Our civilisation and our standard of living depend on an adequate supply of energy. We need energy to light and heat our homes, to cook our food, to drive our transport and power our communications and to provide the motive force that drives the factories. Without energy all this would be impossible on the scale needed, and our civilisation would soon collapse into barbarism. Our dependence on energy is strikingly illustrated by the connection between average life expectancy and energy consumption. People in the poorer countries, especially in Africa and Asia, have an average energy consumption between 0.01 and 0.1 tons of coal equivalent per person per year and have an average life expectancy of between 35 and 45 years. At the other end of the scale, people in the rich well-developed countries in Europe, North America and Japan use between five and ten tons of coal equivalent per person per year and have an average life expectancy between 70 and 75 years. This difference is a measure of the energy that is needed to bring the standard of living of all people to the level now enjoyed by the most favoured ones.
Over the centuries this energy has been obtained in many ways. In ancient times wood was the main fuel, and it provided heat for cooking and warmth. It was often used more rapidly than it was replaced by new growth, and the forests of countries surrounding the Mediterranean were gradually destroyed, followed by the forests of central Europe. In many countries even today wood is the main fuel, but housewives have to walk increasing distances to gather the wood they need. Other energy sources are crop residues and dried animal dung. Ideally these organic residues should be returned to the soil, so burning them gradually reduces its fertility. These are still the main sources of fuel for some two billion people in the developing countries. They provide the energy equivalent of about a billion tonnes of oil each year, about the same as the energy provided by coal in Europe and the USA combined.
The increasing scarcity of wood stimulated searches for alternative energy sources, and soon coal was found, first near the surface and later underground. It has a higher calorific value than wood and can be transported rather more easily. Soon it became the main energy source in many developed countries and provided the power for the industrial revolution, especially in places where iron ore was also available.
During the nineteenth century oil was found, first in the USA and then in many other countries. It has many advantages over coal: it can easily be transported over large distances by pipelines and tankers, and is the basis of the petrochemical industry. During the twentieth century it gradually displaced coal as the favoured energy source. Natural gas was often found in association with oil, and provided a convenient source of lighting and heating.
The nineteenth century also saw the rapid development of the electrical industry for communication, heating and power. Electricity has the advantage of being very easily transported from the generating station to where it is needed. It soon displaced gas as a source of light and became a convenient power source for factories. Electricity is practicable for suburban trains, but long distance trains and ships, which used to be driven by coal, are now mainly driven by oil.
Electricity is generated by turbines driven by steam produced by burning coal or oil. The turbine can also be driven by water, and indeed water wheels have been used since ancient times to rotate the millstones to grind corn. Hydroelectric power is thus another source of electricity.
During the twentieth century the world's economy and population increased more rapidly than ever before and the total energy consumption rose even more rapidly. World population is doubling on the average every 35 years; the rate of increase varies greatly from country to country, it is greatest in Africa and Latin America and almost stationary in more developed areas such as Europe and North America. Together with the increase in the standard of living, this results in the world energy consumption doubling every fourteen years. This is not a measure of the real energy needs, and many billions of people still lack the energy for even some of the basic necessities of life. At present people in the less developed countries are forced to try to survive on a small fraction of the energy used by people in the developed world. The amount of energy needed to raise the standard of living of the people in the poorer countries to that in the developed ones can be estimated from the United Nations Human Development Index. This shows that an acceptable standard of living requires about five thousand kWh per year, or 200,000 megajoules (MJ) per person. Assuming that the world population will rise to eight billion this gives an energy requirement of about 1.6 x 10 (15) MJ. The present population of six billion uses about 0.41 x 10 (15) MJ. Thus world energy production will have to be increased at least fourfold to bring the standard of living of people in the developing countries up to that in the developed ones (Fanchi 2006). This estimate does not take account of the likelihood that most of the increase in population will take place in the poorer countries. It has been estimated that world energy production will grow from 4.43 x 10 (14) MJ in 2003 to about 7.6 x 10 (14) MJ by 2030, which is completely inadequate. Another estimate is an increase from 9.3 billion toe (tonnes of oil equivalent) in 2003 to 15.4 in 2020, with 90% of the increase in the developing countries.
