Tidal Power: From Grinding Grain to Fifteen Gigawatts
Tidal Power has been in the news this month, but for at least 1500 years we have been harnessing the power of the ocean’s tides to improve our lives. Since the Middle Ages, tide mills have been used to grind grain, and beginning in 1966, we have been using tidal generators to generate renewable electricity.
Excitingly, recent developments have greatly increased this technology’s viability. For instance, the UK’s energy minister estimates that tidal power has the potential to meet 20% of Britain’s electricity needs. Despite this promising future however, tidal power has not enjoyed the same attention as other renewable energy types.
Tidal power is the only form of energy production that draws power from the orbital relationships of the earth, moon and sun. These gravitational forces cause our oceans to swell daily. Some places on earth receive two high tides and two low tides, some just one high and one low tide—or in some places, a mix of uneven tides. In any case, tides are predictable and unfailingly regular, making them more reliable than other sources of renewable energy, such as wind and sun.
Currently, there are two forms of tidal power in use at sites around the world. The first is known as a tidal barrage, utilizing what is essentially a dam to allow the waters of a high tide into an inlet using sluice gates. When the tide recedes, the high water level trapped in the inlet by the dam flows out through a series of turbines, which power electricity generators. The first ever tidal barrage, built in 1966 in Brittany, France, continues to power 240,000 homes.
The second form, known as tidal stream generators, work in a very similar way to wind turbines. The process draws power from the kinetic energy of moving water to create electricity. Interestingly, due to the high density of water in comparison to air, water moving at relatively slow speeds through a tidal stream generator can produce equal, or greater amounts of power than a similar sized wind turbine. Projects utilizing this form of tidal power generation are currently being constructed in Maine and Scotland.
While the current forms are promising, the true potential of tidal power lies in the future, with a concept known as Dynamic Tidal Power. DTP is a very large scale operation, involving a T shaped, dam-like structure (at least 15 miles long) built out from land in areas of the world where tides run parallel with the shoreline. This structure would create a difference in water level on either side of the dam, allowing for bi-directional turbines to generate energy during both rising and falling tides. The first proposed DTP facility, if built, will have an installed capacity for 15 gigawatts, and the ability to supply electricity to more than 3 million people. The graphic below shows how a DTP dam slows down a tidal change and creates a significant difference in water levels on either side. This allows turbines capture the the potential energy of the higher water.