Alternative and Renewable Energy - Advanced Materials ...

Spiral 2: 3/28/20116.4.1.4 Types of Hydropower TurbinesDepartment of Defense Energy HandbookAlternative and Renewable Energy Options for DoD Facilities and BasesA turbine may be defined as a device that is used to extract energy from a flowing fluid; in this case, thefluid is water and therefore the turbines are referred to as hydraulic turbines. 90 Turbines can beclassified based upon the direction of flow through the blades: axial, radial, or combined flow. However,it is more common to classify them as either reaction or impulse turbines as most fit into one of thesetwo categories. Table 20 lists the conditions for which each turbine type should be selected.Table 20. Turbine selection. 90Turbine Type Head Flow RateReaction Low 3 – 30 m HighImpulse High > 400 m Low6.4.1.4.1 Reaction TurbinesReaction turbines are most efficient under high flow rate, low head conditions and consist of a rotorthat is enclosed by a casing. With this type of turbine, the water flows within the casing to spin theturbine. 90 A reaction turbine creates power from the action of both the pressure and the motion of thewater. As the water flows over the turbine’s rotor, the angular momentum, velocity, and pressure of thefluid decrease. Because of their design, reaction turbines are most frequently found in hydroelectricplants that utilize dammed rivers. 90For low head (3 to 30 m) applications, reaction turbines have comparatively inexpensive, fixed blades.They also have relatively high conversion efficiencies when used at conditions below the system designlimits. However, the conversion efficiency decreases under partial loading. The average efficiency whenoperating at a third of the full-rated power output is only 50 percent.The two most common types of reaction turbines are the axial-flow Kaplan turbines and the radial- andmixed-flow Francis turbines. The low efficiency under partial loading condition can be improved via theuse of the Kaplan turbine. This turbine has adjustable pitch blades and can produce at an efficiency ofapproximately 90 percent when operating at a third of its full-rated power output. 77 The Kaplan turbinewas developed in the Czech Republic by Victor Kaplan. 90, 91The Francis turbine was developed in Lowell, MA, by an American engineer, James B. Francis, in 1848. 90,91 The radial-flow Francis turbine has inlet guide vanes that are used to direct water into the rotor.These guide vanes may be adjusted for optimum performance. With this type of turbine, the fluid appliesa torque to the rotor in the direction of rotation, and the rotor subsequently extracts energy from thisfluid. 90 For medium head applications (5 to 400 m) mixed-flow, radial turbines are often used, the mostpopular of which is the Francis turbine. This turbine is similar to the fixed-blade propeller type;however, its speed is controlled by adjusting the guide vane angle. 776.4.1.4.2 Impulse TurbinesAn impulse turbine uses a nozzle to convert water pressure into velocity. The resulting stream of waterimpacts a rotor (also known as a runner) that has buckets to capture the kinetic energy of the water. 92The impulse turbine works well for high head applications. 90The most commonly known impulse turbine is the Pelton Wheel, which was designed by an Americanengineer, Lester Pelton, in 1870. 90, 91 (A Pelton Wheel for micro-hydro is illustrated in Figure 24.) Thistype of turbine is typically used when the head is greater than 50 m. At one tenth of its full-rated output,the Pelton Wheel has an efficiency equal to 60%. 77 The Pelton Wheel can have an efficiency as high as90%. 82 The speed is controlled by a variable inlet nozzle to ensure, when used under constant headconditions, that the torque on the generator is proportional to the flow rate. Additionally, it ensuresthat the turbine speed can be maintained for synchronous generation at a specific grid frequency. 77Figure 25 shows a Pelton Wheel installed at a hydropower plant.77