The second category of marine energy that I'll mention is tidal energy. One major advantage of using the (1)__________, rather than waves, as a source of energy is that it's (2)__________: we know the exact times of high and low tides for years to come. For tidal energy to be effective, the difference between high and low tides needs to be at least five metres, and this occurs naturally in only about forty places on Earth. But the right conditions can be created by constructing a tidal (3)__________, an area of sea water separated from the sea. One current plan is to create a tidal lagoon on the coast of Wales. This will be an area of water within a (4)__________ at Swansea, sheltered by a U-shaped breakwater, or (5)__________, built out from the coast. The breakwater will contain sixteen hydro (6)__________, and as the tide rises, water rushes through the breakwater, activating the turbines, which tum a (7)__________ to produce electricity. Then for three hours as the tide goes out, the water is held back within the breakwater, increasing the difference in water level, until its several metres higher within the lagoon than in we open sea. Then, in order to (8)__________ the stored water, gates in the breakwater are opened. It pours powerfully out of the lagoon, driving the turbines in the breakwater in the opposite direction and again generating thousands of megawatts of electricity. As there are two high tides a day, this lagoon scheme would generate electricity four times a day, every day , for a total of around 14 hours in every 24--and enough electricity for over 150, 000 homes. This system has quite a lot in its favour: unlike solar and wind energy it doesn ’ t depend on the weather; the turbines are operated without the need for (9)__________, so it doesn ’ t create any greenhouse gas emissions, and very little maintenance is needed. It's estimated that electricity generated in this way will be (10)__________ cheap, and that manufacturing the components would create more than 2,000 jobs, a big boost to the local economy. On the other hand, there are fears that lagoons might harm both fish and birds. for example by disturbing (11)__________ patterns, and causing a build-up of silt, affecting local ecosystems. There are other forms of tidal energy, but I'll go on to the third category of marine energy: ocean thermal energy conversion. This depends on there being a big difference in temperature between surface water and the water a couple of kilometres below the surface, and this occurs in (12)__________ coastal areas. The idea is to bring cold water up to the surface using a submerged pipe. The concept dates back to 1881. when ...