T206 Long Tour 3: Present Energy Sources and Sustainability








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3. Present Energy Sources and Sustainability

So what are the principal energy systems used by humanity at present, and how sustainable are they?

Until quite recently, human energy requirements were modest and our supplies came either from harnessing natural processes such as the growth of plants, which provided wood for heating and food to energize human or animal muscles, or from the power of water and wind, used to drive simple machinery.

Fossil Fuels

But the nineteenth and twentieth centuries saw a massive increase in global energy use, based mainly on burning cheap and plentiful fossil fuels: first coal, then oil and natural gas. These fossil fuels now supply over 80% of the world’s current energy consumption (see Box 1.1).

Box 1.1 World Primary Energy Consumption

The population of the world rose nearly four-fold during the twentieth century, from 1.6 billion in 1900 to approximately 6.1 billion in 2000. However, world primary energy use increased at a much faster rate. Between 1900 and 2000, it rose more than 10-fold (Figure 1.8). (Primary Energy will be defined in Chapter 2).

Figure 1.8 (a) Growth in world primary energy use, 1850–2000; (b) Growth in world population, 1850–2000
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For most of the nineteenth century the world’s principal fuel was firewood (or other forms of traditional ‘bioenergy’), but coal use was rising fast and by the beginning of the twentieth century it had replaced wood as the dominant energy source. During the 1920s, oil in turn began to challenge coal and by the 1970s had overtaken it as the leading contributor to world supplies. By then, natural gas was also making a very substantial contribution, with nuclear energy and hydro power also supplying smaller but significant amounts.

As Figure 1.9 shows, total world primary energy use in 2000 was an estimated 424 million million million joules, i.e., 424 exajoules, equivalent to some 10 100 million tons of oil. All of these quantities and units are explained in more detail in Section 2.2 of the next chapter.

Figure 1.9 Percentage contributions of various energy sources to world primary energy consumption, 2000. Total consumption in 2000 was 424 exajoules, equivalent to just over 10 000 million tonnes of oil. The average rate of consumption was some 13.4 million million watts (13.4 terawatts). Note that the actual amounts of electricity produced by nuclear and hydro power were almost the same, but due to a statistical convention in the definition of primary energy, the nuclear contribution is multiplied by a factor of 3 (see Chapter 2)
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Figure 1.10 is a photo of a supertanker

Figure 1.10 A typical supertanker used to ship oil around the globe. To transport the c.3500 million tonnes of oil used by the world in 2000 would require some 14 000 tanker journeys, assuming a typical tanker capacity of 250 000 tonnes. Total world primary energy use in 2000 was equivalent to the capacity of some 40 000 supertankers of this size

By the year 2000, oil was still the largest single contributor to world supplies, providing about 35% of primary energy, with gas and coal supplying roughly equal shares at around 21–22%, nuclear providing nearly 7% and hydropower 2%. In 2000, traditional biofuels still supplied an estimated 11%, while more modern forms of ‘bioenergy’ provided around 2%, with other ‘new renewables’ like wind power contributing a very small (though rapidly growing) fraction of world demand.

On average, world primary energy use per person in 2000 was about 70 thousand million joules (70 gigajoules), including non-commercial bioenergy. This is equivalent to about one and two-thirds tonnes of oil per person per year, or about 5 litres (just over one Imperial gallon) of oil per day.

But this average conceals major differences between the inhabitants of different regions. As Figure 1.11 illustrates, North Americans annually consume the equivalent of about 8 tonnes of oil per head (about 20 litres per day), whereas residents of Europe and the former Soviet Union consume about half that amount, and the inhabitants of the rest of the world use only about one-tenth.

Figure 1.11 Per capita primary energy consumption, in tonnes of oil equivalent per year, for different regions of the world and for the world as a whole, 1975–2000. World consumption per person has shown almost no growth over the past 20 years. North American consumption per capita is more than twice that of Europe and the former Soviet Union, and almost 10 times the level in the Rest of the World. Note that these figures include only commercially-traded fuels (i.e. they exclude traditional biofuels)
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Fossil fuels are extremely attractive as energy sources. They are highly concentrated, enabling large amounts of energy to be stored in relatively small volumes. They are relatively easy to distribute, especially oil and gas which are fluids.

