Background

What is tidal stream energy?

A description of tidal stream energy and its characteristics can be found on the British Wind Energy Association website.

Tidal Stream is a hitherto unexploited form of renewable energy that lies in moving bodies of oceanic and estuarial waters. The source of the energy is the interaction between the gravitational fields and relative motions of the sun, moon and earth, which gives rise in many parts of the world to regular patterns of high-speed currents. The speed of these flows can be magnified by local topographical features such as headlands, inlets to inland lakes and straits.

The energy is derived from the kinetic energy of the moving flows using devices that superficially resemble wind turbines, and as such differs from tidal barrages which rely on providing a head of water behind a structure for energy extraction. Tidal stream energy converters extract and convert the mechanical energy in the current into electricity. The main components of a tidal stream energy converter are:

1. the prime mover which extracts the energy from the flow - a rotor of some sort;
2. the foundation which holds the prime mover in the flow and reacts the loads to the seabed;
3. the powertrain (i.e. gearbox & generator), and:
4. the power take-off system (power electrical and control system, and submarine cable to onshore grid connection point).

The power of the sea has been known since the earliest days of seafaring, however it is only recently with the development of offshore engineering technologies coinciding with the need to find significant new renewable energy resources, that tidal stream has become a technically feasible and economically viable possibility.

The advantages of tidal stream energy

The key advantages of tidal stream energy over other renewable forms of generation are:

• High energy intensity ⇒ smaller cheaper rotors for a given power
• Predictable energy capture ⇒ less project risk
• Energy to a timetable ⇒ greater revenue per MWh generated
• Low environmental impact ⇒ low development overheads
• Simple decommissioning ⇒ low back-end risk and cost

Offshore wind is often regarded as the comparator for other marine renewable technologies, because it is a relatively mature technology and there is sufficient resource to provide significant amounts of energy to contribute towards the UK's renewable targets. The advantages of tidal stream technology, however, make it highly cost effective: competitive with (and eventually better than) offshore wind:

• The resource has four times the energy intensity of a good wind site, so tidal turbines need only a quarter of the swept area of a wind turbine - and size has a significant impact on cost.
• Tidal devices can be closely spaced in a string across a flow, since the flow axis is constant and there is no need for wide spacing of the machines to avoid wake interference as there is in wind farms.
• Energy generated to a timetable is more valuable than energy from randomly intermittent and variable sources such as wind, wave and solar, and has a lower impact on the efficiency of displaced thermal generating plant.
• The regular pattern of the tides enables both the amount of energy, and the timetable to which it will be produced for years to come, to be determined with great accuracy, using data gathered over period of only one month. This compares with wind energy, where upwards of two years' meteorological data are required to statistically characterise a site wind regime, and the energy generated is intermittent and variable in a non-predictable manner. Having confidence in the energy production of a renewable installation overcomes one of the biggest hurdles in achieving project finance.
• The degree of over-engineering needed to cope with the "worst case" operating conditions is much lower than for wind or wave powered devices, as conditions under water, for fully submerged devices, even in the severest of weather, do not vary a great deal: you don't get storms and 70mph gusts underwater.

Tidal turbines also have small environmental and social impact:

• they are much less visually intrusive;TGL's machine is totally submerged, and will therefore cause no visual intrusion or bird disturbance;
• they have slow moving rotors (much slower than ship's propellers) and so will cause far less disturbance to sea mammals;
• they are passive energy absorbers, so will produce less underwater noise than ships of a similar power rating, which put energy into the water. Marine life disturbance is therefore minimised;
• TGL's machines maintain an operating blade tip clearance to Low Water of at least 10m; they therefore cause very limited disruption to shipping.

What causes tides?

The source of tidal stream energy is the interaction between the gravitational fields and relative motions of the sun, moon and earth, which gives rise in many parts of the world to regular patterns of currents. There are many good graphical presentations of the origin of tides on the web, including Understanding Tides by David A. Ross.

To understand tides fully, it is necessary to understand the complexities of the orbits of heavenly bodies and of the way water moves over the surface of the earth. For the mathematically inclined, the processes are very well described in the Admiralty Manual of Tides (UKHO NP120), which presents Doodson's classic harmonic analysis. This is an elegant example of a situation where great complexity arises from apparent simplicity: starting with a three planet system (earth-moon-sun), it transpires that over four hundred harmonic constituents are required to accurately model the resulting water movements at specific points on the surface of the earth, by the time all the "wobbles" in lunar orbit and terrestrial phenomena such as resonance of water volumes, estuarial and shallow water effects have been accounted for.

Doodson's analysis requires a steady head for trigonometry - to be honest, it's not generally the sort of book you would leave in the loo - but it nonetheless covers the fundamentals from first principles, and remains the basis of many modern tidal height and tidal stream prediction tools, including ones used worldwide by the British Admiralty.

The tidal stream resource

The basic criteria for cost-effective power generation from tidal streams using TGL's technology are a mean spring peak velocity exceeding about 2.5m/s (5 knots) with a depth of water of >30m. Some potential locations meeting these criteria are shown in the map.

Much effort has been put into tidal stream resource studies, particularly aroundUK and W. European waters. These have identified the existence of an enormous worldwide resource, around 55,000 MW of installed capacity; roughly equivalent to 75% of the total UK installed generating capacity (in 2005). Of this resource, around 25% is found in European waters, and 15% in and around UK waters. The UK is therefore in a unique position with regard to developing tidal stream energy.

In parallel with the B&V study, the DTI published a UK Atlas of Marine Renewable Energy Resources, prepared by ABPMer. This provides a useful picture of the tidal streams around the UK, based on extensive oceanographic modeling, calibrated against long-term measurements from numerous tide gauges around the UK. The figure, taken from the Atlas, shows the predicted mean spring peak current velocities around the UK.

Site-specific studies around UK waters have recently been prepared by Black & Veatch Consulting for the Carbon Trust under the Marine Energy Challenge programme. Their July 2005 report concluded:

"The UK has a significant domestic Tidal Stream resource, representing around half the European resource and around 10-15% of the known global resource. There is around 12TWh/yr of UK Tidal Stream Resource that could be economically exploited. Utilisation of this Economically Extractable UK Resource would require about 3000MW of installed capacity and represent over 3% of UK electricity demand. This suggests that the development of a UK tidal stream industry, through both technology and project development, would contribute meaningfully to UK electricity demand, and place the UK in a strong position to export that industry to other countries."

The report also concluded that in addition to the 12TWh/yr identified above as being immediately economically exploitable, there is a further significant resource that could become economically exploitable in the future, once the technology and necessary installation equipment have been further developed.

This level of resource would support a UK industry worth ∼£240m in average annual machine sales assuming it is exploited over a period of around 20 years. The export market for machines is larger in proportion to the resource, and would support a longer-term industry.

The B&V report breaks the UK resource down by water depth and current velocity - two important parameters in the performance of a tidal stream installation, because they respectively determine the energy capture of a machine and strongly influence the installation technique and the cost of maintenance. The breakdown by depth is shown in the figure:

The report's conclusions strongly support TGL's seabed-mounted deep water tidal stream concept:

"The resource distribution … suggests that UK technology development should be concentrated on devices that are suitable for sites of depth >40m, with the highest focus on devices that are suitable for deep sites with high velocities. There is also merit in developing devices that are suitable for the 30-40m depth range, especially where these may be capable of deployment into greater depths in the future. Although there is a reasonable resource at low speed sites (Vmsp<2.5m/s), it may be more difficult for devices to extract this economically. There appears to be little merit in focusing UK technology development on devices that are only suitable for sites of depth <30m."

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