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Photo: A cutaway model of a steam turbine used to generate electricity ina power plant. This one is an exhibit at the Think Tank science museum in Birmingham, England.
Photo: An old-fashioned windmill. Photo courtesy ofThe Jon B. Lovelace Collection of California Photographs in Carol M. Highsmith''s America Project, Library of Congress, Prints and Photographs Division.
A windmill is the simplest kind of turbine: a machine designed tocapture some of the energy from a moving fluid (a liquid or a gas) soit can be put to use. As the wind blows past a windmill''s sails, theyrotate, removing some of the wind''s kinetic energy (energy ofmovement) and converting it into mechanical energy that turns heavy, rotating stones inside the mill. The faster the windblows, the more energy it contains; the faster the sailsspin, the more energy is supplied to the mill. Adding moresails to the windmill or changing their design so they catch the windbetter can also help to capture more of the wind''s energy. Althoughyou may not realize it, the wind blows just a bit more slowly after it''s passed bya windmill than before—it''s given up some of its energy to the mill!
The key parts of a turbine are a set of blades that catch themoving fluid, a shaft or axle that rotates as the blades move, andsome sort of machine that''s driven by the axle. In a modernwind turbine, there are typically three propeller-like blades attached to anaxle that powers an electricity generator. In an ancient waterwheel,there are wooden slats that turn as the water flows under or overthem, turning the axle to which the wheel is attached and usually poweringsome kind of milling machine.
Turbines work in two different ways described as impulseand reaction—terms that are often very confusingly described (and sometimes completely muddled up) when people try to explain them.So what''s the difference?
In an impulse turbine, a fast-movingfluid is fired through a narrow nozzle at the turbine blades to make them spin around. Theblades of an impulse turbine are usually bucket-shaped so they catchthe fluid and direct it off at an angle or sometimes even back theway it came (because that gives the most efficient transfer of energyfrom the fluid to the turbine). In an impulse turbine, the fluid isforced to hit the turbine at high speed.
Imagine trying to make a wheel like this turn around by kicking soccer ballsinto its paddles. You''d need the balls to hit hard and bounce back well toget the wheel spinning—and those constant energy impulses are the key to how it works. The law of conservation of energy tells us that the energy the wheel gains, each time a ball strikes it, is equal to the energy that the ball loses—so the balls will be traveling more slowly when they bounce back. Also, Newton''s second law of motion tells us that the momentum gainedby the wheel when a ball hits it is equal to the momentum lost by the ball itself; the longer a ball touches the wheel, and the harder (more forcefully) it hits, the more momentum it will transfer.
Artwork: An impulse turbine like this works when the incoming fluid hits the buckets and bouncesback again. The exact shape of the buckets and how the fluid hits them makes a big difference to how much energy the turbine can capture. The buckets also have to be designed to that the action of the jet on one bucket doesn''t affect the next bucket.
Water turbines are often based around an impulse turbine (though some do work using reaction turbines). They''re simple in design, easy to build, and cheap to maintain, not least because they don''t need to be contained inside a pipe or housing (unlike reaction turbines).
Artwork: A Pelton water wheel is an example of an impulse turbine. It spins as one or more high-pressure water jets (blue), controlled by a valve (green), fire into the buckets around the edge of the wheel (red). Lester Pelton was granted a patent for this idea in 1889, from which this drawing is taken.Artwork from US Patent 409,865: Water Wheel by Lester Pelton, courtesy of US Patent and Trademark Office.
In a reaction turbine, the blades sit ina much larger volume of fluid and turn around as the fluid flows past them. Areaction turbine doesn''t change the direction of the fluid flow as drastically as an impulse turbine: it simply spins as the fluid pushes through and past its blades.Wind turbines are perhaps the most familiar examples of reaction turbines.
Artwork: A reaction turbine like this is much more like a propeller. The main difference is that there are more vanes in a turbine (I''ve just drawn four blades for simplicity) and often multiple sets of vanes (multiple stages), as you can see in the photos of the steam and gas turbines at the top of this page.
Photo: A typical reaction turbine from a geothermal power plant.Water or steam flows past the angled blades, pushing them around and turning the central shaft to which they''reattached. The shaft spins a generator that makes electricity.Photo by Henry Price courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
Turbines capture energy only at the point where a fluid touches them, so a reaction turbine(with multiple blades all touching the fluid at the same time) potentially extracts more power than an impulse turbinethe same size (because usually only one or two of its blades are in the path of the fluid at a time).
All have their advantages and disadvantages. The Wells, for example, can turn very fast, but is also noisy and relatively inefficient. The Francis is quieter and more efficient, and very good at coping with the mechanical stresses inside deephydroelectric dams (ones with high "heads" of water), but it''s also slower and mechanically more complex. When they''re operating in air, Darrieus turbines are closer to the ground (so they can do away with a cumbersome tower), but that means they''re less effective at harnessing the wind (which blows faster higher above the ground); generally they''re less efficient and more unstable than other turbine designs (they often have to be steadied with guy ropes) and barely used commercially.
