The quest for renewable energy has led to the widespread installation of solar panel arrays across diverse geographical landscapes. While extensive research exists on the wind loads affecting solar panel structures in flat terrains or on rooftops, their behavior in vast, wind-swept grassland regions remains less explored. These grasslands, often prime locations for solar farms due to high insolation, present a unique aerodynamic environment. The presence of vegetation significantly alters the near-ground wind profile and turbulence characteristics compared to a standard, flat exposure. This study investigates the wind pressure distribution and resulting load effects on isolated, ground-mounted solar panel models under simulated grassland conditions through wind tunnel testing. A direct comparison is made with identical tests conducted under a standard flat-terrain condition to isolate and quantify the interference effects induced by the grassland surface roughness. The analysis focuses on the mean pressure coefficients, local and overall force coefficients, and introduces an interference factor to systematically assess the impact of the grassland environment on the wind loading of the solar panel structure.
The core of this investigation is a series of wind tunnel tests. The experiments were conducted in a closed-circuit boundary layer wind tunnel with a test section of 20m in length, 3m in width, and 2.5m in height. A uniform flow field was utilized for this comparative study. The wind speed at a reference height of 0.5m was maintained at 8 m/s for all test cases. The solar panel was modeled as a rigid structure constructed from acrylic plates. The model, scaled at 1:20, featured a rectangular panel measuring 0.5m by 0.5m, supported by a frame and a central column, allowing for rotation about a vertical axis to change the wind azimuth angle. The panel’s tilt angle (angle of incidence relative to the horizontal) was fixed at 30° for this study. The model surface was instrumented with pressure taps arranged in a grid, allowing for simultaneous measurement of pressures on both the front (windward) and back (leeward) surfaces at corresponding points. This setup enables the calculation of net pressure across the solar panel.
The physical attributes and structural configuration of the solar panel model are critical for interpreting the aerodynamic results. Two primary terrain conditions were simulated: a standard Flat Terrain and a Grassland Terrain. The grassland simulation involved placing a porous barrier upstream of the model to replicate the effect of vegetation. The key parameters for the grassland simulation were: vegetation height (h) = 0.4H (where H is the height of the panel’s leading edge at 30° tilt), distance from model (x) = 8h, and a porosity of 40%. For each terrain condition, the model was tested under wind azimuth angles (β) ranging from 0° to 180° in increments of 30°, where 0° represents wind normal to the panel’s front surface. The test matrix is summarized in Table 1.
| Terrain Condition | Panel Height H (m) | h/H Ratio | x/H Ratio | Porosity (%) | Tilt Angle (°) | Wind Azimuth Angle β (°) |
|---|---|---|---|---|---|---|
| Flat Terrain | 0.65 | 0 | 0 | 0 | 30 | 0, 30, 60, 90, 120, 150, 180 |
| Grassland Terrain | 0.65 | 0.4 | 8 | 40 | 30 | 0, 30, 60, 90, 120, 150, 180 |
The pressure data acquired from the wind tunnel tests were processed into dimensionless coefficients for analysis. The instantaneous pressure coefficient at a tap i is given by:
$$C_{Pi}(t) = \frac{P_i – P_{ref}}{0.5 \rho V_{ref}^2}$$
where $P_i$ is the measured pressure, $P_{ref}$ is the static pressure at the reference point, $\rho$ is the air density, and $V_{ref}$ is the mean wind speed at the reference height. The net pressure coefficient, crucial for evaluating the load on the solar panel, is the difference between the front and rear surface coefficients:
$$C_{Pi}^{net} = C_{Pi}^{front} – C_{Pi}^{back}$$
The mean pressure coefficient $\overline{C_{Pi}}$ is then obtained by time-averaging $C_{Pi}^{net}(t)$. To relate these to structural design codes, the mean local shape coefficient $\overline{\mu_{si}}$ for a point i is calculated, accounting for the wind profile:
$$\overline{\mu_{si}} = \overline{C_{Pi}} \cdot \left( \frac{Z_0}{Z_i} \right)^{2\alpha}$$
where $Z_i$ is the height of the tap, $Z_0$ is the reference height (0.5m), and $\alpha$ is the terrain roughness exponent (taken as 0.15 for open country terrain). The overall shape coefficient $\mu_s$ for the entire panel surface is the area-weighted average of the local coefficients:
$$\mu_s = \frac{\sum_{i} \overline{\mu_{si}} \cdot A_i}{A}$$
where $A_i$ is the tributary area of tap i and $A$ is the total projected area of the solar panel.
To quantitatively assess the effect of the grassland terrain relative to the flat terrain, an Interference Factor (IF) is defined. For the overall force on the solar panel, this factor is:
$$IF(\beta) = \frac{\mu_s^{grassland}(\beta)}{\mu_s^{flat}(\beta)}$$
Similarly, local interference factors can be computed for specific areas of the solar panel. The magnitude of IF indicates the degree of modification: an IF significantly less than 1.0 indicates a reduction in load due to the grassland interference.
