Hydropower is the energy that can be harvested from falling water. Because water is so dense, even a modest drop in elevation can perform significant work. Hydropower has been around since ancient times, most commonly by harnessing the rotary motion provided by water wheels. The water wheel would then be geared to a grain mill, lumber mill, or textile mill.
Waterwheels could be arranged with water flowing over the top - "overshot", or dipped into a fast running current, "undershot".
With the advent electrical power, the waterwheel was eventually abandoned in favor of more efficient and controllable means of capturing the power from running water.
The mechanism that replaced the waterwheel was the turbine. The first turbine entering widespread use was called the "Francis Turbine". It was invented in 1848 for a textile factory in Massachussets. This design is a reaction turbine with quite high efficiency, and it is still widely used today.
Water turbines develop greater power when there is more flow and pressure. The flow of a river varies over time according to the whims of nature, but of course the water can be held in a reservoir and released as needed. The other advantage of a reservoir is that a dam increases the height of the water behind it, increasing the pressure. As the water level rises behind the dam, so does the pressure at the turbine inlet. (In hydropower speak, this pressure is called "head") To maximize the head - the distance between the surface of the water behind the dam and the turbine inlet - turbines are located at the bottom of the dam, with a generators installed just above them in a large machinery hall.
Cutaway of the action: (Image courtesy of the US Army Corps of Engineers)
Below, the installation of a Francis Turbine at Grand Coulee Dam in Washington State, late 1930's. This turbine drives a 125 Megawatt generator.
One of the larger turbines added to Grand Coulee in the 1970's. This drives a 600 MW generator. Grand Coulee can generate up to 6800 MW, making it the largest power station in the U.S.
Below, some of the generators at Grand Coulee Dam.
Below, a panoramic picture of Grand Coulee Dam. The new powerhouse is at the left.
There are other, less massive hydroelectric power plants, of course. Many Hydro power plants are called "run-of-the-river" plants. They will have a dam, to provide a bit of elevation and increase water pressure, but they will not have a reservoir. The output of these hydroelectric power plants is completely at the mercy of the flow of the river. Here is an example of a run-of-the-river dam:
Notice that this dam does not impound a vast amount of water behind it, so when the river runs low, so does the power output. This is not the case with a powerhouse with a reservoir of water.
This is just a brief overview of how hydroelectric power works. If you are more interested, here are a couple of quick links:
History of the LADWP power projects
Everything you need to know about Hydroelectric Energy
I'm not that interested. Hydro is pretty dam (pun intended) boring - except when it fails. That's what I am interested in! You often read about a fire, a meltdown or explosion at a thermal plant. I bet you didn't know that hydro plant failures have killed more people than any other type of power plant. We will look into that more in the next few posts...