Hydropower is power derived from moving water. There are multiple ways to gather the energy of water.

• Hydroelectric Dams
• Diversion Facility
• Tidal Power
• Ocean Waves
• Ocean Thermal Energy

The hydroelectric dam, or "impoundment facility," is the most widely used water-powered technology. Typically, water from a river is impounded behind a barricade, or dam, creating a reservoir. Water is released from the reservoir as needed to flow through a turbine, causing it to spin, thus activating a generator, which produces electricity.

A "diversion facility" uses a conduit to divert parts of a river to turbines which set the generator in motion. Power plants at Niagara Falls, New York, are diversion facilities.

There are three methods available for harnessing of the power of tides.

• Tidal barrages are installed across an ocean inlet or lagoon that forms a tidal basin. The barrages are equipped with devices that can generate power from both the incoming and outgoing tides.
• Tidal turbines are mounted on the ocean floor where there are strong tidal currents. Like wind turbines, tidal turbines convert the flow of tides to energy.
• Tidal fences are rows of vertical tidal turbines mounted on the ocean floor.

Ocean waves are generated by wind blowing over the ocean surface. Wave power is the capture of the energy in those ocean waves. It is estimated that the total energy potential of waves along the coasts of the U.S. are about 66% of the total electricity generated in the U.S. Thus, wave power, were it captured and made available for use, could be a substantial source of renewable energy and could solve many energy needs. There are multiple methods designed and presently under development with the objective of accomplishing that goal; however, there has not been a successful commercial application of wave power to date.

Ocean Thermal Energy Conversion (OTEC) derives energy from temperature differences in the upper and lower levels of the ocean. The surface water of the ocean, which is heated directly by the sun, is substantially warmer than the water in deep areas of the ocean. The objective of OTEC is to use that temperature difference to power turbines that generate electricity. In order for this process to work, there needs to be a temperature difference between the upper and lower water levels of at least 77 degrees F.

There is an application of OTEC technology in Hawaii, run as a research program and administered by the National Energy Laboratory of Hawaii Authority. The laboratory has developed experimental systems capable of supplying electricity to the local Hawaiian electrical grid.

Using the ocean's waves, tides and currents to generate renewable, sustainable energy is leading to interesting experimental projects world-wide. There are many companies with promising technologies who are engaged in the years-long process of building, siting and deploying pilot technologies for long term testing in various parts of the world. So far, though, only a few companies have successfully installed their technologies and linked them to their respective grids.

Ireland's Galway Bay hosts two projects with very different approaches to ocean wave technology. Wavebob's WEC-1 is a floating buoy, an oscillating system capable of "tuning in" to and absorbing the variable wave periods and heights, then maximizing their useful power output. The other, Ocean Energy's OE Buoy works on the oscillating water column principle to spin a turbine which powers a generator. The deployed unit produces .015 MW of power.

Off the coast of Oahu, Hawaii Ocean Power Technologies (OPT) has anchored its PowerBuoy®, another bobbing device that uses its movements to drive a generator, which produces electricity that is sent to shore via underwater cable.

New Energy Corporation makes turbine devices similar to those used to generate wind power that can be used in tidal or fresh water condition. So far they've deployed one 5 kW project in the U.S. (Ruby, Alaska) and several more in Canada. For more information go to New Energy Corporation

Hydro Green Energy's deployed project on the Mississippi River at Hastings, Minnesota, utilizes hydrokinetic turbine power, which is generated from the flow, current or velocity of water. The project currently produces .07 MW of electricity using two turbines, which can be maintained above water level. It is an adjunct to a lock-and-dam owned by the city, and it is the first commercial hydrokinetic power facility approved by the FERC. For more information go to Hydro Green Energy

Verdant Power is now commercially licensed to expand its "Free Flow" tidal system, the RITE Project, in New York's East River, as well as its and its "Free Flow" river system, the CORE Project, in the St. Lawrence River at Cornwall, Ontario. Free Flow's underwater 3-blade, horizontal-axis turbines rotate with the flow of the currents to drive a speed increase which powers a grid-linked generator. For more information go to Verdant Power