Wind Load Analysis on Solar Panels Over Flat Terrain
The wind pressure distribution on the solar panel under flat terrain conditions serves as the baseline for this study. The contours of mean pressure coefficient reveal distinct flow patterns that evolve with the wind azimuth angle β. For wind directions where the panel is primarily windward (β = 0°, 30°, 60°), the surface experiences positive pressure (pushing force). A region of high pressure, known as a stagnation zone, is clearly identifiable. At β = 0° (wind normal to the face), this stagnation zone is centered in the lower-middle section of the panel. The peak mean pressure coefficient in this zone reaches approximately +2.0. The pressure gradually decreases towards the edges and the top of the panel, forming a gradient radiating from the stagnation point, with values tapering to around +1.0.
As the wind direction becomes oblique (β = 30°, 60°), the stagnation zone shifts laterally across the panel surface, following the component of wind normal to the panel’s face. At β = 30°, the high-pressure core moves towards the lower-left corner. By β = 60°, it is located near the left edge. The magnitude of the peak pressure also shows some variation with angle. A critical transition occurs at β = 90°, where the wind flows parallel to the panel’s side edge. At this angle, the flow separates sharply, creating a large area of suction (negative pressure) on the panel face, particularly on the windward side. The pressure coefficients turn predominantly negative, with values around -0.75, marking a shift from overall positive to overall negative net force.
For wind directions where the panel is leeward (β = 120°, 150°, 180°), the entire surface is under suction. The flow separation at the leading edge (now the top edge for β=180°) creates a vortex, resulting in a zone of high suction. Similar to the windward case, this zone moves with changing wind angle. At β = 120° and 150°, the peak suction zone is located on the left-middle and top-middle parts of the panel, respectively. Finally, at β = 180° (wind normal to the back of the panel), the peak suction zone, with coefficients around -1.8, is centered on the upper part of the panel. The pressure distribution for leeward directions also exhibits a gradient from this high-suction core. The key statistics for the flat terrain case are consolidated in Table 2.
| Wind Azimuth β (°) | Dominant Load Type | Location of Peak |Cp| | Approx. Peak |Cp| | Overall Shape Coeff. μs |
|---|---|---|---|---|
| 0 | Pressure (Positive) | Lower Middle | +2.0 | +1.65 |
| 30 | Pressure (Positive) | Lower Left | +1.9 | +1.50 |
| 60 | Pressure (Positive) | Left Edge | +1.7 | +1.25 |
| 90 | Suction (Negative) | Left Edge | -0.75 | -0.55 |
| 120 | Suction (Negative) | Left Middle | -1.5 | -1.20 |
| 150 | Suction (Negative) | Top Middle | -1.7 | -1.45 |
| 180 | Suction (Negative) | Upper Middle | -1.8 | -1.60 |
Wind Load Analysis on Solar Panels Over Grassland Terrain
The introduction of simulated grassland vegetation upstream of the solar panel model significantly alters the incident wind field and, consequently, the surface pressure distribution. The porous barrier representing vegetation disrupts the smooth uniform flow, creating a shear layer that rides over the top of the barrier. This modified flow interacts differently with the mounted solar panel.
For the windward case at β = 0°, a fundamental shift in the stagnation zone is observed. Contrary to the flat terrain, where the stagnation point was low on the panel, the grassland condition sees this high-pressure zone positioned in the upper-middle section of the panel. The peak mean pressure coefficient here is about +1.4, which is notably lower than the +2.0 recorded for flat terrain. This occurs because the upstream vegetation impedes the near-ground flow. A significant portion of the wind stream jumps over the vegetation, effectively raising the height of the impinging flow that first contacts the solar panel. The flow that passes through the porous barrier is slowed and disturbed, reducing its momentum and its contribution to the stagnation pressure on the lower part of the panel.
The general pattern of the stagnation/suction zone moving with wind azimuth angle persists for the grassland solar panel. For β = 30° and 60°, the high-pressure zone shifts towards the left-upper quadrant of the panel. The magnitudes of the pressure coefficients in these windward scenarios remain lower than their flat-terrain counterparts. The transition at β = 90° is more complex. While the overall pressure becomes negative, the distribution appears more fragmented, with smaller, less organized regions of suction rather than one large coherent zone. The peak suction values are also greatly diminished, with local coefficients around -0.06, indicating a very mild suction force compared to the flat terrain case.
