Two Major Types

There are two Major designs for hydroelectric systems: Reservoir Hydro Systems and Streaming (Run of River) Systems. The use of one or the other depends on whether or not there are large bodies of moving water in the vicinity.

Reservoir Hydro Systems

This is the type of hydroelectric system that is usually visualized when some uses the term hydropower. Reservoir systems are optimal when there are large bodies of water in the vicinity. They are easily recognized because of their use of a large dam that holds back water from a lake. The hydroelectric turbine stis at the bottom of the dam, in order to maximize the pressure built up by the dam. The pressure at the bottom of the dam is determined by the height of the dam. We are able to control the amount of water that moves through the turbines at any given time, allowing us to control the amount of electricity produced. The Hoover dam in Navada has te capability to produce 2800 megawatts of electricity. Other well-known reservoir systems include the Grand Coulee Dam  in the United States and the Three Gorges Dam in China. The Three Gorges Dam is an especially interesting case because it is purported to produce about the same amount of power as 15 nuclear dams. Although originally designed in part to reduce the impact of flooding in China’s local coastal region, the project (which is the biggest hydroelectric reservoir system in the world) has come under scrutiny for failing that purpose and endangering local marine life.

High Power Output

Reservoir systems share the characteristic of high electric outputs. During times of low power usage, the amount of water that is allowed to pass through the turbine is reduced to prevent waste. The leftover water is held back in reserve. In peak usage times, this water is released along with the naturally flowing water from the lake, producing more pressure and electricity than the river alone could provide. Because of this feature, reservoir systems typically can produce thousands of watts of electricity; the Three Gorges Dam, for example, can produce 23 thousand megawatts of electricity!

Streaming (Run of River) Systems

Streaming hydroelectric systems are the opposites of resevoir systems in many respects. Rather than using lakes and dams, the only diversion system used in this process is a direct diversion of water from a river or stream using a hydroelectric turbine. Streaming systems are usually employed using smaller streams, and generate far less power than reservoir systems, though they are gaining popularity as they are far cheaper than the former.

How Streaming Systems Work

Water that is diverted from the original stream flows down a pipeline and passes through a hydroelectric turbine to generate electricity, after which it flows back to the original stream. The pipe compresses the water into a tight surface and amplifies the pressure at the bottom of the stream, where the turbine is. Unlike a reservoir system, this does not interrupt the original stream to any great extent. Unfortunately, however, this also creates a few disadvantages: for example, since the stream is only a fraction of the total water available in the stream, the pressure (and thus the power) produced is well below the capacity of the stream. Also, streaming hydroelectric systems cannot account for peak electricity usage times and, since the pressure is much less controlled, cannot be relied on to produce significantly more or less than average electricity depending on need.

However, there are some key advantages

Streaming Hydro does have its advantages, however. For one, this system does not produce the same damage to fish populations as the reservoir system (one of the major complaints against which is that it disrupts migration patterns, leading to a decrease in fish populations). Streaming systems also do not have the same issues with flooding as the reservoir system. Also, since streaming systems are much smaller than reservoir systems, they tend to blend in with the environment, creating much less visual pollution. These systems are ideal for smaller streams and rivers