Short History of CFD and Physical Modeling of ...

Short History of CFD and Physical Modeling of Ballasted Roof Systems for Mounting Solar PV Collectors Rooftops are an ideal location for the placement of photo-voltaic (PV) solar collector modules. Roofs provide access to abundant solar energy, the building structure provides a robust location for the support of such equipment, and it provides a secure region away from the public for the delicate modules and high-voltage equipment. Today there are millions of square feet of PV modules located on both sloped and flat roofs. Unfortunately, along with the abundant sun light and secure support, roofs are also often exposed to high wind speeds that can damage or destroy solar collector systems. To guarantee that PV systems are safe and reliable engineers must consider such wind loads during the design of PV module support systems. Three securement systems are commonly used to fasten PV modules to flat roofs: rack-mounted, adhesive-mounted or ballasted options. Rack-mounted systems often elevate the module off the roof surface exposing the PV module to higher winds and requiring robust support, penetration of the roof surface, and custom design. Ballasted systems have become very popular because of their low cost and fast installation thus reducing labor costs. The ballasted PV module keeps the array in place by means of its own weight and careful aerodynamic design. Of course such systems must be properly designed to resist wind, seismic and thermal loads. Unfortunately, building codes such as the International Building Code (IBC) and the American Society of Civil Engineering Minimum Design Loads for Buildings and Other Structures (ASCE 7-05) were not prepared to correctly estimate loads on permeable and open structures like PV modules. These handbook methods are generally found to under-estimate the actual drag, lift and over-turning moments on the collectors. The currently accepted design method for solar modules is to use physical modeling in wind tunnels and/or numerical modeling by modern computational fluid dynamic (CFD) engineering software programs. Of course any modeling method must be verified for consistency and validated for specific applications (Meroney and Neff (2010), Fritz et al., 2008). This short history provides a review of the application of physical and numerical modeling to the wind resistance of solar modules as performed at Colorado State University in the Wind Engineering and Fluids Laboratory, Civil and Environmental Engineering Department. General Roofing Studies: The Wind Engineering and Fluids Laboratory have conducted research on roofing loads and roof ballast paver performance since 1985. The laboratory has performed general physical modeling of atmospheric interaction with structures since 1955 (see http://www.windlab.colostate.edu/ ). In the beginning physical modeling of the response of roofing materials placed on flat and inclined roofs with various parapet conditions were performed in the large meteorological wind tunnels at Colorado State University. These tunnels were capable of reproducing the characteristics of modeled atmospheric boundary layers for rural, suburban and urban conditions. The tunnels were designed to reproduce the atmospheric boundary layer winds and turbulence Wind Engineering Research and Application Specialists

