the deformation of

the panel frame as a source of strain.

Figure 40. Schematic Diagram of Faux Solar Panel Theory.

64

!" =# $%& '

&'( (6)

Where FR is equal to the net resultant force on the panel, T is the tension force measured

in the tension tie, θ is angle of the tension tie with respect to the horizontal roof surface,

and θp is the panel tilt angle, as shown in Figure 40. The complete derivation of this

equation can be found in the hand calculations of Appendix B.

Pressure Measured from Wind Velocity

The force acting on the net area of the panel surface measured from the recorded

wind velocity is deemed FVP Equipment to measure the barometric pressure was not

available for use on this project therefore the correted daily barometric reported was

converted to the uncorrected value. The calculated forces were also averaged over 3.0

seconds. The equation is shown below and the derivation is presented in the hand

calculations of Appendix B.

!)* = +),-

(7)

Where FVP is equal to the force on the panel, ρ is the air density (not corrected for

altitude), V is the measured wind velocity averaged over 3.0 seconds, and A is the net

area of the surface of the panel.

Coefficient of Force

The Coefficient of Force, CF, is equal to the ratio of the forces on the panel

derived from the measured strain and the measured wind velocity.

. = ./

.01 (8)

65

Where CF is the Coefficient of Force, FR is the net resultant force on the panel and FVP is

the force on the panel from the dynamic pressure of the wind.

Simplifying the above equation results in the following equation.

. = # $%& '

+),-& '( (9)

Where CF is the Coefficient of Force, T is the tension force measured in the tension tie, θ

is the angle of the tension tie with respect to the horizontal roof surface, V is the

measured wind velocity, ρ is the air density (not corrected for altitude), A is the net area

of the surface of the panel, and θp is the panel tilt angle. All of these terms are constant

for each panel, with the exception of the tension and the velocity, which are variables.

Therefore, CF is directly proportional to the change in strain over the change in velocity.

66

CHAPTER VII

RESULTS

Introduction

The following content presents the findings from both faux solar panels. For the

purposes of calculating CF, the strain, wind velocity and wind direction measurements

were measured at one tenth of a second. All data was post processed over three second

rolling averages.

Results

As stated, all measurements were recorded at one tenth of a second, resulting in a

massive amount of data to process. In order to reduce the amount of data requiring

processing, a method of filtering out useless numbers was employed. First, the data was

reduced to periods of time over which the majority of the recorded wind velocity was in

the direction of interest. In the data presented this direction is perpendicular to the back

of the panels, corresponding to 180 degrees. In all cases a tolerance of 10 degrees was

allowed. Next, measurements that were recorded over long periods of time when the

wind velocity was less than 17 miles per hour were removed and the maximum recorded

wind gusts were located. Thus, only data resulting from significant wind speed in the

correct direction was processed. The entire segment of resulting measured strains and

wind speeds are graphed over this period of time during which a significant wind event

occurred. It should be noted that any interval of time over which the data is graphed may

include wind speeds and directions that are less than desireable; the point of this is to

67

represent the behavior that is displayed when the wind speed increases and is

approximately normal to the panels. The criteria used to present corresponding CF values

were much more stringent. The Coefficient of Force is only presented when the wind is

at least 17 miles per hour and in the direction of interest. In addition, the measured strain

and wind velocity must be either increasing or decreasing in synch with one another, so

as to capture the CF that is the direct result of the change in strain. Figure 41 illustrates

the large amount of CF values that are calculated along with the corresponding wind

direction. Much of these CF numbers are ruled out in the filtering process. It should be

noted that only the wind velocity measured from the tallest anemometer was used in the

calculations, however there was good correlation between the readings from all three

anemometers, as can be seen in Figure 42.

68

Figure 41. CF vs. Wind Direction.

Figure 42. Wind Velocities of Each Anemometer.

69

Figure 43. Wind Velocity, Strain and CF Values Data from 10/4/13 02:51AM.

The values presented are from a selected portion of data using a 3-second rolling average.

The strains are plotted on an arbitrary scale.

