Wind energy is energy from moving air, caused by temperature (and therefore pressure) differences in the atmosphere. Irradiance from the sun heats up the air, forcing the air to rise. Conversely, where temperatures fall, a low pressure zone develops. Winds (i.e. air flows) balance out the differences. Hence, wind energy is solar energy converted into kinetic energy of moving air.
Wind Energy Converters (WECs) - or short: wind turbines - capture the air flow by converting it into a rotational movement, which subsequently drives a conventional generator for electricity.
Wind energy has been used for centuries to pump water and grinding. The industrial breakthrough for the generation of electricity, came in the 1980s.
Today, wind energy is the most mature of the renewable energy technologies apart from hydro. In 2010, the global installation might reach 200GW, up 5GW in 1995 - that equates to an annual growth rate of 27%!
Here in Portugal, near the Atlantic coast, each wind turbine has a capacity of 1.5MW, generating electricity for more than 1,000 households.
How much power is in wind energy?
When air mass is flowing through an area A with speed v, the power of that air movement at time t is given by:
where ρ is the density of air, which is around 1.22kg/m³. The energy (kWh) is the product of power and time:
To take account of wind fluctuations, the energy from an air flow over a time period P is made up of the sum of wind speeds of small time intervals. Often, average hourly wind speeds are measured, thus providing 24 time buckets per day.
While the air density is more or less constant, the two parameters to watch out for are the wind swept area, A, and the wind speed v. The latter is even more critical, as it is cubed. A location with double average wind speed has 8 times the power for the same area. Or - to capture the same energy, the blades of the wind turbine in the low wind speed location would have to be almost 3 times as long.
The principles of conversion
There are two forces in play: Lift and Drag. The Lift Force is perpendicular to the wind direction. It is caused by a pressure difference between the air on either side of the blade. The Drag Force is in the same direction as the wind. The ratio between lift and drag largely depends on the shape of the blade and the angle of the main line of the blade (chord line) and the main wind direction - the angle of attack. The lift force is largest for streamlined
Depending on the design of the turbine, either drag or lift moves the blades. Most wind turbines today use the prinicple of lift rather than drag.
The potential of wind energy
How much wind can feasibly extracted and what is the global distribution?
According to a study by the EWEA, the amount that is technically feasible to extract is huge: Western Europe generates around 150TWh per year at the moment. It has the potential to generate 30 times that amount. Eastern Europe, Latin America and Africa, also with significant potential have only started to invest in wind energy assets. Thanks to its wide distribution, degree of maturity and attractive economics, wind energy will play a major role in the next decades.
The Challenges for Wind Energy
Technical
High forces from storms cause thrust on rotor, accelearting wear and tear.
Wind speed fluctuations causing turbulence.
Inertial forces of rotor: Due to its own weight, the rotor doesn't just stop instantly
Higher wind speeds are in higher areas or off-shore, both challenging environments.
Political
Local acceptance: Planning permissions will not be given if local acceptance is low.
Some markets (Germany on-shore) are nearing saturation. Over-doing it may risk losing local acceptance.
Wind farms built in the 90s are coming to their end of life. Re-powering sites requires planning planning permission. Risk is that lots of sites could be lost.
When air mass is flowing through an area A with speed v, the power of that air movement at time t is given by:
