by Chris Woodford. Last updated: September 25, 2016.
We're all used to the idea of boiling water to make tea or coffee, but
what if you had to boil water every time you wanted to do anything?
What if you had to make steam to charge your iPod, watch
TV, or vacuum your carpet? It sounds crazy, yet it's not so far from the
truth. Unless you're using renewable energy from something like a
solar panel or a wind turbine, virtually every watt of power you
consume comes from a power plant that generates electricity from
boiling, hissing, rapidly expanding steam! We might not use
piston-pushing steam engines to power our world anymore, but we still
use their modern equivalents—steam turbines. What are they
and how do they work? Let's take a closer look!
Photo: A one-tenth scale, cutaway model of a steam turbine at Think Tank, the science museum in Birmingham, England. Steam enters from the left through the gray pipe at the top, arriving in the very middle of the
turbine and just above it. Then it simultaneously flows in both directions (to the left and the right) through the low-pressure reaction turbine, which drives the electricity generator on the right.
How does steam provide energy?
If you've ever seen an old-fashioned steam locomotive, you'll have some idea
just how powerful steam can be. A steam locomotive is built around a
steam engine, a complex machine based on a simple idea: you can burn
fuel (coal) to release the energy stored inside it. In a steam engine, coal
burns in a furnace and releases heat, which boils water like a kettle
and generates high-pressure steam. The steam feeds through a pipe
into a cylinder with a tight fitting piston, which moves outward as
the steam flows in—a bit like a bicycle pump working in reverse.
As the steam expands to fill the cylinder, it cools down, loses pressure, and gives up its energy to
the piston. The piston pushes the locomotive's wheels around before returning
back into the cylinder so the whole process can be repeated.
The steam isn't a source of energy: it's an energy-transporting fluid
that helps to convert the energy locked inside coal into mechanical energy
that propels a train.
Photo: The power of steam: a restored locomotive running on the Swanage Railway in England. Expanding steam releases energy that drives the engine's pistons. It's pretty obvious that the steam leaving the chimney stills
contains quite a bit of energy, which is one reason why engines like this are so very inefficient. Steam turbines
usefully capture much more of the energy in steam—and are much more efficient.
Steam engines were great: they powered the world throughout the Industrial
Revolution from the 18th century right up to the middle of the 20th
century. But they were huge, cumbersome, and relatively inefficient.
A simple, steam-driven piston and cylinder is delivering energy to
the machine it powers only 50 percent of the time (during the power
stroke, when the steam is actually pushing it); the rest of the time,
it's being pushed back into the cylinder by momentum ready for
the next power stroke. Another problem is that pistons and cylinders
make back and forth, push-pull, reciprocating motion, when (most of the time) what
we'd really prefer is rotary motion—turning a wheel. To make up for
these problems, steam engines have elaborately complex cylinders that
allow steam in from different directions and heavy levers
(cranks and connecting rods) to convert their push-pull
reciprocating motion into rotary motion. Wouldn't it be better
if we could directly power a wheel with the force of the steam,
cutting out the pistons, cylinders, cranks, and all the rest?
That's the basic idea behind a steam turbine, an energy converting
device perfected by British engineer
Sir Charles Parsons in the 1880s.
What is a turbine?
A turbine is a spinning wheel that gets its energy from a gas or liquid moving
past it. A windmill or a wind turbine takes energy from the wind,
while a waterwheel or water turbine is usually driven by a river
flowing over, under, or around it. Now you can't produce energy out
of thin air: a basic law of physics called the conservation of
energy tells us that a gas or liquid always slows down or changes
direction when it flows past a turbine, losing at least as much
energy as the turbine gains. Blow on the windmill stuck in your
sandcastle and it spins around. What you can't see is that your
breath slows down quite dramatically: on the other side of the
windmill, the air from your mouth is traveling much slower!
Read more in our introduction to turbines.
What is a steam turbine?
Theory of a steam turbine
Artwork: An early steam turbine design developed in 1888 by Swedish engineer
Gustav de Laval (1845–1913).
It works by directing straight-line jets of high-speed steam at a steel paddle wheel,
with reasonable efficiency, so it's an example of an impulse turbine (explained below).
Artwork believed to be in the public domain, from The Steam Turbine by Sir Charles Parsons, Cambridge University Press, 1911.
As its name suggests, a steam turbine is powered by the energy in hot, gaseous
steam—and works like a cross between a wind turbine and a water
turbine. Like a wind turbine, it has spinning blades that turn when
steam blows past them; like a water turbine, the blades fit snugly
inside a sealed outer container so the steam is constrained and
forced past them at speed. Steam turbines use high-pressure steam to
turn electricity generators at incredibly high speeds, so they
rotate much faster than either wind or water
turbines. (A typical power plant steam turbine rotates at 1800–3600
rpm—about 100–200 times faster than the blades spin on a typical
wind turbine, which needs to use a gearbox to drive a generator
quickly enough to make electricity.) Just like in a steam engine, the
steam expands and cools as it flows past a steam turbine's blades,
giving up as much as possible of the energy it originally contained.
But, unlike in a steam engine, the flow of the steam turns the blades
continually: there's no push-pull action or waiting for a piston to
return to position in the cylinder because steam is pushing the
blades around all the time. A steam turbine is also much more compact
than a steam engine: spinning blades allow steam to expand and drive
a machine in a much smaller space than a piston-cylinder-crank
arrangement would need. That's one reason why steam turbines were quickly adopted
for powering ships, where space was very limited.
Parts of a steam turbine
All steam turbines have the same basic parts, though there's a lot of variation
in how they're arranged.
Rotor and blades
Photo: Steam turbine blades look a bit like propeller blades but are made from high-performance alloys because the steam flowing past is hot, at high pressure, and traveling fast. Photo of a turbine blade exhibited at Think Tank, the science museum in Birmingham, England.
