Oceans are noticeably influenced by the
gravitational effect of the sun and moon on the earth. The moon has the strongest influence, which is itself strongest on the side facing it and weakest on the opposite side of the earth. This, combined with the weaker effect of the sun, creates a gravitational field with a differential forcing that causes sea levels to rise and fall around the coastlines of the world. Rising and falling sea levels create a tidal cycle of around 12.5 hours, which is dictated by the orbital period of the moon about the earth and the earths own daily rotation. The tidal range describes the variation in the sea levels as they go through a full tidal cycle. The maximum range is known as a spring tide, which coincides with a full or new moon when the sun and moon are working together to create a strong gravitational pull on the earth. Conversely, the minimum range is known as a neap tide and occurs when the sun and moon are working against each other.
Around the world there exist a number of sites where these tidal patterns produce large local velocities as currents of water force their way through narrow passages between land masses. Tidal cycles are very well predicted making these local velocities easy to track and quantify, presenting a large opportunity for energy extraction by marine current devices. It is estimated in The World Offshore Renewable Energy Report 2002-2007 by the Department of Trade and Industry that there is potentially 3000GW of tidal power available for extraction around the world; however, less than 3% of that resource is located in areas suitable for power extraction. As a result, there are only a few locations around the world which are currently being considered as potential sites for deployment of marine current devices.
The UK has the largest tidal resource in Europe, with 42 sites being considered for large scale energy extraction. Scottish Enterprise has suggested that 34% of the UK’s electrical demand could be met by power generated from tidal currents. Sites are also being considered in other regions of the world including the Straits of Messina in the Mediterranean. Most importantly, marine currents are a renewable source of energy. Realising their potential could make a significant contribution to our efforts to reduce our dependence on fossil fuels and reduce carbon emissions. An additional benefit is that unlike conventional forms of renewable energy such as wind and solar, tidal flows are very well predicted, enabling more efficient grid management, therefore reducing the power demand from the extraction devices.
