Tidal energy is presently one of the more favoured forms of marine renewable energy because, due to its origins in (astronomical) tide generating forces, it is predictable. This is in contrast to other more stochastic renewable energy sources such as wind and wave and, in part, solar. However, regardless of its predictability, tidal energy shares a key feature with the majority of renewable energy sources—it is intermittent, from diurnal (once per day) and semidiurnal (twice daily), to fortnightly (spring-neap) timescales. In this chapter, we explain the origin of the tides, and how tides evolve as they propagate over shelf sea regions. It covers methods of analysing and predicting the tides, and how tides can be used to generate electricity through arrays of tidal stream devices, and tidal range schemes (lagoons). The primary objective of this chapter is to equip those working within or researching marine renewable energy with an understanding of the fundamentals of tidal energy from both oceanographic and engineering perspectives.

Source: Encyclopedia Britannica

Tidal energy, also called tidal power, any form of renewable energy in which tidal action in the oceans is converted to electric power.

Types
There are a number of ways in which tidal power can be harnessed. Tidal barrage power systems take advantage of differences between high tides and low tides by using a “barrage,” or type of dam, to block receding water during ebb periods. At low tide, water behind the barrage is released, and the water passes through a turbine that generates electricity.
Tidal stream power systems take advantage of ocean currents to drive turbines, particularly in areas around islands or coasts where these currents are fast. They can be installed as tidal fences—where turbines are stretched across a channel—or as tidal turbines, which resemble underwater wind turbines.

Electricity Generation Potential
Many tidal power technologies are not available at an industrial scale, and thus tidal energy contributes a negligible fraction of global energy today. There is, however, a large potential for its use, because much usable energy is contained in water currents. The total energy contained in tides worldwide is 3,000 gigawatts (GW; billion watts), though estimates of how much of that energy is available for power generation by tidal barrages are between 120 and 400 GW, depending on the location and the potential for conversion. By comparison, a typical new coal-based generating plant produces about 550 megawatts (MW; million watts). Although total global electricity consumption approached 21,000 terawatt-hours in 2016 (one terawatt [TW] = one trillion watts), energy experts speculate that fully built-out tidal power systems could supply much of this demand in the future. Estimates of tidal stream power—which uses ocean currents to drive underwater blades in a manner similar to wind power generation—in shallow water is capable of generating some 3,800 terawatt-hours per year.

By the early 21st century, some of these technologies had become commercially available. The largest tidal power station in the world is the Sihwa Lake Tidal Power Station in South Korea, which generates 254 MW of electricity. A tidal barrage power station at La Rance in France has been operating since the 1960s, with 240 MW of capacity; its typical output is 0.5 terawatt-hour per year. Larger electricity generation efforts are on the horizon; for example, the first phase of the MeyGen project in Scotland’s Inner Sound generated 700 megawatt-hours of electricity in August 2017. Environmental concerns raised about tidal power stations are largely focused on the tidal barrage systems, which can disrupt estuarine ecosystems during their construction and operation. Tidal fences and turbines are expected to have minimal impact on ocean ecosystems. Tidal fences do have the potential to injure or kill migratory fish, however, but these structures can be designed to minimize such effects.