The tides are the sun and moon’s handiwork, a result of orbital mechanics and Newtonian gravity; the changing position of the three celestial bodies involved determine the height of the seas. High tides (also known as spring tides) occur when all three are aligned, wile low tides (or neap tides) happen when a line between the sun, earth, and moon makes a right angle. In essence, the moon and sun are dragging water around the surface of the Earth. Technically, there is no such thing as a free lunch, and all that moving of water actually slows the earth’s rotation, thus increasing the length of day by about 2.4 milliseconds per century. (Co-incidentally, this is also why the moon is slowly getting farther away; since the earth’s rotation is getting slower, the moon’s orbital radius has to get larger to conserve angular momentum.)
Tidal power harnesses this energy to do work via exploiting either the kinetic or potential (height) energy of the water. Barrage power systems utilize the potential energy via containing the incoming tide behind a dam and releasing the water in a controlled fashion through turbines- not surprisingly, this method suffers from many of the negative environmental impacts associated with dams. A number of major installations were built in the mid- to late- 20th century, and all were based upon the same technology and design principles as more well-known hydroelectric systems. While the technology and construction methods are well-known, the number of natural locations that can host such a system are limited.
Tidal power systems that rely on kinetic energy work in essentially the same way as a windmill, using that kinetic energy to spin the turbine directly; the only difference is that the turbine s underwater.. Some designs involve shrouded fans, while others don’t- still, the principles at work are the same. While this method requires higher current velocities, the potential environmental impact is lower. (In theory, marine organisms such as fish are able to sense the pressure differential and avoid the generators or are pushed away from them by the current. Also, since the density of water is far greater than air, the velocity of the current and the rotational velocity of the turbine/propeller is far lower. ) Some new designs seek to mimic forms found in nature to improve performance and cope with extreme weather conditions.
One of the key advantages of this renewable energy is that the tides exist and do work (erosion of coastlines and the movement of organisms, to name two kinds) ragardless of whether we recover the energy or not- recapturing even a fraction of this energy could to a world of good.
Article two of this series will address the environmental ramifications of this technology.