Category Electricity

Turbines consist of a series of fans, one in front of the other, which drive a shaft when they rotate. The shaft in turn drives a generator. Alternate fans always remain stationary. The position and shape of these fans direct the pressurized steam, or water, onto the rotating fans with the maximum possible force.

At the end of the shaft is a large magnet, which is surrounded by a coil of wire, inside the generator. As the magnetic core rotates, it causes an electric current to flow through the wire coil.

 

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The potential for using the wind to generate electricity is huge. A recent study for the European Community estimated that there were sufficient sites in Europe for about 400,000 big machines – enough to provide three times Europe’s present needs.

Modern wind generators are very different from the old windmills. They are more like giant propellers with two or three blades, called rotors, mounted on top of tall towers of steel or concrete. The rotors turn a shaft which drives an electric generator.

The size of the blades and the height of the tower determine how much electricity the machine can generate. Wind generally gets stronger as you go higher, and the power of the wind you capture depends on the swept area of the blades. Double the length of the blades and the power increases four-fold. More important still is the speed of the wind, for the power that can be extracted goes up as the cube of wind speed – if it blows twice as hard, there is eight times as much power to be had.

However, wind generators do not need, or want, stormy weather. Most machines are designed to operate at wind speeds between Force 3 and Force 10 on the Beaufort Scale – 13 to 60mph (21 to 97km/h). above Force 10 the machines automatically shut down to save themselves from flying apart.

Most machines are designed to produce much the same power throughout their working range, the blades automatically ‘feathering’ as the wind increases so that the machine does not accelerate too much. It is better to have a steady output over a wide range of wind speeds than to be able to take advantage of the few really strong gusts.

Wind generators must point in the right direction, either directly towards the wind or directly away from it. For this reason th rotor is mounted on a turntable and controlled by an electric motor connected to sensors which tell it which way to face.

This problem of wind direction can be avoided completely if the blades are mounted on a vertical rather than horizontal axis. Then it does not matter where the wind is blowing from.

These vertical machines, called Darreius Turbines, have other advantages. The heavy generating machinery that converts the power into electricity can be placed on the ground, rather than at the top of a tower. The rotor is, therefore, subjected to less stress than in the horizontal-axis generators. A disadvantage is that they often need a push to get started, either by hand or by an electric motor.

One of the main problems of using wind turbines is environmental. While people like the idea of wind power, they are less keen on having every hill crowned with a whirling turbine.

Serious examination has been given to placing the turbines out at sea. But there would be problems anchoring them and in transmitting the power back to land. The British Department of energy has estimated that clusters of wind turbines built in shallow water around the coast could produce one and a half times Britain’s present electricity demand, but engineers first want to study the performance of land-based machines.

The people of Fair Isle, off the north coast of Scotland, have already been making use of wind power. They installed a small wind generator in the early 1980s and have cut electricity bills by more than three-quarters from the old diesel engines.

 

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A small handful of uranium provides as much electrical energy as 70 tons of coal or 390 barrels of oil. A power station big enough to supply a city of a million people consumes just 6.6lb (3kg) of uranium a day, so it is by far the most concentrated source of energy used by man.

Uranium is one of the densest naturally occurring elements and each of its atoms teeters on the edge of instability. The heart of the atom, called the nucleus, needs only a tiny ‘push’ to cause it to divide. And when a nucleus splits it releases huge amounts of energy, in a process called nuclear fission.

The ‘push’ can be provided by neutrons, tiny particles much smaller than atoms, which strike the nucleus and cause it to split. In the process of splitting, at least two extra neutrons are produced, which fly off and cause further fissions – so that once the process has started it can continue almost indefinitely.

The energy of fission can be released slowly, bit by bit, and used to heat water. The steam from the water is then used to drive a generator, which produces electricity. This is the principle of the nuclear reactor.

Fuel assemblies

Inside most reactors, the fuel assemblies are made from small pellets of uranium dioxide, loaded into thin tubes. The tubes are usually put into vertical bundles with ‘spacers’ to separate them.

Once inside, a fuel assembly may stay there for as long as three years, but even after that length of time, all the uranium has not been consumed. But by-products begin to accumulate; some are gases like krypton, others are solids like caesium, strontium and plutonium. Before these by-products have built up too much, and water corrodes the fuel tubes, the assemblies are removed. To recover the unburned uranium, the spent fuel may be taken to a special plant where it is reprocessed to separate out uranium, plutonium and waste products.

The plutonium is a useful by-product of the nuclear power industry. It can be used as a fuel in power stations, because plutonium, like uranium, has nuclei that can split and release energy.

Uranium occurs in several different forms, identical chemically but with different-sized nuclei in their atoms. Of these different forms, called isotopes, one is uranium-235, which gets its name from the 235 particles making up its nucleus. Only seven atoms out of every 1000 in naturally occurring uranium are U-235. The rest consist almost entirely of uranium-238.

When U-238 is struck by neutrons it does not split as readily as U-235. It may be converted into a completely now element, plutonium-239. So if a reactor is made using natural uranium as fuel, the danger is that too many neutrons will be absorbed by U-238 before they can hit U-235 atoms and cause further fissions. If this happens the reactor will never get going.

There are two ways around this problem. One is to increase the amount of U-235 in the reactor fuel, by a process called enrichment, from seven atoms to between 30 and 40 in every thousand. This is done before the fuel is manufactured, usually in a centrifuge – a machine that whirls round, separating U-235 from U-238 by the outward pushing forces of high-speed rotation. The second way is to make the very best use of the available neutrons inside the reactor by slowing them down, which increases their chances of causing further fissions.

The way to slow them down is to make them ricochet to and fro off light atoms of an element such as hydrogen or carbon, like balls in a pin-ball machine. The light elements act as a ‘moderator’, because their job is to moderate the speed of the neutrons. Most modern reactors use both enriched fuel and moderators. Some are moderated by water (which, of course, contains hydrogen), while others are moderated by carbon in the form of graphite, which is the hard dark material known as the ‘lead’ in an ordinary pencil.

Obviously, a nuclear reactor produces a great amount of heat, and to stop the reactors from overheating, coolants have to be used. Pressurized water reactors use water as a coolant, so these plants need to be built near rivers or oceans. Advanced gas-cooled reactors, first built in Great Britain, are cooled by carbon-dioxide gas. In Canada, heavy water – in which hydrogen atoms are replaced with an isotope of hydrogen called deuterium – cools fast breeder reactors. France has pioneered the use of liquid sodium as a coolant for their fast breeders.

 

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