Gravity from the moon and sun cause water in the ocean to bulge in a cyclical pattern as the Earth rotates, causing water to rise and fall relative to the land in what are known as tides. Land constrictions such as straits or inlets can create high velocities at specific sites, which can be captured with the use of devices such as turbines. Since seawater is about 800 times denser than air, tidal turbines can collect energy with slower water currents and smaller turbines than wind energy. While tidal currents are very predictable, challenges arise due to the need for devices to collect flow from opposite directions and survive the harsh corrosive marine environment. Power may also be produced by extracting potential energy from the rise and fall of the tides in a manner similar to conventional hydropower.
Most tidal energy converters can be classed into several technology archetypes: axial flow turbine, cross flow turbine, reciprocating device, tidal kite, Archimedes screw, tidal lagoon, and tidal barrage.
Tidal power has been used for centuries to produce mechanical power from paddle wheels to mill grain. In 1966, the first commercial scale tidal energy plant was installed in the estuary of the Rance River in Brittany, France. It uses tidal barrage technology in which a dam-like structure is placed across the tidal flow, allowing water to flow into a bay from the sea during a flood tide. During ebb tide, sluice gates are shut, and water flow is diverted through turbines to generate power. While it is possible to generate power during a flood tide, it is much less efficient.
Modern tidal power generating turbines operate on the same principles as wind turbines. As the moving water passes the current turbine’s blades, the current’s kinetic energy is converted into mechanical energy as the rotating blades spin a drive shaft. The mechanical energy in the drive shaft is then converted to electrical energy using a generator, often through a gearbox. There have been many pre-commercial tidal turbine deployments in North America, Europe and Asia, but these have been limited to single units or small arrays. Some examples include Verdant Power’s East River installation in New York City and Atlantis Resources installation in Pentland Firth, Scotland (Rooney et al. 2013; MeyGen 2017).
Tidal turbines typically operate in restricted passages that limit the height and width of the turbines. The following two primary archetypes have been developed that have direct analogues in wind energy.
These turbines are the most similar to traditional wind turbines, where the kinetic energy of moving water is captured by spinning blades facing the direction of flow. Turbines can be open or ducted (shrouded) and placed anywhere in the water column, though bottom-mounted is the most common. Turbines may use active or passive measures to yaw or vane in the direction of flow. They can have pitching blades allowing them to change their hydrodynamic performance based on flow conditions. As for wind turbines, a number of different electrical generation options may be selected for electricity production, from permanent magnet machines to induction generators directly tied to the grid.
These turbines capture kinetic energy of moving water with spinning blades oriented perpendicular to the direction of flow. They can be mounted in either vertical or horizontal orientations. When mounted vertically, these devices can operate regardless of the direction of flow. They typically have cylindrical cross-sections amenable to placement in confined channels or allowing tight array spacing. Turbines can be open or ducted (shrouded) and placed anywhere in the water column, though bottom-mounted is the most common. The electricity production mechanism is similar to axial-flow turbines.
Reciprocating devices do not have rotating blades, but instead have one or more hydrofoils that translate perpendicular to the flow direction by lift or drag. They may be oriented horizontally or vertically, though like axial-flow turbines, must face the direction of flow for maximum energy extraction. Oscillating devices are the most common form of reciprocating devices. Linear motion of the foils may be converted to rotary motion for electricity generation, or linear generators may be used.
A tidal kite is comprised of a hydrodynamic wing, with a turbine attached, tethered by a cable to a fixed point that leverages flow to lift the wing. As the kite 'flies' loops through the water, the speed increases around the turbine, allowing more energy extraction for slower currents. The kite is neutrally buoyant so as not to fall as the tide changes direction. Electricity production is by means of a generator coupled to the turbine. Power is transferred through a cable coupled to or as part of the tether.
Historically designed to efficiently transfer water up a tube, an Archimedes screw is a helical surface surrounding a ventral cylindrical shaft. Energy is generated as water flow moves up the spiral and rotates the device. The slow rotation implies coupling to a generator through a gearbox.
Tidal lagoons are comprised of retaining walls embedded with reversible low-head turbines that enclose a large reservoir. The rise and fall of tides cause a difference in the water height inside and outside of the reservoir, where a pressure differential drives a turbine, similar to a conventional low-head hydro dam that works in both direction. Electricity production is achieved by coupling generators to the turbines.
Tidal barrages are dams built across the entrance to a bay or estuary that capture potential tidal energy, similar to tidal lagoons. Energy is collected when the height difference on either side of the dam is greatest, at low or high tide. A minimum height fluctuation of 5 meters (16.4 feet) is thought to be required to justify the construction, so only 40 locations worldwide have been identified as feasible. Similar to low-head hydro, electricity production is achieved by coupling generators to the turbines.
Tidal energy technologies remain at the pre-commercial prototype level but are somewhat more advanced than wave technologies. In the U.K., the first commercial scale project is under development in the Pentland Firth, with 269 turbines and a total installed capacity of 398 MW. The first Pentland Firth turbine is expected to be deployed in 2020 (MeyGen 2011; Power Technology 2017). Deployments of pre-commercial systems in the U.S. have been limited to tidal flows in the East River in New York, and Cobscook Bay, Maine. Much like the first wind turbines, tidal technologies need to overcome failures in early designs to increase turbine lifespan. With experience, better design tools and validation, more robust designs can be developed that increase turbine reliability. Accessibility is also an issue in tidal flows as slack tides are short and offer limited windows for installation and access to the turbines.
The cost of tidal turbines is driven by many factors including the size of the machines, the number of machines, and the availability of quality resource. Early estimates indicate that a 10 MW array could have an LCOE of approximately $0.22/kWh and a 100 MW array could have an LCOE of around $0.15/kWh. However, these costs can only be achieved with sufficient industry experience. It is estimated that over 1,000 MW of installed tidal power is needed to gain this experience, which is likely more than a decade away (IEA-OES 2015; Neary et al. 2014).