Green World Trust
Tidal Power & slow turbines

Tidal power (introduction) -
featuring a horizontal turbine

Tidal energy is virtually a blank new field, scarcely utilised at all. Yet the energy potential is huge; and unlike wind and solar power, the tide times are known. So why has tidal energy not been utilised before now? Has it been tried and found to be a non-starter?

The answer is, no.

Historically we have managed quite well enough using fire, air, water, animals, fossil fuels and nuclear energy. But now we are being challenged to find sustainable sources of energy that are more powerful than the pre-industrial ones. A fantastic amount of energy is involved in the ebb and flow of the tides around the UK, in particular the Severn estuary with up to 40 feet tidal range. However, a conventional barrage there would have so many problems that it makes any barrage proposal a non-starter. The worst problems would be the low intermittency of delivered power (around 4 hours in 24), an increased risk of flooding, the silting, the difficulties presented to shipping, substantial changes in rare and protected habitats - and local disaffection. So we are discounting Severn barrages. But there are many other options. Harnessing tidal power is a new challenge, calling for new technology, new ideas, new forms of turbines - and support for research and development.

We see five main possibilities for harnessing tidal power at present.

(1) The Tidal Reef proposal for the Severn estuary (Rupert Armstrong-Evans): a comparatively low barrier that can rise and fall with the tide, using turbines for energy generation on both ebb and flow with only 2 metres difference in sea levels. Atkins Engineering have given it a basic thumbs-up on all counts, and estimates that it would generate considerably more electricity than a barrage; moreover it would operate most of the time rather than lying idle as would a barrage.

(2) Double lagoons or "coffer dams". These can be built completely offshore so that neither shipping nor habitats nor beaches are disturbed. They collect water at high tide, and "space without water" at low tide, and can be set to drive turbines by emptying or by filling, at chosen times. In themselves, lagoons are uncompetitive, but if they ran at the turns of the tide which are the idle times of the Reef, this could possibly be used to ensure a continuous electricity supply.

(3) As yet undeveloped: Why not harvest tidal energy by the direct power of its rise and fall? This would require a very large-area flotation tank driving a generator and would be particularly applicable to the Severn Estuary which has the 2nd highest rise and fall in the world at 40 feet mean difference.

(4) Tidal Stream Turbines harness the horizontal flow of the tide where there is a reasonably fast tidal flow eg Strangford Lough, Ireland. These turbines are not suitable for the Severn estuary except in a few isolated places. They do not dam the water at all, so there is no change of surface level, which means a free channel for ships and fish; silting is scarcely changed; beaches and habitats remain undisturbed. At Strangford Lough, Lynmouth, and elsewhere, such turbines are already in use and generating electricity.

(5) One very interesting option is illustrated below, and our thinking on this was prompted by a visit to Aquascientific in Exeter. This is a horizontal-axis turbine that turns at a slower speed than the prevailing current flow, and has a variable movement of its blades. The turbine blade axles are geared to the main axle to present the most efficient configuration to the flow, thus maximizing the capture of energy. According to our calculations, this happens if each blade is geared to rotate 180º counter-clockwise about its own axle, for every 360º clockwise turn of the main axle, so that it is parallel to the flow when turning against it, and perpendicular to the flow when turning with it. Click to see the animation of the blades turning.

Range of blade positions in the water and effects of the flow. This is only a line diagram to indicate blade positions;
there are actually only 3 blades.
Diagram of turning blades
(click for animation)

This design has a whole host of extremely positive features. It would be suspended from floats, working in the optimum flow which is just under the surface, requires very little depth, and causes virtually no siltation, no change to tide levels or tidal habitats, and no interference to shipping. Its slow speed means that marine life is not at risk. Its neatest asset is its unobtrusiveness. Only the floats would be visible; the areas where it could function efficiently in the Severn estuary are neither close to the shore nor in the shipping lane; it functions best in the mid-ranges. Its flexible size and simple design means it has worldwide potential for both offshore and rivers.

Individual slow-action turbines could be arranged in "farms" over a wide area, and this way it could provide, in the Severn estuary alone, a total power output three times as great as the conventional barrage, and at a fraction of the cost - because of the lack of solid core construction. Around the UK, other "farms" could be built in places with different tide times, so that electricity would be delivered overall at a steady rate. Best of all, because these small units would start producing electricity almost immediately, the cost would be amortized in around two years, unlike the excessively long time needed to amortize barrage costs. Design can be improved continually. The slow horizontal turbine would work equally well in rivers. Maintenance and replacement of parts can be achieved easily. The export potential is enormous.

A tidal fence simply consist of a string of turbines in a line, with gaps between them. It can utilize many different types of turbine, including the horizontal turbine illustrated, but for best effect there may be other types more suitable for the purpose.

See also the excellent 12-page pdf from "Save Our Severn".

Tidal Power News items & Ideas Companies / projects
Tidal Power (intro) and Slow Dual-action Turbines BBC: Sea Energy Tidal Reef (Armstrong-Evans)
Save Our Severn: 3rd Generation Tidal Turbines BBC: Barrage, Lagoons, etc Aquascientific Ltd
Severn Estuary: Tidal Reef vs Barrage BBC: Tidal Fence developments Marine Current Turbines
Tidal Reef   Strangford Lough, Ireland
Summaries from Government Consultation Paper   Lynmouth tidal current turbine
Parrett Estuary Flood Risk Management Strategy   Tidal Generation Ltd


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