The world consumption of energy increased by 4.3% from 2003 to 2004, and this trend is likely to continue. At present the relative amounts due to the various sources are: oil 36.8%; gas 23.7%; coal 27.2%; nuclear 6.1% and hydro 6.2%. Of these coal is the fastest growing and also the most polluting. The main sources of energy at the present time, coal, oil and gas, are limited and indeed are fast being exhausted. Studies of the available resources indicate that in the foreseeable future they will become increasingly difficult to extract in the quantities needed. Furthermore, they are seriously polluting and are already causing great damage to the earth and its atmosphere. Eventually we shall have to learn how to do without them, and the sooner we do so the better.
The generation of electricity is expected to show a similar rise, increasing from 13,290 billion kWh in 2001 to 23,702 in 2025. In the developing countries the rate of increase is 3.5% per year, compared with 2.3% per year for all countries. The proportion of electricity generated by natural gas increased in the same period from 18% to 25%.
What can replace them? There are many possibilities, and they have to be assessed considering their capacity, cost, reliability, safety and effects on the environment. This is done in the following sections. Whenever possible, these assessments must be made numerically because this is the only way to make objective comparisons. As Lord Kelvin once remarked, "unless you can measure what you are talking about, and express it in numbers, your knowledge is of a meagre and unsatisfactory kind". When evaluating the characteristics of the production of energy by different sources these numbers are often subject to many uncertainties, so it is also important to estimate the range of these uncertainties. Approximate numbers are better than no numbers at all. The existence of numerical data is not however a guarantee of its correctness. Many numbers widely quoted are simply wrong, and sometimes numbers that are correct are extrapolated far beyond their range of validity and used as the basis of inaccurate generalised statements. It is thus not enough to collect a few isolated examples and assume that they are representative of global trends. There is no way of avoiding the laborious task of collecting fully representative statistics and recognising that they can only be used to reach valid conclusions for the time and area actually studied.
It is useful to collect and compare numerical estimates of the same quantity obtained by different investigators as, for example is done in Table 3.3 for the costs of electricity from various sources. Physicists know very well how difficult it is to obtain a reliable measurement of some physical quantity such as the charge on the electron, when no one has any motive for preferring one result rather than another. It is quite different when comparing energy sources, because there may be strong political pressure to reach a favoured result. Furthermore, there is an inherent difference between the degree of accuracy that can be reached when measuring physical quantities, and that attainable in a social context. Thus the accuracy of determination of the charge on the electron can presumably be increased without limit, whereas there is an inherent limit to the values of social parameters. Thus we can collect statistics relating to a specific stretch of time at a certain place. We cannot improve the statistics by extending the period of time because the method of energy production may itself have changed; we cannot be sure that the quantity we are measuring is independent of time and place and this inevitably affects the accuracy with which it can be determined. The figures given here are the best I could find, but certainly in many cases better figures will eventually become available.
Energy sources are often divided into renewable and non-renewable. A renewable source is one that is not used up, but what we really want is one that is always available. This is satisfied by a source such as water, the fuel for fusion reactors, that is so plentiful compared with our needs that it will never be exhausted, and also to a lesser extent uranium. Coal and oil are non-renewable because there are limited amounts present in the earth. However the reserves of oil and coal are not like the gold in the Bank of England, where the number of bars can be counted. The amount we can extract depends on the price we are prepared to pay for it. Oilfields, for example, have very different extraction costs. In the Middle East, oil gushes out freely and is cheap and readily available. It is much more expensive to extract it from the North Sea, as oil rigs have to be built in deep water. This consideration applies even more strongly to minerals such as those containing uranium. Rich ores are relatively rare, while poorer ones are very widespread. It is even possible to extract uranium from sea water.
