Wind turbine design is still evolving, and many different designs are available. As wind turbine design is improved and larger numbers of turbines are being manufactured, wind turbine costs are being reduced. Wind turbine design is based on several factors, including available technnology, cost and schedule, and marketability. Wind turbine manufacturers wish to optimize their machines, so that they deliver electricity at the lowest possible cost per kilowatt hour (kWh) of energy. Wind turbine design falls into two basic types: horizontal and vertical axis turbines. These two types will now be described in detail.
Horizontal Axis Wind Turbines (HAWT’s)
A wind turbine in which the axis of the rotor’s rotation is parallel to the wind stream and the ground is called a Horizontal Axis Wind Turbine (HAWT). The horizontal wind turbine design is an affordable and efficient electrical generator. It is considered today’s most popular wind turbine option. This common wind turbine design incorporates multi-blade propellers that rotate about a horizontal axis parallel to the ground. HAWT’s are found in urban, rural, and commercial settings.
Most horizontal axis turbines built today are two or three-bladed, although some have fewer or more blades. The dominant small wind turbine design is essentially a smaller version of the three-blade horizontal axis large turbines used in wind farms. Horizontal axis turbines are generally used in high wind areas. They must be facing directly into the path of the wind, and require sufficient wind speeds to start rotation. Horizontal turbines will utilize winds at ground level, where vertical wind turbines capture the wind that is higher up in the atmosphere.
Vertical Axis Wind Turbines (VAWT’s)
A Vertical Axis Wind Turbine (VAWT) has the generator shaft positioned vertically, with the blades pointing up. While a horizontal axis windmill must turn to face the wind direction, the orientation of a vertical axis wind turbine remains unchanged regardless of wind direction. These stationary wind turbines can be anchored with guy wires, and the generator can be mounted on the ground or on a short tower. The vertical spinning axis helps keep noise low, and the compact size is ideal for both urban and rural areas. However, VAWT’s are not considered as efficient as horizontal axis wind turbines.
There are several variations within the general family of VAWT’s. Savonius vertical wind turbines are used when cost and reliability are considered more important than operational efficiency. The Savonius wind turbine is very simplistic in design, and is self starting in principle. It only requires low volume wind speeds for rotation. Another vertical wind turbine design is the Darrieus, which is compared to an egg beater in appearance. It consists of a number of airfoils (aerodynamic shapes) that are mounted vertically and capture the wind during operation. The Darrieus airfoils are often manufactured in a helix configuration to maximize efficiency.
Wind turbine design is a continuing process, with more efficient and cost-effective turbines being designed and produced every day. Wind turbine designs include two basic variations: (1) the Horizontal Axis Wind Turbine (HAWT), and (2) the Vertical Axis Wind Turbine (VAWT). Both of these design variations have their place in residential and commercial installations. The HAWT is the more popular option, used often in rural settings with open spaces and higher wind speeds. The VAWT is a more compact design that works in lower wind speeds, and is found in urban and residential settings.