by Isaac Asimov
By 1660, England was producing 2 million tons of coal each year, or more than 80 percent of all the coal that was then being produced in the world.
At first, it was used chiefly as a household fuel; but in 1603, an Englishman, Hugh Platt, discovered that if coal were heated in such a way that oxygen did not get at it, the tarry, pitchy material it contained would be driven off and burned. Left behind was almost pure carbon, and this residue was called coke.
At first coke was not of high quality. It was improved with time and eventually could be used in place of charcoal (from wood) to smelt iron ore. Coke burned at a high temperature, and its carbon atoms combined with the oxygen atoms of iron ore, leaving metallic iron behind. In 1709, an Englishman, Abraham Darby, began to use coke on a large scale for iron making. When the steam engine arrived, coal was used to heat and boil the water; and the Industrial Revolution was, in this way, driven forward.
The shift was slower elsewhere. Even in 1800, wood supplied 94 percent of the fuel needs in the young, forest-rich United States. In 1885, however, wood supplied only 50 percent of the fuel needs and, by the 1980s, less than 3 percent. The balance, moreover, has shifted beyond coal to oil and natural gas. In 1900, the energy supplied by coal in the United States was ten times that supplied by oil and gas together. Half a century later, coal supplied only one-third the energy supplied by oil and gas.
In ancient times, the oil used to burn in lamps for illumination was derived from plant and animal sources. Through the long eons of geologic time, however, the oil-rich tiny animals of the shallow seas have sometimes, in dying, escaped being eaten but mingled with the mud and were buried under sedimentary layers. After slow chemical change, the oil was converted to a complex mixture of hydrocarbons and is now properly called petroleum (from Latin, meaning “rock oil”). However, such has been its importance to humanity over the last couple of generations that the simple word oil has come to mean nothing else. We can be sure that when oil hits the headlines it is not referring to olive oil or coconut oil.
Oil is sometimes found on Earth’s surface, particularly in the oil-rich Middle East. It was the pitch that Noah was instructed to dub on his ark inside and out to make it waterproof. In the same way, when Moses was set afloat as a baby in his “ark of bulrushes,” it, too, was daubed with pitch to keep it from sinking. Lighter fractions of the oil (naphtha) were sometimes collected and used in lamps, or for flames used in connection with religious rites.
In the 1850s, inflammable liquids were needed for lamps. There was whale oil, and also coal oil (obtained by heating coal in the absence of air). Another source was shale, a soft material that felt something like wax. When heated, it gave off a liquid called kerosene. Such shale was found in western Pennsylvania; and in 1859, an American railway conductor, Edwin Laurentine Drake, tried something new.
Drake knew that people dug wells to obtain water, and that sometimes people dug even deeper to get brine (very salty water that could be used as a source of salt). Sometimes, an inflammable oily material came up with the brine. There were reports that, in China and Burma two thousand years ago, this oil was burned and the heat used to drive off the water from the brine, leaving the salt behind.
Why not, then, dig for oil? It was used in those days, not only as a fuel in lamps but for medicinal purposes; and Drake felt there would be a good market for anything he might dig up. He drilled a hole 69 feet under the ground at Titusville in western Pennsylvania and, on 28 August 1859, “struck oil.” He had drilled the first oil well.
For the first half-century, oil’s uses were limited; but, with the coming of the internal-combustion engine, oil came to be in great demand. A liquid fraction, lighter than kerosene (that is, more volatile and more easily converted into vapor) was just the thing to burn in the new engines. The fraction was gasoline, and the great oil hunt was on and, over the last century, has never ceased.
The Pennsylvania oil fields were quickly consumed, but much larger ones were discovered in Texas in the early twentieth century; then still larger ones in the Middle East in the middle twentieth century.
