| Producing enough energy to meet our needs has become a serious problem. |
| Demand is rising rapidly, |
| because of the world's increasing population and expanding industry. |
| Burning fossil fuels, like gas, coal and oil, |
| seriously damages the environment |
| and they'll eventually run out. |
| For a number of years now, |
| scientists have been working out how we can derive energy from renewable sources, |
| such as the sun and wind, without causing pollution. |
| Today I'll outline marine renewable energy - also called ocean energy - |
| which harnesses the movement of the oceans. |
| Marine renewable energy can be divided into three main categories: |
| wave energy, tidal energy and ocean thermal energy conversion, |
| and I'll say a few words about each one. |
| First, wave energy. |
| Numerous devices have been invented to harvest wave energy, |
| with names such as Wave Dragon, the Penguin and Mighty Whale, |
| and research is going on to try and come up with a really efficient method. |
| This form of energy has plenty of potential, as the source is constant, |
| and there's no danger of waves coming to a standstill. |
| Electricity can be generated using onshore systems, |
| using a reservoir, or offshore systems. |
| But the problem with ocean waves is that they're erratic, |
| with the winds making them travel in every direction. |
| This adds to the difficulty of creating efficient technology: |
| ideally all the waves would travel smoothly and regularly along the same straight line. |
| Another drawback is that sand and other sediment on the ocean floor |
| might be stopped from flowing normally, |
| which can lead to environmental problems. |
| The second category of marine energy that I'll mention is tidal energy. |
| One major advantage of using the tide, rather than waves, as a source of energy |
| is that it's predictable: |
| 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 lagoon, |
| 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 bay at Swansea, |
| sheltered by a U-shaped breakwater, or dam, built out from the coast. |
| The breakwater will contain sixteen hydro turbines, |
| and as the tide rises, water rushes through the breakwater, |
| activating the turbines, which turn a generator 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 it's several metres higher within the lagoon than in the open sea. |
| Then, in order to release 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 fuel, |
| 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 relatively 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 migration 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 tropical coastal areas. |
| The idea is to bring cold water up to the surface using a submerged pipe. |
| The concept dates back to 1881... |