Wind turbines convert the kinetic energy from wind into mechanical energy. When used with an electrical generator, the rotation of the wind turbine’s blades turns a shaft to produce electricity. There are two basic types of wind turbines: horizontal-axis and vertical-axis.
Horizontal-axis wind turbines (HAWT)
Image Credit: GE Energy - Wind Energy
Horizontal-axis wind turbines (HAWT) have a two or three blade rotor that rotates about a horizontal axis. The rotor is predominant mounted upwind from the tower. The rotor is composed of a hub, wind turbine blades, and a spinner. The rotor can either be directly coupled to the generator, or use a drive train or gearbox to transfer the mechanical energy to the generator. A large aerodynamic enclosure, a nacelle, houses the drive train and generator.
Vertical-axis wind turbines (VAWT)
Image Credit: Wind Turbine Zone
Vertical-axis wind turbines (VAWT) are low-noise, omni-directional wind turbines that use one or more propellers or airfoils that rotate about a vertical axis. Vertical turbines impose less physical stress on the support structure, offer a wide range of operating speeds, and offer easy access to the generator, which is located at the base of the turbine.
Wind Turbine Aerodynamics
Aerodynamics is the study of the physical properties of air flow and the characteristic behavior of how solid objects interact with it. Aerodynamics describes the forces that drive a wind turbine, as well as depict theoretical phenomenon that define available power and maximum attainable efficiencies.
Image Credit: HowStuffWorks, Inc.
Lift and Drag
Lift and drag are two aerodynamic forces exerted on the blades of a wind turbine.
Lift is created as negative pressure builds downwind on the blind side of the turbine blade. As the pressure gradient develops across the wind turbine blades, a vector force perpendicular to the direction of air flow propagates. Lift-based wind turbines typically have rotors, airfoils, or blades that move perpendicular to airflow as illustrated by all HAWT.
Coefficient of Lift
Air Velocity Squared
Rotor Swept Area
Drag is the second force vector that develops parallel to the direction of air flow. It is an aerodynamic force that acts predominantly on blunt objects, decreasing fluid velocity and generating a turbulent flow regime downwind from the affected object. Drag-based wind turbines are less efficient than lift-based wind turbines, although the significant torque available from drag-based wind turbines makes them suitable for water pumping applications.
Coefficient of Drag
Air Velocity Squared
Rotor Swept Area
In 1919, German physicist Albert Bertz published a theorem that no open flow turbine can be more than 59.3% efficient in harnessing kinetic energy from wind. His theorem stated that for there to be airflow through the turbine some amount of kinetic energy must escape the blades of the turbine, thus only 59.3% could be harnessed due to principles of conservation of mass and momentum of the air stream.
In practical applications the efficiency of the wind turbines is even less than 59% due to non-laminar flow patterns, variable air densities, and turbulent flow regimes that propagate downwind. Some amount of airflow is actually deflected and never passes through the rotor swept area as the net combined effect of dampening forces causes wind speeds to diminish prior to reaching the blades of the wind turbine.
Image Credit: Danish Wind Industry Association
Energy from Wind
Wind power is proportional to the cube of wind velocity; therefore fluctuation in wind velocity exponentially impacts the available power. For this reason most wind turbines will not produce any power until the cut-in speed is breached, typically sustained winds of 5m/s. This relationship between wind speed and power also amplifies the importance of proper site selection. Wind turbines are designed to operate in a specific wind speed range. Areas prone to highly variable wind speeds do not prove to be an adequate site for wind turbines.
Onshore Versus Offshore Solutions
In order to provide a viable source of renewable energy wind turbines must be economical in that the power produced must be substantial enough to offset the upstream costs. Onshore and offshore solutions each carry their own pros and cons.
Offshore Wind Farms
Offshore wind farms have the competitive advantage of favorable wind conditions, aesthetics, and minimal impact on migrating birds, which all come with increased upstream costs.
Image Credit: The Bureau of Ocean Energy Management (BOEM)
Studies have shown that offshore locations provide stronger, uniform wind velocities which are two of the primary factors taken into consideration when choosing a site.
Aesthetics are of less debate as offshore locations are pushing toward deeper oceans where appearance is inconsequential.
In-line with the offshore site selection offshore locations also affects smaller bird migration populations.
The biggest drawback to all of the advantages that offshore wind farms offer is the inflated installation and anchoring costs which play into the economics of developing offshore wind farms.
Onshore Wind Farms
Onshore wind farms have been in operation since December of 1980 when the first onshore wind farm was established in southern New Hampshire of the United States of America. Modern onshore wind farms have shown the capability to generate significant energy supply by developing wind farms at ideal locations near coastal boundaries while using micro-siting techniques to further increase energy output.
Image Credit: Power Engineering International (PEi)
The topographical acceleration of wind over near-ocean ridges, where wind power can be substantial and economical to harness, has directed site development along ridglines general three or more kilometers inland from the nearest shoreline.
Micro-siting, the strategic placement of each turbine over finite distances, has shown the ability to double energy output.
Onshore locations offer easy access in order to perform routine maintenance, repair, and testing.
Supporting infrastructure decreases the cost associated with integrating output power to the electrical power grid.
Image Credit: IEEE Power & Energy Society
Wind Turbine Components
Wind turbines contain many parts. The anemometer tracks wind speeds and transfers measurements from the wind vane to the controller, a component which starts the turbine at wind speeds between 8 to 16 miles per hour and shuts off the turbine at about 65 mph. Gears connect the low-speed shaft to the high-speed shaft and increase the turbine's rotational speeds from about 30 to 60 rotations per minutes (rpm) to about 1200 to 1500 rpm. With upwind turbines, motor-powered yaw drives are used to keep the rotor facing the wind. Mechanical, electrical, or hydraulic disc brakes can be used to stop the rotor in the event of an emergency.
Image Credit: National Renewable Energy Laboratory (NREL)
Performance criteria for wind turbines include orientation, number of blades, blade direction, rotor diameter, rated power, maximum power, and temperature range. Cut-in wind speed, cut-out wind speed, rated wind speed, and furling wind speed are additional considerations. In terms of features, wind turbines may provide over-speed protection and blade pitch control. Options include low-end boost, slow-mode operation, electric braking, timed battery equalization, and polarity checking. Predicted energy production can be expressed in tabular format with wind speeds taken at the top of the tower and wind speeds taken at 10 meters (m).
Industry Standards & Regulations
Industrial standards and regulations specify accepted practices for installation, operation, and maintenance of wind turbines.
IEC standards encompass all electro-technologies, including electronics, magnetics and electromagnetics, electro-acoustics, multimedia, telecommunication, and energy production and distribution, in addition to related general disciplines such as terminology and symbols, electromagnetic compatibility, measurement and performance, dependability, design and development, safety, and the environment.
One of the most important developments that NECA works on is the National Electrical Installation Standards (NEIS), which are developed by NECA and in partnership with other industry organizations. NEIS are the standards for electrical construction. In addition to the basic safety requirements available in the National Electrical Code (NEC), the National Electrical Installation Standards clearly define installation instructions of products and systems. It is important to mention that the NEIS are submitted for approval by the American National Standards Institute (ANSI).
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