Power plants (power stations)
by Chris Woodford. Last updated: August 4, 2016.
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
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.
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.
Types of power 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.
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.
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)
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 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.
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.
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
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 2013. 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 slight 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, April 2, 2015. Please be aware that the scale on the two charts is slightly different.
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.