STUDY OF FULL SCALE ROOFTOP SOLAR PANELS SUBJECT TO WIND LOADS ERIN KELLY ANDOLSEK. B.S., University of Colorado Denver, 2010

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1 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

2 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 ii

3 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, C F. The form and content of this abstract are approved. I recommend its publication. Approved: Frederick R. Rutz iii

4 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. iv

5 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 v

6 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. vi

7 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 The Future of Standardized Design Conclusions IV. PROJECT OVERVIEW AND PANEL LOCATION Introduction Faux Solar Panel Test Frame Design Faux Solar Panel Test Frame Construction Faux Solar Panel Frame Installation and Setup vii

8 V. EQUIPMENT Introduction Wind Measurements Thermocouple Strain Transducers Campbell Scientific Datalogger and Accessories Software VI. THEORY Introduction Wind Behavior Pressure Measured from Strain Pressure Measured from Wind Velocity Coefficient of Force VII. RESULTS Introduction Results Discussion VIII. SUMMARY AND CONCLUSIONS Summary Conclusions Possible Sources of Error Recommendations for Further Research REFERENCES viii

9 APPENDIX A. DATALOGGER PROGRAM B. HAND CALCULATIONS ix

10 LIST OF FIGURES Figure 1. Wind Interaction with Building External Pressure Coefficients Net Pressure Coefficients for Monoslope Free Roofs Variables in Solar Panel Wind Load Determination Part one of Figure from SEAOC Publication Figure from SEAOC Publication Initial Panel Placement Project Panel Location Aerial View of Events Center Building and Surroundings Special Wind Region in Colorado Special Wind Region in Colorado Panel A Detailed Section Panel B Detailed Section Panel C Construction Detail Tension Tie Connection Detail Panel Cross Section View in Weak Axis Panel Connection Detail Completed Panel B Angle Connection Shop Drawing Detail Panel A Steel Tube Leg Shop Drawings Panel B Steel Tube Leg Shop Drawings x

11 21. Panel Layout Anemometer Tree RM Young 3101 Anemometer and RM Young 3301 Wind Sentry Vane Strain Transducer Assembly Strain Transducer A Calibration Curve Strain Transducer B Calibration Curve Strain Transducer C Calibration Curve Strain Transducer E Calibration Curve Strain Transducer F Calibration Curve Strain Transducer Placement Diagram Campbell Scientific Measurement and Control Datalogger Solar Panel Providing Power to Datalogger Campbell Scientific SDM-INT8 8-Channel Interval Timer Wind Behavior at Panel Location Streamer Experiment at Roof Edge Streamer Experiment 20 feet From Roof Edge Streamer Experiment 40 feet from Roof Edge Streamer Experiment 60 feet from Roof Edge Streamer Experiment 80 feet from Roof Edge Schematic Diagram of Faux Solar Panel Theory C F vs. Wind Direction Wind Velocities of Each Anemometer Wind Velocity, Strain and C F Values Data from 10/4/1302:51AM Wind Velocity, Strain and C F Values Data from 10/4/1302:51AM xi

12 45. Wind Velocity, Strain and C F Values Data from 10/4/1303:15AM Wind Velocity, Strain and C F Values Data from 10/4/1303:15AM Wind Velocity, Strain and C F Values Data from 10/4/1304:25AM Wind Velocity, Strain and C F Values Data from 10/4/1304:25AM Wind Velocity, Strain and C F Values Data from 10/4/1304:56AM Wind Velocity, Strain and C F Values Data from 10/4/1304:56AM Wind Velocity, Strain and C F Values Data from 10/4/1304:46AM Wind Velocity, Strain and C F Values Data from 10/4/1304:46AM Wind Velocity, Strain and C F Values Data from 10/4/1305:07AM Wind Velocity, Strain and C F Values Data from 10/4/1305:07AM Wind Velocity, Strain and C F Values Data from 10/5/1312:13PM Wind Velocity, Strain and C F Values Data from 10/5/1312:13PM Wind Velocity, Strain and C F Values Data from 10/11/1311:39AM Wind Velocity, Strain and C F Values Data from 10/11/1311:39PM Wind Velocity, Strain and C F Values Data from 10/11/1313:43PM Wind Velocity, Strain and C F Values Data from 10/11/1313:43PM Schematic Time History of Wind and Strain Curves Net Pressure Coefficients for Monoslope Free Roofs xii

13 LIST OF TABLES Table 1. Summary of C F values for Panel B from Figure Summary of C F values for Panel A from Figure Summary of C F values for Panel B from Figure Summary of C F values for Panel A from Figure Summary of C F values for Panel B from Figure Summary of C F values for Panel A from Figure Summary of C F values for Panel B from Figure Summary of C F values for Panel A from Figure Summary of C F values for Panel B from Figure Summary of C F values for Panel A from Figure Summary of C F values for Panel B from Figure Summary of C F values for Panel A from Figure Summary of C F values for Panel B from Figure Summary of C F values for Panel A from Figure Summary of C F values for Panel B from Figure Summary of C F values for Panel A from Figure Summary of C F values for Panel B from Figure Summary of C F values for Panel A from Figure Summary of Net Peak C F Values for Panel B Summary of Net Peak C F Values for Panel A xiii

