Tidal currents

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Whereas the tidal fence is appropriate for capturing fast tidal flow between land masses like an estuary or well-defined channel, other technologies can exploit the energy content of tidal currents that oscillate along coasts. These have to be distinguished from large ocean movements like the Gulf Stream, which are thermodynamic phenomena.

Horizontal-axis underwater turbines are the favoured technology for offshore tidal currents. They are similar to wind turbines but water has an energy density four times greater than air, which means that a rotor 15 m in diameter will generate as much power as a wind

Figure 11.12 Tidal fence vertical axis turbines (courtesy of Blue Energy Canada)

turbine of 60 m diameter. They operate at a minimum tidal velocity of about 2m/second. Since the tidal flow is relatively constant, underwater turbines are subject to much less buffeting than their wind counterparts. According to Peter Fraenkel, director of Marine Current Turbines that have installed a demonstration turbine off the coast of Devon, the best tidal stream sites could generate 10MW per square kilometre (see Fig 11.13).

A demonstration MCT is operating off the north coast of Devon with another planned for Strangford Lough in Northern Ireland. However, the first major deployment of this technology is being considered for the tidal race off the Channel Island of Alderney (see Fig. 11.14).

A problem with tidal energy is that, whilst it is predictable, it produces large bursts of energy at regular intermittent intervals. (see Fig. 11.15). As output approaches gigawatt scale this raises potential problems for the Grid and even more so for the generator if peak output

Figure 11.13 Tidal stream turbines or tidal mills, serviced above water (courtesy of Marine Current Turbines (MCT))
Alderney Tidal Currents
Figure 11.14 Suggested layout of underwater turbines off Alderney (courtesy of the Sustainable Energy Research Group, University of Southampton)

_ Sea surface elevation at Weymouth

Power generated at side diamond F

_ Sea surface elevation at Weymouth

Power generated at side diamond F

Time relative to high water springs (hours)

Figure 11.15 Power output of underwater turbines in the English Channel (courtesy of Southampton University)

Time relative to high water springs (hours)

Figure 11.15 Power output of underwater turbines in the English Channel (courtesy of Southampton University)

coincides with the bottom of the market price. This is a problem experienced by Denmark when its domestic market has no demand for the excess wind power, and it has to export to Norway and Sweden in a buyers' market.

One answer is to use uneconomic power to release hydrogen by the electrolysis of water. This could then serve a fuel cell stack which could deliver energy at the peak of the price curve. This means that electricity could be delivered at around 45% efficiency which is better than the Grid. It does, however, not include the load factor of the turbines since this is, in effect, using surplus power which has a low to zero market value.

Alternatively the Redox Flow Battery is capable of storing multi-megawatt hours of electricity. It comprises very large twin cisterns containing electrolyte. One receives electricity under charge, the other discharges on demand. It is marketed, for example, by VRB Power Systems of Canada which is now developing a higher density system: the RGN flow battery. This technology could go some way to solving the problem of balancing power presented by intermittent renewables (see p. 131).

As fuel cells and electrolysers (which are reverse fuel cells) become more efficient and less expensive this strategy would be particularly appropriate for high-energy-density systems like the tidal energy bridge. It must also be remembered that fossil fuel prices are on a steady and probably irreversible upward trend as the 'peak oil' point is passed at ~2010.

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Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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