Tidal energy uses the natural ebb and flow of coastal tidal waters, which are caused mainly by the interaction of the gravitational fields of the Earth, moon and sun. The coastal water level changes twice daily, filling and emptying natural basins along the shoreline. The currents flowing in and out of these basins can be used to turn mechanical devices to produce electricity.
One type of tidal energy is tidal stream (or marine current) technology. This uses the fast sea currents, which are created by the tides and often magnified by topographical features such as headlands, inlets and straits or by the shape of the seabed when water is forced through narrow channels.
Tidal range technology is another type of technology that converts tidal energy into electricity in a very similar way to that used in traditional hydroelectric power plants. Gates and turbines are installed along a dam or ‘barrage’ that goes across a tidal bay or estuary. When there is an adequate difference in the height of water on either side of the dam, the gates are opened and the ‘hydrostatic head’ that is created causes water to flow through the turbines, turning a generator to produce electricity.
Electricity can be generated by water flowing both into and out of a bay. As there are two high and two low tides each day, electrical generation from tidal power plants goes through periods of maximum generation every six hours.
Another method is for turbines to be used as pumps, to push extra water into the basin behind the barrage during periods of low electricity demand. This water can then be released when demand on the system is at its greatest.
The UK tidal energy resource
As with wave power, tidal and current stream energies are predictable and consistent. It is estimated the UK has around 50% of Europe’s tidal energy resource and a study in 2004 estimated our technical resource is at around 16 TWh/year (4% of supply). Around 64% of this is located in the remote north of Scotland and 20% is in the Channel Islands (which are not part of the UK and are not connected to the UK grid). The majority of the resource is therefore remote from centres of UK demand.
The needs of other sea-users, such as those for fishing and navigation, must be considered when developing tidal energy – tidal currents tend to be near headlands and in channels between islands, which are often busy shipping routes. Such constraints could mean the practical resource is smaller than estimated.
To produce practical amounts of electricity, a difference between high and low tides of at least five metres is required.
Protocols
A number of protocols have been produced on the development and performance of tidal energy devices. These aim to promote industry best practices and enable consistent assessment of the performance of different devices. Read details of these protocols below:
Preliminary Tidal Current Energy Device Performance Protocol
An important objective of the MRDF Wave and Tidal-stream Energy Demonstration Scheme is to produce clear, consistent and meaningful assessments of the performance of tidal devices.
This will enable the Government, industry and the finance and investment community to make informed judgements of the commercial prospects of the technologies being demonstrated.
This is an explicit written protocol on how to assess performance, to ensure consistency.
Assessment of performance for tidal energy conversion systems
This standard is intended to provide a uniform methodology, to ensure consistency and accuracy in measuring and analysing the power performance of tidal energy conversion systems.
Tidal Current Energy Device and Evaluation Protocol
This is a protocol to manage the development of tidal current energy devices, from concept to full-scale demonstration. It aims to support the cost-effective management of research and development in tidal current power technology, while allowing device developers to benefit from the confidence generated by using commonly accepted measures of performance.