Part of a lecture about wind turbines and how they generate

Section 4. You will hear part of a lecture about wind turbines and how they generate electricity. First, you have some time to look at questions 31 to 35. Now listen carefully and answer questions 31 to 35. MAN: Good afternoon. Today we're looking at wind turbines, the tall machines that turn moving air into electricity, and which now supply a rapidly growing share of the world's power. Let me begin with the basic idea, which is beautifully simple, and then look at the difficulties, which are less so. MAN: A wind turbine works, in essence, by reversing what a fan does. A fan uses electricity to make wind, a turbine uses wind to make electricity. The moving air pushes against the long blades and makes them turn. The blades are connected to a shaft, and the spinning shaft drives a generator, which is the component that actually converts this movement into electrical current. Most large turbines have three blades, which experience has shown gives the best balance between efficiency and stability. MAN: Now, one thing that surprises students is that a turbine cannot use all the energy in the wind. If the blades took every scrap of energy out of the air, the air behind them would have to stop completely, and then no more could flow through, so there is a strict physical limit, in fact a turbine can capture at most about sixty per cent of the wind's energy, and real machines manage rather less. The height of the tower matters greatly too, wind blows faster and more smoothly higher up, away from the friction of the ground, which is why modern towers are built so tall. Before you hear the rest of the lecture, you have some time to look at questions 36 to 40. MAN: The single biggest drawback of wind power is obvious: it is intermittent, the wind does not always blow, and a turbine produces nothing when the air is still. This is why wind cannot yet replace conventional power stations entirely, and why so much effort now goes into storage, into batteries that can save electricity generated on windy days for use on calm ones. MAN: A growing trend is to place wind farms offshore, out at sea. This has real advantages, the wind at sea is stronger and steadier than on land, and the huge turbines are far from anyone's home, so complaints about the view and the noise disappear. The drawback is cost, building and maintaining turbines in the sea is far more expensive than on land. MAN: Let me close with the environmental balance, which is overwhelmingly positive. Once it is built, a turbine burns no fuel and releases no carbon, and it typically generates as much energy as went into making it within less than a year. The chief objection raised is the danger to birds, which can be struck by the blades, though modern designs and careful siting have reduced this considerably. On balance, wind is among the cleanest large-scale power sources we have. That's all for today. That is the end of Section 4. You now have some time to check your answers.