Hint:
Wave Dragon -the Penguin -Mighty Whale
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Producing enough energy to meet our needs has become a serious problem.
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Demand is rising rapidly,
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because of the world's increasing population and expanding industry.
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Burning fossil fuels, like gas, coal and oil, seriously damages the environment
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and they'll eventually run out.
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For a number of years now,
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scientists have been working out how we can derive energy from renewable sources,
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such as the sun and wind, without causing pollution.
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Today I'll outline marine renewable energy - also called ocean energy -
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which harnesses the movement of the oceans.
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Marine renewable energy can be divided into three main categories:
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wave energy, tidal energy and ocean thermal energy conversion,
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and I'll say a few words about each one.
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First, wave energy.
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Numerous devices have been invented to harvest wave energy,
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with names such as Wave Dragon, the Penguin and Mighty Whale,
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and research is going on to try and come up with a really efficient method.
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This form of energy has plenty of potential, as the source is constant,
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and there's no danger of waves coming to a standstill.
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Electricity can be generated using onshore systems,
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using a reservoir, or offshore systems.
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But the problem with ocean waves is that they're erratic,
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with the wind making them travel in every direction.
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This adds to the difficulty of creating efficient technology:
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ideally all the waves would travel smoothly and regularly along the same straight line.
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Another drawback is that sand and other sediment on the ocean floor
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might be stopped from flowing normally,
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which can lead to environmental problems.
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The second category of marine energy that I'll mention is tidal energy.
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One major advantage of using the tide, rather than waves, as a source of energy
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is that it's predictable:
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we know the exact times of high and low tides for years to come.
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For tidal energy to be effective,
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the difference between high and low tides needs to be at least five metres,
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and this occurs naturally in only about forty places on Earth.
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But the right conditions can be created by constructing a tidal lagoon,
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an area of sea water separated from the sea.
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One current plan is to create a tidal lagoon on the coast of Wales.
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This will be an area of water within a bay at Swansea,
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sheltered by a U-shaped breakwater, or dam, built out from the coast.
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The breakwater will contain sixteen hydro turbines,
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and as the tide rises, water rushes through the breakwater,
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activating the turbines, which turn a generator to produce electricity.
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Then, for three hours as the tide goes out,
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the water is held back within the breakwater,
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increasing the difference in water level,
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until it's several metres higher within the lagoon than in the open sea.
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Then, in order to release the stored water,
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gates in the breakwater are opened.
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It pours powerfully out of the lagoon,
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driving the turbines in the breakwater in the opposite direction
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and again generating thousands of megawatts of electricity.
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As there are two high tides a day,
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this lagoon scheme would generate electricity four times a day, every day,
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for a total of around 14 hours in every 24,
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and enough electricity for over 150,000 homes.
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This system has quite a lot in its favour:
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unlike solar and wind energy it doesn't depend on the weather;
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the turbines are operated without the need for fuel,
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so it doesn't create any greenhouse gas emissions;
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and very little maintenance is needed.
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It's estimated that electricity generated in this way will be relatively cheap,
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and that manufacturing the components would create more than 2,000 jobs,
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a big boost to the local economy.
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On the other hand,
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there are fears that lagoons might harm both fish and birds,
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for example by disturbing migration patterns,
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and causing a build-up of silt, affecting local ecosystems.
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There are other forms of tidal energy,
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but I'll go on to the third category of marine energy:
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ocean thermal energy conversion.
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This depends on there being a big difference in temperature
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between surface water and the water a couple of kilometres below the surface,
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and this occurs in tropical coastal areas.
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The idea is to bring cold water up to the surface using a submerged pipe.
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The concept dates back to 1881...
