Introduction:
In a typical hydroelectric power generation system, a dam (water reservoir) is built near water body such as a river to create head (height) that enables water to flow at high pressure via a pipe (penstock) to the turbines that drive the power generators. The fast moving water turns the turbine blades at high velocity, and in turn, the turbine turns the electricity generator’s rotor. The wire coils in the generator’s rotor sweep past the stationary coil (stator) in the generator thus producing electricity. This concept was first demonstrated in 1831 by Michael Faraday, who discovered that electricity could be generated by rotating magnets within coils of copper wire.
Hydroelectric turbines are a critical part of hydroelectric power generation systems and greatly impact the overall performance of the entire hydroelectric generation system since their efficiency determines the optimal power produced. Turbines operate on the basic principle of converting the kinetic energy of moving water into mechanical power through impulse or reaction of water with a series of paddles, buckets or blades that are arrayed around a wheel’s or cylinder’s circumference (Pall Corporation, 2006). There are different types of turbine designs, and the choice of turbine depends on the application. For example, Pelton turbines are suitable for high-head applications while Crossflow designs work best in low-head applications with high-flow. Likewise, other types of hydroelectric turbines such as Turgo, Kaplan and Francis each have their own optimum applications. Hydroelectric turbines are broadly categorized as either impulse or reaction turbines (Canyon Hydro Industries, 2016).
Hydroelectric turbine categories:
Impulse turbines:
Impulse turbines operate in air and are driven by one or more water jets delivered at high velocity via a nozzle. These turbines are mostly used in high-head hydroelectric systems and examples include the Turgo and Pelton turbine systems. The impulse turbine blade configuration is shown in Figures 1 (a) and (b).
Figure 1(a) Impulse turbine with a single nozzle; (b) Impulse turbine with four nozzles; (c) Blade arrangement in an impulse turbine; (d) The Pelton wheel (an example of an impulse turbine).
The nozzle is found at the end of a guiding tube while the rotor blades are attached to the rotor which in turn connects to the turbine shaft. The pressure at the nozzle’s inlet is higher than the pressure at its outlet, and this causes most of the water pressure potential energy to be converted into linear kinetic energy. The water emerges out of the nozzle at high velocity and strikes the rotor blades with high force. The applied force produces an impulse on the rotor blades thus transferring most of the linear kinetic energy of water to the rotor blades, but since the rotor blades are attached to a rotating shaft, this linear kinetic energy is converted to rotational kinetic energy that drives the turbine shaft. After flowing through the turbine’s rotor blades, the pressure and velocity of water are lower, and it exits the system via the exit valve (Natural Edge Project, 2008).
Reaction Turbines:
Reaction turbines operate under full immersion in water and are mostly used in low-head hydroelectric systems with a high flow such as Kaplan and Francis. The reaction turbine blade configuration is as shown in Figure 2. In the reaction turbine, the blade is immersed in water, and they rotate as the water flows past them. Reaction turbines do not significantly alter the direction of water flow when compared to impulse turbines since they simply spin as the water come in and pushes the runner blades. After flowing through the turbine’s rotor blades, the water exits the system via an exit valve (Canyon Hydro Industries, 2016). The figure 2 below show illustrates the basic operation of a reaction turbine.
Figure 2 internal structure and working of a reaction turbine.
Components of a typical hydroelectric turbine:
Hydroelectric turbines have fairly simple structural designs and components. Some of the basic components include an intake shaft, runner, water nozzle, the turbine shaft, exit valve, generator and a powerhouse. The intake shaft as its name states is used to connect the piping (penstock) that carries water from the reservoirs to the turbine. The water nozzle shoots a jet of water at high velocity. The runner is the turbine wheel that catches the flowing water with its blades or paddles thus causing the wheel to turn. The turbine shaft is connected vertically or horizontally to the generator shaft, and so when the runner turns, the attached shaft rotates the generator coil. The exit valve is a chute or tube that guides water from the turbine and returns it to the stream it originated from. Finally, the powerhouse is a small enclosure that protects the turbines and power generators from the elements. The flow (speed and amount) of water inside the turbine requires to be controlled, and for this purpose, a wicket gate is used. The figure 3 below shows the internal structure of a typical hydroelectric turbine (Energybible.com, 2012).
Figure 3 the basic components of a hydroelectric turbine and generator system
(Source: (Perlman, 2016).
Types of hydroelectric turbines:
Crossflow turbines:
Crossflow turbines are special turbines that are technically classified as impulse turbines since they are not completely immersed in water. Crossflow turbines are used in low-head, high-flow hydroelectric systems. Water enters through a large rectangular opening to turn the turbine blades instead of using the high-pressure water (nozzle) jets found in Turgo and Pelton turbines. A significant advantage of Crossflow turbines over other turbine types is that it can be used in applications where the flow is significant, but the head pressure is not enough to necessitate the use of a high-head turbine (Energybible.com, 2012; Energy.gov, 2016).
