Renewable Energy - Solar Panels Can... Fly?
For information on tidal turbine hydrodynamics check this link: https://youtu.be/c3VaPgTJh88
Let’s focus on these flat roof systems.
Penetrating the waterproof coverage for anchoring purposes isn’t always an option, many operators just put ballast of weight on the mounting system to keep things down. That avoids a possible leakage problem but adds a new one: extra weight.
A decade ago, when no norms were available yet to determine the right amount of ballast to keep the panels down, some applied more safety than others. And with the financial crisis in full force back then, there was yet another incentive to get the total weight below the capacity limit of the roof to get projects approved and profitable. So sadly, some panels flew away.
Nowadays though, through extensive wind tunnel testing & simulations, norms have been formed and it’s clear & simple: follow the norm. Still, those norms allow for optimization, as they often take the aerodynamic behavior of the system into account. So prior to final wind tunnel testing & certification of a system, it makes a lot of sense to integrate aerodynamic simulations into the R&D phase to optimize the mounting systems.
To illustrate this, we’ve analyzed a single free-standing panel in an airflow of 40 m/s, a pretty decent storm. Under headwind, the vertical lift on the panel is around 240 kg, so the weight of the panel itself, around 20kg, is not nearly enough to keep it down.
This happens because the panel behaves like a wing with ground effect: the air hits the bottom of the panel, creating a high pressure pushing it upwards. At the top, the air separates as it jumps over the panel, creating a large suction effect again pulling the panel upwards. Even when the wind hits the panel under a large angle, the lift force can still be as high as 100kg.
If we look at the same panel receiving headwind, but now surrounded by neighbors in an array, the vertical lift force increases even further. Why? The free-standing panel had an escape route for the air on both sides. But when flanked by two neighboring panels, the only way for the air is up, increasing the pressure build up. The panels on the second, third and fourth-row feature much lower lift values, and sometimes even downforce. That is why very often the ballast for the outer panels of the array is much higher than the ballast for the inner ones.
Now if we rotate the entire array by 45°, we see some other interesting effects. As expected, the panel at the corner feels the highest vertical pull, but it’s lower than the headwind case. As we move away from the corner, the lift on the front row panels decreases significantly. Nevertheless, values are still very high and too much ballast would be needed to keep them down. So to reduce wind loading, plates are usually installed to prevent the air from entering underneath the panel, which reduces the high pressure build up at the bottom side.
We’ve added such a plate to the isolated panel. The air now hits this backplate, pushing the plate itself downwards as the air jumps up. The lift on the total system has been reduced drastically for all angles and even features downforce at some angles. The solar panel itself, however, can still feel the suction effect in the wake of the plate.
To counter that, there are some innovative systems that feature an intended opening at the top of the backplate. The suction effect, which is the largest just at the top of the backplate, will now draw air from underneath the panel. This helps to both reduce the pressure below the panel and to reduce the suction effect at the top. The effect on the vertical forces is quite clear!
Studying the behavior of a system in clean airflow is just the beginning though. In real life, complex wind flow patterns around the building itself, influenced by the surrounding terrain and featuring wind guests, add multiple layers of complexity to the problem. So complementing simulations with wind tunnel testing and adding the right safety factors is key to keep your panels from flying.
Wind tunnel image courtesy of Peutz
The AirShaper videos cover the basics of aerodynamics (aerodynamic drag, drag & lift coefficients, boundary layer theory, flow separation, reynolds number...), simulation aspects (computational fluid dynamics, CFD meshing, ...) and aerodynamic testing (wind tunnel testing, flow visualization, ...).
We then use those basics to explain the aerodynamics of (race) cars (aerodynamic efficiency of electric vehicles, aerodynamic drag, downforce, aero maps, formula one aerodynamics, ...), drones and airplanes (propellers, airfoils, electric aviation, eVTOLS, ...), motorcycles (wind buffeting, motogp aerodynamics, ...) and more!
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