14 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. 1

15 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 C F, is a number that can be compared to the Coefficient of Pressure, or C p, 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. 2

16 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. 3

17 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 7 th Century B.C. used the magnifying glass to create fire (History of Solar 2012). In the 6 th 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 4

18 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 5

19 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. 6

20 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, 7

21 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 8

22 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 9

23 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. 10

24 The same principle can be applied to the incompressible flow of wind. Equation from ASCE7, which determines the velocity pressure of wind with respect to the height above the ground surface, is presented below. = (2) Where q z is the pressure, K z is the velocity pressure coefficient, K zt is the topographic factor, K d is the wind directionality factor, and V is the basic wind speed. The 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 11

25 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. 12

26 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 K z, K zt, and K d 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 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 q h is the velocity pressure evaluated at mean roof height (see Equation 2), GC p is the external pressure coefficient as determined from the appropriate figure, and GC pi is the appropriate interal pressure coefficient determined from Table The internal pressure coefficient is constant for each enclosure classification, but the external pressure coefficient varies with the effective wind area. 13

27 Figure 2. External Pressure Coefficients. Figure from ASCE 7-10 used to determine external pressure coefficients on components and cladding of enclosed buildings (ASCE , used with permission from ASCE). 14

28 For tilted panels, the data for monoslope free roofs in Figure 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, q h is the velocity pressure evaluated at mean roof height (see Equation 2), G is the gust-effect factor, and C N 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). 15

29 Figure 3. Net Pressure Coefficients for Monoslope Free Roofs. Figure from ASCE7-10 used to determine wind pressure on monoslope free roofs (ASCE , used with permission from ASCE). 16

30 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 GC N be multiplied by K corner = 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). 17

31 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 18

32 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 costeffectiveness. 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 19

33 1,000 transducers can be applied (Cochran 2005). The pressure taps convert the windinduced 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 20

34 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 21

35 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 (GC rn ) is the combined net pressure coefficient for solar panels as determined from Figure 5. 22

36 Figure 5. Part one of Figure 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 GC P values as published by SEAOC, August 2012 (SEAOC 2012, used with permission). 23

37 Figure 6. Figure 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 GC P values as published by SEAOC, August 2012 (SEAOC 2012, used with permission). 24

38 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 windtunnel 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. 25

39 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 C F 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. 26

40 Figure 7. Initial Panel Placement. Figure 8. Project Panel Location. 27

41 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 a in ASCE7-10, shown in Figure 10, denotes the basic wind speeds for Risk Cateogry II buildings and locations of Special Wind Regions (ASCE72010). A special 28

42 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 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 a in ASCE7-10. In the absence of guidance on rooftop solar panel design wind pressures, Figure 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 A from ASCE7-10 used to determine Basic Wind Speeds for Occupancy Category II Buildings (ASCE , with permission from ASCE). 29

43 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, 30

44 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 C F 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 C F values would be recorded. Figure 11. Panel A Detailed Section. 31

45 Figure 12. Panel B Detailed Section. Figure 13. Panel C Construction Detail. 32

46 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 33

47 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

48 Figure 14. Tension Tie Connection Detail. Figure 15. Panel Cross Section View in Weak Axis. 35

49 Figure 16. Panel Connection Detail. Figure 17. Completed Panel B. 36

50 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

51 Figure 18. Angle Connection Shop Drawing Detail. 38

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

53 Figure 20. Panel B Steel Tube Leg Shop Drawings. Once Panel A and Panel B were completed they were mobilized for placement on the 40

54 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

55 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 42

56 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. 43

57 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. 44

58 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. 45

59 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. Figure 24. Strain Transducer Assembly. 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 46

60 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 Strain Transducer A Load (lbs) y = x R² = Strain (ue) Figure 25. Strain Transducer A Calibration Curve. 47

61 Strain Transducer B Load (lbs) y = x R² = Strain (ue) Figure 26. Strain Transducer B Calibration Curve. 600 Strain Transducer C Load (lbs) y = x R² = Strain (ue) Figure 27. Strain Transducer C Calibration Curve. 48

62 Strain Transducer E Load (lbs) y = x R² = Strain (ue) Figure 28. Strain Transducer E Calibration Curve. 600 Strain Transducer F Load (lbs) y = x R² = Strain (ue) Figure 29. Strain Transducer F Calibration Curve. 49

63 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. 50

64 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

65 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. 52

66 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). 53