Abstract
Due to the rapid growth of renewable energy demand, solar arrays have been widely installed on roofs of buildings to harness solar energy. However, solar panels are lightweight and prone to damage in strong winds, causing significant economic losses. This study comprehensively examines the wind effects on roof-mounted solar arrays and proposes innovative wind-resistant design strategies. Four main aspects are addressed: (1) wind pressure distributions, (2) flow and aerodynamic mechanisms, (3) wind directionality effects, and (4) code and standard comparisons.
1. Introduction
1.1 Background and Motivation
Solar energy has become an attractive renewable energy source due to its clean and abundant nature. Solar arrays are installed on both ground and building roofs to harness solar radiation. However, these arrays are lightweight and can be easily damaged by wind loads. examples of wind-induced damage to solar arrays around the world.
Solar arrays can be broadly classified into two types: ground-mounted and building-mounted. Building-mounted solar arrays are further subdivided into flat-roof and gable-roof mounted types. The wind aerodynamic characteristics of roof-mounted solar arrays are more complex than ground-mounted arrays due to the presence of vortices generated by the building roof edges and the atmospheric boundary layer turbulence.
1.2 Objective and Outline of the Research
The primary objective of this study is to comprehensively examine the wind effects on roof-mounted solar arrays, focusing on flat-roof and gable-roof configurations.
2. Wind Pressure Characteristics on Solar Arrays by Wind Tunnel Tests
2.1 Introduction
Wind tunnel tests were conducted to investigate the wind pressure distributions on solar arrays mounted on flat and gable roofs. The effects of building parameters such as side ratio (D/B), aspect ratio (H/B), and parapet height (h_p/H) were analyzed.
2.2 Wind Tunnel Experiments
2.2.1 Model Configurations
Geometric scales of 1/50 were adopted for the models to accurately reproduce the solar panel systems. A typical model configuration for flat-roof-mounted solar arrays .
2.2.2 Approaching Flow Characteristics and Data Acquisition
The wind tunnel tests were carried out in a closed-loop boundary layer wind tunnel. The mean wind speed profile and turbulence intensity were calibrated to match the target power law exponent of 0.15. Pressure taps were installed on both the upper and lower surfaces of the solar panels to capture local wind pressures.
2.3 Wind Pressure Distributions on Flat-Roof-Mounted Solar Arrays
2.3.1 Local Wind Pressure Coefficients
The mean and standard deviation (STD) values of local wind pressure coefficients near the centerline of flat-roof-mounted solar arrays for wind directions θ = 0° and 180°.
2.3.2 Area-Averaged Net Pressure Coefficients
The most critical peak area-averaged net pressure coefficients were examined for various tributary areas (A_t). The variation of these coefficients with A_t for different wind directions.
2.4 Effects of Building Parameters
The effects of building side ratio (D/B), aspect ratio (H/B), and parapet height (h_p/H) on wind pressures on flat-roof-mounted solar arrays were analyzed.
2.4.1 Effects of Building Side Ratio (D/B)
The effect of side ratio D/B on array force coefficients for θ = 0° and 180°. It was observed that D/B had no significant effect on array force coefficients for these wind directions.
2.4.2 Effects of Building Aspect Ratio (H/B)
The effect of aspect ratio H/B on mean and peak array force coefficients. The mean and peak force coefficients decreased with increasing H/B.
2.4.3 Effects of Parapet Height (h_p/H)
The effect of parapet height h_p/H on array force coefficients. Increasing parapet height significantly reduced peak force coefficients.
3. Flow and Wind Pressure Characteristics of Flat-Roof-Mounted Solar Arrays for Normal Winds by LES
3.1 Introduction
Computational Fluid Dynamics (CFD) simulations based on Large Eddy Simulation (LES) were conducted to investigate the flow patterns and mechanisms underlying the wind pressure distributions on solar arrays for normal wind directions (θ = 0° and 180°).
3.2 Numerical Simulation Approach
3.2.1 Computational Domain and Mesh
The computational domain and boundary conditions were set up similar to the wind tunnel tests. The computational mesh was refined near the target buildings and solar panels to achieve high resolution.
3.2.2 Numerical Setups
The LES turbulence model with a Smagorinsky SGS model was adopted. The pressure-velocity coupling was handled using the PISO algorithm.
3.3 Validation of LES Results
Comparisons between wind pressure coefficients obtained from LES and wind tunnel tests showed good agreement.
