بسم هللا الرحمن الرحيم

Transcription

1 بسم هللا الرحمن الرحيم - ( العنفات أو التوربينات( Turbines 3 rd Class Dr. Sataa A. Al-Bayati(10-11), )تسريع(, spinning )تدوير(, swirl )توفير( Harness What is Newton s second & third law? Introduction Water wheels have been used for thousands of years for industrial power. Their main shortcoming is size, which limits the flow rate and head that can be harnessed. The migration from water wheels to modern turbines took about one hundred years. Development occurred during the Industrial revolution, using scientific principles and methods. The word turbine was coined by the French engineer Claude Bourdin in the early 19th century and is derived from the Latin word for "whirling" or a "vortex". The main difference between early water turbines and water wheels is a swirl component of the water which passes energy to a spinning rotor. This additional component of motion allowed the turbine to be smaller than a water wheel of the same power. They could process more water by spinning faster and could harness much greater heads. (Later, impulse turbines were developed which didn't use swirl). The first hydroelectric plant was built in the United States in This plant made use of a fast flowing river as its source. Some years later, dams were constructed to create artificial water storage areas at the most convenient locations. These dams also controlled the water flow rate to the power station turbines. Originally, hydroelectric power stations were small in size and were set up at waterfalls in the vicinity of towns because it was not possible at that time to transmit electrical energy over great distances. 1

2 Nowadays the main reason there is large-scale use of hydroelectric power is because electrical energy can now be transmitted inexpensively over hundreds of kilometers to where it is required, making hydro power economically viable. Transmission over long distances is carried out by means of high voltage, overhead power lines called transmission lines. The electricity can be transmitted as either AC or DC. Advantages: 1. Unlike conventional coal-fired power stations, which take hours to start up, hydroelectric power stations can begin generating electricity very quickly. This makes them particularly useful for responding to sudden increases in demand for electricity by customers, known as peak demand". 2. Hydro stations need only a small staff to operate and maintain them, since all essential operating conditions can be remotely monitored and adjusted by a central control facility. 3. The service life of a hydroelectric plant is well in excess of 50 years. 4. As no fuel is needed; fuel prices are not a problem. 5. A hydroelectric power scheme uses a renewable source of energy that does not pollute the environment. Theory of operation Flowing water is directed on to blades of a turbine runner, creating a force on the blades. Since the runner is spinning, the force acts through a distance (force acting through a distance is the definition of work). Therefore, in hydro power plants the kinetic energy of falling water is captured to generate electricity. A turbine and a generator convert the energy from the water to mechanical and then electrical energy. The turbines and generators are installed either in or adjacent to dams, or use pipelines (penstocks) to carry the 2

3 pressured water below the dam or diversion structure to the powerhouse. The power capacity of a hydropower plant is primarily the function of two variables: (1) flow rate expressed in (m 3 /s), (2) the hydraulic head, which is the elevation difference the water falls in passing through the plant. Plant design may concentrate on either of these variables or both. Principals of Hydroelectric Power Stations The amount of electrical energy that can be generated from a water source depends primarily on two things: head & flow rate. Hydroelectric power stations are therefore situated where they can take advantage of the greatest fall of a large quantity of water- at the bottom of a deep and steep sided valley or gorge, or near the base of a dam (Fig. 1). Figure 1 Diagram of hydroelectric scheme Water is collected and stored in the dam above the station for use when it is required. Some dams create big reservoirs to 3

4 store water by raising the levels of rivers to increase their capacity. Other dams simply arrest the flow of rivers and divert the water down to the power station through pipelines. While a water turbine is much more sophisticated than the old water wheels, it is similar in operation. In both cases, blades are attached to a shaft and when flowing water presses against the blades, the shaft rotates. The effect is the same as wind pressing against the blades of a windmill. After the water has given up its energy to the turbine, it is discharged through drainage pipes or channels called the "tailrace" of the power station for irrigation or water supply purposes or, in some parts of the world, even into the ocean. In a conventional coal-fired (thermal) power station each "generating unit" consists of a boiler, a steam turbine, and the generator itself. The greater the height (or head) of the water above the turbine, the more energy each cubic meter of water can impart to spin a turbine (which in turn drives a generator). The greater the quantity of water, the greater the number and size of turbines that may be spun, and the greater the power output of the generators. Types of water turbines Water turbines are divided into two groups; reaction turbines and impulse turbines. Reaction Turbines Reaction turbines are acted on by water, which changes pressure as it moves through the turbine and gives up its energy. 4

