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7/27/2017 by mdc
https://www.forbes.com/sites/jamesconca/2017/07/27/tidal-energy-all-renewable...
Dr. Conca opines that most everyone has heard of hydroelectric energy or hydropower, that uses a dam to store water in a reservoir. Water released from the reservoir flows through turbines, spinning them to generate electricity.

But there are other types of hydropower that harness marine currents, tidal energy and wave energy, also referred to as marine current energy. Tidal stream turbines work much like submerged windmills, but are driven by flowing water rather than by air.

According to Professor Jason Donev at the University of Calgary, tidal power is far more reliable than either wind or solar power as the sun does not always shine and the wind does not always blow. We know how much electricity we are going to generate at any given time, but tidal is still intermittent. Although it is very predictable, it is still non-dispatchable, we can not choose to turn it on or off like a dispatchable natural gas power plant, says Donev. However, the inherent predictability of tidal power is highly attractive for grid management, removing much of the need for back-up plants powered by fossil fuels.

Tidal turbines can be installed in the ocean at places with high-tidal current velocities, or in places with strong and continuous ocean currents. These flows are basically as predictable as the tides. The technology shown above consists of twin axial flow rotors of 15 to 20 meters in diameter, each driving a generator via a gearbox much like hydroelectric and wind turbines. The twin power units of each system are mounted on wing-like extensions on either side of a tubular steel central pillar some ten feet in diameter, set into a hole drilled into the ocean floor.

Marine currents, unlike many other forms of renewable energy, are a consistent source of kinetic energy caused by regular tidal cycles driven by the phases of the moon. Water is over 800 times denser than air so the force of the tidal flow, of only several miles per hour in places like Strangford Lough, Ireland.

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