Table 1. Summary of CF values for Panel B from Figure 43.

CF - 3-second average

4.3 2.7 2.1 1.5 0.7 0.1

4.1 2.6 2.1 1.5 0.7

4.0 2.4 2.1 1.4 0.6

3.7 2.4 2.0 1.4 0.6

3.5 2.3 2.0 1.2 0.6

3.2 2.3 2.0 1.2 0.5

3.1 2.3 1.9 1.2 0.5

3.0 2.2 1.8 1.0 0.4

2.9 2.2 1.7 1.0 0.3

2.7 2.1 1.7 0.9 0.3

70




Table 2. Summary of CF values for Panel A from Figure 44.


4.7 0.9 0.2 2.6 0.6 0.1 2.1 0.6 0.1 1.5 0.5 0.1 1.3 0.5 1.2 0.4 1.1 0.2 1.0 0.2 1.0 0.2 0.9 0.2

71






7.6 1.4 0.7 0.4

6.9 1.3 0.7 0.3

6.5 1.3 0.7 0.3

2.2 1.1 0.7 0.2

2.1 0.9 0.6

2.0 0.8 0.5

1.8 0.7 0.5

1.7 0.7 0.5

1.6 0.7 0.5

1.5 0.7 0.4

72






7.8

0.2

0.1

0.3

0.2

0.3

0.8

0.1

0.3

0.3

73






3.2 0.7

3.1 0.7

1.9 0.7

1.6 0.6

1.4 0.6

1.2 0.5

1.0 0.4

1.0 0.1

0.9

74






1.9

1.5

1.2

0.6

0.5

0.2

0.1

0.1

0.1

75






10.6 3.9 2.7 1.1 0.8 0.8 0.5 0.2

8.6 3.8 2.5 1.1 0.8 0.7 0.5

8.1 3.6 2.5 1.1 0.8 0.7 0.5

6.1 3.4 2.2 1.1 0.8 0.7 0.5

5.2 3.3 2.0 1.1 0.8 0.7 0.4

4.9 3.1 2.0 1.0 0.8 0.6 0.3

4.9 3.0 1.8 1.0 0.8 0.6 0.3

4.4 3.0 1.6 1.0 0.8 0.6 0.3

4.2 2.9 1.5 0.9 0.8 0.5 0.2

4.2 2.8 1.2 0.8 0.8 0.5 0.2

76






4.5 0.6 0.4

3.1 0.6 0.3

2.3 0.6 0.3

1.7 0.6 0.3

1.3 0.5 0.2

1.0 0.5 0.1

0.9 0.5

0.9 0.5

0.8 0.5

0.7 0.5

77






16.3 1.7 0.8 0.1

10.3 1.7 0.8 0.1

5.9 1.7 0.7 0.1

3.2 1.5 0.6 0.1

2.6 1.5 0.4

1.9 1.2 0.4

1.9 1.2 0.3

1.9 1.0 0.2

1.8 0.9 0.2

1.8 0.9 0.2

78






14.9 0.4

10.6 0.4

10.4 0.3

4.9 0.3

4.0

3.5

0.6

0.4

0.4

79




80



7.8 4.2 2.1 1.4 1.3 1.0 0.7 0.4 0.3

7.4 3.9 2.1 1.4 1.3 1.0 0.7 0.4 0.3

7.3 3.8 1.9 1.4 1.2 1.0 0.6 0.4 0.3

7.0 3.5 1.9 1.4 1.2 1.0 0.6 0.4 0.2

6.6 3.3 1.8 1.4 1.2 0.9 0.6 0.4 0.2

6.1 3.1 1.7 1.4 1.2 0.9 0.6 0.4 0.2

5.3 2.9 1.7 1.3 1.2 0.9 0.6 0.4 0.1

4.9 2.8 1.6 1.3 1.2 0.9 0.5 0.4

4.9 2.8 1.6 1.3 1.2 0.8 0.5 0.4

4.9 2.7 1.6 1.3 1.1 0.8 0.5 0.4

4.8 2.6 1.6 1.3 1.1 0.8 0.5 0.4

4.8 2.6 1.6 1.2 1.1 0.7 0.5 0.4

4.8 2.5 1.6 1.2 1.1 0.7 0.5 0.4

4.6 2.4 1.5 1.2 1.1 0.7 0.4 0.3

4.4 2.4 1.5 1.2 1.0 0.7 0.4 0.3

4.2 2.3 1.5 1.2 1.0 0.7 0.4 0.3

