Conversion of the Kinetic Energy of Flowing Water Into Rotating Energy - Coursework Example

Hydraulic Turbines Name Institution Lecturer Course Date How Hydraulic Turbines Convert the Kinetic Energy of Flowing Water into a Rotating Energy Introduction Water is considered one of the most crucial elements of life, being used for drinking, cleaning, transportation and energy production. This paper focuses on the use of water for producing electric energy using hydro-power plants. Electricity generation using water as the source of energy is an old technology that has advanced with time resulting to more enhanced systems that harness the static energy contained in flowing water. In several countries, water is being used as the main source of power for producing electric energy, which has resulted into extensive investments in hydroelectric power plants especially the construction of big dams capable of producing several megawatts of electricity. Basically, hydro electric power plants produce energy by converting the static energy of flowing water into a rotation motion (mechanical energy) of a turbine. The turbine’s mechanical energy is then converted to electric energy using an electric generator (comprises of rotor and stator as the main components). The electric energy is then distributed, via aerial or underground transmission lines (electric conductors) for use throughout the country. The generated electric energy may also be stored in various storage devices, such as capacitors, for later use. This paper concentrates on hydraulic turbines, in particular how hydraulic turbines convert the kinetic energy of flowing water into a rotating energy. Hydraulic turbines The turbine is the main component of a hydroelectric power plant that converts water energy to rotation motion (mechanical energy) that is then converted, via an electric generator, to electric energy. According to Wagner and Mathur (2011), hydraulic turbines can be classified based on direction of water flow, water pressure, turbine shape and orientation, and turbine construction (71-76). Hydraulic turbines, irrespective of the classification, are comprised of a set of blades that are fitted to a shaft. The blades are placed in the flow path of a stream of water. As water flows, it strikes the turbine blades, which makes the turbine shaft to rotate as long as the kinetic energy of the flowing water exceeds the resistance to motion arising from the weight of the turbine and friction on the bearings and other components. Once the turbine starts rotating, the mechanical energy maintains the desired rotation speed by overcoming resistance to motion. Figure 1: Principle of operation of a hydraulic turbine for a horizontally placed turbine Figure 2: Principle of operation of a hydraulic turbine for a vertically placed turbine As shown in figures 1 and 2, water, at high velocity and pressure, runs through the penstock and strikes the turbine blades, which makes the turbine shaft (turbine) to rotate. According to the principle of moments, water pressure and velocity drops as it passes through the hydraulic turbine. Before striking the turbine blades, water has a high pressure and velocity, which depends on the height of fall and the rate of flow (the quantity of water flowing or contained in a dam). The higher the flow rate and height of fall of the water, the higher the velocity and pressure of water. Initially, the turbine is stationary. As water strikes the blades of the turbine, the turbine develops a torque and starts to rotate owing to the impact, on turbine blades, from the flowing water. Since mass of water and turbine remains constant (there are no water losses or loss in turbine shaft and blade), the velocity and pressure of flowing water reduces as velocity of the turbine increases. Equation 1 uses the principle of conservation of moments to explain the interaction between flowing water and the turbine in which the kinetic energy of the flowing water is converted to mechanical energy through changes in their respective velocities.  -------------------------------------------------------------------- 1 Where M1 and M2 are the masses of water and turbine respectively, and V1 and V2 are the velocities of water and turbine respectively. Since the masses remain constant, a reduction in V1 is associated with an increase in V2 to satisfy the requirements of the equation. According to the law of conservation of energy, it is impractical to create or destroy energy. It is only possible to convert energy from one form to another. The same principle applies to hydraulic turbines, where kinetic energy of flowing water is converted to mechanical energy (rotation motion) of the turbine. As water flows through the penstock, it possesses high kinetic energy owing to the high flow velocity. Once it strikes turbine blades, which are initially considered to be static, some (a majority) of the kinetic energy of the water is transmitted to the blades due to moment changes between the water and the turbine. Since the blades are firmly attached to the turbine shaft, the turbine rotates as the kinetic energy of the flowing water is converted to mechanical energy at the turbine blades. However, as aforementioned, the turbine will only rotate after resistance to motion, due to the weight of the entire turbine and friction forces (at the bearings), has been overcome. Now that the