Previous work into the extraction and utilisation of power from tides has centred on large barrier type system, such as that frequently proposed for the Severn Estuary. In this type of scheme, water is trapped at high tide and then released through low head turbines when the tide has fallen, converting potential energy given to the water by the tides. A summary of the various methods and strategies for optimum energy extraction using this type of scheme, along with major existing and proposed sites worldwide is available . In tidal stream energy projects, however, kinetic energy is extracted from the mass of water as it moves naturally between high and low tides, eliminating the need to construct barrages. Relative to both wave energy and tidal energy, tidal stream energy is quite a young technology with comparatively little research completed to date. This is due, in part, to the conclusions of a 1993 report which acknowledged the large resource available to the UK yet concluded that the cost of capture was prohibitively high, in the region of 17 - 39 p/kWh for the two most promising sites in the UK . That report, funded by the Energy Technology Support Unit on behalf of the Department of Energy, led to the withdrawal of support from the UK Renewable Energy Programme for any tidal stream energy proposal. Several other independent organisations continued to be interested and research programmes continued. Their judgement was ratified by the conclusion of a more recent Department of Trade and Industry report which gave a cost estimate for the same site as 4.56 p/kWh . This estimate, with an 8% discount rate over 29 years, has again renewed interest and government support from the renewable energy program for this energy source.
Based on sites with a mean spring tide greater than 2ms-1, it has been estimated that 10 % of the UK's electricity capacity may be provided by expected recovery rates of energy from tidal streams .
This condensed form of energy is proportional to the cube of the water velocity, for example 1.89kWm-2 in a 1.5 ms-1 flow. The theoretical maximum energy which can be extracted from a tidal stream is known as the Betz limit and equal to 59% of that available.
A crucial advantage of tidal stream energy over many other renewable resources is the predictable nature of its output, allowing accurate predictions of its annual yield and giving utilities ample warning of periods of low energy (i.e. neap tides).
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Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.