3.9 2.3 1.5 1.2 1.0 0.7 0.4 0.3

3.8 2.2 1.4 1.2 1.0 0.7 0.4 0.3

3.5 2.2 1.4 1.1 1.0 0.7 0.4 0.3

81






4.8 2.2 0.4 0.2

4.7 0.9 0.3 0.2

3.9 0.9 0.3 0.2

3.6 0.8 0.3 0.2

3.2 0.7 0.3 0.2

3.1 0.6 0.3 0.2

2.9 0.6 0.3 0.1

2.4 0.6 0.3 0.1

2.4 0.5 0.3

82

Figure 55. Wind Velocity, Strain and CF Values Data from 10/5/13 12:13PM.




CF - 3-second Average

4.0 0.5 0.4 0.2 0.2 0.2 0.1

0.6 0.5 0.4 0.2 0.2 0.2 0.1

0.6 0.5 0.4 0.2 0.2 0.2

0.6 0.5 0.3 0.2 0.2 0.2

0.6 0.5 0.3 0.2 0.2 0.2

0.6 0.5 0.3 0.2 0.2 0.1

0.6 0.5 0.3 0.2 0.2 0.1

0.6 0.5 0.3 0.2 0.2 0.1

0.5 0.4 0.3 0.2 0.2 0.1

0.5 0.4 0.3 0.2 0.2 0.1

83






3.2 0.8 0.3 0.2

2.6 0.6 0.3 0.2

1.6 0.6 0.3 0.1

1.4 0.5 0.3 0.1

1.3 0.5 0.3 0.1

1.2 0.5 0.3 0.1

1.1 0.4 0.2 0.1

0.9 0.4 0.2

0.9 0.3 0.2

0.9 0.3 0.2

84






10.1 1.0 0.5 0.2

6.3 0.9 0.4 0.2

4.3 0.9 0.4 0.2

2.2 0.9 0.4 0.2

2.1 0.9 0.3 0.1

1.9 0.6 0.3 0.1

1.9 0.6 0.3 0.1

1.7 0.6 0.3 0.1

1.6 0.6 0.3 0.1

1.1 0.5 0.2 0.1

85






10.1 1.0 0.4

6.3 0.9 0.3

5.3 0.9 0.3

4.7 0.6 0.3

4.3 0.6 0.2

3.4 0.5 0.2

1.3 0.4

1.1 0.4

1.1 0.4

1.0 0.4

86






12.6 1.1 0.6 0.3 0.2 0.1

3.7 1.0 0.5 0.3 0.2 0.1

3.7 0.9 0.5 0.3 0.2

2.4 0.9 0.5 0.3 0.2

1.8 0.8 0.4 0.3 0.2

1.6 0.8 0.4 0.3 0.2

1.6 0.7 0.4 0.3 0.1

1.3 0.6 0.4 0.3 0.1

1.3 0.6 0.4 0.2 0.1

1.2 0.6 0.3 0.2 0.1

87






12.6 0.6 0.1

3.7 0.5 0.1

1.8 0.4 0.1

1.6 0.4

1.6 0.3

1.3 0.3

1.1 0.3

1.0 0.2

0.7 0.2

0.6 0.2

88

Discussion

The time over which the wind velocity, strains and corresponding CF values are

graphed was chosen arbitrarily. The intervals that were selected for each graph included

in the time histories were chosen because they enveloped the highest wind gusts that

occurred in the direction of interest over that particular time period. It is observed that

the wind velocity and strain measurements are highly variable over a short amount of

time. For this reason, the averages of each strain measurement and the wind velocity

measurement are included in the time histories.

