Wind is caused by uneven heating of the earth from the sun making wind a renewable and free source of energy. Wind turbines are an alternate source of energy that harnesses this renewable wind power to make electricity. Since wind turbines run solely on wind, they cause no pollution making them environmentally friendly. Basically, wind turns blades that are connected to a generator, the generator then makes electricity (more on this later). There are two main types of wind turbines, horizontal and vertical axis. A wind turbine applicable for urban settings was also studied. All three types of wind turbines have varying designs, and different advantages and disadvantages.
Horizontal axis wind turbines are the most common type used (see figure 1). All of the components (blades, shaft, generator) are on top of a tall tower, and the blades face into the wind. The shaft is horizontal to the ground. The wind hits the blades of the turbine that are connected to a shaft causing rotation. The shaft has a gear on the end which turns a generator. The generator produces electricity and sends the electricity into the power grid. The wind turbine also has some key elements that adds to efficiency. Inside the Nacelle (or head) is an anemometer, wind vane, and controller that read the speed and direction of the wind. As the wind changes direction, a motor (yaw motor) turns the nacelle so the blades are always facing the wind. The power source also comes with a safety feature. In case of extreme winds the turbine has a break that can slow the shaft speed. This is to inhibit any damage to the turbine in extreme conditions.
Figure 1: Horizontal axis wind turbine
• Blades are to the side of the turbines center of gravity, helping stability
• Ability to wing warp, which gives the turbine blades the best angle of attack
• Ability to pitch the rotor blades in a storm to minimize damage
• Tall tower allows access to stronger wind in sites with wind shear
• Tall tower allows placement on uneven land or in offshore locations
• Can be sited in forest above tree-line
• Most are self-starting
• Difficulty operating in near ground winds
• Difficult to transport (20% of equipment costs)
• Difficult to install (require tall cranes and skilled operators)
• Effect radar in proximity
• Local opposition to aesthetics
• Difficult maintenance
In vertical axis turbines the shaft the blades are connected to is vertical to the ground (see figure 2). All of the main components are close to the ground. Also, the wind turbine itself is near the ground, unlike horizontal where everything is on a tower. There are two types of vertical axis wind turbines; lift based and drag based. Lift based designs are generally much more efficient than drag, or ‘paddle’ designs.
Figure 2 : Vertical axis wind turbine (lift type)
• Easy to maintain
• Lower construction and transportation costs
• Not directional
• Most effective at mesas, hilltops, ridgelines and passes
• Blades constantly spinning back into the wind causing drag
• Less efficient
• Operate in lower, more turbulent wind
• Low starting torque and may require energy to start turning
Ducted Wind Turbines
Ducted wind turbines are positioned at the edge of the roof of a building and utilize the airflow along a building’s side. The air flows upwards, hugging the building wall then enters the front of the duct. Turbine blade diameter is usually around 600 mm. The devices are relatively small leaving little visual impact to the building. They are positioned on a building as shown in figure 3 below.
Figure 3: Ducted Wind Turbine
• Less visual impact on buildings architecture than traditional HAWT or VAWT turbines
• Make use of unused roof space in cities
• Allows energy need to be met on-site avoiding transmission losses associated with centralized energy generation
• Suitable for urban environments, but not households (only effective on urban high-rise buildings)
• Uni-directional. Fixed position and are dependent upon wind blowing in the correct direction
• Much more research and development is needed. Research in this field is growing as people become more interested in urban wind generation.
• Research has to be done to determine energy production potential
A. Guzzetta, G. Myers, A. Purse
4. “Industrial Development of a Building Integrated Ducted Wind Turbine Module”, http://www.carbontrust.co.uk