4. Renewable Energy Sources

Fossil and nuclear fuels are often termed non-renewable energy sources. This is because, although the quantities in which they are available may be extremely large, they are nevertheless finite and so will in principle ‘run out’ at some time in the future.

By contrast, hydropower and bioenergy (from biofuels grown sustainably) are two examples of renewable energy sources – that is, sources that are continuously replenished by natural processes. Renewable energy sources are essentially flows of energy, whereas the fossil and nuclear fuels are, in essence, stocks of energy.

World-wide, there has been a rapid rise in the development and deployment of renewable energy sources during the past few decades, not only because, unlike fossil or nuclear fuels, there is no danger of their ‘running out’, but also because their use normally entails no (or few) greenhouse gas emissions and therefore does not contribute to global climate change.

The companion volume, Renewable Energy, describes in more detail the renewable energy sources, which range from solar power in its various forms, through bioenergy and hydro to wind, wave, tidal and geothermal energy. (Figure 1.26)

Figure 1.26 The various forms of renewable energy depend primarily on incoming solar radiation, which totals some 5.4 million Exajoules (EJ) per year. Of this, approximately 30% is reflected back into space. The remaining 70% is in principle available for use on Earth, as shown, and amounts to approximately 3.8 million EJ. This is some 10 000 times the current rate of consumption of fossil and nuclear fuels, which in 2000 amounted to some 360 EJ. Two other, non-solar, renewable energy sources are shown in the figure. These are the motion of the ocean tides, caused principally by the moon’s gravitational pull (with a small contribution from the sun’s gravity); and geothermal heat from the earth’s interior, which manifests itself in convection in volcanoes and hot springs, and in conduction in rocks
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The general nature and scope of the various ‘renewables’ can be briefly summarized as follows, beginning with the most important renewable source, solar energy.

Solar Energy

Solar energy, it should firstly be stressed, makes an enormous but largely unrecorded contribution to our energy needs. It is the sun’s radiant energy, as noted in Box 1.2, that maintains the Earth’s surface at a temperature warm enough to support human life. But despite this enormous input of energy to our civilization, the sun is virtually ignored in national and international energy statistics, which are almost entirely concerned with consumption of commercial fuels.

Figure 1.27 Radiation of energy to and from the earth
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The sun has a surface temperature of 6000 °C, maintained by continuous nuclear fusion reactions between hydrogen atoms within its interior. These nuclear reactions will gradually convert all of the hydrogen into heavier elements, but this is a relatively slow process and the sun should continue to supply power for another 5 billion years.

The sun radiates huge quantities of energy into the surrounding space, and the tiny fraction intercepted by the Earth’s atmosphere, 150 million km away, is nonetheless equivalent to about 15 000 times humanity’s present rate of use of fossil and nuclear fuels. Even though approximately one-third of the intercepted energy is reflected away by the atmosphere before reaching the earth’s surface, this still means that a continuous and virtually-inexhaustible flow of power amounting to 10 000 times our current rate of consumption of conventional fuels is available in principle to human civilization.

Solar energy, when it enters our buildings, warms and illuminates them to a significant extent. When buildings are specifically designed to take full advantage of the sun’s radiation, their needs for additional heating and for artificial lighting can be further reduced.

Solar power can also be harnessed by using solar collectors to produce hot water for washing or space heating in buildings.

Such collectors are in widespread use in sunny countries such as Israel and Greece, but are also quite widely used in less sunny places such as Austria. Even in cloudy Britain there are more than 40 000 roof-top solar water heating systems.

Figure 1.28 The roof of this solar house in Oxford has a grid-linked 4 kW array of photovoltaic panels. These generate enough electricity to supply its annual requirements, plus a surplus which is used to provide part of the power to run a small electric car. The roof also incorporates a 5 m² array of solar water heating panels which provide three-quarters of the house’s hot water requirements. The house is well-insulated and includes a conservatory that contributes ‘passive’ solar energy to space heating in Spring and Autumn, supplemented by a natural gas boiler and a small wood-burning stove used on very cold days
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In regions such as Southern California, where solar radiation levels are more than twice those of the UK and skies are clearer, the sun’s rays are strong enough to make it practicable to generate high-temperature steam using arrays of concentrating mirrors. The steam can then be used to power a turbine that drives a generator to produce electricity (Figures 1.29 a and 1.29b).