Each figure above represents the data for Panel B and Panel A over corresponding

time intervals. For example Figure 43 displays the data for Panel B and Figure 44

displays the data for Panel A. In both cases, the time at which the graph begins is 2:51am

on October 4, 2013. Figure 43 Figure through 60 display CF values that occurred on each

panel when the wind was approximately perpendicular to the panel and in each case the

data was averaged over a three second interval. Note that there is an apparent response in

each strain measurements when the wind velocity changes; the response of the strains on

Panel A mirror the response of the strains on Panel B, and vice versa. This is evidence

that the strain data is valid.

It can be observed that, in general, more CF values are reported for Panel B than

are reported for Panel A. It can also be noted that in most cases the maximum CF value

reported for Panel B is greater than the maximum CF value reported for Panel A. This

behavior is expected because Panel B is taller and more exposed than Panel A. Notice

also that the CF values of each panel occur at about the same moment in time, suggesting

that both panels are reacting to a wind force and the coefficients are in fact legitimate.

89

The Coefficients of Force that are summarized in the tables above range from 0.1

to 16.3. The majority of the CF values fall within the 0.1 to 5 range, but values as high as

10 were found with both panels. A total of 29 CF values are greater than 5. Four CF

values range from 5.0-5.9, seven range from 6.0-6.9, six values ranger from 7.0-7.9, two

values range from 8.0-8.9, six values range from 10.0-10.9, two values range from 12.0-

12.9, one value ranges from 14.0-14.9, and one value ranges from 16.0-16.9.

In some rare cases a rather large CF value was calculated. It should be noted that

pressure coefficeints as high as 10 are reported to “often occur” and up to 20 have

occasionally been measured; however these values are associated with corner vortices

(Holmes 2007). Careful consideration was given to the validity of large CF values. CF is

simply the ratio of a measured force, FR, over a calculated force due to velocity pressure,

FVP. The FVP term is always in the denomitor of this ratio, therefore the square of the

velocity is always in the denominator as well. Because of this, if the change in velocity

was somewhat small while the change in strain was relatively large, the magnitude of the

CF term would be large. Conversley, if the change in the wind speed was large and the

change in strain was small, the Coefficeint of Force calculated would be small. As

mentioned before, the requirements for acceptable CF values were strict. In addition to

wind direction and wind velocity criteria, it was decided that CF values would only be

relevant when the slope in the curves for the change in strain and the change in wind

velocity were in the same direction. Specifically, both curves had to be positive or both

curves had to be negative. A schematic illustration of this condition is shown in Figure

61. Ultimately numerous data points were considered irrelevant. In order to be

considered extraneous several other factors were considered. If a large CF value was

90

calculated but there was no corresponding peak wind gust, the coefficient was discarded.

Similarly, if there was not a correlation in the change in strain the value was removed

from consideration. In addition, if the CF value was not reflective of synchronized

behavior between the panels, it was rejected. Conversely, if there was good agreement

between two large CF values between the panels, it was accepted.

Figure 61. Schematic Time History of Wind and Strain Curves.

As mentioned, in some cases large CF values were calculated that could not be

ruled out. In particular these values include 16.3, 14.9, 12.6, and several hovering around

10. These values were found to occur on both panels, suggesting a trend that could not

be ignored. Since the panels are placed at an interior location on a flat roof, there is not

much in the way of an explanation for these large Coefficient of Force numbers. One

91

could reason that the higher CF numbers are evidence that on rare occasions the code

calculated values are exceeded. The vast majority of the reported CF values are within

reasonable proximity to Cp values presented in ASCE 7-10. In particular, the panel set up

that has been created can be compared to Figure 27.4-4 in ASCE 7-10. Figure 27.4-4

displays an open, monoslope roof over the surface of the earth. Because the panels are

placed two building heights from the edge of the roof, it is practical to believe that

monoslope solar panels over the surface of the roof would behave similarly. Using this

figure assuming θ is 30 degrees, γ is zero degrees, and clear wind flow, Cp values ranging

from 0.5 to 2.5 are provided. A good majority of CF values presented fall within this

range, reinforcing the previously stated hypothesis. Compared to results presented in

Study of Wind Loads on Rooftop Mounted Solar Panels, the CF values offered in this

research are considerably lower (Harris 2013). This confirms the theory that expected

wind loads and Coefficients of Force at the edge of the roof are more severe than

conditions at a distance of two building heights from the edge.

