Sources of Tidal Power

It’s impossible to spend any amount of time along an ocean shoreline and fail to be impressed by the ebb and flow of the tide and its power. But what are the sources of tidal power? What drives the twice daily motion of all that water? Tidal energy is unique in that it derives from the gravitational pull of the moon and, to a much lesser extent, the sun. This is unique because virtually all other forms of energy on planet earth derive ultimately radiation energy from the sun. (Nuclear energy is the only other exception I can think of at the moment.) If you question that, remember that hydro power requires the sun’s energy for the hydrologic cycle to continue (essentially, the sun evaporates water from the ocean to form clouds, it condenses and falls as rain and snow on the mountains and runs back to the sea as streams and rivers from which the energy can be captured.

Biomass depends on photosynthesis driven by the sun to grow the biomass.

Coal and oil derive from ancient biomass.

Wind derives from weather patterns and temperature differentials caused by the sun.

But tidal power is different. Tides derive from the pull of gravity. As the earth revolves on its axis and the moon orbits around the earth, different parts of the earth are closest to the moon. Gravitational force changes with the square of the distance between two objects (two object 1 mile apart will experience 16 time the gravitational attraction then they would if they were 4 miles apart), so the part of the earth closest to the moon experiences a much stronger gravitational force. As you’re no doubt aware, ¾ of the surface of the earth is covered with water. Water, of course, is fluid so it responds fluidly to the gravitational pull.

Over the oceans, this results in a small “bulge” of water toward the moon that moves across the water’s surface in concert with the motion of the celestial bodies. When this bulge reaches shore, it creates high tide. As it passes, the waters ebb.

On average, the height of this bulge in open ocean is about 3 feet. However local conditions along a coastline influence the tidal effect dramatically. In some regions the difference between high tide and low tide may only be a few inches. In other costal areas it may be as much as 40 or 50 feet!

Most tidal ranges obviously fall somewhere in between. The motion of that much water represents a tremendous amount of energy. Humans have harvested it to some extent for millennia, but haven’t tried to apply contemporary technology to it until recently. Current energy concerns have us looking once again at the tides as a power source. You can read more about the history of harnessing tidal power as well as current projects at other pages on this site.


Tidal Power


Solar Energy

Wind Power

Geothermal Energy

Biomass Energy

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