Oil has many advantages over coal. Human beings do not have to go underground to gouge oil out of the ground; nor do innumerable freight cars have to be loaded with it; nor does it have to be stored in cellars and shoveled into furnaces; nor does it leave ashes to dispose of. Oil is pumped out of the ground, distributed by pipes (or tankers over sea), stored in underground tanks, and fed into furnaces automatically, with flames that can be started and stopped at will, leaving no ash behind. Particularly after the Second World War, the world as a whole shifted vastly from coal to oil. Coal remained a vital material in the manufacture of iron and steel and for various other purposes, but oil became the great fuel resource of the world.
Oil includes some fractions so volatile that they are vapors at ordinary temperature. This is natural gas which is now referred to, usually, simply as gas, as petroleum has become simply oil. Gas is even more convenient than oil, and its use has been growing even more rapidly than that of the liquid fractions of oil.
And yet these are limited resources. Gas, oil, and coal are fossil fuels, relics of plant and animal life eons old, and cannot be replaced once they are used up. With respect to the fossil fuels, human beings are living on their capital at an extravagant rate.
The oil, particularly, is going fast. The world is now burning over 4 million barrels of oil each hour; and despite all efforts at conservation, the rate of consumption will continue to rise in the near future. Although nearly a trillion barrels remain in the earth, this is not more than a thirty-year supply at present levels of use.
Of course, additional oil can be formed by the combination of the more common coal with hydrogen under pressure. This process was first developed by the German chemist Friedrich Bergius in the 1920s, and he shared in the Nobel Prize for chemistry in 1931 as a result. The coal reserve is large indeed, perhaps as large as 7 trillion tons; but not all of it is easy to mine. By the twenty-fifth century or sooner, coal may become an expensive commodity.
We can expect new finds. Perhaps surprises in the way of coal and oil await us in Australia, in the Sahara, even in Antarctica. Moreover, improvements in technology may make it economical to exploit thinner and deeper coal seams, to plunge more and more deeply for oil, and to extract oil from oil shale and from subsea reserves.
No doubt we shall also find ways to use our fuel more efficiently. The process of burning fuel to produce heat to convert water to steam to drive a generator to create electricity wastes a good deal of energy along the way. Most of these losses could be sidestepped if heat could be con.verted directly into electricity. The possibility of doing this appeared as long ago as 1823, when a German physicist, Thomas Johann Seebeck, observed that, if two different metals are joined in a closed circuit and if the junction of the two elements is heated, a compass needle in the vicinity will be deflected, indicating that the heat is producing an electric current in the circuit (thermoelectricity). Seebeck misinterpreted his own work, however, and his discovery was not then followed up.
With the coming of semiconductor techniques, the old Seebeck effect underwent a renaissance. Current thermoelectric devices make use of semiconductors. Heating one end of a semiconductor creates an electric potential in the material: in a p-type semiconductor, the cold end becomes negative; in an n-type it becomes positive. Now if these two types of semiconductor are joined in a U-shaped structure, with the n-p junction at the bottom of the U, heating the bottom will cause the upper end of the p branch to gain a negative charge and the upper end of the n branch to acquire a positive charge. As a result, current will flow from one end to the other, and will be generated so long as the temperature difference is maintained (figure 10.1). (In reverse, the use of a current can bring about a temperature drop, so that a thermoelectric device can also be used as a refrigerator.)
Figure 10.1. The thermoelectric cell. Heating the conductor causes electrons to flow toward the cold end of th
e n-type semi-conductor and from the cold to the warm region of the p-type. If a circuit is formed, current Rows in the direction shown by the arrows. Thus heat is converted to electrical energy.
The thermoelectric cell, requiring no expensive generator or bulky steam engine, is portable and can be set up in isolated areas as a small-scale supplier of electricity. All it needs as an energy source is a kerosene heater. Such devices are reported to be used routinely in rural areas of the Soviet Union.
Notwithstanding all possible increases in the efficiency of using fuel and the likelihood of new finds of coal and oil, these sources of energy are definitely limited. The day will come, and not far in the future, when neither coal nor oil can serve as an important large-scale energy source.