As evidenced by Figures 49 and 50, some drifting in the strain readings did occur.

This is most likely the result of thermal effects on the strain transducer. In order to

negate these effects on the results of this study only the change in strain over a short

period of time, in this case three seconds, was considered when calculating CF values. It

is not likely that the temperature changed drastically so as to produce a significant change

in strain over three seconds. In addition, only the difference in the strain values is

utilized, thus the force coefficient is independent of any thermal influences.

In order to validate the findings of this study a repeatability study was performed.

The objective of a repeatability study is to provide evidence of consistency in CF values

92

calculated at different times under similar conditions. The peak net Coefficient of Force

for each time history was calculated by averaging the top 10 maximum CF values over

that period of time (Harris 2013). The results of the repeatability studies are shown in

Tables 19 and 20.

Table 19. Summary of Net Peak CF Values for Panel B.

CFpeak Figure Table Panel

3.5 43 1 B

3.4 45 3 B

1.6 47 5 B

6.1 49 7 B

4.8 51 9 B

6.2 53 11 B

0.9 55 13 B

3.3 57 15 B

3.1 59 17 B

Table 20. Summary of Net Peak CF Values for Panel A.

CFpeak Figure Table Panel

1.8 44 2 A

1 46 4 A

0.7 48 6 A

1.7 50 8 A

5 52 10 A

3.3 54 12 A

1.5 56 14 A

3.9 58 16 A

2.6 60 18 A

93

The Coefficients of Force obtained from this study are reasonable. The values for

Panel B are found to be greater than the values for Panel A. The coefficeints of Panel B

also are more erratic than those of Panel A. The overall average CFpeak value for Panel A

is 2.4 and 3.7 for Panel B.

94

CHAPTER VIII

SUMMARY AND CONCLUSIONS

Summary

As the need for guidance in the area of solar panel design for wind loading

increases, a full scale faux solar panel study was developed in order to provide results for

comparison to wind tunnel studies and code calculated values. In particular, force

coefficients were calculated for direct comparison to pressure coefficients. In addition, it

was desired to provide results for comparison to a previous research project that was

conducted at the University of Colorado Denver by Jennifer Harris at the edge of the roof

(Harris 2013). For the purpose of this study, the faux solar panels were placed at a

distance of two building heights from the edge of a flat roof. It was expected that the

wind velocities experienced at this location would not be highly significant, resulting in

smaller force coefficients than those found in the previous study at the roof edge. For the

limited range of wind directions investigated in this research, it was found that the largest

force coefficients were generated when the wind direction was approximately normal to

face of the panel. As expected, the peak net force coefficients of this study were

generally less than those reported in the previous research. The largest peak force

coefficients obtained were 16.3 and 14.9. Force coefficients rarely exceeded 6 and the

majoritywere found to be between 0.1 and 5. The repeatability study provides evidence

that there is good correlation between the force coefficients resulting from this research.

95

Conclusions

Force coefficients from this study were found to be less significant than when the

panels were placed near the edge of the roof, as anticipated. The most significant force

coefficients were found to occur when the wind direction was perpendicular to the face of

the panel. When compared with the repeatability study for Panel B from Study of Wind

Loads on Rooftop Mounted Solar Panels the overall CFpeak value for Panel B presented in

this research is lower. The CFpeak value calculated when Panel B was located four feet

from the edge of the roof was 4.9 compared to 3.7 when the panel was located 80 feet

from the edge of the roof. It appears that the initial prediction of the response of Panel B

is confirmed. The fact that high peak coefficients are being calculated in this research

could possibly be attributed to the fact that data was measured at one tenth of a second,

thus amplifying the response of Panel B when compared to data measured at one second.