During the twentieth century, these unique advantages enabled the development of increasingly-sophisticated and effective technologies for transforming fossil fuel energy into useful heat, light and motion; these ranged from the oil lamp to the steam engine and the internal combustion engine. Today, at the beginning of the twenty-first century, fossil fuel-based systems reign supreme, supplying the great majority of the world’s energy.

The fossil fuels we use today originated in the growth and decay of plants and marine organisms that existed on the earth millions of years ago. Coal was formed when dead trees and other vegetation became submerged under water and were subsequently compressed, in geological processes lasting millions of years, into concentrated solid layers below the earth’s surface. Oil and associated natural gas originally consisted of the remains of countless billions of marine organisms that slowly accreted into layers beneath the earth’s oceans and were gradually transformed, through geological forces acting over aeons of time, into the liquid and gaseous reserves we access today by drilling into the earth’s crust. The fossil fuels are composed mainly of carbon and hydrogen, which is why they are called hydrocarbons.

Figure 1.12 is a photo of Parc Colliery taken in 1960

Figure 1.12 Parc Colliery in Cwm Park, Rhonda Valley, Glamorgan, South Wales, (photo, 1960). Coal was the fuel that powered the industrial revolution. Its combustion produces relatively large amounts of carbon dioxide (CO₂) compared with other fuels. It also results in particulates (soot), and sulphur dioxide emissions, though these can be reduced by various techniques. The use of coal in UK homes and industry has now been largely superseded by natural gas, but it is still used for electricity generation. Huge world-wide coal reserves remain, enough for several hundred years’ use at current rates

Figure 1.13 is a photo of a North Sea oil drilling platform structure

Figure 1.13 A North Sea oil drilling platform. Oil is the world’s leading energy source. Its high energy density and convenience of use are particularly advantageous in the transport sector, where it is the dominant fuel. Oil combustion produces less CO₂ per unit of energy released than burning coal, but more CO₂ than burning natural gas. Proven world oil reserves are sufficient for about 40 years of use at current rates

Figure 1.14 is a photo of a natural gas drilling platform

Figure 1.14 The offshore rig Semac 1, a natural gas pipelaying barge in the North Sea. Natural gas combustion produces significantly lower CO₂ emissions per unit of energy than the combustion of other fossil fuels. Emissions are also free from sulphur dioxide or particulates. The relative cleanliness and convenience of natural gas have made it the preferred fuel for heating and, increasingly, for electricity generation in Western Europe. Proven world gas reserves are sufficient for about 60 years of use at current rates

Since the fossil fuels were created in specific circumstances where the geological conditions were favourable, the largest deposits of oil, gas and coal tend to be concentrated in particular regions of the globe (see Figure 1.15) – although less appreciable deposits are remarkably widespread. The majority of the world’s oil reserves are located in the Middle East and North Africa, while the majority of our natural gas reserves are split roughly equally between the Middle East/North Africa and the former Soviet Union. (BP, 2002) Although coal deposits are rather more evenly spread throughout the world, three-quarters of world coal reserves are concentrated in just four countries: Australia, China, South Africa and the United States of America. (United Nations, 2000; BP, 2002) (Figure 1.15).

Figure 1.15 Proven world fuel reserves, 2001: (a) oil reserves; (b) natural gas reserves; (c) coal reserves
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Although human society now consumes fossil fuels at a prodigious rate, the amounts of coal, oil and gas that remain are still very large. One simple way of assessing the size of reserves is called the reserves/production (R/P) ratio – the number of years the reserves would last if use continued at the current rate.

Coal has the largest R/P ratio. Present estimates suggest the world has more than 200 years’ worth of coal left at current use rates. For oil, current R/P estimates suggest a lifetime of about 40 years at current rates. For gas, the R/P ratio is somewhat higher, at around 60 years. (BP, 2002) (Figure 1.16)

Figure 1.16 Reserves/production (R/P) ratios (in years) for oil, natural gas and coal, 2000, for various regions of the world and the world as a whole
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Fossil fuel reserves/production ratios need to be interpreted with great caution, however. They do not take into account the discovery of new proven reserves, or technological developments that enable more fuel to be extracted from deposits or improve the economic viability of ‘difficult’ deposits.

Despite these developments, it seems likely that, at least in the case of oil from conventional sources, world production will reach a peak in the first decade of this century. From then on, although vast quantities of conventional oil will still remain, the resource will be on a declining curve. (United Nations, 2000; Campbell and Laherrere, 1998). This seems likely to lead to increased instability and potential for conflict as the twenty-first century proceeds.