The use of fossil fuels will have to be curtailed, in all probability long before the supplies actually run out, for their increasing use has its dangers. Coal is not pure carbon, and oil is not pure hydrocarbon; in each substance, there are minor quantities of nitrogen and sulfur compounds. In the burning of fossil fuels (particularly coal), oxides of nitrogen and sulfur are released into the air. A ton of coal does not release much; but with all the burning that takes place, some 90 million tons of sulfur oxides were being discharged into the atmosphere each year in the course of the 1970s.
Such impurities are a prime source of air pollution and, under the proper meteorological conditions, of smog (that is, “smoky fog”), which blankets cities, damages lungs, and can even kill people who already have pulmonary disease.
Such pollution is washed out of the air by the rain, but this is a solution that merely creates a new and possibly worse problem. The nitrogen and sulfur oxides, dissolving in water, turn that water very slightly acid, so that what falls to the ground is acid rain.
The rain is not acid enough to bother us directly, but it falls into ponds and lakes and acidifies them—only slightly, but enough to kill much of the fish and other water life, especially if the lakes do not have beds of limestone which might in part neutralize the acid. The acid rain also damages trees. This damage is worst where coal burning is greatest and the rain falls to the east, thanks to prevailing westerly winds. Thus, eastern Canada suffers from acid rain due to coal burning in the American Midwest, while Sweden suffers from the coal burning in western Europe.
The dangers of such pollution can become great indeed if fossil fuels continue to be burned and in increasing volume. Already, international conferences are being held in connection with the problem.
To correct this, oil and coal must be cleaned before being burned—a process that is possible but that will obviously add to the expense of the fuel. However, even if coal that was pure carbon, and oil that was pure hydrocarbon, were burned, the problems would not end. Carbon would burn to carbon dioxide, while hydrocarbon would burn to carbon dioxide and water. These are relatively harmless in themselves (though some carbon monoxide—which is quite poisonous—is bound to be formed as well), and yet the matter cannot be dismissed.
Both carbon dioxide and water vapor are natural constituents of the atmosphere. The quantity of water vapor varies from time to time and place to place, but carbon dioxide is present in constant amounts of about 0.03 percent by weight. Additional water vapor added to the atmosphere by burning fossil fuel finds its way into the ocean eventually and is, in itself, an insignificant addition. Additional carbon dioxide will dissolve, in part, in the ocean and react, in part, with the rocks, but some will remain in the atmosphere.
The quantity of carbon dioxide in the atmosphere has increased by half again its original amount since 1900, thanks to the burning of coal and oil, and is increasing measurably from year to year. The additional carbon dioxide creates no problem where breathing is concerned and may even be considered as beneficial to plant life. It does, however, add somewhat to the greenhouse effect and raises the overall average temperature of the earth by a small amount. Again, it is scarcely enough to be noticeable, but the added tempera ture tends to raise the vapor pressure of the ocean and to keep more water vapor in the air, on the whole, and that, too, enhances the greenhouse effect.
It is possible, then, that the burning of fossil fuels may trigger a large enough rise in temperature, to begin melting the ice caps with disastrous results to the continental coastlines. It may also result in long-range climatic changes for the worse. There is even a small possibility that it may initiate a runaway greenhouse effect which would push Earth in the direction of Venus, although we need to know a great deal more about atmospheric dynamics and temperature effects before any predictions we make can be more than guesses.
In any case, however, the continued burning of fossil fuels must be treated with considerable caution.
And yet our energy needs will continue and even be far larger than those of today. What can be done?
SOLAR ENERGY
One possibility is to make increasing use of renewable energy sources: to live on the earth’s energy income rather than its capital. Wood can be such a resource if forests are grown and harvested as a crop, though wood alone could not come anywhere near meeting all our energy needs. We could also make much more use of wind power and water power, though these again could never be more than subsidiary sources of energy. The same must be said about certain other potential sources of energy in the earth, such as tapping the heat of the interior (as in hot springs) or harnessing the ocean tides.