Nonetheless, the CF values seem to suggest that solar panels experience intensified wind

loads when placed closer to the edge of the roof.

The overall CFpeak value for Panel B of 3.7 is greater than the overall CFpeak value

for Panel A of 2.4. This implies that a solar panel that is higher above the roof surface is

subject to more predominant wind flow than a solar panel that is closer to the roof

surface. This indicates that it would be advisable to keep solar panels as close to the roof

surface as possible.

As previously discussed, Figure 27.4-4 of ASCE 7-10, shown in Figure 62, is

often used to estimate wind loads on solar panels. The values presented in this figure,

however, relate to a monoslope roof over ground surface which is not the same as a

monoslope solar panel over a roof surface. The majority of the coefficients for Panel B

96

fall below 4.0, but still over half of the peak coefficients are consistently higher than the

max value of 2.5 presented in the figure. The overall CFpeak value is significantly larger at

3.7. The 2.5 value corresponds to a wind direction that is perpendicular to the back of the

sloped roof, a roof angle of 30 degrees, and clear wind flow, all of which are comparable

to the conditions of the solar panels. The results of Panel A seem to be more agreeable

with the values presented in the figure, with less than half of the coeffeicents exceeding

2.5 and the CFpeak value at 2.4. Therefore, if Figure 27.4-4 was used to estimate wind

pressures on a solar panel similar to Panel A, the results would be acceptable. If,

however, Figure 27.4-4 was used to estimate wind pressures on a solar panel similar to

Panel B, the results would be unconvservative.

There were a small fraction of force coefficients that were significantly larger

than both expected values and pressure coefficient values found in ASCE 7-10.

Repeatability studies suggest that the findings are consistent over different variances in

time. The data provided within this report is offered for comparison to the findings of

wind tunnel studies and other full scale experiements, past, present and future.

97

Figure 62. Net Pressure Coefficients for Monoslope Free Roofs.

Figure 27.4-4 from ASCE7-10 shown for comparison (ASCE 7-10 2010, used with

permission from ASCE).

98

Possible Sources of Error

There are several factors that could introduce error within this research, as

described below.

In order to prevent damage to the roof, the faux solar panel frames were not

mechanically attached to the roof surface. Therefore, sandbags and sections of scrap

metal were used as a means of providing resistance to uplift forces on the panels. While

every effort was made to strategically place these items, it is possible that these objects

could have had some effect on the wind flow in the region surrounding the panels.

It is possible that the 3/8 inch plywood that was used to construct the surface of

each panel was flexible enough to respond to some wind velocities that were experienced.

In the event that this did occur, the plywood surface could have deflected to such a

degree that the strain measurments were affected. The deflection of the panel surface

would contribute to the forces measured in the tension tie, possibly affecting the CF

values.

When wind collides with the faux solar panels, it is likely that vibration is induced

on the panel surface. This vibration could ultimately result in inertial forces that could

impact the measured strain from the diagonal tension ties. In an effort to correct for the

inertial effects, acelerometers could be installed on the panel frames in the future.

The post processing technique could be improved and standardized in the future.

The data of this study was measured at one tenth of a second and compared with data that

was measured at one second.

99

The conclusions of this study are based on measurements taken at wind velocities

that were less than design wind speed. This could possibly have an effect on the

precision of the results which could contribute to error.

Recommendations for Further Research

The following items are suggested for the use of further exploration on the

research of wind loads on solar panels.

To expand upon this particular research, it seems sensible to move the panels to

an entirely new location on the same roof and employ similar methods of recording and

processing data. For instance, it would be valuable to study the effects of wind loads on

the faux solar panels when placed near a roof corner in order to investigate the influence

of corner vortices. Doing so would result in a relatively complete study of wind loads on

the full scale panels on a flat roof which would include data near the edge, center and

corner of the roof. In addition, useful results could come from studying the effects of

wind from various other directions. The panel orientation could be adjusted to account

for various wind directions.