distributions measured under various terrain roughness conditions. Initial measurements of roof pressures and model paver displacements were reported by Bienkiewicz and Meroney (1985,1986). This work represented an extension of a long history of research on roof coverings begun as early as 1951 and pursued at various institutions up to 1985. These studies allowed us to predict the primary failure mechanisms of roofs covered by various combinations of single-ply membranes, tiles, pavers, insulation boards and gravels. Subsequently, pressure distributions both above and below the pavers were measured such that uplift forces and drag could be predicted, Sun and Bienkiewicz (1992a, 1992b, 1993). Computational models were validated that permitted the prediction of paver uplift for different roof height, roof exposure, roof orientations, distance from roof edges, and even the effects of paver clips and connectors. Between 1989 and 2001 Dr. Meroney co-directed the National Science Foundation Cooperative Wind Engineering Project at Colorado State and Texas Tech Universities. This project generated extensive field and wind tunnel data concerning wind loads on low-rise flat-roofed building structures. (Dr. Meroney was also Director of the Wind Engineering and Fluids Laboratory between 1985 and 2000. See http://www.engr.colostate.edu/~meroney/index.html ) The data was subsequently integrated into the current ASCE-7 building code used country wide for designing low-rise buildings. Solar Collector and Heliostat Studies: Over this same period a number of research projects examined wind loading on ground-mounted solar collector systems, see Peterka et al. (1987, 1989). But in the late 1990s and early 2000s attention switched to modeling inclined solar collectors mounted in arrays on flat roofs. A series of both full size and scaled solar photo-voltaic panels were examined to determine the effect of panel orientation, array size, edge baffling, panel design, and roof placement affected wind uplift, drag and over-turning moments. The driving force behind the design process was to produce the lightest possible and most economical support structure that could withstand severe winds. Competition among collector manufactures meant that the lightest weight and most material economical designs would be most profitable. Combined CFD and Physical Model Protocol: This search for an optimum collector design led to a desire to examine many combinations of possible construction arrangements. But to test every configuration in the wind tunnel with physical modeling would become time-consuming and uneconomical. A hybrid approach to augment the design process was proposed (Meroney, 2005). First, a computational fluid mechanic simulation protocol would be validated by comparing calculated lift, and drag forces against full scale wind-tunnel measurements in the laboratory. Once the methodology was verified, then the CFD method could be used to simulate efficiently many alternative design configurations. Configurations that responded poorly during numerical simulation were discarded, but optimal configurations were chosen for limited physical simulation in the CSU wind tunnels. See Meroney and Neff (2003, 2010), Meroney (2011). This led to an extensive but proprietary test series over several years for different clients. Wind Engineering Research and Application Specialists

Caveats: The CFD methodology was validated by Meroney and Neff (2003, 2010) to the extent that steady state pressure distributions and lift, drag and moment coefficients were reproduced during physical model experiments in a meteorological wind tunnel. These data provide a baseline for assurance that the methodology produces reasonable and trustworthy results in the situations compared. However, there remain many possible design configurations that have not been specifically validated; hence, there will continue to be uncertainties associated with the design process. These uncertainties can be diminished by careful analysis, conservative design and cautious extensions of proven data to new situations, but there still remain untested issues: •

Tests to date only consider a limited range of system orientation angles without the additional complications of roof configuration, presence of parapets, upwind structures or landscaping, adjacent terrain or vegetation effects, and presence of nearby auxiliary and/or heating and ventilating structures.

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Since wind loads are highest at corners and edges of the roof, PV modules placed in these regions may require additional securement. Consequently, it is generally advised to locate the solar array as close to the middle of the roof as possible to reduce exposure.

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All tests were performed for stationary conditions without the effects of wind gusts or wind veering. These weather conditions are known to exacerbate wind loads significantly. Even physical modeling in wind tunnels sometimes under estimate these effects if great care is not taken to reproduce the scale and spectra of atmospheric turbulence (Fritz et al., 2008). For this reason building codes such as the ASCE 7-05 protocols suggest adding a gust-effect factor (GEF) or loading factor to account for higher 3-second gusts when estimating drag, lift or moments.

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Finally, there remain strong differences of opinion concerning the maturity of CFD modeling of atmospheric loading phenomenon primarily by the physical modeling community. Pessimistic reviews have been made by Cochran and Derickson (2011) and Kopp et al. (2011). It is my own contention that given adequate care with inflow boundary conditions, proper spatial discretization near corners and other hot-spots, and the use of modern Large Eddy Simulation (LES), Detached Eddy Simulation (DES) and/or embedded boundary layer (EBL) turbulence methods CFD can routinely and faithfully reproduce atmospheric flow around solar module structures in unsteady wind conditions, certainly within known statistical variations. For now even the RANS type models permit comparative evaluation of alternative design configurations. Sincerely yours,

