What is tidal energy?
Tidal energy is a form of hydropower that harnesses the movement of the ocean’s tides, using turbines to produce large amounts of clean, renewable energy.
Where do tides come from?
Tides are caused by the moon’s gravity pulling on the ocean. The Sun’s gravity also has an effect on tides, however it is much further away from Earth and so its effect on the tides in much less despite its size.
The largest tides are called ‘spring tides’ and happen when the Sun and the Moon are lined up in a row. This happens every two weeks during a new moon or a full moon.
Small tides are called ‘neap tides’ and they happen when the sun and moon form a right angle to each other. This causes the sun and moon to pull water in two different directions. Neap tides happen during a quarter or three-quarter moon.
So how do you generate power from tidal energy?
There are two main types of tidal power: tidal stream and tidal range.
Tidal stream
Tidal streams are currents in the sea that flow as the tide moves in and out. Tidal stream turbines are similar to wind turbines but use fast moving tidal streams instead of the wind to generate electricity. Sea water is 832 times denser than air and so a 5 knot ocean current has more kinetic energy than a 350 km/h wind. As a result, ocean currents have a very high energy density and therefore tidal stream turbines tend to be smaller in size than wind turbines. Electricity is generated by the turning of a turbine and is brought to shore by a cable.
An example of a tidal stream project is Atlantis Resources’ MeyGen scheme in Scotland, intended to become the largest tidal stream project in the world. Following the deployment of a demonstration array of up to six turbines, a maximum of 86 turbines (86MW) will be deployed in phase one, with the project ultimately reaching up to 398MW of tidal stream capacity.
Tidal range
The difference between high tide and low tide is known as tidal range. The range in the Severn Estuary can be over 15 metres making it the second largest in the world.
Until recently, the only way electricity has been harnessed from tidal range is through construction of tidal barrages containing turbines across the mouth of estuaries that have large tides. The first large tidal barrage to be built was La Rance in Brittany, France. This project began producing electricity in 1966 and is currently the second largest tidal power station in the world.
Tidal lagoons work in a similar way to barrages by capturing a large volume of water behind a man-made structure, which is then released to drive turbines and generate electricity. Unlike a barrage, where the structure spans an entire river estuary in a straight line, a tidal lagoon encloses an area of coastline with a high tidal range behind a breakwater, with a footprint carefully designed for the local environment.