Running through the center of the turbine is
a sturdy axle called the rotor, which is what takes power from the
turbine to an electricity generator (or whatever else the turbine is driving).
The blades are the most important part of a turbine:
their design is crucial in capturing as much energy from the steam as
possible and converting it into rotational energy by spinning the
rotor round. All turbines have a set of rotating blades attached to
the rotor and spin it around as steam hits them. The blades and the
rotor are completely enclosed in a very sturdy, alloy
case (one capable of withstanding high pressures and temperatures).
Impulse and reaction turbines
In one type of turbine, the rotating blades are shaped like buckets.
High-velocity jets of incoming steam from carefully shaped nozzles
kick into the buckets, pushing them around with a series of impulses,
and bouncing off to the other side at a similar speed but
much-reduced pressure (compared to the incoming jet).
This design is called an impulse turbine
and it's particularly good at extracting energy from high-pressure steam.
(The de Laval turbine illustrated up above is an example.)
In an alternative design called a reaction turbine, there's a second
set of stationary blades attached to the inside of the turbine case.
These help to speed up and direct the steam onto the rotating blades
at just the right angle, before it leaves with reduced temperature
and pressure but broadly the same speed as it had when it entered.
In both cases, steam expands and gives up some of its energy as it passes through the turbine.
In an ideal world, all the heat and kinetic energy lost by the steam would be gained
by the turbine and converted into useful kinetic energy (making it spin around). But, of course, the turbine will heat up somewhat, some steam might leak out, and there are various other reasons why turbines (like all other machines) are never
100 percent efficient.
Photo: Impulse and reaction. Left: This Pelton water wheel is an example of an impulse turbine. It spins as high-pressure water jets fire into the buckets around the edge. Steam
impulse turbines work a bit like this. Photo courtesy of Wonderferret, published on Flickr
under a Creative Commons licence. See more of Wonderferret's photos. Right: A reaction turbine turns when steam hits its curved blades. Photo by Henry Price courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
Apart from the rotor and its blades, a turbine also
needs some sort of steam inlet (usually a set of nozzles that
direct steam onto either the stationary or rotating blades).
Steam turbines also need some form of control mechanism that regulates their speed, so they
generate as much or as little power as needed at any particular
time. Most steam turbines are in huge power plants
driven by enormous furnaces and it's not easy to reduce the
amount of heat they produce. On the other hand, the demand (load)
on a power plant—how much electricity it needs to make—can
vary dramatically and relatively quickly. So steam turbines need to
cope with fluctuating output even though their steam input may be
relatively constant. The simplest way to regulate the speed is using valves that release some of the steam that would otherwise go through the turbine.
Practical steam turbines
"I therefore decided to split up the fall of pressure of the steam into small fractional
expansions over a large number of turbines in series so that the velocity of the steam
nowhere should be great."
Sir Charles Parsons, The Steam
Photo: The multiple stages in a typical steam turbine. This model is at Think Tank, the science museum in Birmingham, England.
In practice, steam turbines are a bit more complex than we've suggested so far.
Instead of a single set of blades on the rotor, there are usually a
number of different sets, each one helping to extract a little bit
more energy from the steam before it's exhausted. Each set of blades
is called a stage and works by either impulse or reaction, and
a typical turbine can have a mixture of impulse and reaction stages,
all mounted on the same rotor axle and all turning the generator at
the same time. Often the impulse stages come first and extract energy
from the steam when it's at high pressure; the reaction stages come
later and remove extra energy from the steam when it's expanded to a bigger
volume and lower pressure using longer, bigger blades. The multi-stage approach, invented by Charles Parsons,
means each stage is slowing or reducing the pressure of the steam by
only a relatively small amount, which reduces the forces on the
blades (an important consideration for a machine that may have to run
for years without stopping) and greatly improves the turbine's overall power output.
Photo: Multiple stages (closeup): Zooming in on the photo above, you can see how each stage is bigger than the previous one: bigger in diameter, more widely spaced, with bigger blades set at greater angles that have wider openings in between them. This allows the steam to expand gradually on its journey through the turbine, which is how it gives up its energy. You can see the same thing even more clearly if you check out this wonderful
photo of an old dual-flow Siemens steam turbine from a nuclear plant.
Condensing and noncondensing
Turbines also vary in how they cool the steam that passes through them.
Condensing turbines (used in large power plants to generate
electricity) turn the steam at least partly to water using condensers
and giant concrete cooling towers. This allows the steam to expand
more and helps the turbine extract the maximum energy from it, making
the electricity generating process much more efficient. A large
supply of cold water is needed to condense the steam, and that's why
electricity plants with condensing turbines are often built next to
large rivers. Noncondensing turbines don't cool the steam so much,
and use the heat remaining in it to make hot water in a system known
as combined heat and power (CHP or cogeneration).
Practical steam turbines come in all shapes and sizes and produce power ranging
from one or two megawatts (roughly the same output as a
single wind turbine) up to 1,000 megawatts or more (the output from a
large power plant, equivalent to 1000 or so wind turbines). In the biggest
turbines, in large fossil-fuel power plants, the steam pressure can be as high as 20–30MPa (3000–4000 psi or about 200–270 times atmospheric pressure). A small,
10 megawatt steam turbine is roughly the same size as a Greyhound
bus (a large single-deck passenger coach).
Photo: Cooling towers like these help a steam turbine condense steam to extract more energy. If you're uncertain why we need cooling towers, check out The Energy Cost of Heat by David MacKay, an extract from his excellent book Sustainable Energy: Without Hot Air.
Photo by Dave Parsons courtesy of US Department of Energy/NREL.