CACTUS MOON EDUCATION, LLC ENERGY FROM THE WIND WIND ENERGY TECHNOLOGIES EDUCATION MODULE www.cactusmooneducation.com TEACHER S NOTES (wnd01tn) _ Cactus Moon Education, LLC.
ENERGY FROM THE WIND WIND ENERGY TECHNOLOGIES EDUCATION MODULE Slide 1 Slide 2 Slide 3 Title Slide Index Historical Use of Wind Energy Wind energy is one of the oldest energy sources man has used. From the earliest sailing ships to modern electricity generating turbines, the energy in the wind has been put to good use even though it is not 100% reliable. On a day that the wind doesn t blow sailing ships don t move and wind turbines produce no electricity! Slide 4 Where does the Wind Come From? Air molecules moving in the same direction make what we call the wind. Air that is in contact with the warm earth becomes warm. It expands and rises allowing cooler air to move in to take its place. It is this air moving in that constitutes the wind. Slide 5 Betz s Law We can only ever extract about 60% of the energy in the wind using a wind turbine. The energy in the wind is the kinetic energy of the moving air molecules. The blades of a wind turbine extract some of that energy leaving the air molecules that go beyond the turbine blades with a lower energy, that is, a lower speed. So, the speed of the wind after the turbine blades is lower than the speed of the wind in front of the turbine blades. If the turbine blades extracted 100% of the energy from the wind, then the air behind the blades would have zero energy and the air molecules would be stationary. This would be like building the wind turbine in front of a large brick wall the solid wall would prevent any air passing through the turbine blades and the turbine would not work! A German physicist, Albert Betz, calculated that the maximum energy that turbine blades could extract from the wind was 59.3% of the energy in the incoming air. This allowed the air downstream of the turbine blades to get out of the way and let new air pass through the blades. Wind turbine generators are about 70% efficient. They can convert 70% of the extracted energy into electricity. A modern wind turbine generator can therefor convert about 40% of the incident wind energy into electricity. 2
Slide 6 Wind Turbine Power Curve The power contained in wind is proportional to the cube of the wind speed. If the wind speed doubles, the power in the wind increases 8 times. Wind turbines typically start to operate at a wind speed of about 12 mph and reach their rated output power at a wind speed of about 30 mph. As the wind speed increases above this, control systems change the pitch of the turbine blades so they don t extract as much energy from the wind which prevents the turbine from rotating faster and faster. This keeps the turbine producing its rated power output even though the wind speed may be well above 30 mph. At wind speeds above 50 mph the pitch of the turbine blades is reduced to zero to stop the turbine from spinning and possibly being damaged. Slide 7 Wind Turbine Development Over the last 30 years or so, significant advances have been made in the design of wind turbines and turbine/generator technology. In the nineteen eighties, the power output from wind turbines was only in the tens of kilowatts. Today, wind turbines can produce almost 50 times that power and larger machines are in the development phase. Slide 8 Wind Machines 1 There are two basic types of wind turbine turbines that have a horizontal rotating axis and turbines that have a vertical rotating axis. The horizontal axis (HAWT) wind turbines are the ones we re most familiar with and look like conventional windmills. Vertical axis (VAWT) wind turbines tend to be experimental or ornamental in nature and have blades that resemble an egg-beater. An advantage of the vertical axis turbine is that the turbine and generator machinery can be located on the ground rather than at the top of a high tower as in the case of the horizontal axis turbine. However, horizontal axis machines can be much more powerful than their vertical axis counterparts since their blades are located high off the ground where wind speeds are generally higher. Another advantage of vertical axis machines is that they will work with wind from all directions. A horizontal axis machine has to be turned so that the blades face the wind. Slide 9 Wind Machines 2 There are two types of VAWT designs the Darrieus and the Savonius turbines. The Darrieus Turbine was patented by Georges Jean Marie Darrieus, a French aeronautical engineer in 1931 and consists of a number of curved airfoil blades mounted on a vertical rotating shaft or framework. 3
The Savonius Turbine was invented in 1922 by Sigurd Savonius in Finland and uses air drag technology to spin the turbine blades. Although more complex than the vertical axis designs, horizontal axis machines have more control and can produce more power. Slide 10 Wind Turbine Components There are three major components to a modern wind turbine, the tower, the nacelle and the blades. The tower holds the nacelle and blades high off the ground away from the slow, turbulent ground wind currents. The nacelle houses the gearbox and electrical generator. The blades turn in the wind and drive the equipment in the nacelle. Slide 11 The Tower (Diagram) A modern wind turbine tower can be 300 feet or so in height 1. The tower is manufactured in three sections which are delivered to the project site on flatbed trucks 2. The base section of the tower is bolted to the pre-prepared concrete foundation 3. The other two sections of the tower are hoisted into position using a crane and bolted together 4. Generally, the higher the tower the higher the wind speed that will be encountered by the turbine 5. From a distance, a wind turbine looks fairly small. However, when you get up close to one it quickly becomes apparent just how tall these things are! Slide 12 The Nacelle (Diagram) The nacelle is hoisted by a crane and placed on top of the tower. 1. Inside the nacelle the shaft that the turbine blades are attached to powers a gearbox in which the relatively slow speed of the blade shaft, 20 to 30 RPM, is increased to 1,200 to 1,800 RPM to power the electric generator. 2. The high speed shaft turns an electric generator which makes the electricity. Some nacelles are large enough for a helicopter to land on! Slide 13 The Blades The power output of a wind turbine is proportional to the area swept out by the turbine blades. Doubling the length of the blades would increase the potential power output of the turbine by a factor of 4. The blades are manufactured from fiberglass or epoxy-based composite materials and can be over 140 feet long. 4
Special flatbed trucks are used to transport the blades to the wind farm where they are attached to the nacelle. Slide 14 Wind Farms Windfarms are groups of wind turbines located in areas that have a wind resource that (ideally) has steady, continuously blowing wind. The electrical output from each of the turbines is collected and fed to the electric power grid which can be some tens of miles away. The designer of the wind farm has to be careful that a wind turbine placed in front of another does not create turbulence or cause wind shading that will reduce the performance of the downwind machine. Typical rules of thumb are that turbines should not be placed closer than 10 times the blade length away from each other and no closer than 20 times the blade length behind each other. This means that a wind farm can occupy a lot of land. At least 60 acres of land are required for a Megawatt of wind power generation. That is more than 10 times the area required for 1 MW of solar generation. Slide 15 A Look into the Future Wind turbines are now being located several tens of miles off-shore where the wind resource is superior to that on land and where the turbines cannot be seen from land. The turbines are getting even larger than those used on land with machines capable of generating 8 MW and above currently under development. The gearbox in the nacelle is one of the highest maintenance items in a wind turbine. New turbines are being developed without gearboxes increasing their reliability. 5