How Hydropower Works

Why water contains energy - and how we benefit?

Green hydropower continues to generate electricity when the sun goes down and the wind stops blowing
Only 3% of installed dams are used to generate electricity
Streaming hydropower systems do not use a dam, and are usually smaller and more difficult to spot
Reservoir hydropower syatems are easily distinguished by a large dam and lake behind it
Hydropower extracts energy from water as it flows by

Hydropower captures the energy produced by moving water. The methods for harnessing this power are remarkably simple; in fact, our use of water power goes back hundreds of years. Today’s modern hydroelectricity plants may be many times more sophisticated, but they rely on the same principles used to power mankind’s earliest machines.

The most important thing to remember is that water must be moving to generate power. A quiet lake may have the potential to generate power, but nothing happens until the water moves. Fortunately, Mother Nature does a terrific job of moving water.

Remember learning about the Water Cycle when you were a kid? It’s the natural process we rely on for hydropower. Water evaporates from the ocean into the atmosphere and forms clouds as it cools. Then, while we duck under cover, the water returns to the earth as rain and snow, eventually forming streams and rivers as it heads back to the ocean to start the process all over again.

Thanks to gravity, the water moving down those streams and rivers can pick up tremendous force – energy we can put to work as hydropower.

That’s the essence of Hydropower: extracting energy from water as it flows by. A simple example is shown with the water-powered mill wheel. Water coming down a stream is diverted to the top of the water wheel. As each bucket fills, the added weight carries it to the bottom, turning the wheel as it goes. Notice that the water wheel is connected to a shaft, which connects to machinery inside the mill. The water flows and the machines turn – all by capturing a little energy from water as it follows its natural path back to the ocean.

It’s easy to see why water power can be so compelling. As long as water is flowing down the hill (thanks to the Water Cycle), energy can be extracted from it. Nothing is consumed; the water continues on. Water is a 100% clean, renewable resource – but with some substantial advantages: streams and rivers still flow when the sun goes down and the wind stops blowing. Plus, electricity from water power is far less expensive to produce than solar or wind power.

The vintage water wheel is rarely used today, having been replaced with more efficient water turbines that are typically used to generate electricity. As shown in the diagram, however, the fundamentals of water power are the same. Water flows across the turbine, turning a shaft connected to a generator that produces electricity.

How Much Power?

Any moving water can produce hydropower. Add more moving water, or Flow, and you can produce more power. You can tell by looking that a raging river contains more energy than a small stream.

But if you add pressure to the water, you can get a lot more energy from it. (Ever see how far a fire hose can squirt water?) As it turns out, it’s pretty easy to add pressure to water, simply by putting it in a container.

Water weighs quite a bit, and the sheer weight of a container of water puts pressure on the bottom. The water in a medium-sized aquarium puts about 1 pound per square inch (psi) of pressure on the bottom of the aquarium. Now imagine an aquarium 100 feet tall. The water pressure at the bottom of that tall aquarium will be about 43 psi – roughly the same as the average outdoor faucet.

Hydropower systems build pressure either by containing the water behind a dam, or within a pipe that runs down a hill. The weight of the water behind the dam or in the pipe creates pressure at the bottom. More height creates more pressure, and more pressure means we can get more power from the flow of water. (The technical term for this vertical distance is Head.)

The Two Major Designs for Hydropower

Most hydroelectric systems can be classified into one of two groups:

Reservoir Hydro Systems

Most people visualize a Reservoir system when they hear the term “hydroelectric project.” Reservoir systems can easily be recognized by the large dam that creates a sizeable lake behind it. Examples include Srisailam Dam & Nagarjuna sagar Dam in Andhra Pradesh, and the massive Three Gorges Dam in China.

Run -of-the-River (Run of River) Systems

In many respects, Run -of-the-River Hydro (also known as Run of River) systems are the opposite of Reservoir systems. There are no dams and lakes, only diversion systems that direct a portion of a stream or river through the hydroelectric turbine. Run -of-the-River systems are typically installed on smaller streams and rivers, and generate less power than large Reservoir systems. They are rapidly gaining popularity due to their ease of installation and small ecological footprint.

In contrast to Reservoir designs, Run -of-the-River Hydro projects use a completely different approach. First, there is no dam or reservoir. Instead of a dam, Run -of-the-River Hydro systems use a diversion to channel some of the water from a stream into a pipe that supplies the water turbine. The rest of the stream water continues past the diversion down its natural path.

How Run -of-the-River Hydro Works

The diverted water flows down a pipeline (known as a penstock), passing through a turbine to generate electricity, and then recombining with the original stream. As we discussed earlier, this pipeline is used to contain the water to build high pressure at the bottom where it enters the turbine.

In effect, Run -of-the-River Hydro systems “borrow” a portion of the stream’s water to produce power, returning it to the stream after the energy is extracted. Unlike the Reservoir system, Run -of-the-River Hydro does not change the natural course of the stream or store water for future use.

This creates a couple of disadvantages. First, without a reservoir there is no reserve capacity for peak load periods. Second, because not all the water from the stream is being used to generate electricity, the output of the hydroelectric system is less than it could be with a Reservoir system.

But Run -of-the-River Hydro has many advantages. First, these systems help resolve the two major disadvantages of the Reservoir system: fish migration and flooding. The stream still flows in parallel with the hydro system, providing an unencumbered path for fish migration. And since there is no reservoir, habitat flooding is not an issue.

Another example of a Run -of-the-River Hydroelectric powerhouse. Notice the pipeline feeding into the rear of the powerhouse.

Because most Run -of-the-River Hydro systems are smaller – typically less than 30 megawatts – they occupy very little space and tend to blend into the environment. This makes them ideal for smaller streams and rivers where a Reservoir system wouldn’t be appropriate. A side benefit is that transmission lines are much shorter; in fact, they often reduce transmission losses by providing locally generated power instead of requiring lines from large generating plants that may be hundreds of miles away.

These are important considerations today, as potential sites for large reservoir systems are extremely limited. There are thousands of smaller streams that could be used for Run -of-the-River Hydro with minimal visual and environmental impact.