Power plants (power stations)

A single large power plant can generate enough electricity (about 2 gigawatts, 2,000 megawatts, or 2,000,000,000 watts) to supply a couple of hundred thousand homes, and that's the same amount of power you could make with about 1000 large wind turbines working flat out. But the splendid science behind this amazing trick has less to do with the power plant than with the fuel it burns. The real magic isn't that power plants turn fuel into electricity: it's that even small amounts of fossil fuels contain large amounts of energy. A kilogram of coal or a liter of oil contains about 30MJ of energy—a massive amount, equivalent to a good few thousand 1.5-volt batteries! A power plant's job is to release this chemical energy as heat, use the heat to drive a spinning machine called a turbine, and then use the turbine to power a generator (electricity making machine). Power plants can make so much energy because they burn huge amounts of fuel—and every single bit of that fuel is packed full of power.

Unfortunately, most power plants are not very efficient: in a typical old plant running on coal, only about a third of the energy locked inside the fuel is converted to electricity and the rest is wasted. Newer designs, such as combined cycle power stations (which we'll explore in a minute) may be up to 50 percent efficient. As the chart here shows, even more electricity is squandered on the journey from the power plant to your home. Adding all the losses together, only about a fifth of the energy in the fuel is available as useful energy in your home.

How does a power plant work?

A power plant's a bit like an energy production line. Fuel feeds in at one end, and electricity zaps out at the other. What happens in between? A whole series of different steps, roughly along these lines:

Artwork showing the steps involved in how a power plant makes electricity

1. Fuel: The energy that finds its way into your TV, computer, or toaster starts off as fuel loaded into a power plant. Some power plants run on coal, while others use oil, natural gas, or methane gas from decomposing rubbish.
2. Furnace: The fuel is burned in a giant furnace to release heat energy.
3. Boiler: In the boiler, heat from the furnace flows around pipes full of cold water. The heat boils the water and turns it into steam.
4. Turbine: The steam flows at high-pressure around a wheel that's a bit like a windmill made of tightly packed metal blades. The blades start turning as the steam flows past. Known as a steam turbine, this device is designed to convert the steam's energy into kinetic energy (the energy of something moving). For the turbine to work efficiently, heat must enter it at a really high temperature and pressure and leave at as low a temperature and pressure as possible.
5. Cooling tower: The giant, jug-shaped cooling towers you see at old power plants make the turbine more efficient. Boiling hot water from the steam turbine is cooled in a heat exchanger called a condenser. Then it's sprayed into the giant cooling towers and pumped back for reuse. Most of the water condenses on the walls of the towers and drips back down again. Only a small amount of the water used escapes as steam from the towers themselves, but huge amounts of heat and energy are lost.
6. Generator: The turbine is linked by an axle to a generator, so the generator spins around with the turbine blades. As it spins, the generator uses the kinetic energy from the turbine to make electricity.
7. Electricity cables: The electricity travels out of the generator to a transformer nearby.
8. Step-up transformer: Electricity loses some of its energy as it travels down wire cables, but high-voltage electricity loses less energy than low-voltage electricity. So the electricity generated in the plant is stepped-up (boosted) to a very high voltage as it leaves the power plant.
9. Pylons: Hugh metal towers carry electricity at extremely high voltages, along overhead cables, to wherever it is needed.
10. Step-down transformer: Once the electricity reaches its destination, another transformer converts the electricity back to a lower voltage safe for homes to use.
11. Homes: Electricity flows into homes through underground cables.
12. Appliances: Electricity flows all round your home to outlets on the wall. When you plug in a television or other appliance, it could be making a very indirect connection to a piece of coal hundreds of miles away!

Types of power plants

Steam turbine plants

Most traditional power plants make energy by burning fuel to release heat. For that reason, they're called thermal (heat-based) power plants. Coal and oil plants work much as I've shown in the artwork above, burning fuel with oxygen to release heat energy, which boils water and drives a steam turbine. This basic design is sometimes called a simple cycle.

Cutaway model of a steam turbine

Photo: An excellent cutaway model of a steam turbine and electricity generator. Steam flows into the turbine through the huge gray pipes at the top, turning the windmill-like turbine in the middle. As the turbine spins, it turns the electricity generator connected to it (the blue cylinder you can just see on the right). This model lives in Think Tank, the science and engineering museum in Birmingham, England.

Gas turbine plants

Natural gas plants work in a slightly different way that's quite similar to how a jet engine works. Instead of making steam, they burn a steady stream of gas and use that to drive a slightly different design of turbine (called a gas turbine) instead.

Combined designs

Every power plant ever built has had one main objective: to get as much useful electricity as possible from its fuel—in other words, to be as efficient as possible. When jet engines scream through the sky firing hot gases like rocket jets in their wake, they're wasting energy. There's not much we can about that in a plane, but we can do something about it in power station. We can take the hot exhaust gases coming from a gas turbine and use them to power a steam turbine as well in what's called a combined cycle. That allows us to produce as much as 50 percent more electricity from the fuel compared to an ordinary, simple cycle plant. Alternatively, we can improve the efficiency of a power plant by passing waste gases through a heat exchange so they heat up water instead. This design is called combined heat and power (CHP) or co generation, and it's rapidly becoming one of the most popular designs (it can also be used for very small-scale power production in units roughly the same size as car engines).

Nuclear plants

Nuclear power plants work in a similar way to simple cycle coal or oil plants but, instead of burning fuel, they smash atoms apart to release heat energy. This is used to boil water, generate steam, and power a steam turbine and generator in the usual way. For more details, see our main article on how nuclear power plants work.

Hydro plants

While all these types of power plants are essentially thermal (generating and releasing heat to drive a steam or gas turbine), two other very common types don't use any heat whatsoever. Hydroelectric and pumped storage plants are designed to funnel vast amounts of water past enormous water turbines (think of them as very efficient water wheels), which drive generators directly. In a hydroelectric plant, a river is made to back up behind a huge concrete dam. The water can escape through a relatively small opening in the dam called a penstock and, as it does so, it makes one or more turbines spin around. For as long as the river flows, the turbines spin and the dam generates hydroelectric power. Although they produce no pollution or emissions, hydroelectric stations are very damaging in other ways: they degrade rivers by blocking their flow and they flood huge areas, forcing many people from their homes (the Three Gorges Dam in China displaced an estimated 1.2 million people).

Pumped storage generates electricity in a similar way to a hydroelectric plant, but shuttles the same water back and forth between a high-level lake and a lower one. At times of peak demand, the water is allowed to escape from the high lake to the lower one, generating electricity at a high price. When demand is lower, in the middle of the night, the water is pumped back up again from the low lake to the high one using low-rate electricity. So pumped storage is really a way of taking advantage of how electricity is worth more at some times than at others.