5 They must be encased to contain the water pressure (or suction), or they must be fully submerged in the water flow. Newton's third law describes the transfer of energy for reaction turbines. (For every action there is an equal & opposite re-action.) Most water turbines in use are reaction turbines. They are used in low and medium head applications. Types: Francis Kaplan, Propeller, Bulb, Tube, Straflo Tyson Water wheel FRANCIS TURBINE Fig. 2 Francis water turbine Fig. 3 Detail of Francis turbine 5

6 In the great majority of cases (large and small water flow rates and heads) the type of turbine employed is the Francis or radial flow turbine. The significant difference in relation to the Pelton turbine is that Francis (and Kaplan) turbines are of the reaction type, where the runner is completely submerged in water, and both the pressure and the velocity of water decrease from inlet to outlet. The water first enters the volute, which is an annular channel surrounding the runner, and then flows between the fixed guide vanes, which give the water the optimum direction of flow. It then enters the runner, which is totally submerged, changes the momentum of the water, which produces a reaction in the turbine. Water flows radially i.e., towards the centre. The runner is provided with curved vanes upon which the water impinges. The guide vanes are so arranged that the energy of the water is largely converted into rotary motion and is not consumed by eddies and other undesirable flow phenomena causing energy losses. The guide vanes are usually adjustable so as to provide a degree of adaptability to variations in the water flow rate and in the load of the turbine. The guide vanes in the Francis turbine are the elements that direct the flow of the water, just as the nozzle of the Pelton wheel does. The water is discharged through an outlet from the centre of the turbine. In design and manufacture, Francis turbines are much more complex than Pelton turbines, requiring a specific design for each head/flow condition to obtain optimum efficiency. Runner and housing are usually cast, on large units welded housings, or cast in concrete at site, are common. With a Francis turbine, downstream pressure can be above zero. Precautions must be taken against water hammer with this type of turbine. 6

7 Under the emergency stop, the turbine overspeeds. One would think that more water is going through the turbine than before the trip occurred since the turbine is spinning faster. However, the turbine has been designed to work efficiently at the design speed, so less water actually flows through the turbine during overspeed. Pressure relief valves are added to prevent water hammer due to the abrupt change of flow. Besides limiting pressure rise, the pressure relief valve prevents the water hammer from stirring up sediment in the pipes. With a big variety of designs, a large head range from about 30 m up to 700 m of head can be covered. The most powerful Francis turbines have an output of up to 800 MW and use huge amounts of water. KAPLAN TURBINE For very low heads and high flow rates a different type of turbine, the Kaplan or Propeller turbine is usually employed. In the Kaplan turbine the water flows through the propeller and sets the latter in rotation. In this turbine the area through the water flows is as big as it can be the entire area swept by the blades. For this reason Kaplan turbines are suitable for very large volume flows and they have become usual where the head is only a few meters. Figure 4 Kaplan and propeller type turbine 7

8 Fig. 5 Kaplan turbine and electrical generator cut-away view. Figure 6 Detail of Kaplan turbine The flow rate of the water through the turbine can be controlled by varying the distance between the guide vanes; the pitch of the propeller blades must then also be appropriately adjusted. Each setting of the guide vanes corresponds to one particular setting of the propeller blades in order to obtain high efficiency. Important feature is that the blade speed is greater than the water speed as much as twice as fast. This allows a rapid rate of rotation even with relatively low water speeds. Kaplan turbines come in a variety of designs. Their application is limited to heads from 1 m to about 30 m. 8

9 Under such conditions, a relatively larger flow as compared to high head turbines is required for a given output. These turbines therefore are comparatively larger. The Kaplan or propeller type turbines can be mounted at almost any angle, but this is usually vertical or horizontal. Impulse Turbines ( Free-jet Turbines) Impulse turbines change the velocity of a water jet. The jet impinges on the turbine's curved blades which reverse the flow. The resulting change in momentum (impulse) causes a force on the turbine blades. Since the turbine is spinning, the force acts through a distance (work) and the diverted water flow is left with diminished energy. Prior to hitting the turbine blades, the water's pressure (potential energy) is converted to kinetic energy by a nozzle and focused on the turbine. No pressure change occurs at the turbine blades, and the turbine doesn't require housing for operation. Newton's second law describes the transfer of energy for impulse turbines. (Acceleration is produced when a force acts on a mass, F=ma.) Impulse turbines are most often used in very high head applications. Types: Pelton Turgo Michell-Banki (Crossflow or Ossberger turbine) PELTON TURBINE The principle of the old water-wheel is embodied in the modern Pelton turbine. This turbine has a similar look and physical principle like a classic water wheel. A Pelton turbine is used in cases where large heads of water are available (more than 40 m). It is used for heads up to 2000 m. Below 250 m, mostly Francis turbines are given preference. 9

