The latest Renewable Energy Acceleration Act aims to identify the most suitable areas for wind and solar power installations.
This mapping will enable us to take full advantage of the energies available in each region.
While this identification exercise has become the norm for large-scale installations, photovoltaic solar panels are the benchmark solution for rooftops, in the absence of a democratized alternative .
However, just as in the field, the yield of a photovoltaic solar panel on a roof varies according to sunlight conditions. Some areas are better suited to wind power, some to solar power and others to both. With the emergence of hybrid technologies, there's no need to choose!
Rooftop electricity generation solutions
Rooftop electricity generation solutions
Photovoltaic solar panels
The capacity of a solar panel is defined by its power, expressed in peak watts (Wp ). It corresponds to the maximum power delivered by the solar panel under standard sunlight conditions (1,000 W/m² at 25°C).
By way of illustration, the map opposite shows the ratio between the energy produced and the installed capacity of photovoltaic solar panels.
In Lille, between 800 and 1,000 hours of operation at maximum power. In Marseille, between 1,200 and 1,400 hours of operation at maximum power. The energy yield of a watt-peak of photovoltaic solar panel installed on a roof is therefore between 0.8 and 1.4 on average in France.
Rooftop wind turbines
Energy efficiency is calculated as the ratio between installed power and annual energy output. The calculation is more complicated: the efficiency of a wind turbine depends on the area, the height of the building, its exposure and its environment. Using the power curve opposite and the wind rose for a given location, we can calculate the electricity generated over a year.
The WindBox turbine produces between 1,000 kWh and 2,000 kWh per year, with a peak power of 1,500 W.
The energy yield of a WindBox wind watt installed on a roof is therefore between 0.67 and 1.3 on average, in France.
Towards a rooftop electricity mix?
Overcoming the seasonal nature of renewable energies
Solar energy is a seasonal energy source. Production from solar panels peaks in the summer months, and declines in the winter months, when sunshine is less frequent and less intense. Wind power is also seasonal in most geographical areas. The production curve is the opposite of that for solar energy: wind power production peaks in winter and declines in the spring and summer months.
By combining wind and solar power, hybrid modules like the WindBox can smooth out the production curve.
To illustrate this, let's take the example of two cities with different characteristics (Hamburg and Marseille) and compare solar, wind and hybrid production on a building in the city. For each test case, we assume that the installation criteria have been met.
Test case 1:
Optimized solar production
Installation of two photovoltaic solar panels, each 375 Wp (surface area equivalent to a WindBox), tilted and optimally oriented in relation to the area.
Test case 2:
Wind generation
Installation of a single 1,500 W wind turbine (surface area equivalent to that occupied by a WindBox).
Test case 3:
Hybrid production
Installation of a 2,250 W WindBox hybrid module with panels oriented at a maximum of 10° in the same direction as the wind turbine.
Monthly production in Hamburg: solar, wind, hybrid
Solar and wind power have inverted production curves
Optimized solar production
An optimum installation is considered on the roof with panels facing south at a 40° angle.
No Data Found
Average solar production is 700 kWh per year and 60 kWh per month, with a production variability of 45% in relation to this average, depending on the month considered. The most productive months are the summer months.
Wind generation
We consider the installation of a wind turbine on the west-facing facade of a 20 m-high building.
No Data Found
Output from the wind turbine is 1,340 kWh per year and 110 kWh per month on average, with a mean deviation of 23%. The most productive months are winter.
Hybrid production
In this case, the photovoltaic panels are installed at 5°, which results in lower production (610 kWh per year vs. 700 kWh with an optimal tilt).
No Data Found
Hybrid production is balanced between seasons
Optimized solar production (%)
No Data Found
Wind generation (%)
No Data Found
Hybrid production (%)
No Data Found
Monthly production in Marseille: solar, wind, hybrid
By combining solar and wind power, total production peaks in summer
Optimized solar production
An optimum installation is considered on the roof with panels facing south at a 40° angle.
No Data Found
Average solar production is 1,170 kWh per year and 100 kWh per month, with a variability in production of 28% in relation to this average, depending on the month considered. The most productive months are the summer months.
Wind generation
We consider the installation of a wind turbine on the west-facing facade of a 20 m-high building.
No Data Found
In Marseille,wind turbine production averages1,915 kWh per year, with an average variation of 13%. This low seasonal variation is typical of the Mediterranean basin, and can be explained by strong northerly winds all year round.
Hybrid production
In this case too, the photovoltaic solar panels are installed at 5°, resulting in lower production (965 kWh per year vs. 1,170 kWh with an optimal tilt).
No Data Found
Overall, balanced hybrid production between seasons
Optimized solar production (%)
No Data Found
Wind generation (%)
No Data Found
Hybrid production (%)
No Data Found
Day and night energy production
Whatever the type of building, it consumes electricity day and night:
- three-shift operation of industrial and logistics buildings;
- need to keep the cold chain for the tertiary sector;
- or power elevators and ventilation systems in residential buildings.
Solar generation is concentrated exclusively in the daytime, with a peak around midday. Wind generation is more evenly distributed throughout the day, with a slight peak in the late morning/early afternoon. To ensure constant production, it is therefore vital to diversify the sources of renewable energy on the roof .
Let's take the previous examples and detail the hourly profile of solar, wind and hybrid generation over the four seasons.
Hourly production in Hamburg: solar, wind, hybrid
Hourly production in Marseille: solar, wind, hybrid
Solar production has the same hourly profile as in Hamburg, operating from 10 to 14 hours depending on the season, with a peak around midday. Wind generation is continuous and stable (mean deviation between 0.5 and 1.1 points depending on the season). Here too, hybrid generation guarantees a minimum production heel.
Increasing production in constrained spaces
The final argument in favor of hybrid solutions is the question of available roof space. Rooftops present a number of disadvantages for renewable energy installations:
- The structure of the buildings was not designed to accommodate technical equipment.
- Where the structure allows, the available space is reduced because other technical equipment is often installed there.
Space being at a premium, the most productive technologies must be favored to maximize production. On the other hand, to prevent structural problems, one solution is to prefer technologies that are installed at the edge of the building: the roof is more resistant there. As space is not infinite, the most efficient solutions should be placed there.
This is the case with hybrid solutions. In terms of power, a 4m² WindBox represents the equivalent of 12 m² of solar panels. In terms of production, the results depend on the weather conditions of the building in question.
Let's return to the examples developed earlier.
Hamburg
Marseille
