Tidal Energy

Tides are caused by the gravitational pull of the moon and the sun acting on the oceans of the rotating earth. This causes the regular and periodical raising and lowering of ocean surfaces, which produces the strong tidal streams or currents that are prevalent around Scotland.

The strength of the tidal current varies depending on the position of the site on the earth, the shape of the coastline, and the bathymetry (shape of the sea bed). Areas that have high tidal currents are in narrow straits, between islands, around headlands, entrances to lochs, bays and large harbours. This flow is cyclical, first increasing in velocity and then decreasing before switching to the opposite direction.

The generation of energy from tidal currents is therefore very site specific. The World Offshore Renewable Energy Report 2002-2007 , released by the DTI, suggests that while a staggering 3000GW of tidal energy is estimated to be available, less than 3% of this is located in areas suitable for power generation. A number of possible sites have been identified in Scotland. The Pentland Firth has been described as the Saudi Arabia of the world's future tidal industry, which is capable of providing up to 10% of the UK's energy demand alone. Scottish Enterprise has estimated that 34% of the UK's electricity demand could be generated by tidal currents.

However, tidal current energy converters are at an early stage of development, with only a small number of prototypes being tested to date and there are no commercial units operating with grid connection. The first permanent offshore tidal energy turbine to generate electricity was launched in 2003 by Marine Current Turbines Ltd.

This tidal energy turbine uses the same principles of operation found in wind turbine but under water, comprising of rotor blades and a generator. They use the kinetic energy of the moving water and transfer it into useful rotational energy and electricity. The turbines will have rotor blades with variable pitch so that they are bidirectional, which means that electricity can be generated from both flow directions. The turbines are smaller than wind turbines yet they can produce the same power, because a tidal current has four times the energy density of a good wind site. The rotor blade turns slowly, between 10 to 20 rpm, so the technology does not pose any serious threat to fish or marine mammals. In comparison a ship propeller runs about 10 times faster.

© Marine Current Turbines Ltd

Under water turbines need to have much sturdier structural components than wind turbines, because of the higher forces acting on the turbine due to low flow rates and higher density of water. This may increase the capital costs of the turbines but this should be offset by a higher energy yield.                                                                                         

As there are no commercial turbines currently in operation, it is difficult to estimate the cost of energy and competitiveness with other energy sources. At the moment the costs of installing tidal current turbines are high but as the technology matures and it achieves economies of scale, it is likely that the costs will reduce significantly.

Tidal current energy is predicable both in amount and timing, making it more attractive than other forms of renewables. Unlike wind, solar or wave energy, it is not affected by weather conditions. It also has less visual impact and is unobtrusive. Tidal current energy has the potential for exploitation to equal or exceed wind energy in its importance.