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. As seawater is about 800 times denser than air, tidal turbines can collect energy with slower water currents and smaller turbines than wind energy. Modern tidal power generating turbines operate on the same principles as wind turbines. While the moving water passes the turbine’s blades, the kinetic energy of moving water 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. Power may also be produced by extracting potential energy from the rise and fall of the tides in a manner similar to conventional hydropower.
Axial Flow Turbine
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 or control settings.
The main environmental concern is collision between turbine blades and marine organisms due to natural animal movements, attraction to the device, or inability to avoid the turbines within strong currents. There is also concern that noise from turbines can affect animals that use sound for communication, social interaction, orientation, predation, and evasion. As with all electricity generation, there is a slight concern that electromagnetic fields generated by power cables and moving parts of the turbines may affect animals that use Earth's natural magnetic field for orientation, navigation, and hunting. Likewise, chemicals such as anti-corrosion paint and small amounts of oil and grease may enter the waterbody during spills, though some turbine designs do not require lubrication, and affect water quality. Large-scale tidal changes in flow (from arrays) may disrupt natural physical systems to cause degradation in water quality or changes in sediment transport, potentially affecting ecosystem processes.
Photo Credit: BALAO-SABELLA
Cross Flow Turbine
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.
There is typically less environmental concern for collision between turbine blades and marine organisms because, depending on the design, blades are spinning in the same direction to the flow of water. Concerns about noise, electromagnetic fields, changes in flow, and impacts on water quality are similar to that of axial flow turbines.
Photo Credit: Ocean Renewable Power Company (ORPC)
Reciprocating devices do not have rotating components and instead have a hydrofoil that is pushed back and forth transverse to the flow direction by lift or drag. Oscillating devices are the most common form of reciprocating devices. Oscillating hydrofoils operate via passive or active manipulation of one or more foils to induce hydrodynamic lift and drag forces due to pressure differences on the foils. They may be oriented horizontally or vertically, though like axial-flow turbines, they must face the direction of flow for maximum energy extraction. Linear motion of the foils may be converted to rotary motion for electricity generation, or linear generators may be used.
Reciprocating devices often move slower than turbines, but move more freely in the water, resulting in some concern for collision. Depending on the design and generator, reciprocating devices often produce little noise. Concerns about electromagnetic fields, impacts on water quality, and changes in flow are similar to that of other tidal devices.
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.
Collision risk may be of some concern with tidal kites. Although animals are more likely to collide with the tether than the kite itself, little is known about the ability of animals to detect the free movement of some tidal kites. Tidal kites can emit noise over a larger frequency than horizontal axis turbines depending on the design and generator. Concerns about electromagnetic fields, impacts on water quality, and changes in flow are similar to that of other tidal devices.
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.
The helical turbine moves very slowly relative to other tidal technologies and is likely to have little collision risk. Archimedes screws often produce little noise, though this depends on the design and generator. Concerns about electromagnetic fields, impacts on water quality, and changes in flow are similar to that of other tidal devices.
Tidal lagoons are comprised of retaining walls embedded with low-head turbines that surround a large reservoir of water. Functioning similar to a hydroelectric dam, tides cause a difference in the water height inside and outside the walls of tidal lagoons. The ecosystem within the reservoir undergoes significant transformation, potentially yielding positive impacts with a more diverse seabed, depending on site selection.
Tidal barrages capture water in a holding area, making use of the difference in water height from one side of the barrage to the other. Water is then released through a large turbine or turbines as it flows out with the ebb of the tide. They are typically built across the entrance to a bay or estuary and generate electricity using the difference in water height inside and outside of the structure. A minimum height fluctuation of 5 meters (16.4 feet) is typically required to justify the construction of tidal barrages, so only 40 locations worldwide have been identified as feasible.
Installing a tidal barrage impacts bay or estuary ecosystems due to changes in flow and can have negative effects such as changing the shoreline and important tidal flats. Inhibiting the flow of water in and out of the bay, may also lead to less flushing of the bay or estuary, altering the water quality, and potentially causing additional turbidity (suspended solids) and less saltwater, which may result in the death of fish that act as a vital food source to birds and marine mammals. Migrating fish may also be unable to access breeding streams, and may attempt to pass through the turbines and risk collision. Impacts from noise depend on turbine selection, similar to tidal lagoons. Decreasing shipping accessibility can become a major socio-economic issue, though locks can be added to allow slow passage. However, the barrage may improve the local economy by increasing land access when used as a bridge and allowing for more recreation and tourism opportunities due to calmer waters.