Hydropower, or hydroelectric power, is the most common and least expensive source of renewable electricity in the United States today. According to the Energy Information Administration, more than 6% of the country's electricity was produced from hydropower resources in 2008, and about 70% of all renewable electricity generated in the United States came from hydropower resources.



Hydropower technologies have a long history of use because of their many benefits, including high availability and lack of pollution emissions. The term “hydro” comes from an ancient Greek word for water. For centuries, waterwheels provided the energy to grind grain and saw lumber.


In the United States, the first hydropower plant was built in 1880 in Grand Rapids, Michigan. Large scale construction of these power plants occurred in the 1930s and ‘40s as a part of Franklin D. Roosevelt’s “New Deal” programs. These efforts were intended to provide electricity to remote areas of the country where private utility companies were absent. Since rivers are abundant, hydroelectricity is widely used by both rural and urban settlements.


Hydropower uses water to power machinery or make electricity. Water constantly moves through a vast global cycle, evaporating from lakes and oceans, forming clouds, precipitating as rain or snow, and then flowing back down through our natural water systems. The energy of this water cycle, which is driven by the sun, can be tapped to produce electricity or for mechanical tasks like grinding grain. Hydropower uses a fuel—water—that is not reduced or used up in the process. Because the water cycle is an endless constantly recharging system, hydropower is considered a renewable energy.

The cycle of water

When flowing water is captured and turned into electricity, it is called hydroelectric power. There are several types of hydroelectric facilities; they are all powered by the kinetic energy of flowing water as it moves downstream. Turbines and generators convert the energy into electricity, which is then fed into the electrical grid to be used in homes, businesses, and by industries. As an example, the force of the flow of a medium-size river is equal to several million horsepower. One million horsepower, if converted to electricity, would equal the power of 746 megawatts (MW). One megawatt can produce enough electricity to service approximately 1,000 homes in the U.S.


Green Avenger Fast Fact

W = Watt = rate of energy conversion of 1 joule per second
kW = kilowatts = 1,000 watts
MW = megawatts = 1,000 kilowatts
GW = gigawatts = 1,000 megawatts

Joule = passing an electric current of one ampere through a resistance of 1 ohm for one second.


Tidal, Wave and Ocean Energy

Oceans cover more than 70% of the Earth's surface. As the world's largest solar collectors, oceans contain thermal energy from the sun and produce mechanical energy from tides and waves. Even though the sun affects all ocean activity, the gravitational pull of the moon primarily drives tides, and wind powers ocean waves. The circular motion of water molecules swirl throughout water systems with loads of kinetic energy. The most productive or high-energy producing waves (generating about 100 MW) are located on western coastlines facing the open sea such as California and the Pacific Northwest, Chile, northern Europe, South Africa, Australia, and New Zealand.

Ocean wave energy is captured directly from surface waves or from pressure fluctuations below the surface. Wave energy can be converted into electricity by offshore or onshore systems. To learn more visit:


An image of ocean hydropower


Green Avenger Fast Fact
There are currently no commercial ocean energy power plants in the U.S. Only about 300 megawatts of tidal energy is currently produced worldwide.


A picture of Tidal Barrage in action!

Tidal Power

Tidal power, or tidal energy, is a form of hydro power that exploits the tidal movements of the ocean as water flows back and forth. Tidal energy technologies include barrages or dams, tidal fences, and tidal turbines. Tidal power can be harnessed in a couple of ways: kinetic energy that powers turbines as the water moves between full and ebb tide, and; potential energy in which barrages are used to exploit the difference between high and low tide.

When there is a substantial amount of water that rushes in and out of some rivers and inlets it is possible to harness the energy created to drive generators to produce electricity. To tap this energy a barrage is built across the mouth of the river. Water turbines sit in the barrage wall and as the water rushes through, the turbines generate electricity.


A process called ocean thermal energy conversion (OTEC) uses the heat energy stored in the Earth's oceans to generate electricity. OTEC is an energy technology that converts solar radiation to electric power. OTEC systems use the ocean's natural thermal gradient—the fact that the ocean's layers of water have different temperatures—to drive a power-producing cycle.


Graph of Thermal energy absorbed by the ocean


Green Avenger Fast Fact
The heat energy from the sun that is absorbed each day in the ocean is equal to the energy stored in 250 billion barrels of oil.


As long as the temperature between the warm surface water and the cold deep water differs by about 20°C (36°F), an OTEC system can produce a significant amount of power. The oceans are thus a vast renewable resource, with the potential to help us produce billions of watts of electric power. This potential is estimated to be about 1013 watts of baseload power generation, according to some experts. The cold, deep seawater used in the OTEC process is also rich in nutrients, and it can be used to culture both marine organisms and plant life near the shore or on land.

Learn more about OTEC


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