Sailing boats, windmills, etc. Humans have made use of wind energy for centuries. Thanks to technological progress, today it is possible to produce “green” electricity without using fossil resources. We take a closer look at its development prospects.
- Wind energy: a few definitions
- How does a wind turbine work?
- From wind turbines on land to offshore wind turbines
- Advantages and challenges of wind energy
A wind turbine is a machine used to convert kinetic energy from the wind into mechanical energy, in turn converted into electricity. When several wind turbines are installed on the same site, this is called a “wind park” or “wind farm”.
The first wind turbines used to produce electricity date back to the 1970s. In France today, wind power is the second most used renewable energy source behind hydropower. It supplies almost 5% of national electricity requirements.
In France, wind power supplies almost 5% of national electricity requirements.
Wind energy is produced thanks to the force exerted by the wind on the blades of a rotor.
The rotor generally has 2 or 3 blades rotating around a horizontal axis. The sweep diameter of these blades ranges from 80 to almost 200 meters. The longer the blades, the greater the electricity production capacity.
As they rotate, the blades drive a generator that produces electric energy. A multiplier is often installed between the rotor and the electric generator, since the latter requires a rotation speed of 1000 to 2000 rotations per minute (rpm), whereas the blades turn more slowly (10 to 25 rpm). Recently designed variable rotation speed systems are often direct drive turbines and therefore do not have a multiplier. The assembly composed of the blades, the rotor, the multiplier and the generator forms the nacelle.
The nacelle is installed at the top of a tower that is between 50 and over 100 m high. A rotation system turns the rotor and blades so that they are always facing into the wind. Wind turbines generally operate at wind speeds of between 10 and 90 km/h. Beyond this maximum speed, they stop automatically to avoid any accidents.
The electric energy produced by the generator is transported down the tower via cables to a transformer, before being injected into the electricity grid by underground cables.
Two wind power sectors have developed in parallel with one another: land wind turbines, currently the most numerous and the most mature, and offshore wind turbines, installed in the sea, where the wind is more powerful and more regular.
Europe has been a pioneer in the development of wind energy and is still the second-placed region in the world in terms of wind turbine growth. France is ranked 3rd in Europe, behind Germany and Spain. On a worldwide scale, China and the USA have the biggest wind farm sectors.
Land wind turbines
Easier to operate, land wind turbines were used first.
However, the potential of wind power on land is limited by:
- a maximum power only just above 3 to 4 MW
- a sometimes poor acceptability to society (visual and noise pollution, impact on flora and fauna)
- a lack of available sites (for example in Denmark, which produces over 40% of its electricity via wind turbines, but where the landscape is saturated).
Offshore wind turbines
Offshore wind power has been taking off in the past few years. Larger and more powerful (6 to 10 MW, or even 12 MW for some models to come), wind turbines installed in the sea supply more energy per machine than wind turbines on land. They have a limited impact on the landscape, making it possible to have larger wind parks, with more turbines.
There are two types of offshore wind turbines depending on their location:
- Fixed wind turbines: at depths of up to 50 meters, with the wind turbines fixed to the seabed. Fixed offshore wind power technologies are already available, but bigger and more powerful turbines will be needed to reduce operating costs. In Europe, the UK leads the way in terms of installed capacity, followed by Germany, the Netherlands and Denmark.
- Floating wind turbines: at depths of over 50 meters, fixing to the seabed becomes too expensive and difficult. The wind turbines are therefore connected to a floating support on the surface of the water, or just below it. They can be located further from the shore and therefore benefit from stronger and more regular winds. This sector is still in the demonstration phase on pilot farms, since the design is more complex, but the first commercial wind parks are anticipated in the next few years. Reducing the costs is also a major challenge.
The future looks bright for wind energy. It uses a renewable resource (the wind) and its operation does not produce any greenhouse gases. In Europe, the installed power capacity supplies the electricity needs of some 10 million people and avoids the production of 24 million tonnes of CO2 per year. Offshore wind turbines offer some interesting development prospects despite their still high costs.
However, a number of challenges are yet to be addressed, including:
- the continued reduction of costs related to manufacture, installation and production, with land and fixed offshore wind turbines already producing electricity in market conditions, depending on the sites,
- social acceptability, including for offshore wind turbines, which can come into conflict with fishing and leisure usages,
- the integration of offshore wind turbines in the electricity network, since it is not dimensioned to accept the high powers supplied,
- the development of electricity storage technologies to ensure a balance between supply and demand on the electricity system, since wind power has variable production.