flowing water has less kinetic energy, flow velocity, after the turbine, reduces. Proper turbine selection is paramount to the success of the hydro-power plant. According to Wagner and Mathur (2011), turbine selection depends on water flow and head. By selecting and using the most appropriate hydraulic turbine, it is possible to achieve optimum output of a hydro-power plant. According to Khan, different types of hydraulic turbines perform differently at different flow rates, but at constant water head (2009: 351). Therefore, it is crucial to select the most appropriate turbine for a given flow characteristic to achieve maximum plant output. For example, figure 3 shows the efficiencies of the various turbines at different part flow conditions. Part flow is the ratio of the current flow to the maximum flow. For example, if the maximum flow (design flow) of a given hydro-power plant is 20 cubic meters per second, the part flow rate when the plant is operating at 10 cubic meters per second is 0.5. Figure 3: A graph showing efficiencies of the various turbines at different part flow conditions (Khan 2009: 351) From the chart, it is apparent different turbines perform differently at different flow characteristics. For instance the graph shows that cross-flow, Pelton and Turgo turbines are highly efficient when running operating below design flow rates. The efficiencies of Pelton and Turgo turbines shoot to about 80 percent when operating at less than 0.1 part flow rate (less than 10 percent of the design flow rate). Maximum efficiency is achieved at about 0.6 part flow rate (60 percent of the design flow rate). However, this efficiency reduces gradually to less than 80 percent when operating at 100 percent flow rate (1 part flow rate). On the other hand, a Francis turbine operates best at high flow rates. In fact maximum efficiency is achieved at 90 percent flow rate operating conditions. At less than about 0.2 part flow rate, a Francis turbine will not operate. According to Khan, Francis turbines are among the few hydraulic turbines that rotate at reasonable speeds at certain combinations of water head and power requirements, thereby giving optimum electric generation. If an impulse turbine was to be operated under similar head and power combinations as the Kaplan turbine, one would require a far much larger turbine, which would be more expensive than a Kaplan turbine of the same efficiency. The Kaplan and Propeller turbine also operate at high flow ratios of more than about 0.37. The operating efficiencies of the Kaplan and Propeller turbines increase progressively to maximum efficiency when operating at the design flow rate. For instance, at about 40 percent flow rate, the propeller turbine would have an efficiency of less than 10 percent. This efficiency increases progressively to a maximum of about 90 percent when operating at 100 percent flow rate. Turbine selection for a given application is usually based on three key parameters: the power required (P), the available head (H) and the discharge rate (Q). Head is the height difference between the uppermost water level in the reservoir (usually a dam) and the turbine blades. For a given head, hydraulic turbines operate most effectively at a given rotation speed, which requires a given flow rate, which will provide the necessary impact force to run the turbine. The chart shown in figure 4 shows the estimated power, discharge and flow ranges for different turbines. From the chart, it can be seen that at a head of 10 m, a flow rate of about 5 cubic meters per second is enough for the Kaplan turbine to convert the kinetic energy of the flowing water to a rotation motion (mechanical energy) of the turbine to produce about 200 kW of electric power. Figure 4: A chart showing estimated power output, net head and discharge rate for different turbines (Khan 2009: 352) Conclusion The hydraulic turbine is the main component of a hydro-power plant that converts the kinetic energy of the flowing water to mechanical energy (rotation motion of the turbine). This mechanical energy is further converted to electric energy using a generator. A hydraulic turbine is comprised of two main components: a shaft and a set of blades attached to the shaft. The flowing water strikes the blades. Following the principles of energy and momentum conservation, velocity and pressure of water are reduced upon striking the turbine blades while the velocity of the turbine increases (if the turbine is considered stationary). Consequently, kinetic energy of the flowing water is converted to rotation motion of the turbine. If the turbine is considered in motion, the converted energy is used to maintain the turbine rotation by overcoming resistance to rotation motion arising from friction forces and the weight of the of the turbine (all its components assembled). Proper turbine selection is paramount to the success of the hydro-power plant. Turbine selection depends on the required power output, water flow rate and head. By selecting and using the most appropriate hydraulic turbine, it is possible to achieve optimum output of a hydro-power plant. Bibliography Khan, B.H. 2009. Non-Conventional Energy Resources, 2nd Edition. New Delhi, India: Tata McGraw-Hill, pp. 351-352. Wagner, H. & Mathur, J. 2011. Introduction to Hydro Energy Systems: Basics, Technology and Operation, London: Springer, pp. 71-76. Read More