The massive use by our society of coal, oil and gas has, literally, fuelled enormous increases in material prosperity – at least for the majority in the industrialized countries. But it has also had numerous adverse consequences. As already mentioned, these include air and water pollution, mining accidents, fires and explosions on oil or gas rigs, conflicts over access to fuel resources and, perhaps most profoundly, the global climate change that is likely to be the result of increasing atmospheric carbon dioxide concentrations caused by fossil fuel combustion (see Box 1.2).

Box 1.2. The Greenhouse Effect and Global Climate Change

The greenhouse effect in its natural form has existed on the planet for hundreds of millions of years and is essential in maintaining the Earth’s surface at a temperature suitable for life. Without it, we would all freeze.

The sun’s radiant energy, as it falls on the earth, warms its surface. The earth in turn re-radiates heat energy back into space in the form of infra-red radiation. The temperature of the earth establishes itself at an equilibrium level at which the incoming energy from the sun exactly balances the outgoing infra-red radiation.

If the earth had no atmosphere, its surface temperature would be approximately minus 18 °C, well below the freezing point of water. However our atmosphere, whilst largely transparent to incoming solar radiation in the visible part of the spectrum, is partially opaque to outgoing infra-red radiation. It behaves in this way because, in addition to its main constituents, nitrogen and oxygen, it also contains very small quantities of ‘greenhouse gases’. Put simply, these enable the atmosphere to act like the panes of glass in a greenhouse, allowing the sun to enter but inhibiting the outflow of heat, so keeping the earth’s surface considerably warmer than it would otherwise be. The average surface temperature of the earth is in fact around 15 °C, some 33 °C warmer than it would be without the greenhouse effect.

Figure 1.17 A Simplified Depiction of how the Greenhouse Effect Raises the Earth’s Temperature
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The most important greenhouse gases are water vapour, carbon dioxide and methane, though other gases such as the Chlorofluorocarbons (CFCs) also play significant but lesser roles.

Water vapour evaporating from the oceans plays a major part in maintaining the natural greenhouse effect, but human activities have very little influence on the vast processes through which water cycles between the oceans and the atmosphere.

Carbon dioxide (CO₂) is also primarily generated by natural processes. These include the process of respiration, in which organisms ‘breathe out’ carbon dioxide; and the emissions of CO₂ that occur when organisms die and the carbon compounds of which they are composed decay. But since the industrial revolution, the burning of fossil fuels by humanity has been adding substantial quantities of CO₂ to our atmosphere. The fossil fuels are essentially compounds of carbon and hydrogen. Coal consists mostly of carbon, the chemical symbol for which is C. Natural gas, the chemical name for which is methane, consists of carbon and hydrogen. Each carbon atom is surrounded by four hydrogen atoms, so in chemical shorthand its symbol is CH₄. Oil is a more complex mixture of many different hydrocarbon molecules. When any of these fuels is burned, carbon dioxide is produced, along with water.

The concentration of CO₂ in the atmosphere in pre-industrial times was around 280 parts per million by volume (ppmv) but levels have been steadily rising since then, reaching some 360 ppmv in 2000.

Figure 1.18 (a) Atmospheric concentrations of carbon dioxide (CO₂), 1854–2000; (b) estimated global mean temperature variations, 1860–2000. Carbon dioxide data from 1958 were measured at Mauna Loa, Hawaii; pre-1958 data are estimated from ice cores
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The other main greenhouse gas, methane, is given off naturally when vegetation decays in the absence of oxygen – for example, under water. However various human activities, including increasing rice cultivation, which causes methane emissions from paddy fields, and leaks of fossil methane from natural gas distribution systems, have caused the levels of methane in the atmosphere to increase sharply. Concentrations have risen from about 750 parts per billion by volume (ppbv) in pre-industrial times to around 1750 ppbv in 2000.

These additional emissions of carbon dioxide and methane are the main causes of the so-called anthropogenic – that is, human-induced – greenhouse effect. Unlike the operation of the natural greenhouse effect, which is benign, the anthropogenic greenhouse effect is almost certainly the cause of a global warming trend that could have very serious consequences for humanity. Though a small minority dissents, the majority of scientists now believe that the anthropogenic effect, acting to enhance the natural processes, has already caused the mean surface temperature of the earth to rise by about 0.6 °C during the twentieth century (Intergovernmental Panel on Climate Change, 2001). Moreover, if steps are not taken to limit greenhouse gas emissions, atmospheric CO₂ levels will probably rise by 2100 to between 540 and 970 ppmv (depending on the assumptions made). These levels would be between two and three times the pre-industrial CO₂ concentration, and would be likely to lead to rises in the earth’s mean surface temperature of between 1.4 and 5.8 °C by the end of the century. Such temperature rises would be unprecedented since the ending of the last major Ice Age, more than 10 000 years ago.

