Tidal Stream

The worlds oceans and seas are in constant motion creating currents that circumnavigate the globe to local streams caused by the ever moving tide. Tidal streams occur when a body of sea water is moving from one area to another and has to pass through a narrowing gap. As the mass of water is forced into this neck, usually formed by two neighbouring areas of land, the speed of all that water has to accelerate. It is in these areas of accelerated water where there is a lot of energy which can be tapped to produce electricity.

When we talk about tidal stream energy, it is about using the power contained within these naturally occurring tidal currents. Tides and currents are formed by a combination of the Earth's rotation, gravitational pull of the moon, weather conditions, land geography and temperature differences of the water. Ocean currents can be deep sea, slow moving affairs which may travel less than one kilometre an hour up to surging tide streams which pass between land masses and travel over 15 kilometres an hour.

Tidal stream is one of the newer forms of renewable energy. Tidal barrages have been around producing energy (not necessarily electricity) for hundreds of years and wave power has been in existence for several decades. Tidal stream has really only got going in the last ten years and the technology is moving forward at a fast pace.

When the sea's currents flow they produce a stream much like a river. Within these flows are placed devices which can catch the power of the moving water, a windmill type structure being the current favourite design. These water turbines act in the same fashion as wind turbines do on land only instead of the air driving them it is sea water. Tidal streams tend to move slower than the wind so water turbines rarely rotate faster. At first thought you might feel they will therefore produce less electricity. Water has an advantage though, it is much denser which means there is more weight pushing on a water turbine blade.

It is after all, much easier to push your hand through the air than water (water is 832 times denser than air). This effect allows water turbines to turn much slower yet out perform wind turbines. Because there is more weight loaded onto the blade of a water turbine, although the rotation is slower, that extra weight provides more torque which can in turn gives more power to produce electricity.

Underwater turbines also have little environmental impact. The rotation of the blade is slow enough to avoid damaging sealife and excepting a small footprint where they are attached to the sea bed, have little intrusion to their surroundings. They are also out of vision which helps to avoid upsetting the vista for some. They are deep enough down in the water to avoid colliding with ships though fishing nets and lines may be a hazard. Submarine owners will also have to be wary!

The cost of installing a tidal turbine is quite costly. The actual structure in itself is no more costly than a wind turbine but the expense of locating it on the sea floor and the cabling needed to transfer the electricity back to land are big expenses. One cost saving though which is often overlooked is that the permissions required to site a tidal turbine are considerably less than a land sited generator and once the sea turbine is in place, apart from general maintenance, the cost to run is very low. As the energy required to drive the turbine is free from the flow of the tide, there is no cost to provide fuel power it.