One suggestion is to conduct similar tests using actual full scale solar panels

rather than the faux full scale solar panels. Then CF values could be compared and

validated.

Another option is to move the fuax solar panels to entirely new location. The

same type of studies would be useful if conducted on a completely different shaped roof

structure, in different exposure conditions or in a location that generally experiences

100

higher wind speeds. Predictions on CF values could be made and compared to the results

presented in previous research.

The use of pressure transducers would prove to be a valuable component of this

type of research, particularly in reducing the amount of data processing and the inherent

possible error. Pressure tranducers could be installed either on the surface of the faux

solar panels or on actual solar panels. The applied pressure could be directly measured

and used to generate Cp values that could then be compared to the findings of wind tunnel

studies and code calculated values.

101

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Banks, David, (2011). “How to Calculate Wind Loads on Roof Mounted Solar Panels in

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Campbell Scientific. (2001). CR5000 Measurement and Control System Operator’s

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Cermak, Jack E., (2003). “Wind-tunnel development and trends in applications to civil

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Cochran, Leighton, (2006). “Wind-wise: Wind engineering for structural design”.

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APPENDIX A

Datalogger Program

'CR5000

'Created by Short Cut (2.9)

'Declare Variables and UnitsPublic BattVPublic FCLoadedPublic PTemp_CPublic CRepsPublic ZModePublic QBSSModePublic CIndexPublic CAvgPublic LCountPublic Strain (3)

Public Vr1000 (3)

Public GFAdj(3)

Public BrZero (3)

Public CKnown(3)

Public CReps_2Public ZMode_2Public QBSSMode_2Public CIndex_2Public CAvg_2Public LCount_2Public Strain_2 (4)

Public Vr1000_2 (4)

Public GFAdj_2 (4)

Public BrZero_2 (4)

Public CKnown_2(4)

Public WS_mphPublic WindDirPublic WS_mph_2Public Temp_FPublic GFsRaw(3)=2.115,2.115,2.115

Public GFsRaw_2(4)=2.115,2.115,2.115,2.115

Public Int8(5)

Public PulseCh (2)

Dim I

Units BattV =Volts

Units PTemp_C=Deg C

Units Strain =microstrain

Units Vr1000 =mV/V

Units GFAdj=unitless

Units BrZero =mV/V

Units Strain_2 =microstrain

Units Vr1000_2 =mV/V

Units GFAdj_2 =unitless

Units BrZero_2 =mV/V

Units WS_mph=miles/hour

Units WindDir =degrees

Units WS_mph_2=miles/hour

Units Temp_F=Deg F

'Define Data TablesDataTable(Wind7,True,-1)

DataInterval(0,100,mSec,10)

CardOut(0,-1)

Sample(1,PTemp_C,FP2)

Sample(1,Strain (1),IEEE4)

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Program: Wind7_4.CR5

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Sample(1,Strain_2 (3),IEEE4)

Sample(1,Strain_2 (4),IEEE4)

Sample(1,WindDir ,FP2)

Sample(1,Temp_F,FP2)

Sample(1,Int8(3),FP2)



EndTable

DataTable(Table2,True,-1)

DataInterval(0,1440,Min,10)

Minimum(1,BattV ,FP2,False,False)

EndTable

'Calibration history tableDataTable(CalHist,NewFieldCal,10)

CardOut(0,10)

SampleFieldCal

EndTable

'Main ProgramBeginProg

'Initialize calibration variables for

'Quarter Bridge Strain, 3-wire 350 ohm with 4WFBS35 0 TIM measurement 'Vr1000()'

CIndex =1 : CAvg=1 : CReps=3

For LCount = 1 To 3

GFAdj(LCount )=GFsRaw(LCount )

Next

'Initialize calibration variables for

'Quarter Bridge Strain, 3-wire 350 ohm with 4WFBS35 0 TIM measurement 'Vr1000_2()'

CIndex_2 =1 : CAvg_2=1 : CReps_2=4

For LCount_2 = 1 To 4

GFAdj_2 (LCount_2 )=GFsRaw_2(LCount_2 )

