Maximizing production from rooftop wind turbines
Before installing rooftop wind turbines, the first step is to ensure a high level of production. WindBox productivity depends on four criteria, evaluated in in-house studies, but which can be gauged fairly quickly:
- wind energy ;
- Facade orientation;
- the height of the targeted building ;
- and urban density.
The wind resource: favouring windy areas
The roof-top turbine starts up at 3 m/s (10.8 km/h) and begins generating power at 4 m/s (14.4 km/h). It reaches its maximum output at 18 m/s (64.8 km/h). Selecting areas with good wind potential is therefore a natural first step in identifying suitable sites.
In France, the Mediterranean basin, the Rhone valley, the north and north-western quarter of France, Normandy and Brittany are the areas to focus on. Northern Europe is also an ideal area for wind power (UK, Benelux, northern Germany, for example). To the south, the coasts of Portugal and Spain, for example, are attractive, as are southern Italy and the Greek islands.
Let's take the example of three cities in France: Bordeaux, Dunkirk and Marseille, with different wind energy resources.
In Bordeaux, at a height of 20 m, the average wind speed is 4.7 m/s. Winds of less than 4 m/s account for more than 45% of all winds. Winds above 10 m/s, which boost production, are in the minority.
In Dunkirk, at a height of 20 m, the average wind speed is 9.1 m/s. Winds over 4 m/s account for almost 90% of the city's winds, and those over 10 m/s for over 30%.
In Marseille, at a height of 20 m, the average wind speed is 8.4 m/s. Winds over 4 m/s account for 80% of the city's winds, and those over 10 m/s for almost 30%.
Choose wind-friendly orientations
Orientation has a direct influence on production. The WindBox captures winds over a range of 120° on the rose. Installing the WindBox on the facade facing the prevailing winds will optimize production. A strong misalignment between the facades and the prevailing winds will, on the contrary, reduce production.
Wind roses are used to determine the optimum orientation and the facade(s) on which to place the wind turbines.
It's worth noting that while winds can vary seasonally, these variations are more or less significant depending on the geographical area.
Favour tall buildings
The height of the building has a direct impact on the production level of rooftop wind turbines. This is due to the atmospheric boundary layer, the principle whereby winds are slowed down less by the roughness of the ground at higher altitudes. When a building is tall, the wind is less constrained by obstacles on the ground - and therefore less slowed down .
In all three cities studied, wind speeds increase with altitude. Note that the average wind speed in Bordeaux at 30 m altitude (5.3 m/s) is slower than at 10 m in Marseille (6.4 m/s) or Dunkirk (9.6 m/s).
For WindBoxes more specifically, height plays a dual role: positioning the turbines at the edge of the roof allows accelerated winds to be picked up. This effect, also known as the corner-flow "increases the output of rooftop turbines. The animation below shows how winds behave on the facade of a building: winds accelerate upwards along the facade, then take off over the roof. To capture these strong winds, it is therefore necessary either to place very high wind turbines in the center of the roof - which can damage the building structure - or to position oneself at the edge, to catch the strong winds before they take off - this is the choice made by WIND my ROOF.
Prefer sparsely populated areas for installations
The last criterion to be assessed before installing rooftop wind turbines is the building's environment .
Roughness characterizes the state of a ground surface in relation to the presence of obstacles. It has an impact on wind speed.
An area with high roughness is full of obstacles of varying heights and densities (such as cities or forests). Wind flows with difficulty and speed is reduced. In these environments, you need to select tall buildings that allow you to gain speed quickly.
Conversely, lower roughness, associated with open country or the seashore, allows the wind to flow more freely and retain its initial speed. Variation in height will have less impact than in an area with high roughness. WindBoxes can also be installed on lower buildings.
Urban density can also lead to masking effects. For example, a taller building in front of the installation façade will block the wind. The geometry of the building of interest and the surrounding neighborhood must therefore be taken into account when selecting sites for rooftop wind turbines.
Variation of wind speed with height in an open area and in a dense area
The graph below shows the speed gains/losses induced by height (base: 10m), depending on the environment: open area and dense area.
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In a nutshell
The ideal spot for maximizing production is multi-faceted and depends on the specific wind conditions in each area, in terms of wind intensity, frequency and direction. Depending on the location of the installation - in the city, on the outskirts of town or in the open countryside - you'll be looking for more or less height to maximize production.
Once these criteria have been met, the second phase of the analysis begins: ensuring the technical feasibility of installing the WindBoxes on the roof.
Verify the technical feasibility of rooftop wind turbine installations
- stabilize WindBoxes in heavy storms;
- maintain watertightness;
- distribute the weight of the system on the roof.
- identification of the building's load-bearing elements ;
- dimensioning of the metal framework.
Identification of load-bearing elements on buildings
To limit pressure on the roof, concrete studs are placed on the roof's load-bearing elements. These are used to ballast the metal framework on which the WindBoxes will rest.
The weight of the studs depends on weather conditions and the dynamic pressure generated by their movements. The weight of the stud takes into account the action that the wind could have on it, which could lead to the WindBoxes sliding or tipping over, for example. Weather conditions are defined according to wind exposure(Eurocode 1 - Design bases and actions on structures Part.1.4 Wind actions) and environmental roughness. The height of the studs is obtained using similar reasoning
Consider, for example, three identical 21m-high buildings in the surrounding countryside of Bordeaux, Dunkirk and Marseille.
- As Marseille and Dunkirk have the same wind classification (Zone 3), the weight of the studs will be the same: with 4 studs for 1 wind turbine, we'll need studs weighing 163 kg at the rear and 221 kg at the front.
- Bordeaux is in Zone 4, with 4 studs for 1 wind turbine, so we'll need studs weighing 72 kg at the rear and 113 kg at the front.
Note that the number of studs per wind turbine will decrease with the number of WindBoxes installed (4 studs for 2 wind turbines, 8 studs for 5 wind turbines, etc.).
Sizing the metal framework
The metal framework connects the concrete blocks and distributes the weight of the WindBoxes over the roof. It comprises beams and bars, and is made to measure. Due to its size, it cannot be assembled in a workshop. It is mounted directly on the roof.
And what about the ideal building?
The study phase of rooftop wind turbine installations therefore comprises two stages, carried out either simultaneously or one after the other, depending on the building: calculation of producibility and technical verification of the installation. WIND my ROOF supports installers in both phases with feasibility studies, but it's also possible to assess your building yourself, with a few simple questions:
- Is my building big enough for an installation? Is there enough roof space?
- Is there any interest in wind power in my area (windy zone, building height, facade orientation)? The Global Wind Atlas website provides a quick overview of average winds and main wind directions. Alternatively, ask us for a diagnosis by filling in the contact form!
- Is urban density a good place to settle?