While all flat roofed buildings are eligible for a rooftop wind turbine installation, in reality two steps are necessary to identify the ideal building. The first step is to select sites with high production potential according to certain criteria; the second step is to verify the technical feasibility of the installation.
This article summarizes the criteria for selecting a building for installation and presents one of the methods for mounting rooftop wind turbines.
Maximizing production from rooftop wind turbines
Before installing the rooftop wind turbines, the first step is to verify the building’s features ensure a high level of production. The productivity of the WindBoxes depends on four criteria, evaluated in in-house studies, but which can be gauged fairly quickly:
- the wind field;
- the orientation of the facades;
- the height of the targeted building;
- and urban density.
The wind resource: favouring windy areas
The turbine starts moving at 3 m/s (10.8 km/h) and producing at 4 m/s (14.4 km/h). It reaches its maximum power at 18 m/s (64.8 km/h). The selection of areas with a good wind potential is therefore a natural first step in the identification of relevant sites.
The areas to be favored in France are the Mediterranean Basin, the Rhone Valley, the north and northwest quarter of France, Normandy and Brittany. The north of Europe is also an area conducive to wind power (United Kingdom, Benelux, northern Germany for example). In the south, the Portuguese and Spanish coasts are interesting, as well as southern Italy and the Greek islands.
Let’s take the example of three cities in France: Bordeaux, Dunkerque and Marseille, with different wind energy deposits. (note: “vitesse des vents” means wind velocity)
Bordeaux
In Bordeaux
at 20 m height, the average wind speed is 4.7 m/s. Winds of less than 4
m/s represent more than 45% of the winds. Winds above 10 m/s, which make the
production take off, are in the minority.
Dunkerque
In Dunkerque, at 20 m height, the average wind speed is 9.1 m/s. Winds of more than 4 m/s represent almost 90% of the winds in the city, those of more than 10 m/s more than 30% of the winds in the city.
Marseille
In Marseille, at a height of 20 m, the average wind speed is 8.4 m/s. Winds of more than 4 m/s represent 80% of the winds in the city, and those of more than 10 m/s represent nearly 30% of the winds.
Choosing orientations that are favorable to the wind
The orientation directly influences the production. The WindBox captures the wind over a range of 120° on the rose. The production of the WindBox will be optimized it is installed on the façade that is best oriented to the prevailing winds. A strong misalignment between the facades and the prevailing winds will reduce the production.
The calculation of wind roses allows to determine the optimal orientation and the facade(s) on which to place the wind turbines.
Note that if the winds have seasonal variations, they are more or less important depending on the geographical area.
Bordeaux
In Bordeaux, the wind rose over the year is fragmented and no clear majority axis emerges.
This trend is repeated monthly, with different majority currents each month. It will be complicated in this city to identify a front of interest.
Dunkerque
In Dunkerque, the majority of the wind corridor is southwest. This axis remains dominant regardless of the season, with a difference in intensity between the winter months (strong winds) and the summer months (slower winds). This seasonality will result in a higher production in winter and a lower production in summer, compensated by the addition of solar panels.
Marseille
In Marseille, the majority of the wind corridor is north-west all year round. The monthly wind roses remain almost identical throughout the year, both in frequency and intensity. This will result in little variation in production over the year. The addition of the solar panels will increase the density of electricity produced by the rooftop turbines.
Favouring high buildings
The height of the building has a direct impact on the production level of the rooftop wind turbines. This is due to the atmospheric boundary layer, the principle that winds are slowed down less by the roughness of the ground at higher altitudes. When a building is high, the wind is less constrained by obstacles on the ground – and therefore less slowed down.
Bordeaux
Dunkerque
Marseille
In the three cities studied, wind speeds increase with altitude. Note that the average wind speed in Bordeaux at 30 m altitude (5.3 m/s) remains slower than at 10 m in Marseille (6.4 m/s) or in Dunkerque (9.6 m/s).
For WindBoxes more specifically, height plays a double role: the positioning of the turbines at the edge of the roof allows the accelerated winds to be recovered. This effect, also called “corner-flow“, increases the production of the rooftop turbines. The animation below shows the behavior of the winds on the facades of a building: the winds go up by accelerating along the facade then take off by passing well above the roofs. 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 structure of the building – or to place oneself at the edge, to capture the strong winds before they take off – this is the choice of WIND my ROOF
Prefering low-density areas for installations
The last criterion evaluated before a rooftop wind turbine installation is the environment of the building.
Roughness allows us to characterize the state of a soil in relation to the presence of obstacles. It has an impact on wind speed.
An area with a high roughness is full of obstacles of various heights and densities (such as cities or forests). The wind flows with difficulty and its speed is reduced. In these environments, high buildings should be selected to gain speed quickly.
On the other hand, a lower roughness, associated with an open country or the seashore, allows the wind to flow more freely and to maintain its initial speed. The variation of the height will have a lesser impact compared to an area with a high roughness. A WindBox installation can be done on lower buildings.
Urban density can also induce masking effects. For example, a taller building in front of the installation will block the wind. Therefore, the geometry of the building of interest and the surrounding neighborhood must be considered 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 represents the speed gains/losses induced by the height (base: 10m), according to the environment: open area and dense area.
