What is a TURBINE ?
machine designed to capture some of the energy from a moving fluid (a liquid or a gas) so it can be put to use. As the wind blows past a windmill’s sails, they rotate, removing some of the wind’s kinetic energy (energy of movement) and converting it into mechanical energy that turns heavy, rotating stones inside the mill. The faster the wind blows, the more energy it contains; the faster the sails spin, the more energy is supplied to the mill. Adding more sails to the windmill or changing their design so they catch the wind better can also help to capture more of the wind’s energy. Although you may not realize it, the wind blows just a bit more slowly after it’s passed by a 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 the moving fluid, a shaft or axle that rotates as the blades move, and some sort of machine that’s driven by the axle. In a modern wind turbine, there are typically three propeller-like blades attached to an axle that powers an electricity generator. In an ancient waterwheel, there are wooden slats that turn as the water flows under or over them, turning the axle to which the wheel is attached and usually powering some kind of milling machine.
Mechanical uses of turbine power go back to ancient Greece. The first wind wheels relied upon gearing and shafts to power machinery
How Turbines works ?
- An Impulse turbine, which is driven by a high-velocity jet (or multiple jets) of water.
- A Reaction turbine. The rotor of a reaction turbine is fully immersed in water and is enclosed in a pressure casing. The runner blades are profiled so that pressure differences across them impose lift forces, just as on aircraft wings, which cause the runner to rotate faster than is possible with a jet.
- A Gravity turbine is driven simply by the weight of water entering the top of the turbine and falling to the bottom, where it is released – for example, an overshot waterwheel. These are inherently slow-running machines.
In an impulse turbine, a fast-moving fluid is fired through a narrow nozzle at the turbine blades to make them spin around. The blades of an impulse turbine are usually bucket-shaped so they catch the fluid and direct it off at an angle or sometimes even back the way it came (because that gives the most efficient transfer of energy from the fluid to the turbine). In an impulse turbine, the
fluid is forced to hit the turbine at high speed. Imagine trying to make a wheel like this turn around by kicking soccer balls into its paddles. You’d need the balls to hit hard and bounce back well to get 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 gained by 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. 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
In a reaction turbine, the blades sit in a much larger volume of fluid and turn around as the fluid flows past them. A reaction 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. If an impulse turbine is a bit like kicking soccer balls, a reaction turbine is more like swimming—in reverse. Let me explain! Think of how you
do freestyle (front crawl) by hauling your arms through the water, starting with each hand as far in front as you can reach and ending with a “follow through” that throws your arm well behind you. What you’re trying to achieve is to keep your hand and forearm pushing
against the water for as long as possible, so you transfer as much energy as you can in each stroke. A reaction turbine is using the same idea in reverse: imagine fast-flowing water moving past you so it makes your arms and legs move and supplies energy to your body! With a reaction turbine, you want the water to touch the blades smoothly, for as long as it can, so it gives up as much energy as possible. The water isn’t hitting the blades and bouncing off, as it does in an impulse turbine: instead, the blades are moving more smoothly, “going with the flow.”
The Archimedes Screw has been used as a pump for centuries, but has only recently been used in reverse as a turbine. It’s principle of operation is the same as the overshot waterwheel, but the clever shape of the helix allows the turbine to rotate faster than the equivalent waterwheel and with high efficiency of power conversion (over 80%).
However they are still slow-running machines, which require a multi-stage gearbox to drive a standard generator. A key advantage of the Screw is that it avoids the need for a fine screen and automatic screen cleaner because most debris can pass safely through the turbine. The Archimedian screw is proven to be a ‘fish-friendly’ turbine.