ocean energy

tidal power Tidal power can be harnessed by constructing a dam or barrage across an estuary or bay with a tidal range of at least five metres. Gates in the barrage allow the incoming tide to build up in a basin behind it. The gates then close so that when the tide flows out the water can be channelled through turbines to generate electricity. Tidal barrages have been built across estuaries in France, Canada and China but a mixture of high cost projections coupled with environmental objections to the effect on estuarial habitats has limited the technology’s further expansion.

wave and tidal stream power In wave power generation, a structure interacts with the incoming waves, converting this energy to electricity through a hydraulic, mechanical or pneumatic power take-off system. The structure is kept in position by a mooring system or placed directly on the seabed/seashore. Power is transmitted to the seabed by a flexible submerged electrical cable and to shore by a sub-sea cable.

In tidal stream generation, a machine similar to a wind turbine rotor is fitted underwater to a column fixed to the sea bed; the rotor then rotates to generate electricity from fast-moving currents. 300 kW prototypes are in operation in the UK.

Wave power converters can be made up from connected groups of smaller generator units of 100 – 500 kW, or several mechanical or hydraulically interconnected modules can supply a single larger turbine generator unit of 2 – 20 MW. The large waves needed to make the technology more cost effective are mostly found at great distances from the shore, however, requiring costly sub-sea cables to transmit the power. The converters themselves also take up large amounts of space. Wave power has the advantage of providing a more predictable supply than wind energy and can be located in the ocean without much visual intrusion.

There is no commercially leading technology on wave power conversion at present. Different systems are being developed at sea for prototype testing. The largest grid-connected system installed so far is the 2.25 MW Pelamis, with linked semi-submerged cyclindrical sections, operating off the coast of Portugal. Most development work has been carried out in the UK.

Wave energy systems can be divided into three groups, described below.

• shoreline devices are fixed to the coast or embedded in the shoreline, with the advantage of easier installation and maintenance. They also do not require deep-water moorings or long lengths of underwater electrical cable. The disadvantage is that they experience a much less powerful wave regime. The most advanced type of shoreline device is the oscillating water column (OWC). One example is the Pico plant, a 400 kW rated shoreline OWC equipped with a Wells turbine constructed in the 1990s. Another system that can be integrated into a breakwater is the Seawave Slot-Cone converter.
• near shore devices are deployed at moderate water depths (~20- 25 m) at distances up to ~500 m from the shore. They have the same advantages as shoreline devices but are exposed to stronger, more productive waves. These include ‘point absorber systems’.
• offshore devices exploit the more powerful wave regimes available in deep water (>25 m depth). More recent designs for offshore devices concentrate on small, modular devices, yielding high power output when deployed in arrays. One example is the AquaBuOY system, a freely floating heaving point absorber system that reacts against a submersed tube, filled with water. Another example is the Wave Dragon, which uses a wave reflector design to focus the wave towards a ramp and fill a higher-level reservoir.