Turbines
by Chris Woodford. Last updated: January 13, 2022.
What does a windmill standing on a sandcastle have in
common with a massive ocean liner, a hydroelectric dam, or a
transatlantic jet? Answer: They all use turbines—machines
that capture energy from a moving liquid or
gas. In a sandcastle windmill, the curved blades are designed to catch the wind's energy
so they flutter and spin. In an ocean liner or a jet, hot burning gas
is used to spin metal blades at high speed—capturing energy that's
used to power the ship's propeller or push the plane through the sky.
Turbines also help us make the vast majority of our
electricity: turbines driven by steam are used in virtually every major
power plant, while wind and water turbines help us
to produce renewable energy. Wherever energy's being harnessed for human needs, turbines are usually somewhere nearby. Let's take a closer look at these handy
machines and find out how they work!
Photo: A cutaway model of a steam turbine used to generate electricity in
a power plant. This one is an exhibit at the Think Tank science museum in Birmingham, England.
What is a turbine?
Photo: An old-fashioned windmill. Photo courtesy of
The 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 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.
Impulse and reaction turbines
Turbines work in two different ways described as impulse
and reaction—terms that are often very confusingly described
(and sometimes completely muddled up) when people try to explain them.
So what's the difference?
Impulse turbines
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.
Artwork: An impulse turbine like this works when the incoming fluid hits the buckets and bounces
back 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.
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.
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.
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."
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're
attached. 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 turbine
the same size (because usually only one or two of its blades are in the path of the fluid at a time).
Types of reaction turbines
Some common designs of reaction turbines are:
- Wells—which looks much like a propeller, with airfoil-shaped blades turning around a horizontal axis.
- Francis—typically, with large V-shaped blades, often turning on a vertical axis inside a sort of giant, spiral snail shell. The Francis is by far the most common type of water turbine; McCormick, Kaplan, and Deriaz turbines are essentially improvements of the original Francis design.
- Darrieus—with airfoil blades turning around a vertical axis.
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 deep
hydroelectric 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.
Photo: A giant Francis reaction turbine (the orange wheel at the top) being lowered into position at the
Grand Coulee Dam in Washington State, USA.
Water flows past the angled blades, pushing them around and turning the shaft to which they're
attached. The shaft spins an electricity generator that makes power.
Photo by courtesy of US Bureau of Reclamation.
Thinking backwards
You might have noticed that wind turbines look just like giant
propellers—and that's another way to think of turbines: as
propellers working in reverse. In an airplane, the
engine turns the propeller at high speed, the propeller creates a
backward-moving draft of air, and that's what pushes—propels—the plane
forward. 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, you
have a spinning paddle wheel that sucks water in through one pipe and
throws 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 can
see the internal paddle wheel (which is called an impeller)
is very similar to what you'd find inside a water turbine. The difference is
that a pump uses energy to make a fluid move, while a turbine
captures the energy from a moving fluid.
Turbines in action
Broadly speaking, we divide turbines into four kinds according to
the type of fluid that drives them: water, wind, steam, and gas.
Although all four types work in essentially the same way—spinning
around as the fluid moves against them—they are subtly different and
have to be engineered in very different ways. Steam turbines, for
example, turn incredibly quickly because steam is produced under
high-pressure. Wind turbines that make electricity turn relatively
slowly (mainly for safety reasons), so they need to be huge to capture
decent amounts of energy. Gas turbines need to be made from specially
resilient alloys because they work at such high temperatures. Water turbines
are often very big because they have to extract energy from an entire river,
dammed and diverted to flow past them. They can turn relatively slowly, because is water
is heavy and carries a lot of energy (because of its high mass) even when it flows at low speeds.
Water turbines
Water wheels, which date back over 2000 years to the time of the
ancient Greeks, were the original water turbines.
Photo: A water wheel is an ancient form of turbine.
In 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.
Today, the same principle is used to make electricity in hydroelectric power plants.
The basic idea of hydroelectric power is that you dam a river to
harness its energy. Instead of the river flowing freely downhill from
its hill or mountain source toward the sea, you make it fall through
a height (called a head) so it picks up
speed (in other words, so its potential energy is converted to kinetic energy), then channel
it through a pipe called a penstock past a
turbine and generator. Hydroelectricity is effectively a three-step energy conversion:
- The river's original potential energy (which it has because it starts from high ground) is turned
into kinetic energy when the water falls through a height.
- The kinetic energy in the moving water is converted into mechanical energy by a water turbine.
- The spinning water turbine drives a generator that turns the mechanical energy into electrical energy.
Different kinds of water turbine are used depending on the
geography of the area, how much water is available (the flow), and the distance over which it can be made to fall (the head).
Some hydroelectric plants use bucket-like impulse turbines (typically Pelton wheels);
others use Francis, Kaplan, or Deriaz reaction turbines. Impulse water turbines (like the Pelton wheel) can be
completely open to the air—so sometimes you can actually see the water jet hitting the turbine. Reaction water turbines,
on the other hand (like the Francis), must be completely enclosed inside the channel or passage through which
the water flows. As mentioned above, while an impulse turbine is capturing energy at only the single point where the water
jet is hitting it, a reaction turbine is capturing energy across the whole wheel at once—which is why
a reaction turbine in a hydroelectric plant can produce more power than an impulse turbine of the same size.
That, in turn, explains why most modern hydroelectric plants use reaction turbines.
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.
Wind turbines
These are covered in much more detail in our separate article on
wind turbines.
Photo: A typical wind turbine, in Staffordshire, England.
The tower is ~50m (~150ft) off the ground because the wind moves faster when
it'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
Steam turbines evolved from the steam
engines that changed the
world in the 18th and 19th centuries. A steam engine burns coal on an
open fire to release the heat it contains. The heat is used to boil
water and make steam, which pushes a piston in a cylinder to power a
machine such as a railroad locomotive. This is quite inefficient (it
wastes energy) for a whole variety of reasons. A much better design
takes the steam and channels it past the blades of a turbine, which
spins 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 motorboat
called Turbinia in 1889. Since then,
they've been used in many
different ways. Virtually all power plants generate electricity using
steam turbines. In a coal-fired plant, coal is burned in a furnace
and used to heat water to make steam that spins high-speed turbines
connected to electricity generators. In a nuclear power plant, the
heat that makes the steam comes from atomic reactions.
Unlike water and wind turbines, which place a single rotating
turbine in the flow of liquid or gas, steam turbines have a whole
series of turbines (each of which is known as a stage)
arranged in a sequence inside what is effectively a closed pipe. As
the steam enters the pipe, it's channeled past each stage in turn so
progressively more of its energy is extracted. If you've ever watched
a kettle boiling, you'll know that steam expands and moves very
quickly if it's directed through a nozzle. For that reason, steam
turbines turn at very high speeds—many times faster than wind or
water turbines.
Read more in in main article on steam turbines.
Gas turbines
Airplane jet engines are a bit like steam turbines in that they
have multiple stages. Instead of steam, they're driven by a mixture
of the air sucked in at the front of the engine and the incredibly
hot gases made by burning huge quantities of kerosene
(petroleum-based fuel). Somewhat less powerful gas turbine engines
are also used in modern railroad locomotives and industrial machines.
See our article on jet engines for more
details.
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.