Next

'Load the most recent calibration values from the C alHist table

FCLoaded=LoadFieldCal(True)

'Main ScanScan(100,mSec,1,0)

'Default Datalogger Battery Voltage measurement 'Ba ttV'Battery(BattV )

'Default Wiring Panel Temperature measurement 'PTem p_C'PanelTemp(PTemp_C,250)

'Quarter Bridge Strain, 3-wire 350 ohm with 4WFBS35 0 TIM measurement 'Vr1000()'BrFull(Vr1000 (),3,mV20,1,1,3,5000,True,True,0,250,1,0)

'Calculated strain result 'Strain()' for

'Quarter Bridge Strain, 3-wire 350 ohm with 4WFBS35 0 TIM measurement 'Vr1000()'StrainCalc(Strain (),3,Vr1000 (),BrZero (),-1,GFAdj(),0)

'Quarter bridge strain shunt calibration for

'Quarter Bridge Strain, 3-wire 350 ohm with 4WFBS35 0 TIM measurement 'Vr1000()'FieldCalStrain(13,Strain (),1,GFAdj(),0,QBSSMode,CKnown(),CIndex ,CAvg,GFsRaw(),0)

'Zeroing calibration for

'Quarter Bridge Strain, 3-wire 350 ohm with 4WFBS35 0 TIM measurement 'Vr1000()'FieldCalStrain(10,Vr1000 (),CReps,0,BrZero (),ZMode,0,CIndex ,CAvg,0,Strain ())

'Quarter Bridge Strain, 3-wire 350 ohm with 4WFBS35 0 TIM measurement 'Vr1000_2()'BrFull(Vr1000_2 (),4,mV20,4,2,4,5000,True,True,0,250,1,0)

'Calculated strain result 'Strain_2()' for

'Quarter Bridge Strain, 3-wire 350 ohm with 4WFBS35 0 TIM measurement 'Vr1000_2()'StrainCalc(Strain_2 (),4,Vr1000_2 (),BrZero_2 (),-1,GFAdj_2 (),0)

'Quarter bridge strain shunt calibration for

'Quarter Bridge Strain, 3-wire 350 ohm with 4WFBS35 0 TIM measurement 'Vr1000_2()'

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FieldCalStrain(13,Strain_2 (),1,GFAdj_2 (),0,QBSSMode_2,CKnown_2(),CIndex_2 ,CAvg_2,GFsRaw_2()

'Zeroing calibration for

'Quarter Bridge Strain, 3-wire 350 ohm with 4WFBS35 0 TIM measurement 'Vr1000_2()'FieldCalStrain(10,Vr1000_2 (),CReps_2,0,BrZero_2 (),ZMode_2,0,CIndex_2 ,CAvg_2,0,Strain_2 ())

'03001 Wind Speed & Direction Sensor measurements ' WS_mph' and 'WindDir'PulseCount(WS_mph,1,1,1,1,1.677,0.4)

If WS_mph<0.41 Then WS_mph=0

BrHalf(WindDir ,1,mV5000,17,3,1,5000,True,0,250,355,0)

If WindDir >=360 Then WindDir =0

'03101 Wind Speed Sensor measurement 'WS_mph_2'PulseCount(WS_mph_2,1,2,1,1,1.677,0.4)

If WS_mph_2<0.41 Then WS_mph_2=0

'Type T (copper-constantan) Thermocouple measuremen ts 'Temp_F'TCDiff(Temp_F,1,mV20C,8,TypeT,PTemp_C,True,0,250,1.8,32)

'measure 03101 on SDMINT8 channel 1 through channel 5SDMINT8(Int8(),0,0002,2222,0002,2222,32768,1,1667,0.4)

'For I=1to5

'If Int8(I)<0.21 Then INT8(I)=0

'nextI

'Call Data Tables and Store DataCallTable(Wind7)

CallTable(Table2)

CallTable(CalHist)

NextScan

EndProg

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APPENDIX B

Hand Calculations

STUDY OF FULL SCALE ROOFTOP SOLAR PANELS ERIN KELLY...

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