Francis turbines:
The Francis turbine is the most popular hydroelectric reaction turbine, and its entire wheel assembly is completely immersed in water and covered with a pressure casing. The pressure casing in this turbine has a tapered spiral shape to facilitate uniform water distribution around the runner’s entire perimeter. The Francis turbine uses guide vanes to ensure water enters the system and is fed into the runner at the required angle. The reaction turbine example in Figure 2 is a Francis turbine (Energybible.com, 2012; Energy.gov, 2016).
Propeller turbines:
Propeller turbines are hydroelectric reaction turbines that use a runner shaped like a boat’s propeller to drive the generator. Usually, propeller turbines have six vanes and water makes contacts with all six blades constantly to ensure that the runner stays balanced. The pitch of a propeller turbine’s blades can be adjustable or fixed. When the pitch of the blades in propeller turbines is adjustable, the turbine is known as a Kaplan turbine. Propeller turbines are often used in large-scale hydroelectric power generation plants, and their major advantage is that they can be used in low-head conditions that have significant flow. Other variations of the propeller turbine include the bulb, Straflo and tube turbines (Energybible.com, 2012; Energy.gov, 2016).
Turgo turbines:
Turgo turbines are impulse turbines where water jets are emanating from a nozzle strike the turbine blades. A Turgo turbine is designed much like a jet engine turbine with a runner hub surrounded by curved vanes that catch the water jets as they flow through the turbine causing the hub and turbine shaft to turn. The major advantage of Turgo turbines is that they are designed to rotate at very high speeds compared to other turbines such as the Pelton turbine. Turgo turbines also have a smaller diameter than Pelton turbines and are manufactured exclusively in England by the Gilkes Company (Energybible.com, 2012; Energy.gov, 2016).
Pelton turbines:
A Pelton turbine is an impulse turbine with a unique design. The runner hub does not have blades or vanes; instead, the Pelton turbine has a series of buckets or cups for catching water from the nozzles. The cups are split into two halves to eliminate dead spots within the central area which could render the system incapable of deflecting water away from the incoming water jets. The cutaways made on the lower lips of each cup/bucket allow a cup to move further before it cuts off the jet that propels the bucket ahead. This also allows for smoother entry of the cup into a water jet. The Figure 4 below shows some of the popular turbines discussed above.
Overall the best turbines for high-head applications are the Pelton and Turgo impulse turbines, while Crossflow, Francis, Turgo and multi-jet Pelton turbines are preferred for medium head applications. For low-head but significant flow applications, Cross-flow and propeller turbines are used (Energybible.com, 2012; Energy.gov, 2016).
Figure 4(a) Pelton Wheel turbine; (b) and (c) Francis turbine; (d) Kaplan Turbine
(Source: Manno, 2013).
Comparison of hydroelectric turbines with other types of turbines:
There is no significant difference between hydroelectric turbines and other turbines other than the mode of propulsion. For example, wind turbines are driven by wind currents while steam driven turbines are driven by steam under pressure. However, wind-driven and steam/gas turbines are made of a lighter material for efficiency as compared to hydroelectric turbines which use robust materials to ensure they can handle high-pressure water currents.
References:
Canyon Hydro Industries,. (2016). Guide to Hydropower: An Introduction to Hydropower Concepts and Planning (1st ed., pp. 6-8). Canyon Hydro Industries. Retrieved from http://www.asociatiamhc.ro/
Energy.gov. (2016). Types of Hydropower Turbines | Department of Energy. Energy.gov. Retrieved 19 January 2016, from http://energy.gov/eere/water/types-hydropower-turbines
Energybible.com. (2012). Water Turbines. Energybible.com. Retrieved 21 January 2016, from http://energybible.com/water_energy/water_turbines.html
Manno, M. (2013). Hydraulic Turbines and Hydroelectric Power Plants (1st ed., p. 11). Department of Industrial Engineering, University of Rome Tor Vergata. Retrieved from http://didattica.uniroma2.it/assets/uploads/corsi/144713/hydro.pdf
Natural Edge Project. (2008). Electricity- Innovative Technologies towards Sustainable Development (1st ed., pp. 8-11). Brisbane, AU: The Natural Edge Project (NEP) and Griffith University. Retrieved from http://www.naturaledgeproject.net/Documents/SLC/SLC%20Subject%20Supplement%20-%20Lesson%206%20Final.pdf
Pall Corporation. (2006). Power Generation (1st ed., pp. 1-2). New York: Pall Corporation. Retrieved from http://www.pall.com/pdfs/Power-Generation/PGHYDEN.pdf
Perlman, H. (2016). Hydroelectric Power: How it works, USGS Water-Science School. Water.usgs.gov. Retrieved 20 January 2016, from http://water.usgs.gov/edu/hyhowworks.html