For the leeward wind directions (β = 120° to 180°), the solar panel surface is again under suction. The movement of the primary suction zone from the left side to the top of the panel is consistent with the flat terrain behavior. However, the magnitude of suction is dramatically reduced across all these angles. At β = 180°, the peak suction coefficient is approximately -1.0, which is only about 56% of the peak value observed for the flat terrain solar panel at the same angle. This substantial reduction highlights the sheltering or disrupting effect the upstream vegetation has on the flow structures (like vortices) responsible for creating high suction on the leeward side of the structure. Table 3 provides a comparative summary for the grassland case.
| Wind Azimuth β (°) | Dominant Load Type | Location of Peak |Cp| | Approx. Peak |Cp| | Overall Shape Coeff. μs | Interference Factor (IF) |
|---|---|---|---|---|---|
| 0 | Pressure (Positive) | Upper Middle | +1.4 | +1.05 | 0.64 |
| 30 | Pressure (Positive) | Upper Left | +1.3 | +0.96 | 0.64 |
| 60 | Pressure (Positive) | Left Edge | +1.25 | +0.70 | 0.56 |
| 90 | Suction (Negative) | Distributed | -0.06 | -0.04 | 0.07 |
| 120 | Suction (Negative) | Left Middle | -0.4 | -0.30 | 0.25 |
| 150 | Suction (Negative) | Top Middle | -0.7 | -0.55 | 0.38 |
| 180 | Suction (Negative) | Upper Middle | -1.0 | -0.65 | 0.41 |
Quantification of Interference Effects and Design Implications
The Interference Factor (IF) provides a clear, quantitative measure of how the grassland environment modifies the wind load on the solar panel. As calculated using the overall shape coefficients and presented in Table 3, the IF is consistently less than 1.0 for all wind angles, confirming an overall reduction in net wind force. The trend of IF versus wind azimuth angle β shows a pronounced decrease, reaching minimal values around β = 90° before increasing slightly for leeward directions but still remaining well below 0.5. For the critical design cases of maximum positive pressure (β ≈ 0°) and maximum suction (β ≈ 180°), the IF values are 0.64 and 0.41, respectively. According to the classification where IF ≤ 0.6 indicates a “significant reduction” and 0.6 < IF < 0.8 indicates a “moderate reduction,” the grassland terrain causes a moderate reduction in peak positive pressure and a significant reduction in peak suction on the solar panel.
This has direct implications for the structural design and optimization of solar panel support systems in grassland regions. While the local peak pressure coefficient of +1.4 observed at β=0° for the grassland case is lower than the flat terrain value, it still represents a significant localized load. Design codes typically provide global shape coefficients. The presence of such localized high-pressure zones, even in reduced magnitude, suggests that designers should consider local pressure effects or introduce local pressure coefficients for cladding and connection design to prevent localized failure of the solar panel or its attachments. The more dramatic reduction in suction forces, however, could allow for some optimization in the anchoring and ballasting systems designed to resist overturning, particularly for the critical leeward wind conditions. The force balance on the entire solar panel structure can be expressed as:
$$F(\beta) = \mu_s(\beta) \cdot q_H \cdot A$$
where $q_H$ is the dynamic wind pressure at the panel’s characteristic height H. The reduction in $\mu_s$ due to grassland interference directly reduces the design force $F(\beta)$.
Furthermore, the shift in the stagnation zone to a higher position on the panel for windward winds alters the distribution of the resulting bending moment on the support arms. This change in load pattern may influence the fatigue analysis and detailing of the mounting structure for the solar panel. The highly disturbed and attenuated flow at β=90° indicates that cross-wind loads may be less severe in grassland settings than predicted by standard flat-terrain models.
Conclusion
This comparative wind tunnel study elucidates the distinct wind load characteristics acting on a ground-mounted solar panel in a simulated grassland environment versus a standard flat terrain. The presence of upstream vegetation, modeled as a porous barrier, fundamentally alters the flow-structure interaction. Key findings are systematically concluded as follows:
1. Altered Flow Impingement: For wind normal to the panel face (β=0°), the grassland condition shifts the stagnation point upward on the solar panel surface due to the flow jumping over the vegetation. The local peak pressure coefficient is reduced from approximately +2.0 (flat) to +1.4 (grassland).
2. Systematic Load Reduction: The grassland terrain consistently reduces the overall wind load on the solar panel across all wind directions. This is quantified by an Interference Factor (IF) less than 1.0. The reduction is most extreme for oblique winds (β=90°, IF≈0.07), indicating highly disrupted flow.
3. Significant Suction Attenuation: The most pronounced load reduction occurs for suction forces on the leeward side of the solar panel. For wind normal to the back (β=180°), the peak suction coefficient drops from -1.8 to -1.0, and the overall shape coefficient reduces by about 59% (IF=0.41).
4. Design Implications: While global forces may be lower, the localized high-pressure zones necessitate attention in the design of solar panel cladding and connections. The overall reduction in net force, particularly uplift/suction, suggests potential for optimizing support structures in grassland-based solar farms. The modified pressure distribution also affects the internal bending moment pattern in the mounting system.
In summary, applying standard flat-terrain wind load coefficients to a solar panel installation in a grassland region is likely to be conservative, potentially leading to over-design. This study provides initial quantitative evidence and a methodological framework (using interference factors) for assessing terrain-specific wind loads, contributing to more efficient and cost-effective engineering of solar energy infrastructure in non-standard environments. Future work should investigate the effects of different vegetation densities, heights, and the mutual interference effects within large arrays of solar panel structures in such terrains.