Figure 1.29 The largest ‘solar thermal-electric’ installation of its kind in the world, the Luz project in California’s Mojave Desert, has a peak output of some 350 megawatts and occupies several square kilometres of land

Harnessing solar energy to provide electricity directly involves the use of a different and more sophisticated technology called solar photovoltaics (PV). Photovoltaic ‘modules’ are made of specially-prepared layers of semi-conducting materials (usually silicon) that generate electricity when photons of sunlight fall upon them. Arrays of PV modules are normally mounted on the roofs or facades of buildings, providing some or all of their electricity needs. (Figures 1.28 and 1.30)

Photovoltaic technology is growing very rapidly and several countries have initiated major development and demonstration programmes. Germany, for example, plans to install 100 000 PV roofs and building facades by the end of 2003.

Figure 1.30 This 3500 m² solar office building at the Doxford International Business Park near Sunderland in the UK incorporates 646 m² of photovoltaic modules. These have a peak power output of 73 kW and generate some 55 000 kWh of electricity per year. The building is well insulated and uses passive solar design to maximise the use of natural daylight and to minimise space heating and air conditioning needs. It is also designed for natural ventilation and night-time cooling

Photovoltaics may well make a significant contribution to world needs in coming decades, but at present its share of world consumption is extremely small. This is mainly due to the very high cost of PV modules, which are currently produced in relatively small quantities. Studies have shown that if the annual output of the manufacturing plants that produce PV modules were increased by a factor of about 20, the cost of PV-generated electricity could be reduced to a point at which it would be competitive with electricity from conventional sources in many industrialized countries.

Indirect Use of Solar Energy

The above examples illustrate the direct harnessing of the sun’s radiant energy to produce heat and electricity. But the sun’s energy can also be harnessed via other forms of energy that are indirect manifestations of its power. Principally, these are bioenergy and hydropower, already discussed in Section 1.3 above, together with wind energy and wave power.

Wind Energy

When solar radiation enters the earth’s atmosphere, because of the curvature of the earth it warms different regions of the atmosphere to differing extents – most at the equator and least at the poles. Since air tends to flow from warmer to cooler regions, this causes what we call winds, and it is these air flows that are harnessed in windmills and wind turbines to produce power.

Wind power, in the form of traditional windmills used for grinding corn or pumping water, has been in use for centuries. But in the second half of the twentieth century, and particularly in the past few decades, the use of modern wind turbines for electricity generation has been growing very rapidly. Installed wind generating capacity has doubled every two and a half years since 1991, and at the end of 2001 the world total was over 23 000 MW. (Windpower Monthly, 2002) Denmark derives more than 15% of its electricity from wind, and in other countries such as Germany, Spain and the United States of America turbines have in recent years been installed at a rate of rate of more than a thousand megawatts per year.

Figure 1.31 This wind farm, at Carno in mid-Wales, is one of the largest in Europe. It incorporates 56 wind turbines, each with a rotor diameter of 44 metres and a tower height of 31.5 metres. The total installed capacity is 33.6 MW, sufficient to provide power for some 25 000 homes

At present, most of these turbines have been installed on land. But several countries have ambitions plans to install thousands of wind turbines offshore. Denmark, for example, has three offshore wind farms and plans many more, as part of its aim of deriving 30% of its electricity from wind by 2020– though these plans are subject to future political approval.

Figure 1.32 The Middelgrunden wind farm, completed in 2001, is located in the sea just off Copenhagen harbour in Denmark. It includes 20 two megawatt wind turbines, which provide 3% of the electricity consumption of the Copenhagen municipality

The UK, too, has ambitious offshore wind power proposals. Britain’s first two offshore wind turbines were installed off Blyth harbour in Northumberland in 2000, and sites have been identified for 13 offshore wind farms that could be built in the coming decade. These would have a total installed capacity of 1600 MW.

Figure 1.33 Britain’s first offshore wind turbines, located 1 km away from the coast at Blyth harbour, Northumberland. The twin 2 MW turbines were installed in 2000 by a consortium including AMEC, Border Wind, Shell Renewables and the Dutch electricity utility Nuon

Figure 1.34 Proposed locations of Britain’s first 13 offshore wind farms. Also shown is the area of the North Sea that would be needed for offshore wind farms to produce 10% of the UK’s current annual electricity demand
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Wave Power

When winds blow over the world’s oceans, they cause waves. The power in such waves, as they gradually build up over very long distances, can be very great – as anyone watching or feeling that power eventually being dissipated on a beach will know.

Various technologies for harnessing the power of waves have been developed over the past few decades, of which the ‘oscillating water column’ (OWC) is perhaps the most widely used. In an OWC, the rise and fall of the waves inside an enclosed chamber alternately blows and sucks air through a special kind of air turbine, which is coupled to a generator to produce electricity.

