Wind at work
February 16, 2017
The energy industry is changing. As technology advances, the use of electricity delivered by renewable energy sources is growing. Many Americans are interested in harnessing energy from the sun through the use of solar panels, but you might be surprised to learn that wind, as a renewable energy source, is a much larger contributor to America’s diverse energy mix. Wind accounts for 4.7 percent of our nation’s fuel mix.
So how is wind harvested? In simplest terms, the wind turns a propeller that is connected to a generator via a gear box; these parts are contained in a housing called a nacelle. This mechanical connection increases the revolutions of the blades from a leisurely 15-20 revolutions per minute (rpm) to 1,800 rpm at the generator, where wind becomes electricity.
As the wind changes direction, the nacelle turns the blades to continue generating. When wind farms are laid out, the placement of the turbines is strategically planned so the turbulence from one turbine does not interfere with the operation of others behind it. The turbines also have protective mechanisms built in that will furl the blades once a certain wind speed is reached to prevent the turbine from spinning itself to pieces.
Like everything else, technology is driving the development of larger capacity wind turbines. Earlier models of turbines had the capacity to produce 660 kW (kilowatts) to 1 MW (megawatts) of power. Current models have the capacity to produce 1.2 to 2 MW. And turbines able to produce 12 to 21 MW are currently being tested and developed. Larger capacity is critical to production because of Metz’s Law. This theory was developed in 1919 by Albert Metz and stated that a wind generator would be able to convert a maximum of 59.3 percent of wind energy into electricity. Larger capacity equates to more output.
Next, why are there typically three blades on a turbine? Single blade turbines have been found to be unstable in operation. Adding a second blade increases output by 10 percent. Adding a third blade increases output by 5 percent. Each additional blade increases the output, but the increase is considered small––and the increased cost of materials and construction make it uneconomical––so, three blades has become the norm.
Because of the enormous stresses the blades face––and the need for lighter weight––the blades are typically built from resin impregnated composite materials. The most common form of construction is molding epoxy soaked fiberglass into the desired shape with cores of balsa wood. Anyone who has ever built a balsa wood model airplane will question this, as those assemblies are extremely fragile. However, balsa’s light weight and composition make it an excellent contributor to the stability and durability of these monster blades.
The largest blade being produced today is 75 meters (m) in length, just a bit less than the wingspan of an Airbus A380!
In the wind generation game, height is a critical consideration. Near the surface of the earth, wind conditions become unstable and erratic as the sun warms the ground. The temperature difference between the ground and the air creates effects like wind shear, which can make efficient operation difficult. At higher levels, undesirable ground effects rapidly diminish and wind speed becomes much more consistent.
The U.S. Government and other agencies produce wind speed maps at a number of heights. Today’s standard wind speed map uses a height of 80 meters. When a company looks to develop a commercial wind farm, they use these maps to locate areas where they can find a consistent 13 mph wind speed or higher. [Readers can learn more about wind speed maps here: http://www.nrel.gov/gis/wind.html]
A key challenge facing wind and solar energy is variability. The output of solar and wind, for example, can vary significantly over short periods, like when the wind stops blowing or the sun goes behind a cloud. One way to deal with that issue is energy storage, an advancing technology that will equip electric co-ops to beat peak energy prices and save members money.
For now, wind and solar are best deployed as components of a diverse energy portfolio that also includes traditional generating resources, but continued technological developments will ensure more reliable power from renewable resources in the future.