How Electricity Is Generated At Power Plants

Power plants (power stations)

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by Chris Woodford. Last updated: May 17, 2017.

Not so long ago, alchemists dreamed of turning cheap and ugly metals into valuable ones like gold. Power plants (also called power stations) pull off a similar trick, converting lumps of coal and drops of oil into zaps of electric current that can cook your dinner or charge your phone. If it weren't for power plants, I wouldn't be writing these words now—and you wouldn't be reading them. In fact, most of the things we do every day and much of the stuff we use owes a hidden debt of gratitude to these gigantic energy factories, which turn "fossil fuels" (coal, natural gas, and oil) into electric power.

This energy-alchemy is a pretty amazing trick—and quite a recent one too, since the very first practical power station was built in only 1882 (by Thomas Edison). Yet amazement is often the last thing we feel when we think about generating electricity at the start of the 21st century. In an age when caring for the environment is (quite rightly) more important than ever, it's fashionable to sneer at power plants as evil, dirty places pumping pollution into our air, land, and water. One day, we might be able to make all our electricity in a completely clean and green way. Until then, power plants are vital for keeping our schools, hospitals, homes, and offices light, warm, and buzzing with life; modern life would be impossible without them. How do they work? Let's take a closer look!
Photo: A typical coal-fired power plant. Photo by Warren Gretz courtesy of US DOE/NREL (US Department of Energy/National Renewable Energy Laboratory).

The magical science of power plants

Chart: Large, centralized fossil-fueled power plants are very inefficient, wasting about two thirds of the energy in the fuel. Here's a typical scenario: About 62 percent is lost in the plant itself as waste heat. A further 4 percent disappears in the power lines and transformers that carry electricity from a power plant to your home. Once the electricity has arrived, your home appliances waste a further 13 percent. All told, only 22 percent of the original energy in the fuel (green slice) turns into energy you can actually use. Source: Figures from "Decentralizing Power: An Energy Revolution for the 21st Century," Greenpeace, 2005.
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. 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:

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

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.

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

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 exchanger so they heat up water instead. This design is called combined heat and power (CHP) or cogeneration, 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

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

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.

Photo: Ice Harbor hydroelectric dam produces electricity when water rushes through its turbines. Photo by US Army Corps of Engineers courtesy of US DOE/NREL (US Department of Energy/National Renewable Energy Laboratory).

How electricity gets to your home

One of the great things about electricity is that we can make it almost anywhere and transmit it vast distances along power lines to our homes. That makes it possible for us to power huge cities without building enormous dirty power plants right in the middle of them or to site power plants where there are convenient coal deposits or fast-flowing rivers to feed them. Now it takes energy to send an electric current down a wire, because even the very best wires, made from substances like gold, silver, and copper, have what's called resistance—they obstruct the flow of electricity. The longer the wire, the greater the resistance, and the more energy that's wasted. So you might think sending electricity down enormously long power cables would be a very stupid and wasteful thing to do.
There is a simple way around this, however. It turns out that the bigger the current flowing through a wire, the more energy gets wasted. By making the current as small as possible, we can keep the energy to a minimum—and we do that by making the voltage as big as possible. Power stations produce electricity at something like 14,000 volts, but they use transformers (voltage increasing or decreasing devices) to "step up" the voltage by anything from three to fifty times, to roughly 44,000–750,000 volts, before sending it down power lines to the towns and cities where it'll be consumed. Generally, power is transmitted over long distances using overhead lines strung between supporting frames called pylons; it's much quicker and cheaper to do that than to bury lines underground, which is commonly done in towns and cities. The pylons supply substations, which are effectively mini supply points devoted to powering perhaps a large factory or a small residential area. A substation uses "step-down" transformers to convert the high-voltage electricity from the power line to one or more lower voltages suitable for factories, offices, homes, or whatever it has to supply.

Photo: Left: Power station transformers. Right: Transmission line (pylons). Both photos courtesy of US Department of Energy.

How the power grid works

Substations get their name from the time when power stations supplied very clearly defined local areas: each station fed a number of nearby substations, which passed the power on to homes and other buildings. The trouble with this arrangement is that if a power station suddenly fails, lots of homes have to go without electricity. There are other problems with running power stations independently. One power station might be able to make electricity very cheaply (perhaps because it's very new and using natural gas) while another one (using old technology based on coal) could be much more expensive, so it might make sense to use the cheaper station whenever possible. Unfortunately, power stations aren't like car engines: they have to keep going all the time; generally, they can't start and stop altogether, whenever we want them to. For these and various other reasons, electricity utilities have found that it makes sense to connect all their power stations into a vast network called a grid. Highly sophisticated, computerized control centers are used to raise or lower the output of stations to match the demand from minute to minute and hour to hour (so more stations will be working flat out in the evening, for example, when most people cook their dinner).

What does the future hold for power plants?