Figure 1.35 The 500 kW ‘Limpet’ wave energy plant installed in 2001 on the Scottish island of Islay

Wave energy technology is not as fully developed as wind power or photovoltaics, but its potential has recently been re-emphasized by several governments, including that of the UK. Rapid advances in developing and demonstrating the technology can be expected over the coming decade.

All of the renewable energy sources described above – solar, bioenergy, hydropower, wind and wave – are, as we have seen, either direct or indirect forms of solar energy. However there are two other renewable sources, tidal and geothermal energy, that do not depend on solar radiation.

Non-solar renewables

Tidal Energy

The energy that causes the slow but regular rise and fall of the tides around our coastlines is not the same as that which creates waves. It is caused principally by the gravitational pull of the moon on the world’s oceans. The sun also plays a minor role, not through its radiant energy but in the form of its gravitational pull, which exerts small additional effect on tidal rhythms.

The principal technology for harnessing tidal energy essentially involves building a low dam, or barrage, across the estuary of a suitable river. The barrage has inlets that allow the rising sea levels to build up behind it. When the tide has reached maximum height, the inlets are closed and the impounded water is allowed to flow back to the sea in a controlled manner, via a turbine-generator system similar to that used in hydroelectric schemes.

The world’s largest tidal energy scheme is at La Rance in France, which has a capacity of 240 MW.

Figure 1.36 The 240 MW tidal barrage installed at the Rance Estuary in France

There are a few other, smaller, tidal plants in various countries, including Canada, Russia and China. The United Kingdom has one of the world’s best potential sites for a tidal energy scheme, in the Severn Estuary. If built, its capacity would be around 8600 MW, much larger than any other single power plant, and it could provide about 6% of current UK electricity demand. But the scheme has not yet been implemented, mainly due to its very high capital cost and concerns about the effects on wildlife in the Severn estuary.

Another, newer tidal energy technology involves the use of underwater turbines (rather like submerged wind turbines) to harness the strong tidal and oceanic currents that flow in certain coastal regions. A 10 kW prototype tidal current turbine was tested at Loch Linne, in Scotland, in 1994, and a larger, 300 kW prototype was tested off the Devon coast in 2002.

The technology is still under development, but its prospects are promising.

Figure 1.37 Artist’s impression of an array of undersea tidal current turbines. The twin-rotor turbines can be raised to the surface to avoid the need for underwater maintenance
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Geothermal Energy

Geothermal energy is another renewable source that is not derived from solar radiation. As the name implies, its source is the earth’s internal heat, which originates mainly from the decay of long-lived radioactive elements. The most useful geothermal resources occur where underground bodies of water called aquifers can collect this heat, especially in those areas where volcanic or tectonic activity brings the heat close to the surface. The resulting hot water, or in some cases steam, is used for electricity generation where possible, for example in Italy, New Zealand and the Philippines, and for direct heating use in more than 60 other countries. Geothermal energy is already making a minor but locally useful contribution to world energy supplies.

Figure 1.38 One of the geothermal power plants at Larderello, Italy, used to provide electricity and hot water

If geothermal heat is extracted in a particular location at a rate that does not exceed the rate at which it is being replenished from deep within the earth, it is a renewable energy source. But in many cases this is not so: the geothermal heat is in effect being ‘mined’ and will ‘run out’ locally in perhaps a few years or decades.

Sustainability of Renewable Energy Sources

Renewable energy sources are generally sustainable in the sense that they cannot ‘run out’ – although, as noted above, both biomass and geothermal energy need wise management if they are to be used sustainably. For all of the other renewables, almost any realistic rate of exploitation by humans would be unlikely to approach their rate of replenishment by nature, though of course the use of all renewables is subject to various practical constraints.

Renewable energies are also relatively ‘sustainable’ in the additional sense that their environmental and social impacts are generally more benign than those of fossil or nuclear fuels. However, the deployment of renewables in some cases entails significant environmental and social impacts. Renewable energy sources are generally much less concentrated than fossil or nuclear fuels, so large areas of land (or building surfaces) are often required if substantial quantities of energy are to be collected. This can lead to a significant visual impact, as in the case of wind turbines.

Also, the monetary costs of many renewable sources are at present considerably higher than those of conventional fuels. Until this imbalance is reduced, either by reducing the costs of renewables or through increases in the costs of conventional sources, renewables may be unable to succeed in capturing a substantial fraction of the world market.

Renewables may seem attractive in many ways, but how large a contribution might they make to world energy needs in the future? This is an important question to which we shall return, initially in the final section of this introductory overview, and in more detail in the companion volume, Renewable Energy.

Next: 5. Energy Services and Efficiency Improvement