Wind Energy Basics
The Earth is surrounded by the atmosphere, which is made up of air. Air is a mixture of gas, and solid and liquid particles. Energy from the sun heats up the atmosphere and the Earth unevenly. Cold air contains more air particles than warm air. Cold air is therefore heavier and sinks down through the atmosphere, creating high pressure areas. Warm air rises through the atmosphere, creating low pressure areas. The air tries to balance out the low and high pressure areas – air particles move from areas of high pressure (cold air) to areas of low pressure (warm air). This movement of air is known as the wind.
The wind is also influenced by the movement of the Earth. As it turns on its axis the air does not travel directly from areas of higher pressure to areas of lower pressure. Instead, the air is pushed to the west in the northern hemisphere and to the east in the southern hemisphere. This is known as the Coriolis force.
The Earth’s surface is marked with trees, buildings, lakes, sea, hills and valleys, all of which also influence the wind’s direction and speed. For example, where warm land and cool sea meet, the difference in temperature creates thermal effects, which causes local sea breezes.
How can you measure the wind?
Wind is usually measured by its speed and direction. Wind atlases show the distribution of wind speeds on a broad scale, giving a graphical representation of mean wind speed (for a specified height) across an area. They are compiled by local meteorological station measurements or other wind-related recorded data. Traditionally, wind speed is measured by anemometers – usually three cups that capture the wind rotating around a vertical axis. The wind direction is measured with weather vanes.
After measuring wind data for at least one year, the mean annual wind speed can be calculated. Wind speed and wind direction statistics are visualised in a wind rose, showing the statistical repartition of wind speed per direction.
Wind statistics show the best sites to locate wind farms according to the best wind resources. They also provide further information on how the turbines should be positioned in relation to each other and what the distance between the turbines should be.
What is a Wind Turbine?
A wind turbine transforms the kinetic energy of the wind into mechanical or electrical energy.
Wind turbines consist of a foundation, a tower, a nacelle and a rotor. The foundation prevents the turbine from falling over; it is usually 13m across and 1-2m deep. The tower holds up the rotor and a nacelle (or box). The nacelle contains large primary components such as the main axle, gearbox, generator, transformer and control system. The rotor is made of the blades and the hub, which holds them in position as they turn. Most commercial wind turbines have three rotor blades. The length of the blades can be more than 60 metres.
How big is a wind turbine?
The average size of onshore turbines being manufactured today is around 2.5-3 MW. One 2.5 MW onshore turbine produces power for over 1,500 average EU households.
The largest onshore turbine is a 7 MW turbine with a rotor diameter of 127 m.
Offshore turbines can reach up to 5 MW or more, with a rotor diameter of 120 metres – longer than a football field and powering around 3000 average EU households.
What is a Wind Turbine Made of?
The towers are mostly tubular and made of steel or concrete, generally painted light grey. The blades are made of glass-fibre reinforced polyester or wood-epoxy. They are light grey because it is inconspicuous under most lighting conditions. The finish is matt, to reduce reflected light.
How is a wind farm designed?
There are many factors at play when designing a wind farm. Ideally, the area should be as wide and open as possible in the prevailing wind direction, with few obstacles. Its visual influence needs to be considered – few, larger turbines are usually better than many smaller ones. The turbines need to be easily accessible for maintenance and repair work when needed. Noise levels can be calculated so the farm is compatible with the levels of sound stipulated in national legislation. The turbine supplier defines the minimum turbine spacing, taking into account the effect one turbine can have on others nearby – the ‘wake effect’. Then, the right type of turbine must be chosen. This depends on the wind conditions and landscape features of the location, local/national rules such as on turbine height, noise levels and nature conservation, the risk of extreme events such as earthquakes, how easy it is to transport the turbines to the site and the local availability of cranes.
How long does it take to build a wind farm?
Construction time is usually very short – a 10 MW wind farm can easily be built in two months. A larger 50 MW wind farm can be ready in 18 months to two years. Most of that time is needed for measuring the wind and obtaining construction permits. Building the wind farm itself usually takes only 6 months.
What are the costs of building a wind farm?
Costs vary but the major cost is the turbine itself. This is a capital cost that has to be paid up front and typically accounts for 75% of the costs. Once it is up and running there are few costs – and of course no fuel and carbon costs.
The cost of installing wind turbines is assumed to be 1,225 €/kWh.
How efficient are wind turbines?
The theoretical maximum energy which a wind turbine can extract from the wind blowing across is just under 60%.
Why do some wind turbines have two and others have three blades?
The optimum number of blades for a wind turbine depends on the job the turbine has to do. Turbines for generating electricity need to operate at high speeds, but do not need much turning force. These machines generally have three or two blades. On the other hand, wind pumps need turning force but not much speed and therefore have many blades.
The majority of modern commercial wind turbines have three blades, as this design has been found to have a greater aesthetic appeal.
Two bladed machines are cheaper and lighter, with higher running speeds which reduces the cost of the gearbox, and they are easier to install. However they can be noisier and are not as visually attractive, appearing ‘jerky’ when they turn.
Why do some of the turbines in a wind farm stand still?
Turbines sometimes have to be stopped for maintenance, for repairing components or if there is a failure that needs to be checked. Another reason can be too little or too much wind: if the wind is too strong, the turbine needs to be shut down because it could be damaged.
How much space does a wind farm need?
In a wind farm the turbines themselves take up less than 1% of the land area. Existing activities like farming and tourism can take place around them and animals like cows and sheep are not disturbed.
Could I put a turbine in my garden or on the roof of my house?
More and more householders, communities and small businesses are interested in generating their own electricity by using small scale wind turbines, either on their roofs or in their back gardens. If you are interested in how you can power your home or business with your own turbine, then contact your national wind energy association for more information on how this works in your country.
Why don’t we all put wind turbines out to sea?
At present, onshore wind is more economical than development offshore. Furthermore, offshore wind farms take longer to develop, as the sea is inherently a more hostile environment. To expect offshore to be the only form of wind generation allowed would therefore be to condemn us to miss our renewable energy targets and commitment to tackle climate change. However, in the coming years, as offshore turbines are manufactured on a larger scale, prices will come down, making offshore wind energy increasingly competitive. Enough wind blows over European seas to power Europe seven times over, making offshore wind a highly viable option to exploit.
How many wind turbines are there in the EU?
Across the EU there are around 60,000 turbines. Some countries have far more wind power capacity installed per km2 than others. Denmark, Germany, the Netherlands and Spain have the highest wind power densities, while Slovakia, Romania, Slovenia, Malta and Cyprus have very low turbine densities.
How long does a wind turbine work for?
Wind turbines can carry on generating electricity for 20-25 years. Over their lifetime they will be running continuously for as much as 120,000 hours. This compares with the design lifetime of a car engine, which is 4,000 to 6,000 hours.
How much of the time does a turbine work?
Wind turbines start operating at wind speeds of 4 to 5 metres per second and reach maximum power output at around 15 metres/second. At very high wind speeds, i.e. gale force winds, (25 metres/second) wind turbines shut down. A modern wind turbine produces electricity 70-85% of the time, but it generates different outputs depending on the wind speed.
Over the course of a year, it will typically generate about 30% of the theoretical maximum output (higher offshore). This is known as its capacity factor. The capacity factor of conventional power stations is on average 50%. Because of stoppages for maintenance or breakdowns, no power plant generates power for 100% of the time.
How fast do the blades turn?
The blades rotate at anything between 15-20 revolutions per minute at constant speed. However, an increasing number of machines operate at variable speed, where the rotor speed increases and decreases according to the wind speed.
View the full article here: www.ewea.org