A propeller turbine belongs to a group of machines that convert fluid or gas flow energy into rotational motion. As this description indicates, these machines fall into two basic categories: wind or water propeller turbines. The rotational force that these machines produce is most commonly used to generate electrical power and, to a lesser extent, deliver mechanical work. Common examples of mechanical work variants of the propeller turbine are windmills and certain types of water-driven mills. Due to the plentiful, renewable, and cheap nature of the propeller turbine power source, it is one of the most cost effective and environmentally friendly methods of energy production ever devised.
A propeller turbine applies conventional propeller theory in reverse to harness the latent kinetic energy in gas and fluid flows. Propellers consist of a central shaft with a minimum of two opposed, airfoil shaped blades or vanes attached to it. These are generally turned by an external energy source to produce thrust by pushing or displacing air or liquid over the blades. In a propeller turbine, this principle is flipped; a flow or air or liquid displaces the blades causing them to turn the shaft.
Wind turbines are used extensively across the globe to harness wind power to generate electricity, run mills, or pump water. Wind driven propeller turbines may be of horizontal or vertical axis design. The most readily recognized variant is the horizontal axis turbine which includes traditional windmills and wind generators with aircraft type propellers. Equally effective are the newer generation of vertical axis designs which feature flat or curved vanes driving a vertical shaft. These include the curved vane Savonius, the flat vane Giromill, and the distinctive Darrieus “eggbeater” types.
Older horizontal turbines require the turbine head to be kept turned into the wind at all times. In the case of smaller examples, a simple weather vane style rudder turns the pivoting turbine head. Larger turbines utilize a system of wind sensors and servo motors to keep the propeller turned into the wind. Most wind driven propeller turbine designs make use of a gearbox to drive the generator or pump crank at the correct speed.
Water driven propeller turbines are commonly associated with large hydroelectric generation plants although there are several smaller industrial and agricultural applications. These turbines function in the same way as their wind driven siblings although their basic design differs substantially. These machines are generally far larger and feature blade designs that are typically shorter than wind driven variants. The most common of these larger water driven turbines is the Kaplan turbine. Kaplan turbines are low head, high flow, reaction units used in most large hydro-electric installations.
The Kaplan variant features pitch adjustable blades leading to efficiency levels typically in excess of 90 percent in a wide range of water level and flow conditions. A large part of the efficiency is achieved by a carefully designed water flow path which causes the outlet fluid to decelerate. This deceleration leads to a transfer of the maximum amount of kinetic energy to the propeller mechanism. Kaplan turbines can produce power outputs of 100 megawatts (100,000,000 watts) or more.
The propeller turbine taps into power sources that are renewable and either free or extremely cheap when compared to fossil fuel options. Advances in the technologies used in these devices are continuously extending the boundaries of their efficiency and capacity, and they may turn out to be viable replacement for conventional fuel in the near future. Propeller turbine technology is also becoming increasingly accessible which further enhances its role in the realm of a cleaner, more ecologically friendly energy supply scenario.