10 Today the maximum output lies at around 200 MW. Fig. 6 Layout of Pelton turbine Fig. 7 Typical Pelton turbine Fig. 8 Pelton wheel 10

11 In Pelton turbines, the available head is converted to kinetic energy at atmospheric pressure and partial admission of flow into the runner. It was invented around 1880 by the American Pelton, after whom it got its name. The largest Pelton wheels have a diameter of more than 5 m and weigh more than 40 t. The wheel must be placed above the tailrace water level, which means a loss of static head, but avoids watering of the runner. In order to avoid an unacceptable raise of pressure in the penstock, caused by the regulating of the turbine, jet deflectors are sometimes installed. The deflector diverts the jet, or part of it, from the runner. Hydroelectric dams and run-of-the-river systems Depending on their configurations, hydroelectric station can be divided into 2 major types. i.e. hydroelectric dam type and the run-ofthe-river type. Fig. 9 Types of layout Hydroelectric dam type This type of hydroelectric station involves the construction of a dam to store water in a reservoir to create the water head difference. 11

12 Water flows by gravity through a channel (called penstock) to reach a turbine at a lower level to make it spin. The spinning turbine then drives an electric generator to produce electricity. Fig. 10 Typical hydroelectric dam Although hydroelectric dams are usually built for large hydro stations of hundreds and even thousands of mega watts in capacity, they are also built for small hydro stations. Hydroelectric dams can store water, and then release the water for power generation when needed. Thus this type of system is more suitable than other intermittent renewable energy sources to provide peak load power. Besides, dams also serve other practical functions of flood control, irrigation and navigation. Run-of-the-river type This type of hydroelectric station is normally located on swift flowing streams or rivers, and extracts the energy from the water as it passes through the station. Run-of-the-river stations are usually built on small dams or diversion weirs, and in some occasions these structures are not required. Water collected by the diversion weir falls by gravity through a penstock to drive a turbine at a lower level, which is coupled to a generator to produce electricity. The water then joins back to the normal river channel. Since such systems do not involve significant amount of water storage, they have 12

13 much smaller impact on the surrounding environment than the hydroelectric dam systems. Fig. 11 Configuration of a run-of-the-river system Fig. 12 Detail of a run-of-the-river system Run-of-the-river systems are usually mini or micro hydro systems, and small hydro systems in the lower end of the capacity range. Since this type of system does not store a large amount of water, the electricity output will be affected by the seasonal flow of the river. 13

14 Pico hydro turbines Pico hydro turbines are small water turbines with capacities of a few hundred watts to 5 kw. Pico hydro turbines can be used to harness hydro power from streams, rivers, irrigation canals and waterfalls, and require only a constant water supply with a very low water head. Pico hydro turbines are usually axial-flow turbines. They are portable, & are suitable for village applications in developing countries. Apart from electricity generation, some pico hydro turbines, such as the water mill, can also be used to provide mechanical power for various applications such as grinding flour and irrigation. Environmental impact Water turbines have had both positive and negative impacts on the environment. They are one of the cleanest producers of power, replacing the burning of fossil fuels and eliminating nuclear waste. They use a renewable energy source and are designed to operate for decades. They produce significant amounts of the world's electrical supply. There have also been negative consequences: Most water turbines require a dam that can interrupt the natural ecology of rivers, killing fish, stopping migrations, and disrupting peoples' livelihoods. For example, American Indian tribes in the Pacific Northwest had livelihoods built around salmon fishing, but aggressive dam-building destroyed their way of life. Since the late 20th century, it has been possible to construct hydropower systems that divert fish and other organisms away from turbine intakes without significant damage or loss of power; such systems require less cleaning but are substantially more expensive to construct. In the United States, it is now illegal to block the migration of fish so fish ladders must be provided by dam builders. )سد( dam,, ( خانق( gorge, ( وادي( valley,blade, penstock, ( أنتقال( migration )مرجل( boiler,, impinge, propeller, تقط ع( swept (, وضع( pitch (, swift,)أحفوري) fossil,)سريع( Reference: 14

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