Some of the oldest ocean energy technologies use tidal power. All coastal areas experience two high tides and two low tides over a period of slightly more than 24 hours. For those tidal differences to be harnessed into electricity, the difference between high and low tides must be more than 16 feet (or at least 5 meters). However, there are only about 40 sites on Earth with tidal ranges of this magnitude.
Currently, there are no tidal power plants in the United States, but conditions are good for tidal power generation in the Pacific Northwest and the Atlantic Northeast regions.
Tidal Energy Technologies
Tidal energy technologies include barrages or dams, tidal fences, and tidal turbines.
Barrages or Dams
A barrage or dam is typically used to convert tidal energy into electricity by forcing water through turbines, which activate a generator. Gates and turbines are installed along the dam. When the tides produce an adequate difference in the level of water on opposite sides of the dam, the gates are opened. The water then flows through the turbines. The turbines turn an electric generator to produce electricity.
Tidal fences look like giant turnstiles. They can reach across channels between small islands or across straits between the mainland and an island. The turnstiles spin via tidal currents typical of coastal waters. Some of these currents run at 5–8 knots (5.6–9 miles per hour) and generate as much energy as winds of much higher velocity. Because seawater has a much higher density than air, ocean currents carry significantly more energy than air currents (wind).
Tidal turbines look like wind turbines. They are arrayed underwater in rows, as in some wind farms. The turbines function best where coastal currents run between 3.6 and 4.9 knots (4 and 5.5 mph). In currents of that speed, a 49.2-foot (15-meter) diameter tidal turbine can generate as much energy as a 197-foot (60-meter) diameter wind turbine. Ideal locations for tidal turbine farms are close to shore in water 65.5–98.5 feet (20–30 meters) deep.
Environmental and Economic Challenges
Tidal power plants that dam estuaries can impede sea life migration, and silt build-ups behind such facilities can affect local ecosystems. Tidal fences may also disturb sea life migration. Newly developed tidal turbines ultimately may prove to be the least environmentally damaging of the tidal power technologies because they do not block migratory paths.
It does not cost much to operate tidal power plants, but their construction costs are high, which lengthens payback periods. As a result, the cost per kilowatt-hour of tidal power is not competitive with conventional fossil fuel power.