Tidal power makes use of the compound gravitational effects of the moon travelling around the Earth and the Earth travelling around the sun, raising and lowering the water level and generating strong currents with very high-power density compared to air. There are four main types of tidal power systems: - Tidal barrages make use of strong tidal flows in existing natural estuaries to create temporary hydropower dams. Effectively, the estuary is blocked with a dam that lets seawater in, the dam is closed, and the seawater is let out through a turbine. The power generated is a function of the volume of water and the height that the water falls. - Tidal stream generators are more akin to wind turbines than tidal barrages, which are more similar to hydropower dams. They sit “in-stream” in the tide without holding it back, generating power using different types of reciprocating device, for example, a turbine or reciprocating flap. The reciprocating action moves slowly enough that in theory it should not disturb passing aquatic life. - Dynamic tidal power is a newer concept that takes advantage of differential tide states along a coast to create pressure or head differences from which power can be generated. Discrete differential tide states are created by long barrages (~30km) that extend into the ocean without entrapping any water bodies, in theory preserving the existing dynamic tidal ecosystems. - Tidal lagoons are simple concepts in that effectively an artificial encircled barrage is built in open water. Existing ecosystems are not affected. Only very small tidal lagoons have been piloted to date. The potential contribution of tidal power to climate mitigation efforts is highly localized to maritime regions. No new large tidal barrages are likely to be allowed for environmental reasons.