3.4 Wind Flow Characteristics
Mean streamlines and vorticity contours were analyzed to understand the flow mechanisms. the mean streamlines and vorticity contours for θ = 0°.
3.5 Gap Effect
The effect of gap height h_G1 between the solar panels and the roof on flow and pressure characteristics was investigated. the mean streamlines and vorticity contours for different gap heights.
4. Flow and Wind Pressure Characteristics of Flat-Roof-Mounted Solar Arrays for Oblique Winds by LES
4.1 Introduction
Oblique winds (e.g., θ = 45° and 135°) generate conical vortices that significantly influence the wind pressures on solar arrays. LES simulations were conducted to investigate these effects.
4.2 Grid Systems and Numerical Setups
Similar computational domain, mesh, and numerical setups as for normal winds were adopted.
4.3 Flat-Roof Building without Solar Arrays
Wind pressure distributions and flow fields around the flat-roof building without solar arrays were analyzed to validate the LES approach. the mean pressure coefficients and Q-criterion isosurface for θ = 45°.
4.4 Flat-Roof Building with Solar Arrays
Wind pressures on solar arrays for θ = 45° and 135° were analyzed, and comparisons were made with wind tunnel test data. Flow fields were examined to understand the underlying mechanisms.
4.4.1 Wind Pressure Coefficients on Solar Arrays
The mean wind pressure coefficients on solar arrays for θ = 45° obtained from LES and wind tunnel tests.
4.4.2 Flow Fields
Instantaneous Q-criterion isosurfaces and mean streamlines were analyzed to understand the flow patterns. the mean streamlines and vorticity contours for θ = 45°.
4.5 Gap Effect
The effect of gap height h_G1 on wind pressures and flow fields was analyzed for oblique winds. the gap effect on mean lift coefficients for θ = 45° and 135°.
5. Wind Directionality Effects
5.1 Introduction
Accurate estimation of extreme wind loading on solar arrays requires consideration of the randomness and directionality of wind speed and wind loading coefficients.
5.2 Methodology
A probabilistic approach based on joint probability distributions of multiple extremes was adopted. The Gaussian copula was used to model the joint probability distribution of directional yearly maximum wind speeds and extreme wind loading coefficients.
5.3 Modelling of Wind Climate and Wind Loading Coefficients
In-situ wind speed measurements from meteorological stations and wind loading coefficient measurements from wind tunnel tests were used to model the wind climate and loading coefficients.
5.4 Results and Discussions
The effects of uncertainty in extreme loading coefficients, structural orientation, and wind climate with different directionality characteristics on extreme wind loading were investigated. the probability distribution of yearly maximum wind loading considering directionality.
5.5 Conclusions
Considering the directionality effect in the estimation of extreme wind loading on solar arrays leads to more accurate and economic designs.
6. Comparisons of Design Wind Pressures with Codes and Standards
6.1 Introduction
Wind loading provisions for roof-mounted solar arrays in various codes and standards were compared with the experimental results.
6.2 Overview of Codes and Standards
Relevant codes and standards such as ASCE 7-16 and JIS C 8955:2017 were reviewed.
6.3 Comparisons with ASCE 7-16
Comparisons were made between the wind pressures obtained from wind tunnel tests and those specified in ASCE 7-16 for flat-roof-mounted and gable-roof-mounted solar arrays.
6.4 Comparison with JIS C 8955:2017
Similar comparisons were made with JIS C 8955:2017. Recommendations were made for the refinement of codes and standards.
7. Conclusions and Recommendations
7.1 Conclusions
Comprehensive studies on wind effects on roof-mounted solar arrays were conducted, yielding the following conclusions:
- Wind Pressure Distributions: Wind pressure distributions on solar arrays differ significantly from those on roof components and cladding.
- Flow Mechanisms: The flow mechanisms were clarified through LES, revealing the role of conical and columnar vortices in determining wind pressures.
- Wind Directionality Effects: Considering wind directionality in the estimation of extreme wind loading leads to more accurate and economic designs.
- Code and Standard Comparisons: Recommendations were made for the refinement of codes and standards to better reflect experimental results.
7.2 Recommendations for Future Work
Future work could focus on:
- Flow and Wind Pressures on Gable/Hip-Roof-Mounted Solar Arrays: Further studies are needed to investigate the aerodynamic characteristics of solar arrays mounted on gable/hip roofs.
- Wind Load Reduction Techniques: Develop and evaluate wind load reduction techniques for solar arrays.
- Wind and Snow Load Combinations: Investigate the combined effects of wind and snow loads on solar arrays.