Tidal power is one of the most predictable of renewable energy sources (although it is still intermittent due to the tides coming in and out approximately twice per day). Tidal flows are relatively well understood in terms of their daily, monthly and yearly variations, and have the potential to yield large amounts of energy. There are strong tides around the UK coasts, with the Severn estuary having the second largest tidal range in the world.

There are two types of concepts to capture tidal energy: tidal stream and tidal range. Tidal stream devices extract the kinetic (ie moving) energy of a tidal flow and convert it into electricity. The vast majority of the devices being developed for tidal stream use a similar concept to a wind turbine, but underwater. Tidal range projects take advantage of the potential energy stored in a head of water (ie a difference in water level) and generate electricity by releasing this water through turbines almost identical to those used in low head hydro plants.

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