Robert N. Meroney, Ph.D and P.E. Professional Engineer, State of Colorado

Wind Engineering Research and Application Specialists

References: Meroney, R.N., “Hurricane Effects on Roof-mounted Solar Collector Arrays,”, 5th Intl Symp on Wind Effects on Buildings and Urban Environment, Tokyo, Japan, March 7-8, 2011, 2 pp. Cochran, L.S. and Derickson, R.G., "A Physical Modeler's View of Computational Wind Engineering", Journal of Wind Engineering and Industrial Aerodynamics, Volume 99, Number 4, pages 139-153, 2011. Kopp, G., Maffei, J., and Tiley, C., “Rooftop Solar Arrays and Wind Loading: A Primer on Using Wind Tunnel Testing as a Basis for Code Compliant Design for ASCE 7", prepared for and published by SunLink, Boundary Layer Wind Tunnel Laboratory, Universityof Western Ontario, Canada, 11 pp. Meroney, R.N. and Neff, D.E., “Wind effects on roof-mounted solar photovoltaic arrays: CFD and wind-tunnel evaluation,” Fifth Int. Symp. On Computational Wind Engineering (CWE2010), Chapel Hill, NC, May 23-27, 2010, 8 pp. Fritz, W., B. Bienkiewicz, B. Cui, O. Flamand, T.C.E. Ho, H. Kikitsu, C.W. Letchford, and E. Simiu, "International Comparison of Wind Tunnel Estimates of Wind Effects: Test-related Uncertainties", Journal of Structural Engineering, ASCE, Vol. 134, No. 12, 2008, pp. 1887-1890. Meroney, R.N., Wind Tunnel and Numerical Simulation of Pollution Dispersion: A Hybrid Approach”, Proceedings of Croucher Advanced Study Institute, Hong University of Science and Technology, 6-10 December 2004, and again 7-8 December 2005, 66 pp ASCE 7-05, Minimum Design Loads for Buildings and Other Structures, American Society of Civil Engineers, Reston, Virginia, 2006. Neff, D.E. and Meroney, R.N., 2003, “Wind Performance of Photovoltaic Arrays”, Final Report, Wind Engineering and Fluids Laboratory, Colorado State University, 181 pp (Proprietary) Lee, S. and Bienkiewicz, B., 1995, “Wind Engineering Study of PowerGuard Roofing System”, Technical Report for PowerLight Corporation, Wind Engineering and Fluids Laboratory, Colorado State University, Fort Collins,192 pp. (Proprietary) Neff, D.E. and Bienkiewicz, B., 2000, “Wind Tunnel Study of PowerGuard RT Arrays”, Technical Report for PowerLight Corporation, Wind Engineering and Fluids Laboratory, Colorado State University, Fort Collins, February, 175 pp. (Proprietary) Sun, Y. and B. Bienkiewicz, "Numerical Simulation of Pressure Distribution Underneath Roofing Paver System", Journal of Wind Engineering and Industrial Aerodynamics, Vol. 46 & 47, 1993, pp. 517-526.* Bienkiewicz, B. and Y. Sun, "Local Wind Loading on the Roof of a Low-Rise Building", Journal of Wind Engineering and Industrial Aerodynamics, Vol. 45, 1992, pp. 11-24. Wind Engineering Research and Application Specialists

Bienkiewicz, B. and Y. Sun, "Wind-tunnel Study of Wind Loading on Loose-laid Roofing Systems", Journal of Wind Engineering and Industrial Aerodynamics, Vol. 41-44, 1992, 1817-1828. Peterka, J. A., Z. Tan, J. E. Cermak and B. Bienkiewicz, "Mean and Peak Wind Loads on Heliostats", Journal of Solar Energy Engineering, May 1989, Vol. 111, pp. 158-164. Bienkiewicz, B. and R. N. Meroney, "Wind Effects on Roof Ballast Pavers", Journal of Structural Division, Proceedings American Society of Civil Engineers, Vol. 114, No. 6, June 1988, pp. 1250-1267. Peterka, J. A., B. Bienkiewicz, N. Hosoya and J. E. Cermak, "Heliostat Mean Wind Load Reduction", Energy - The International Journal, Vol. 12, No. 3/4, pp. 261-267, 1987.

Wind Engineering Research and Application Specialists