Reservoir: a large lake, often artificial, used for water supply
Sluice gates: a controlled gate in front of a water channel (in this case, the penstock)
Penstock: the water intake valve that runs from the reservoir to the downstream outlet
How It Works
Hydroelectricity operates on the principles of gravity and motion. A hydroelectric dam is built on a large river with a drop in elevation. As a result, the dam builds up a reservoir behind it, where it can draw upon the water. The sluice gates block the penstock but open to allow water to come through, providing an easy way to halt generation. When the water goes through the penstock, it increases in speed then rushes through a series of turbines to generate electricity. Afterward, the water empties into a downstream river outlet.
How It Is Renewable Energy
Because a hydroelectric system operates on the basic principles of physics and motion, it does not require extra energy to make the water go faster. Furthermore, as with other forms of renewable energy, the source — water — never runs out.
However, some hydroelectric systems need to use electricity to pump water to the top of the dam so that it can fall over the edge and gain speed. To achieve this, many use a pumped storage system to pump extra water up to the top of the dam if there is unused electricity to generate more the next day.
One of the most important aspects of hydroelectricity is that it generates electricity at all times of the day; a river will always flow and is not dependent on outside influences, unlike wind, solar PV, or CSP.
Advantages and Disadvantages
The primary advantage of hydroelectricity is its invariability, as aforementioned. Rivers flow at a naturally constant rate, unlike solar PV, CSP, or wind energies. Similarly, hydroelectric plants can go from zero to full capacity very quickly. As a result, hydroelectricity can be used to supplement other types of renewable energy, particularly in times of high demand. Hydroelectricity also has relatively low operational and maintenance costs.
While hydroelectricity is an essential renewable energy system, it does have some negative impacts on the environment. Firstly, and most severe, creating reservoirs requires much flooding of the surrounding land. This flooding can lead to deforestation (which is a major threat to carbon sinks), habitat loss, and loss of agricultural land, leading to more land cleared for agriculture, which also has negative environmental consequences. Furthermore, hydroelectric reservoirs have forced many communities to relocate as their homes are too near to, or are in, the flooding radius.
In addition to reservoir flooding, there are other impacts on wildlife, particularly aquatic ecosystems. Many fish can get caught in the penstock and are injured or killed by the turbines. However, some facilities are attempting to fix this situation with recent innovations, such as a fish ladder. A fish ladder, though not always effective, is a series of small pools that fish swim through to avoid the dam.
Most significantly, hydroelectric plants can emit 0.02 to 0.06 pounds of carbon dioxide per kilowatt-hour. While these emissions are important, hydroelectric plants are still much better than the 0.92 to two pounds of carbon dioxide per kilowatt-hour for natural gas, the cleanest fossil fuel.
Conclusion: Our Take
Hydroelectricity is the second-most prevalent source of renewable energy. However, the solar and wind industries are growing much more rapidly than hydroelectricity. In 2019, wind overtook it as the leading source of renewable energy. Furthermore, hydroelectricity's negative impacts are some of the greatest of any type of renewable energy. Therefore, though hydroelectricity is important to supplement other forms of renewable energy, we do not think it should not be used as a primary source of electricity.
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“What Is a Fish Ladder?” NOAA's National Ocean Service. National Oceanic and Atmospheric Administration, June 1, 2013. https://oceanservice.noaa.gov/facts/fish-ladder.html.
Waldman, John. “Blocked Migration: Fish Ladders On U.S. Dams Are Not Effective.” Yale Environment 360. Yale University, April 4, 2013. https://e360.yale.edu/features/blocked_migration_fish_ladders_on_us_dams_are_not_effective.
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“Hydroelectric Power Water Use.” United States Geological Survey. Department of the Interior. Accessed January 23, 2021. https://www.usgs.gov/special-topic/water-science-school/science/hydroelectric-power-water-use?qt-science_center_objects=0#qt-science_center_objects.
“Wind Has Surpassed Hydro as Most-Used Renewable Electricity Generation Source in U.S.” U.S. Energy Information Administration. Department of Energy, February 26, 2020. https://www.eia.gov/todayinenergy/detail.php?id=42955.
Image: Spiderskidoo/Getty. Slovenian Hydroelectric Dam. National Geographic. National Geographic Society. Accessed January 23, 2021. https://media.nationalgeographic.org/assets/photos/261/013/c45d76a4-4605-4e28-a35a-98a9f56cedc2.jpg.