You might have noticed that wind turbines look just like giantpropellers—and that''s another way to think of turbines: aspropellers working in reverse. In an airplane, theengine turns the propeller at high speed, the propeller creates abackward-moving draft of air, and that''s what pushes—propels—the planeforward. With a propeller, the moving blades are driving the air;with a turbine, the air is driving the blades.
Photo: Turbines and propellers work in exactly opposite ways. Propellers use energy to make a fluid move (air, in the case of a plane, or water, in a ship or submarine); turbines harness energy when a moving fluid flows past them. Left: Propeller photo by Tech. Sgt. Justin D. Pyle courtesy of US Air Force.
Photo: Turbine blades are shaped in a similar way to propeller blades but are typically made from high-performance alloys because the fluid flowing past them can be very hot. Photo of a turbine blade exhibited at Think Tank, the science museum in Birmingham, England.
Turbines are also similar to pumps and compressors. In a pump, youhave a spinning paddle wheel that sucks water in through one pipe andthrows it out from another so you can move water (or another liquid)from one place to another. If you take a water pump apart, you cansee the internal paddle wheel (which is called an impeller)is very similar to what you''d find inside a water turbine. The difference isthat a pump uses energy to make a fluid move, while a turbinecaptures the energy from a moving fluid.
Water wheels, which date back over 2000 years to the time of theancient Greeks, were the original water turbines.
Photo: A water wheel is an ancient form of turbine this overshot wheel, the water is chaneled over the top by the wooden boards and pushes the wheel round as it flows down.Photo by Edouard E. Exline courtesy of Historic American Buildings Survey (HABS TENN,5-CADCO,1--4), Library of Congress Prints and Photographs Division.
Photo: A Pelton water turbine. Notice how each bucket is, in fact, two buckets joined together.The water jet hits the "splitter" (the place where the buckets join in the middle), dividing it into two jets that exit cleanly either side. Photo by Benjamin F. Pearson courtesy of Historic American Buildings Survey/Historic American Engineering Record, US Library of Congress.
These are covered in much more detail in our separate article onwind turbines.
Photo: A typical wind turbine, in Staffordshire, England.The tower is ~50m (~150ft) off the ground because the wind moves faster whenit''s clear of ground-level obstructions.The rotor blades are ~15m (50ft) in diameter and, with a huge sweep, capture up to 225kW (kilowatts) of energy.
Steam turbines evolved from the steamengines that changed theworld in the 18th and 19th centuries. A steam engine burns coal on anopen fire to release the heat it contains. The heat is used to boilwater and make steam, which pushes a piston in a cylinder to power amachine such as a railroad locomotive. This is quite inefficient (itwastes energy) for a whole variety of reasons. A much better designtakes the steam and channels it past the blades of a turbine, whichspins around like a propeller and drives the machine as it goes.
Steam turbines were pioneered by British engineer Charles Parsons(1854–1931), who used them to power a famously speedy motorboatcalled Turbinia in 1889. Since then,they''ve been used in manydifferent ways. Virtually all power plants generate electricity usingsteam turbines. In a coal-fired plant, coal is burned in a furnaceand used to heat water to make steam that spins high-speed turbinesconnected to electricity generators. In a nuclear power plant, theheat that makes the steam comes from atomic reactions.
Unlike water and wind turbines, which place a single rotatingturbine in the flow of liquid or gas, steam turbines have a wholeseries of turbines (each of which is known as a stage)arranged in a sequence inside what is effectively a closed pipe. Asthe steam enters the pipe, it''s channeled past each stage in turn soprogressively more of its energy is extracted. If you''ve ever watcheda kettle boiling, you''ll know that steam expands and moves veryquickly if it''s directed through a nozzle. For that reason, steamturbines turn at very high speeds—many times faster than wind orwater turbines.
Read more in in main article on steam turbines.
Airplane jet engines are a bit like steam turbines in that theyhave multiple stages. Instead of steam, they''re driven by a mixtureof the air sucked in at the front of the engine and the incrediblyhot gases made by burning huge quantities of kerosene(petroleum-based fuel). Somewhat less powerful gas turbine enginesare also used in modern railroad locomotives and industrial machines.See our article on jet engines for moredetails.
Photo: A prototype gas turbine produced for a high-efficiency power plant. Each of the metal wheels is a separate turbine stage designed to extract a bit more energy from a high-speed gas. You can see how big this turbine is by looking at the little man dressed in white sitting on the middle of the machine. Photo taken at the National Energy Technology Laboratory, Morgantown courtesy of US Department of Energy.
Photo: Imagine your hand could capture the energy in running water and your body your transform it into a more useful form. That would make you a kind of turbine.
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