These temperature rises would be very likely to result in significant changes to the earth’s climate system. Such changes would probably include more intense rainfall, more tropical cyclones, or long periods of drought, all of which would disrupt agriculture. Moreover, ecosystems might be damaged with some species unable to adapt quickly enough to such rapid changes in climate.

In addition, due to thermal expansion of the oceans, sea levels would be expected to rise by around 0.5 metres during the twenty-first century, sufficient to submerge some low-lying areas and islands. In the longer term, further sea level rises would result if the Antarctic ice sheets were to melt significantly.

Nuclear Energy

Nuclear energy is based on harnessing the very large quantities of energy that are released when the nuclei of certain atoms, notably uranium-235 and plutonium-239, are induced to split or ‘fission’. The complete fission of a kilogram of uranium-235 should produce, in principle, as much energy as the combustion of over 3000 tonnes of coal. In practice, the fission is incomplete and there are other losses, but nevertheless nuclear fuels are more highly-concentrated sources of energy than fossil fuels.

The heat generated by nuclear fission in a nuclear power station is used to raise high-pressure steam which then drives steam turbines coupled to electrical generators, as in a conventional power station.

Figure 1.19 shows Queen Elizabeth II at the opening ceremony at Calder Hall , 1956

Figure 1.19 Queen Elizabeth II opening Calder Hall nuclear power station in Cumbria, 1956

The development of ‘peaceful’ nuclear electricity generation after its use for military purposes in World War 2 was initially heralded as ushering-in a new era of virtually-limitless, clean power that some predicted would be ‘too cheap to meter’. In practice, however, nuclear electricity has proved to be more expensive than that from fossil fuels. Since the UK opened the world’s first grid-connected nuclear power station at Calder Hall in Cumbria in 1956, nuclear electricity generation has expanded to a point where it now accounts for nearly 7% of world primary energy, and for over 17% of the world’s electricity. In some countries, it is the principal source of electricity generation. France, for example, derives three-quarters of its electricity from nuclear power.

Figure 1.20 shows the two large cooling towers at Civaux

Figure 1.20 The Civaux Pressurized Water (PWR) nuclear Reactor near Poitiers, Vienne, France. In 2000, France produced 77% of its electricity from nuclear power

A major advantage of nuclear energy is that the operation of nuclear power plants results in no emissions of CO₂ emissions or of other ‘conventional’ pollutants like sulphur dioxide. However, there are some emissions from the fossil fuel used in uranium mining, nuclear fuel manufacture, and the construction of nuclear power plants.

There seems little danger of the world ‘running out’ of nuclear fuel in the near future. Uranium reserves have been identified in many countries and are sufficient for many decades of use at current rates, and there are probably enough additional deposits to extend this to several centuries. Furthermore, advanced nuclear technologies such as the ‘fast breeder reactor’ (FBR) could enable uranium deposits to be used even more effectively, thus extending the lifetime of reserves. In an FBR, the plentiful but non-fissile isotope uranium-238 is transformed into fissile plutonium-239, which can then be used as reactor fuel. But the development of FBRs has been inhibited by substantial technical and safety problems, and by the low price of uranium which currently makes the technology un-competitive economically.

Although the majority of nuclear reactors in most countries have operated without serious safely problems, a number of major accidents, like those at Windscale in the UK in 1957, Three Mile Island in the USA in 1979 and Chernobyl in the Ukraine in 1986, have created widespread public unease about nuclear technology in general – despite the opinion of nuclear-industry experts who argue that such anxieties are irrational.

Figure 1.21 is an aerial photo showing the devastation at Chernobyl following the accident in 1986

Figure 1.21 The Chernobyl nuclear power plant following the accident in 1986

Less spectacular are the continued releases of harmful radioactive by-products, in small but insidious and cumulative quantities, to the atmosphere and oceans during the routine operation of nuclear power plants and fuel manufacturing or reprocessing facilities.

