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Potential of Tidal power generation
Tidal power 90 years ago could have provided all the power in the UK and today could provide a sizable proportion. Many other countries with a coast line will also have considerable potential.
Tidal power therefore has advantages over wind power, 1) predictability so engineers can make adjustments to grid power flows in advance of tide changes; 2) the load factor of 75% instead of around 30% (wind) and 3) water being 800 times denser than air there is more power for given size of turbine.
However as power is proportional to the cube of flow a variation between spring and neap tides of 2X results in a potential power difference of 8X.
Tidal power, barrages, dams, making use of tidal 'range.' (ref 186)
Use is made of the 'potential' energy in the water, captured at high tide behind a dam in an estuary. Water flows to the lower level as the tide recedes driving turbines which in turn drive electric generators. The system could work both ways. A typical tidal power 'barrage' may look like this:-
Examples are the Severn Barrage project proposal in the UK to generate up to 8.600 MW with a 14 metre tide (ref 123 & 234) depending on barrage location; or La Rance in France, finished in 1967, 240 MW, with a 8 metre tide.
20 places have been identified around the world as potential barrage sites, eight being around the UK, including the Humber, Dee, Solway estuaries.
Tidal lagoon
A further possibility in a tidal and shallow area such as the European continental shelf is to construct a vast lagoon which fills up at high tide and drives turbines while emptying and from low tide when refilling. A study by Atkins consultants of a 5km sq area off Swansea UK concluded feasibility for 60 MW generation, tides 4.1 to 8.5m 36% load factor. The lagoon walls would be designed for a 11.1 m rise in future.
Tidal turbine farms, making use of tidal flow- predictable tidal flows 18-20 hours a day.
Tidal turbines make use of the kinetic energy in tides where many turbines are mounted in the path of the tidal flow. The energy in a tidal current is proportional to the cube of the water velocity and there is thus 8 times the tidal stream power in 'spring tides' compared to 'neap tides'(a two week cycle) and no power in periods of 'slack water, when the tide is turning. There are several different approaches being tested.
Sea Gen (above) are supplying 3 X 1.2MW turbines at Campbell River, West Canada; total potential in area 4000MW.
Shrouded tidal stream turbines.
The shroud produces accelerated flow through the turbine and therefore increased power from a given size. This is suitable for slow tide movement, shallow water eg rivers. A wide angle diffuser should help if the turbine axis is not aligned with the tidal flow. Turbine assemblies are bi directional or multidirectional.
Underwater kite idea increases flow into turbines by 10 X
Minesto (Saab Group offshoot) provide "Deep Green" an underwater turbine-kite system tethered to the sea floor suitable for locations of light tide flow in deep water. The assembly rides at least 20 m deep and is light in weight for the output.
The system moves around in a figure of eight pattern to increase flow and thus power sterred by a rudder. The claim is that this will almost double the available power from tidal flow turbines.
Vertical mounted tidal turbine
Blue Energy manufacture. Several would be locked together in the path of the tide:-
Gorlov vertical helical turbines are planned in Korea
These generate 1 MW each; a group of 3600 MW are planned.
Hydrofoils
The Sea Snail is an alternative concept being developed by CRE+E consists of hydrofoils which induce a down force from the stream flow and counteract the overturning moment of the tidal current. Devices oscillate to generate electricity.
Overall potential of tidal power.
Tidal power can provide 3 watts/sqm from lagoons or 6 w/sqm sea bed space in tidal streams. The Pentland Firth (N Scotland) would provide considerably more power than other areas around the UK.
Ocean power plant in development by Lockheed Martin using the Ammonia cycle.
Ammonia is pressurized, then evaporated using warm sea water in a heat exchanger, passes through a turbine loosing heat & pressure, driving an electrical generator and finally a condenser using cold water (taken from 500 to 1000 m depth.) Compare the cycle with the steam cycle in a coal or nuclear power station. (see steam cycle on electrical page.).
Locations such as at Hawaii are being considered, with deep cold nearby water, as large depth and thus temperature difference is needed between the warm and cold water used. The cold pipes consist of two strong concentric continuous sheets with fibreglass blanket rolls in between.