In summary
The ideal spot to maximize production is multiple and will depend on the specificities of the wind field of each area, in terms of intensity, frequency and orientation of the winds. Depending on the environment of the installation – in the city, in a peri-urban area or in the open country – we will look for more or less height to maximize the production.
Once these criteria are met, the second phase of analysis begins: ensuring the technical feasibility of installing the WindBoxes on the roof.
Verifying the technical feasibility of rooftop wind turbine installations
The minimum project size is five WindBoxes, which requires a facade of at least 15 m in length. The WindBoxes are installed in groups of five: 30 m for 10 WindBoxes, 45 m for 15, etc.
In most cases, the fixation system chosen for existing buildings is a combination of concrete studs and a metal frame. This combination allows to :
- stabilize the WindBoxes in case of strong storms
- to preserve the waterproofing;
- distribute the weight of the installation on the roof.
The mounting brackets are made-to-measure. Other methods can be studied, especially for new buildings. The dimensioning of the supports goes through two steps:
- identification of the building’s load-bearing elements;
- the dimensioning of the metal frame.
All calculations must be validated by a structural design office.
The minimum project size is five WindBoxes, which requires a facade of at least 15 m in length. The WindBoxes are installed in groups of five: 30 m for 10 WindBoxes, 45 m for 15, etc.
In most cases, the fixation system chosen for existing buildings is a combination of concrete studs and a metal frame. This combination allows to :
- stabilize the WindBoxes in case of strong storms
- to preserve the waterproofing;
- distribute the weight of the installation on the roof.
The mounting brackets are made-to-measure. Other methods can be studied, especially for new buildings. The dimensioning of the supports goes through two steps:
- identification of the building’s load-bearing elements;
- the dimensioning of the metal frame.
All calculations must be validated by a structural design office.
The minimum project size is five WindBoxes, which requires a facade of at least 15 m in length. The WindBoxes are installed in groups of five: 30 m for 10 WindBoxes, 45 m for 15, etc.
In most cases, the fixation system chosen for existing buildings is a combination of concrete studs and a metal frame. This combination allows to :
- stabilize the WindBoxes in case of strong storms
- to preserve the waterproofing;
- distribute the weight of the installation on the roof.
The mounting brackets are made-to-measure. Other methods can be studied, especially for new buildings. The dimensioning of the supports goes through two steps:
- identification of the building’s load-bearing elements;
- the dimensioning of the metal frame.
All calculations must be validated by a structural design office.
Identification of load-bearing elements on buildings
To limit the pressure on the roof, concrete blocks are placed on the load-bearing elements of the roof. They serve to ballast the metal frame on which the WindBoxes will rest.
The weight of the blocks depends on the weather conditions and the dynamic pressure generated by their movements. The weight of the blocks takes into account the effect of the wind, which can lead to the WindBoxes sliding or tilting, for example. Weather conditions are defined according to the exposure to wind (Eurocode 1 – basis of calculation and actions on structures Part.1.4 Wind actions, only in French) and the roughness of the environment. The height of the studs is obtained with a similar reasoning.
To limit the pressure on the roof, concrete blocks are placed on the load-bearing elements of the roof. They serve to ballast the metal frame on which the WindBoxes will rest.
The weight of the blocks depends on the weather conditions and the dynamic pressure generated by their movements. The weight of the blocks takes into account the effect of the wind, which can lead to the WindBoxes sliding or tilting, for example
. Weather conditions are defined according to the exposure to wind (Eurocode 1 – basis of calculation and actions on structures Part.1.4 Wind actions, only in French) and the roughness of the environment. The height of the studs is obtained with a similar reasoning.
Consider for example three identical 21m high buildings in the surrounding countryside of Bordeaux, Dunkerque and Marseille.
- Marseille and Dunkerque have the same wind classification (Zone 3), so the weight of the poles will be the same: with 4 poles for 1 wind turbine, poles of 163 kg will be needed at the back, 221 kg at the front.
- Bordeaux is in Zone 4, with 4 poles for 1 wind turbine, it will take poles of 72 kg at the back, 113 kg at the front.
Please note that the number of poles per wind turbine will decrease according to the number of WindBoxes installed (4 poles for 2 wind turbines, 8 poles for 5 wind turbines, etc.).
Dimensioning of the metal frame
The metal frame connects the concrete blocks and distributes the weight of the WindBoxes on the roof. It consists of beams and bars and is made to measure. Due to its size, it cannot be assembled in a workshop. It is assembled directly on the roof.
And what about the ideal building?
The study phase of rooftop wind turbine installations therefore includes two steps, carried out simultaneously or one after the other, depending on the building: calculation of the producibility and technical verification of the installation. WIND my ROOF supports installers in both steps with feasibility studies, but it is also possible to evaluate your building yourself with simple questions:
- Is my building big enough for an installation? Is there enough space on the roof?
- Is there an interest for wind energy in my region (windy area, height of the building, orientation of the facades)? The Global Wind Atlas website allows you to quickly find out the average wind speed and the main wind direction. Otherwise, ask us for a diagnosis by filling in the contact form!
- Is the urban density favorable to an installation?
And if you have any doubt, you can always send us an email with the description of your building! The more precise you are, the more our teams will be able to decide on the relevance of an installation.