There is also the problem of how, ultimately, to dispose of nuclear waste products, some of which remain hazardous for many thousands of years; and the problem of proliferation of nuclear materials such as plutonium-239 and uranium-235, which could fall into the hands of ‘rogue states’ or ‘terrorists’ capable of creating crude but devastating atomic weapons from them. Nuclear power stations and reprocessing facilities may themselves be vulnerable to terrorist attacks, which could result in the release of very large quantities of radioactive substances into the environment.

Despite these difficulties, the nuclear industry is attempting to develop more advanced types of nuclear reactor which, it claims, will be cheaper to build and operate, and inherently safer, than existing designs. These are being promoted as an improved technological option for generating the carbon-free electricity that will be required later in the twenty-first century if global climate change is to be mitigated.

Another potentially important nuclear technology is that of nuclear fusion. As its name implies, this involves the fusing together of atomic nuclei, in this case those of deuterium (so-called ‘heavy’ hydrogen). This process, similar to that underlying the generation of energy within the sun, also results in the release of very large amounts of energy. However, in order to create fusion on earth it is necessary to create conditions in which the nuclei of special forms (called isotopes) of hydrogen interact in an extremely confined space at extremely high temperatures, and so far scientists have only been able to make this happen for a few seconds.

Moreover, the energy required to power the process currently greatly exceeds the energy generated. Research into fusion power continues, with substantial funding, but most experts consider that the technology, even if eventually it can be demonstrated successfully, is very unlikely to become commercially available for many decades.

Figure 1.22 shows the inside of the experimental fusion reactor at Culham

Figure 1.22 The Joint European Torus (JET) experimental nuclear fusion reactor, located at Culham, Oxfordshire, UK

Bioenergy

Not all fuel sources are, of course, of fossil or nuclear origin. From prehistoric times, human beings have harnessed the power of fire by burning wood to create warmth and light and to cook food.

Wood is created by photosynthesis in the leaves of plants. Photosynthesis is a process powered by solar energy in which atmospheric carbon dioxide and water are converted into carbohydrates (compounds of carbon, oxygen and hydrogen) in the plant’s leaves and stems. These, in the form of wood or other ‘biomass’, can be used as fuels – called biofuels, which are sources of bioenergy.

Wood is still very widely used as a fuel in many parts of the ‘developing’ world. In some countries, other biofuels such as animal dung (ultimately also derived from the growth of plants) are also used. As described in Box 1.1, such traditional biofuels are estimated to supply some 11% of world primary energy, though the data are somewhat uncertain.

If the forests that provide wood fuel are re-planted at the same rate as they are cut down, then such fuel use should in principle be sustainable. When forests are managed sustainably in this way, the CO₂ absorbed in growing replacement trees should equal the CO₂ given off when the original trees are burned. However, this is only true when complete combustion of the wood occurs and all the carbon in the wood is released as carbon dioxide. Although near-complete combustion can be achieved in the best available wood stoves and furnaces, most open fires and stoves are not so efficient. This means that not only is carbon dioxide released (albeit in somewhat smaller quantities if the combustion is incomplete) but other combustion products are also emitted, some of which are more powerful greenhouse gases than CO₂. In particular, these can include methane, which on a molecule-for-molecule basis has 20 times the global warming potential of CO₂ over a 20 year period. The incomplete combustion of wood can therefore release a mixture of greenhouse gases with a greater overall global warming effect than can be offset by the CO₂ absorbed in growing replacement trees. This suggests an urgent need to improve the efficiency of traditional wood burning processes (Smith et al., 2000). However it should be stressed that the overall global effect of greenhouse gas emissions arising from incomplete biomass combustion in developing countries is probably much less than that of emissions from burning fossil fuels, which occurs mostly in the ‘developed’ countries.

A further problem is that in many ‘developing’ countries wood fuel is being used at a rate that exceeds its re-growth, which is not only unsustainable but also results in villagers having to travel ever-increasing distances, often involving great hardship, to gather sufficient firewood for their daily needs. Also, when it has been gathered, firewood is often burned very inefficiently in open fires – as was the case in Britain and many other ‘developed’ countries until quite recently. This not only results in excess greenhouse gas emissions, as we have seen, but also gives much less effective warmth that if an efficient stove were used. Moreover, it usually results in high levels of smoke pollution, with very detrimental health effects.

