Hydroelectric Power and the City of Ember
Power up! An underground hydroelectric generator powers the entire City of Ember. This week, Ask Science explains the science behind the popular series.
Lee Falin, PhD
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Hydroelectric Power and the City of Ember
According to the Book of the City of Ember, “The City of Ember is the only light in the dark world. Beyond Ember, the darkness goes on forever in all directions.” And just what is the reason for that light? If you’ve read the book or seen the movie (and if you’ve only seen the movie I strongly recommend you forget all about it and read the book instead), you know that the source of Ember’s light is an underground hydroelectric generator. So this week, let’s take a look at how hydroelectric generators work, and where they’re used today.;
Generators
To understand how hydroelectric generators work, we first need to understand electromagnetic induction. Back in the 1800s, a man named Michael Faraday discovered this interesting fact: If you take a conductor (like metal wire) and subject it to a fluctuating magnetic field, you can induce, or create, an electrical current in the conductor.
In a typical generator design, magnets are mounted on a disk called a rotor. This rotor spins around inside of a container (called a stator) with conductors mounted on the outside. As the rotor spin around inside the stator, the magnets continually pass the conductors, inducing electricity.
Turbines
Now that we know how generators work, we just need to find a way to turn the rotor. As you might remember from the episode on potential and kinetic energy, energy can’t be created from nothing; it can only be converted from one form to another. We’re trying to create electrical energy using mechanical energy, the energy of movement. But where can we get the mechanical energy?
The most common method for getting the mechanical energy to turn the rotor is via a device called a turbine. A turbine takes energy from a flowing fluid and converts it to mechanical energy. Remember that when scientists talk about fluids, we mean not only liquids, but gases as well. Most turbines attached to generators work by harnessing the energy in flowing steam.
Power Plants
Fossil fuel and nuclear power plants both use steam turbines in their generators. First they heat some water by either burning fossil fuels or using the heat energy from a nuclear reaction. That heated water turns into steam, which then flows past a turbine, causing it to spin. The spinning turbine is attached to a metal shaft, which is connected to a rotor, and this means the rotor spins when the turbine spins. As the rotor spins inside of the stator, electricity is generated through induction.
A hydroelectric power plant uses the same principles, but instead of using heat energy or nuclear energy to convert the water into steam, it uses the mechanical energy of falling water to turn the turbine.
Most hydroelectric plants involve building a dam across a river to create a large reservoir of water. There’s a small tunnel through the dam called a penstock, which has a turbine at the other end. Gravity pulls the water through the penstock, converting its gravitational potential energy into the kinetic energy of movement. As it flows past the turbine, it causes the turbine to spin, generating electricity just like the generators in other power plants.
Wind generators work the same way except they use wind instead of falling water or steam to spin the turbine.
I’m a Big Fan
You might be surprised to learn that most electric fans are in fact just miniature wind power generators running in reverse. Electrical energy enters the coils on the stator, which causes a fluctuation in magnetic field, which then exerts a force on the magnets, causing the rotor to spin. The spinning rotor turns the turbine, which converts the rotating mechanical energy into wind.
If you’re interested, you can find all kinds of projects on the Internet that show you how to turn old electric fans into miniature generators.
Conclusion
So now you know how hydroelectric generators (and all generators) work. Anytime you see a mechanical device generating electricity, be it via wind, steam, coal, or river power, there’s a pretty good chance that these same steps are involved.
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Michael Faraday and Steam Turbine images from Shutterstock
