You will hear a lecturer discussing ways of producing renewable energy from the oceans.
First, you have some time to look at questions 31 to 40.
Now listen carefully and answer questions 31 to 40.
Providing sufficient energy for our needs has become a major challenge.
Demand is climbing quickly because the world's population is increasing and industry is expanding.
At the same time, burning fossil fuels such as gas, coal and oil causes severe environmental damage and those resources will eventually be exhausted.
Scientists have therefore spent years exploring ways to obtain power from renewable sources, including sunlight and wind, without producing pollution.
In this talk, I'll introduce marine renewable energy, also known as ocean energy, which uses movement within the oceans.
There are three broad forms of marine renewable energy.
Wave energy, tidal energy and ocean thermal energy conversion.
I'll briefly consider each of them, starting with wave energy.
Many machines have been designed to capture power from waves, with names such as Wave Dragon, Penguin and Mighty Whale, and researchers are still looking for a truly efficient system.
Wave power has considerable promise because the source is constant, the waves are never going to stop altogether.
Electricity can be produced by offshore installations or by systems on land that use a reservoir.
The difficulty is that ocean waves behave irregularly, wind can send them in any direction, which makes efficient equipment much harder to design.
Ideally, the waves would move steadily along the same straight path.
There is also an environmental concern.
Devices may prevent sand and other sediment on the ocean floor from moving in its usual way.
The next type is tidal energy.
Compared with waves, tides have one important advantage.
They are predictable, since the times of high and low tide can be calculated many years ahead.
For tidal power to work well, there normally needs to be a difference of at least 5 meters between high and low tide, and only around 40 locations worldwide have that naturally.
However, suitable conditions can be engineered by building a tidal lagoon, which is a body of seawater separated from the open sea.
A proposed scheme in Wales would place such a lagoon in a bay at Swansea.
A U-shaped breakwater, or dam, would extend from the coast around the water.
16 hydro turbines would be installed in it.
As the tide comes in, water would pass rapidly through the breakwater and turn the turbines, which would drive a generator.
When the tide begins to fall, water would be retained behind the breakwater for 3 hours.
This would increase the difference in level until the lagoon stood several meters above the sea outside.
The gates in the breakwater would then be opened, allowing the stored water to surge out.
Its force would drive the turbines in the reverse direction and generate thousands of megawatts of electricity again.
With two high tides each day, the lagoon could produce power four times daily, for roughly 14 hours out of every 24, supplying more than 150,000 homes.
The design offers several benefits.
Unlike wind and solar systems, it is not controlled by the weather.
The turbines require no fuel, so they release no greenhouse gases, and they need very little maintenance.
Forecasts suggest that power produced in this way would be fairly inexpensive.
In addition, manufacturing the necessary parts could create more than 2,000 jobs and provide a substantial benefit to the local economy.
There are concerns, however, that lagoons could damage fish and bird populations by disrupting migration patterns and allowing silt to accumulate, with consequences for nearby ecosystems.
Other tidal technologies exist, but I'll now turn to the third category, ocean thermal energy conversion.
This method requires a large temperature difference between surface water and water about two kilometers below it, a condition found in tropical coastal regions.
Cold water is carried up through a submerged pipe.
The principle was first proposed in 1881 when...
