Types of Turbines
The definition of a turbine is any rotary mechanical device that extracts energy from a flowing fluid. That is a pretty broad definition, made even broader by the fact that a fluid, in this context, is generally any matter that flows. So, a jet of steam or air can both be fluids just as easily as water or some other liquid. So, turbines are found in everything from wind mills to nuclear power plants.
Of course, the properties of the fluid being dealt with vary from gases like air and steam to liquids like water or ammonia. Because of these differences in properties of fluids, different turbine designs have been devised. To begin, there are two general types of turbine, the impulse and reaction turbines.
The impulse turbines change the direction of flow of a fluid and the resulting change in inertia is used to turn the turbine. This results in a loss of energy in the fluid itself, but no pressure change. This is beneficial because the turbine does not require any special pressure casement.
A reaction turbine develops torque as a result of pressure or mass effects from the fluid it is in contact with. As a fluid passes through the turbine blades, it changes pressure and transfers some of that energy to the turbine. If the turbine is not fully immersed in the fluid, then it requires a casement to contain the fluid. Otherwise, the fluid would take the path of least resistance and bypass the turbine. Most steam turbines used in power generation as well as wind turbines and most underwater turbines are reaction types.
Water Turbine Designs
In this section we will look at various types of water turbines, paying special attention to their design characteristics.
The Pelton wheel is a type of impulse turbine first invented in the later 1870s. The real innovation for the Pelton wheel was that it extracted energy from a jet of moving water rather than from the weight of falling water like most water wheels of the time. The key to the Pelton Wheel is its unique geometry, which allowed it to extract most of the energy from the water stream. The wheel basically just contains troughs with cutaway end section that allow the water stream to enter. The diagram below illustrates three-quarter and side-on views of the Pelton Wheel.
Pelton Wheels work best in high pressure, low flow settings and worst in low pressure, high flow settings. Tides tend to be high flow, but relatively low pressure, so Pelton Wheels are not ideal for tidal power schemes that use stream generation. In general, fifteen meters or more of head is needed to make a Pelton Wheel efficient. So, Pelton Wheels could find application in barrage type systems where head is greater than 15 meters. The problem is, as head drops with the outgoing tide, the pressure will drop and the wheel will be less efficient. Thus, it is better to maintain high head at all times, something that is easier to attain with traditional hydropower systems that damn rivers with a constant flow as opposed to barrage systems in which the tide waxes and wanes.
The Francis turbine was first created in Lowell Massachusetts and is a type of reaction turbine. Because it is a reaction turbine, the Francis wheel requires a casement around its outside. When completely constructed a Francis turbine looks much like a Conch shell. Francis turbines are the most popular type of turbine used in hydroelectric plants where they range in size from 10 to 750 megawatts. They work well in heads from 10 to over 700 meters.
Water enters on one side of the spiral and then exits at the center of the wheel. In many cases, the blades themselves can be adjusted for lower or higher flows of water. Water entering at the spiral is high pressure while that exiting at the center is lower pressure. Francis turbines could not be used in barrage-type tidal power schemes as they do require at least 10 meters of head.
The Kaplan turbine is a propeller type reaction turbine that is usually immersed completely in the fluid it derives energy from. A modified Kaplan turbine is easily seen in wind mills where it is often referred to as a Horizontal Axis Wind Turbine or HAWT. A Kaplan turbine is beneficial in that it is able to operate in lower pressure situations where Pelton or Francis turbines cannot.
Kaplan turbines operate well in head ranges of 10 to 70 meters. When modified with longer blades, similar to those found on wind turbines, Kaplan turbines do very well in tidal stream generator schemes where there is low head, but high flow. Kaplan turbines are expensive to design and manufacture due to the fact that they are often custom-made for specific settings and applications.
The Gorlov helical turbine is very new, having been patented in 2001. It was initially designed as a vertical axis wind turbine, but has also found applications in tidal stream settings. The advantages of this turbine are:
- Simple design that is inexpensive to manufacture and has low maintenance needs
- Works well in low flow and low head settings.
- It is not dependent on the direction of current flow, so it works when the tide is coming and when it is going.
While there are a number of other turbine designs, those mentioned above are the most commonly used in water settings and the most likely to be used in future tidal applications, though in various modified forms. Tidal power generation is generally no different from either hydroelectric or wind power generation. The major differences have to do with the corrosion of parts, so materials advances will be important in turbine construction moving forward.