Category Electricity

The energy that reaches the Earth in the form of sunlight is immense – more than 12,000 times greater than the world’s fuel consumption. The sunshine falling each year on the surface of America’s roads alone contains twice as much energy as all the coal and oil used every year in the entire world.

But collecting and storing this abundant supply of free energy is difficult and expensive. The Sun sheds its rays thinly over a vast area and its heat must be collected and concentrated before it can be used in homes or power plants.

In domestic hot-water systems that use the Sun’s energy, solar collectors (panels) are mounted on roofs facing the Sun. they have glass or plastic panels behind which water circulates in pipes painted black to absorb maximum heat. The heated water is then pumped into the hot-water tank.

Japan has 3 million solar panels on its roofs, and half the houses in Israel have them. They are popular in California, but cloudier Europe, which gets only half the sunshine of Israel or California, has far fewer. Only a fraction of energy collected in direct sunlight can be trapped on an overcast day.

Solar energy is also used to generate electricity. For direct uses of the Sun high temperatures are needed, and to achieve them, sunlight must be concentrated by focusing.

Mirrors, rather than lenses, are arranged in a semicircle, reflecting the sunlight towards a concrete ‘power tower’. The concentrated sunlight shines on a receiver at the top of the tower and heats a fluid which circulates through pipes. If the fluid is water, the high-pressure steam that is produced is used to drive electricity generators.

The largest power in the world is near Barstow, California, in the Mojave Desert, which has 300 days of sunshine a year. Its reflector covers 100 acres (40 hectares) and consists of 1818 mirrors in concentric circles focused on a boiler at the top of a tower that is 255ft (78m) high.

Europe’s biggest solar energy plant is in France, at Themis in the western Pyreness. Built in June 1981, it has a generating capacity of 2.5mW.

Virtually every spacecraft and satellite has depended on solar cells for its electricity since the US Satellite Vanguard in 1958. Solar cells exploit the discovery, made in 1887 by the German physicist Heinrich Hertz, that certain substances generate electricity when exposed to light-the photo-voltaic effect.

Cells are made from a thin layer of silicon placed next to an even thinner layer of silicon impregnated with boron, which alters the electrical behaviour of the silicon. Light falling on the outer layer causes electrons to migrate into the silicon backing, creating a voltage between the two layers. A series of cells must be connected together so their output adds up to a usable amount. Although silicon is cheap – it is the basic constituent of sand and rock – converting it into the single crystals necessary for solar cells is expensive. And huge numbers of cells are usually needed.

The Solar Challenger, an aeroplane powered by 16,128 solar cells generating 2.5kW, crossed the English Channel in 1981. Solar cars carry batteries, but only to store solar energy for use when it is cloudy or when the car is climbing hills.

Everyday applications of solar energy, like solar-powered watches and calculators, are widespread. Solar-heated swimming pools are becoming popular.

The first solar-cell power station of significant size – with an output of 1mW – was completed near Victorville, California, in 1982. One of the largest solar-cell projects in Europe is on the island of Pellworm, off the West German coast, where 17,500 solar cells covering an area as big as two football pitches provide the electricity for the island’s health spa.

 

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The nearer you get to the Earth’s centre the hotter it becomes. Nuclear reactions, caused by the decay of radioactive materials, constantly heat the molten core to 7200ºF  (4000ºC). Because of this geothermal energy it is several degrees warmer at the bottom of a mine than it is at the top.

In some places hot rocks lie quite near the surface, causing hot springs, geysers or steam to rise out of the ground. These can be used to produce electricity.

The first geothermal power station was built in 1904, at Larderello in northern Italy, where steam was coming out of the ground at temperatures between 280ºF  and 500ºF  (140ºC  and 260ºC). The steam was piped to turbines which powered generators.

In New Zealand, the Philippines, California and Mexico, power stations have been built where the Earth’s heat reaches the surface naturally. In some cases there may be o water present at all, just dry hot rocks, whose heat can only be used if water is pumped down to them and then recovered as steam. The steam is then used to drive turbines and generate electricity.

The granites of Cornwall are a source of geothermal energy that has recently been tested. Some 6500ft (1980m) beneath Camborne in Cornwall, the rocks reach temperatures of about 158ºF (70ºC).

To extract energy two boreholes would have to be drilled, cold water pumped down one and pressurized hot water returned up the other. The water would flow from one borehole to the other through fissures in the rock created by blasting it with explosives. Although the water is at 390ºF (200ºC), the pressure it is under prevents it from boiling. But when it is returned to normal atmospheric pressure at the surface, it instantly “flashes’ into steam – ready to drive the turbines.

Like other sites where geothermal energy could be tapped, Camborne has several problems. Minerals will have to be removed from the hot water, otherwise they could fur up pipes and corrode turbines. Tests have also shown that only one-third of the water pumped down finds its way back to the surface – the rest is lost. The third problem will be drilling deep enough.

If all these problems can be solved, the potential is enormous. It has been calculated that the Cornish granites alone contain as much energy as the whole of Britain’s coal reserves.

More and more countries are looking into geothermal energy as an alternative to fossil fuels. A major power station has been started in New Mexico, and a joint French and German project is being carried out near Strasbourg.

 

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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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The tides have been used to provide power for hundreds of years. In the 18th century, the coast of Europe was dotted with tidal mills, which let the incoming tide into a reservoir through open sluices. At high tide the sluices were closed and the only way the water could escape as the tide fell was by passing through and propelling a waterwheel, so providing turning power.

The same principle was used in a power station built in France in the 1960s. a dam was built across the estuary of the River Rance at St Malo in Brittany, with 24 machines that could be used as turbines in either direction.

As the tide comes in, it is allowed to build up against the dam until there is a difference of 5ft (1.5m) between one side and the other. Then it is allowed to pass through the turbines, driving them and generating electricity. When the tide begins to fall, the turbine blades are reversed, and the water generates electricity again.

The amount of electricity generated depends on the ‘head’ of water – the difference in the level of the water between one side of the dam and the other. The larger the head, the greater the amount of electricity that will be generated, because the water is under greater pressure and so turns the turbines with more force.

At high tide the sluices are shut and extra water is pumped from the sea into the estuary. The water level in the estuary is raised above high tide, so when the sea falls back to low tide the difference in levels has been accentuated.

Once all the water has been allowed to flow into the sea – driving the turbines as it does so – extra water is pumped out to make the level in the estuary artificially low.

When it is high tide again, the turbines are reversed, water flows back into the estuary, and the cycle starts once more. Of course, pumping consumes electricity, but the additional heads produce considerably more electricity than the pumps use.

The scheme at La Rance generates 240 megawatts at peak output – sufficient for a medium-sized city such as Rennes or Caen, but it has had few followers. The immense cost of building the dams and the lack of suitable sites have discouraged everybody except the Russians and Canadians.

The Bay of Fundy in Nova Scotia has the biggest tidal range in the world, with up to 59ft (18m) height difference between tides.

A successful pilot plant was opened across an inlet of the bay at Annapolis Royal in 1984. If the power of the tides across the whole bay could be harnessed it would produce ten times more energy than could be used locally. The surplus electricity could be used in New England and New York Experts believe that it is just a matter of time before the project goes ahead.

 

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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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