How steam turbines work - A simple introduction

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 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.

The steam engine Manston with tender

Photo: The power of steam: a restored steam locomotive called Manston running on the Swanage Railway in England.

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 travelling much slower! Read more in our introduction to turbines.

What is a steam turbine?

Theory of a steam turbine

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.

Metal turbine blade exhibited in a science museum.

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

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 steel outer case (one capable of withstanding high pressures and temperatures).

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 travelling fast. Photo of a turbine blade exhibited at Think Tank, the science museum in Birmingham, England.

Impulse and reaction turbines

In one type of turbine, the rotating blades are like deep buckets. High-velocity jets of incoming steam from carefully shaped nozzles kick into the blades, pushing them around with a series of impulses, and bouncing off to the other side with a similar pressure but much-reduced velocity. This design is called an impulse turbine and it's particularly good at extracting energy from high-pressure steam.

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 velocity as it had when it entered. This design is known as a reaction turbine. Both sets of blades have to be made from incredibly tough materials capable of rotating at very high speeds with high-pressure steam blowing at them the whole time.

Disused Pelton water wheel. A reaction turbine

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 Laborayory (DOE/NREL).

Other parts

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

Closeup cutaway model of a steam turbine showing blades and stages.

Multiple stages

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: The multiple stages in a typical steam turbine. This model is at Think Tank, the science museum in Birmingham, England.

Condensing and noncondensing

Cooling towers on a power plant billowing out steam.

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 condensors 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). 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. Photo by Dave Parsons courtesy of US Department of Energy/NREL.

Steam turbines