We'll always need energy and especially electricity—a very versatile kind of energy we can easily use in many different ways—but that doesn't mean we'll always need power plants like the ones we have today. Environmental pressures are already forcing many countries to close coal-fired power plants that produce the greatest carbon dioxide emissions (responsible for climate change and global warming). Although nuclear plants might offer the cleanest route to a low-carbon future, there are grave concerns over whether we can build them fast enough or overcome people's fears about pollution and safety (whether those fears are rational or not).

Dash for gas

In the short term, it's fairly clear what the future holds: there's a worldwide "dash for gas." The majority of new electric power generating plants now use natural gas, which is significantly cheaper, relatively abundant (for now), and produces lower emissions than other fossil-fueled stations. Natural gas stations are also quicker and cheaper to build than more complex alternatives like nuclear plants, and face less public opposition. In 2011, the United States made about a quarter of its electricity from natural gas; by 2040, that's predicted to rise to about a third.

CHP

Other trends are also becoming important, notably a shift toward smaller plants driven by combined heat and power (CHP). A 2016 report by the US DOE's Energy Information Administration suggested the United States has the potential to build almost 300,000 small CHP plants (many just powering individual buildings or complexes), which would avoid the need to construct about 100 large coal or nuclear plants. Since some of these will be fueled by biomass (such as trees or "energy crops" grown specifically for the purpose) or waste, that illustrates three different trends at work: the shift to smaller plants and more of them, and the transition from fossil fuels to renewables.

Renewables

In the longer term, the future must be renewable because fossil fuel supplies will either run out or (more likely) be deemed too dirty or expensive to use. We've already seen a huge expansion of wind power over the last couple of decades and solar power is likely to increase dramatically in coming years. The big drawback, as I mentioned earlier, is that you need about 1000 wind turbines (rated at 2MW) or 400,000 solar roofs (rated at 5kW) to make the same power as one large power plant (2GW), so if we're going to switch from power plants to green energy, we need an awful lot of it covering a massive area. Whatever drawbacks power plants might have, they certainly use land very efficiently (though you could argue that the vast land-take of coal mines or oil and gas fields should be considered as well).

Charts: The changing nature of power plants. These two charts break down the total population of US electric power industry power plants by the type of fuel or other energy that they use for 2003 and 2015. Fossil fueled plants are shown in blue, nuclear plants in orange, renewable plants in green, and other power plants in yellow. You can see that there has been a significant reduction in coal and petroleum plants, a slight increase in natural gas plants, and a huge increase in renewables (though hydro plants remain about the same). Drawn using data from How many and what kind of power plants are there in the United States?, US Energy Information Administration, December 1, 2016.

Efficiency and demand management

Some argue that we can save our way out of building power plants through energy efficiency, for example, by using more efficient home appliances and better insulation. Many utility companies have embraced this idea with simple initiatives like giving out free energy-saving lightbulbs to householders. In theory, if you give out 50 million low-energy lamps and they each save 50 watts of power, you completely avoid the need to build one large (2.5GW) power plant. (This idea is sometimes called "negawatts," a word coined by Amory Lovins of the Rocky Mountain Institute.) We can also reduce the need for new power plants by storing energy more sensibly and managing demand so we don't have such huge peaks in power use. Unfortunately, this approach only takes us so far. The problem is that our total energy needs are constantly growing—and our need for electricity is bound to grow too as we shift from fossil-fueled automobiles and diesel trains to electric alternatives. Moreover, there's the issue of growing energy needs in developing countries: people in those countries cannot save energy they're not already using, and it would be immoral to try to stop them using energy to climb out of poverty. Ultimately, the world as a whole is going to need to harness much more energy and much more electricity and, though efficiency has a crucial part to play, it's only a small part of the solution.
In the short term, the dash for gas helps if it shifts us away from coal. CHP also helps if it improves efficiency, but not if it locks us into fossil fuels for decades to come. Carbon-capture and storage (CCS) might help us make older, coal-fired plants more environmentally friendly, but it remains largely unproven and expensive. The long-term future must certainly be a renewable one and energy efficiency could make a greener future, powered by the sun and the wind, easier to achieve. Even so, for now and for decades to come, conventional, fossil-fueled power plants will remain the bedrock of our energy and electricity supply. We should admire them, respect them for powering our lives, and make them as clean and green as we possibly can.

Don't be a fool, stay cool—and keep away from power plants!

Electricity is brilliant, but it's also very dangerous.
As we've just seen, power plants and transmission lines carry electricity at incredibly high voltages—thousands of times greater than those used in your home.
Playing in, on, or anywhere near power equipment is extremely stupid and dangerous. Touch a power line and you'll very likely to be burned to death in a particularly slow and horrible way. Don't fly a kite near power lines or play soccer nearby. If you happen to kick a ball or something like that into a substation, leave it there and forget it. Your life is worth more than a silly bit of plastic.
Working near power lines can also be much more dangerous than you might think—so take care! According to the US CDC, electrocution is the fifth leading cause of work-related death and the second most common cause of death in the construction trade. Research by the US National Institute for Occupational Safety and Health (NIOSH) discovered that at least 154 people were killed between 1992 and 2005 by touching power lines with metal ladders they were using or working with.