Not all bioenergy use is in the form of traditional biofuels. As noted in Box 1.1, a significant contribution to world supplies now comes from so-called ‘modern’ bioenergy power plants. These feature the clean, high-efficiency combustion of straw, forestry wastes or wood chips from trees grown in special plantations. The heat produced is either used directly or for electricity generation, or sometimes for both purposes.

Figure 1.23 is a photograph of the building housing the Arbre power station

Figure 1.23 The ‘Arbre’ power station at Eggborough, Yorkshire, UK. Fuelled by wood chips, it has an electrical generating capacity of 10 MW

Municipal wastes, a large proportion of which are biological in origin, are also widely used for heat or electricity production. However, there is considerable controversy over whether or not energy from waste should be regarded as ‘sustainable’. Waste-to-energy plants have been opposed by some environmental groups on the grounds that, in order to be economically viable, they need to be fed with a steady stream of waste over many years, which discourages better solutions to the waste problem, such as material re-use or recycling. There are further concerns over possible emissions of dioxins, which are carcinogenic, from the combustion of chlorine compounds present in municipal waste.

Another modern source of bioenergy is alcohol (ethanol) produced by fermenting sugar cane or maize, which is quite widely used in vehicles in Brazil and some states of the USA. The alcohol is often blended with conventional petroleum to form a mixture known as ‘Gasohol’.

Figure 1.24 shows a car being filled up with fuel based on alcohol

Figure 1.24 A substantial proportion of vehicles in Brazil are fuelled by alcohol derived from sugar cane

Hydroelectricity

Another energy source that has been harnessed by humanity for many centuries is the power of flowing water, which has been used for milling corn, pumping and driving machinery. During the twentieth century, its main use has been in the generation of hydro-electricity, and hydropower has grown to become one of the world’s principal electricity sources. It currently provides some 2.3% of world primary energy. However, the relative contribution of hydroelectric power (and of other electricity-producing renewables) is under-stated by a factor of about three in most national and international compilations of energy statistics. This is due to a convention whereby the heat produced by thermal power stations (both fossil and nuclear-fuelled) is included as part of their primary energy contribution, even though that heat is normally wasted. (A more detailed explanation of such conventions is given in Chapter 2.) The annual electricity outputs of the world’s hydro and nuclear power plants are actually about the same, but due to this statistical convention the primary energy contribution of nuclear is calculated to be about 7% whereas that of hydro is only about one-third of this. Hydro power in 2000 contributed over 17% of world electricity.

Figure 1.25 shows the dam wall holding back the water at Glen Canyon

Figure 1.25 The large hydro-electric power plant at Glen Canyon Dam, Lake Powell, Arizona, which has an electrical generating capacity of 1300 MW. Lake Powell was formed with the construction of the Glen Canyon Dam in 1963, on the Colorado River in Page, Arizona. It is the second largest man-made lake in the United States and is 187 miles long

The original source of hydroelectric power is solar energy, which warms the world’s oceans, causing water to evaporate from them.

In the atmosphere, this forms clouds of moisture which eventually falls back to earth in the form of rain (or snow). The rain flows down through mountains into streams and rivers, where its flow can be harnessed using water wheels or turbines to generate power.

When harnessed on a small scale, hydropower creates few, if any, adverse environmental impacts.

However, many modern hydro installations have been built on a very large scale, involving the creation of massive dams and the flooding of extremely large areas. This often entails the re-location of many thousands of indigenous residents who are usually, to say the least, reluctant to move from their homes. Other impacts include adverse effects on fish and other wildlife, reductions in water-borne nutrients used in agriculture downstream, increases in water-borne diseases – and not least, the rare but catastrophic effects of dam failures. A further problem with large dams is that in certain circumstances trees and other vegetation trapped below water when a reservoir is flooded can decay ‘anaerobically’ (i.e. in the absence of oxygen). This produces methane which, as we have seen, is a more powerful greenhouse gas than the CO₂ that would have been produced if the tree had decayed normally in the presence of oxygen from the atmosphere.

However, the current consensus is that greenhouse gas emissions from hydropower generation are likely to be at least an order of magnitude lower than those from fossil fuel generated electricity (United Nations, 2000).

Summary

This section has described how fossil fuels provide the majority of the world’s energy requirements, with bioenergy, nuclear energy and hydropower also making major contributions. The other ‘renewable’ energy sources currently supply only a small fraction of world demand, although the contribution of these ‘renewables’ seems likely to grow rapidly in coming decades, as we shall see in the following section.

Next: 4. Renewable Energy Sources