It might be thought that the lifetimes of availability of an energy source can be obtained by dividing the estimated reserves by the yearly consumption. This gives about 42 years for oil, 65 for natural gas and 217 for coal. Another way of expressing this is to divide the total recoverable resource of 1260 TWy (terawatts per year) by the world consumption, which amounted to 14.1 TWy in 2003. This gives about 90 years. By 2030 the world consumption is expected to be about twice this, halving the expected lifetime (Avery 2007, pp. 107, 113). However the situation is not as bad as this because there are several competing effects that occur when any raw material becomes more difficult to obtain. Initially the price rises, providing an incentive to reduce consumption and to look for alternatives sources. Improved technology and the higher price make it economic to bring into operation sources previously considered to be exhausted. The Stone Age ended because bronze and iron were discovered, not because the supply of flints ran out. This process continues: steel and other metals are replaced by plastics and composites. In other cases the incentive for change comes from other motives, such as the replacement of domestic coal fires by electric heaters and much correspondence by e-mail. The net result of all this is that, contrary to what might have been expected, it is found for coal and oil that the ratio of reserves to annual consumption and the cost both remain remarkably steady as a function of time (Lomborg 2001/2004), as shown in Figures 1.1 and 1.2. Of course this cannot go on for ever, but it is difficult to estimate the lifetimes, although prices are certainly expected to rise.
It is also difficult to obtain accurate estimates of the cost of energy from various sources. It is relatively easy to determine the cost of construction of a power station, but not to foresee its working life. A power station may last fifty years or more, and it is not easy to determine the lifetime in advance. During that time the value of the currency is likely to change substantially which makes it difficult to calculate the true cost of the electricity generated.
For most industrial and domestic uses, particularly when electricity is being used, it is essential that the supply is reliable. The use of computers is spreading, and an interruption of electrical power immediately brings many vital activities to a standstill. Industrial processes controlled by computers come to a stop, electronic communications are cut, and supermarkets have to close. The cost of such interruptions, even if quite short, can be very high. If the breakdown is prolonged, households relying on electricity are in severe difficulties as heaters, lights and refrigerators fail. We are so used to relying on our power supplies that we are thrown into disarray when they cease. In New York a few years ago, a power station was overloaded and so automatically cut out, throwing more load on another power station, which also cut out and so on until the whole electrical grid was down. The system was so complicated that it took several days before power was restored. Such events are very rare, but power breakdowns are everyday occurrences in many countries, so that it is not possible to carry out lengthy computer calculations or delicate manufacturing processes.
It is neither possible nor necessary for every power station to be able to operate indefinitely, but they should at least normally operate for several months at a time, and this is the case for coal, gas, nuclear and hydroelectric power stations. Their electrical output is fed into a national grid, so if one power station breaks down the load can be carried by the others without interrupting the supply. From time to time power stations must be shut down for routine maintenance, and the grid allows this to be done without difficulty. The demand for electricity fluctuates, but usually in a predictable way. The greatest demand is in the winter, so most maintenance shutdowns are scheduled for the summer. The greatest demand comes during sharp cold spells, so the maximum number of power stations are made available at times when this may happen.
There are some applications where the reliability of the energy source is not important. For example, a farmer may need to have water available for irrigation, so he installs a storage tank and uses a windmill to pump water from below ground to fill it. Providing the tank is large enough, and the wind blows now and then, there is always enough water when needed, and it is topped up whenever the wind is blowing. In this way the unreliability of the energy source is converted to reliability of the system. This is only possible when the amount of energy to be stored is relatively small, as it is uneconomic and often impossible to store very large amounts of energy.
Another requirement is that the energy is available in concentrated form, that is at a higher temperature than the surroundings. Every room contains a large amount of energy, but it is useless for boiling water in a kettle since the availability of energy depends on the temperature difference between source and surroundings. Energy exists in many form, kinetic, potential, electrical and chemical, and since conversion from one form to another always takes energy it should be in the form needed whenever possible. As MacKay (2008) remarks, 'you can't power a TV with cat food, nor can you feed a cat with energy from wind turbines'.
Safety is an essential requirement for energy generation. Perfect safety is impossible, so it is necessary to ensure that any source is a safe as reasonably possible. Increasing safety is costly, so a balance has to be struck between conflicting demands. The same applies to the requirement that adverse environmental effects should be minimised.
Energy is very often wasted, and it is frequently urged that the energy crisis can be solved by increasing the efficiency of energy use and eliminating waste. This is discussed in the next section and in the following sections the possible alternative energy sources are assessed according to the five criteria proposed above.
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