“Moon, Turn the Tides” is one of the more evocative selections on an appropriately titled album, Electric Ladyland, by Jimi Hendrix. The selection and the album title are appropriate as the moon does turn the tides, and the tides can be used to generate electrical energy.
Tidal energy is one of the rare exceptions to the rule that all the exploitable energy sources on earth are directly or indirectly solar power. Fossil fuels are solar power stored via photosynthesis and collected over millions of years. Wind and hydropower are also derived from solar power. Modern architects incorporate passive solar design into homes in the same way that the American Indians of the Southwest chose south or southeast facing cliffs to exploit passive solar energy input.
The tides are unique in that they are driven by the gravitational pull of the moon on large bodies of water. Smaller bodies of water such lakes or even a glass of water will be impacted by tidal forces but the effects are ridiculously small. In locations where local geography allows, the tidal movement of water can be exploited as an energy source to generate electrical energy.
Two methods can be used to capture tidal power, turbines placed in the path of a rising and falling tide and tidal lagoons which are natural areas where a barrage (a small dam) can create a lagoon from the tidal flow. Release of the impounded water then turns turbines. Turbines that utilize the flow of the tide are simpler to operate but only produce power when the tides are moving whereas tidal lagoons are more complex but allow for time shifting to release the water, better matching energy supply with demand.
Because of the unique set of circumstances required for the exploitation of tides, capacity is limited. In North America the Bay of Fundy in Nova Scotia, Canada has a tidal lagoon with a capacity of 18 megawatts. This relatively small power production site is meant mainly as a test facility to examine turbine design and environmental effects of tidal power
The largest currently operating tidal power project is the Rance Tidal Power Station in Brittany, France. This is a 240 megawatt facility. Much larger facilities are in construction or planned around the world. In Great Britain, the Severn Barrage is planned to yield 8.66 gigawatts. Estimates suggests that twenty per cent of Great Britain’s electrical energy needs can be met through tidal power.
The largest project being considered is a huge barrage, covering some 20,000 square kilometers in Penzhinskaya Bay, Russia. If this facility comes to fruition, it will have a peak power of 87 gigawatts, equal to the output of over 50 modern coal fired power plants.