Far more important, for the long run, is the possibility of directly tapping some of the vast energy pouring on the earth from the sun. This insolation produces energy at a rate that is some 50,000 times as great as our current rate of energy consumption. In this respect, one particularly promising device is the solar battery, or photovoltaic cell, which makes use of solid-state devices to convert sunlight directly into electricity (figure 10.2).
Figure 10.2. A cell. Sunlight striking the thin wafer frees electrons, thus forming electron-hole pairs. The p-n junction acts as a barrier, or electric field, separating electrons from holes. A potential difference therefore develops across the junction, and current then Rows through the wire circuit.
As developed by the Bell Telephone Laboratories in 1954, the photovoltaic cell is a Hat sandwich of n-type and p-type semiconductors that is part of an electric circuit. Sunlight striking the plate knocks some electrons out of place—the usual photoelectric effect. The freed electrons move toward the positive pole and holes move toward the negative pole, thus constituting a current. Not much current is produced as compared with an ordinary chemical battery, but the beauty of the solar battery is that it has no liquids, no corrosive chemicals, no moving parts: it just keeps on generating electricity indefinitely merely by lying in the sun.
The artificial satellite Vanguard I, launched by the United States on 17 March 1958, was the first to be equipped with a photovoltaic cell to power its radio signals; and those signals were continued for years since there was no “off” switch.
The amount of energy falling upon one acre of a generally sunny area of the earth is 9.4 million kilowatt-hours per year. If substantial areas in the earth’s desert regions, such as Death Valley and the Sahara, were covered with solar batteries and electricity-storing devices, they could provide the world with its electricity needs for an indefinite time—for as long, in fact, as the human race is likely to endure, if it does not commit suicide.
One catch is, of course, expense. Pure silicon crystals out of which thin slices can be cut for the necessary cells are expensive. To be sure, since 1958, the price has been cut to 1/250th of what it originally was, but solar electricity is still about ten times as expensive as oil-generated electricity.
Of course, photovoltaic cells may get cheaper still and more efficient, but collecting sunlight is not as easy as it sounds. Sunlight is copious but dilute; and as I mentioned, two paragraphs back, vast areas may have to be coated with them, if they are to serve the world. Then, too, it is night for half the time; and even in the daytime, there may be fog, mist, or cloud. Even clea
r desert air absorbs a sizable fraction of the solar radiation, especially when the sun is low in the sky. Maintenance of large, exposed areas on Earth would be expensive and difficult.
Some scientists suggest that such solar power stations be placed in orbit about the earth under conditions where nearly unbroken sunlight with no atmospheric interference could increase production per unit area as much as sixtyfold, but this is not likely to come to pass in the immediate future.
The Nucleus in War
Between the large-scale use of fossil fuels in the present and the large-scale use of solar energy in the future, there is another source of energy, available in large quantities, which made its appearance rather unexpectedly, less than half a century ago, and which has the potentiality of bridging the gap. This is nuclear energy, the energy stored in the tiny atomic nucleus.
Nuclear energy is sometimes called atomic energy, but that is a misnomer. Strictly speaking, atomic energy is the energy yielded by chemical reactions, such as the burning of coal and oil, because they involve the behavior of the atom as a whole. The energy released by changes in the nucleus is of a totally different kind and vastly greater in magnitude.
THE DISCOVERY OF FISSION
Soon after the discovery of the neutron by Chadwick in 1932, physicists realized that they had a wonderful key for unlocking the atomic nucleus. Since it had no electric charge, the neutron could easily penetrate the charged nucleus. Physicists immediately began to bombard various nuclei with neutrons to see what nuclear reactions could be brought about; among the most ardent investigators with this new tool was Enrico Fermi of Italy. In the space of a few months, he had prepared new radioactive isotopes of thirty-seven different elements.