5 Biomass conversion of solar energy

Photosynthesis in the geological past was responsible for all fossil fuel reserves, but its products are buried about 2000 times more slowly than we use them at present. The total carbon content of all biomass growing on land is estimated to be 5.6 × 1014 kg and, as Figure 10 shows, about one-fifth of this mass is renewed each year. Figure 6 shows how modern plant biomass is distributed across the continents. Clearly, biological conversion of solar energy to a chemical form in com
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

1 Solar energy

The Sun will radiate energy until it ceases thermonuclear fusion, in around 5 billion years. The solar power that enters the Earth's system is 1.1 × 105TW (0.3 × 105 TW to atmospheric heating and 0.8 × 105 TW absorbed at the surface – Figure 1). This is equivalent to a global e
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

Introduction

Energy from sources other than fossil or nuclear fuels is to a large extent free of the concerns about environmental effects and renewability that characterise those two sources. Each alternative source supplies energy continually, whether or not we use it. Many alternative sources of energy have been used in simple ways for millennia, e.g. wind and water mills, sails, wood burning – but only in the last two centuries has their potential begun to be exploited on an industrial scale. Except
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

References

Sheldon, P. (2005) Earth’s Physical Resources: An Introduction (Book 1 of S278 Earth’s Physical Resources: Origin, Use and Environmental Impact), The Open University, Milton Keynes
Smith, S. (2005) Water: The Vital Resource (Book 3 of S278 Earth’s Physical Resources: Origin, Use and Environmental Impact), The Open University, Milton Keynes

Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

7 The pros and cons, and future of geothermal energy

Geothermal energy is renewable but the fluids emit gases such as CO2, H2S, SO2, H2, CH4 and N2 when used for electricity generation. However, geothermal power plants are usually sited in areas of natural geothermal activity, where such emissions occur anyway. Other potential pollutants are various ions dissolved in the geothermal fluids, but these are almost always returned to the reservoir when the spent fluids are re-injected
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

3 Hot dry rock (HDR) fields

Heat flow through some parts of the continental crust can be well above normal locally because the underlying rocks contain abnormally high concentrations of uranium, thorium and potassium, which generate considerable heat. To add significantly to surface heat flow and thereby create high-temperature anomalies at shallow depths requires a large volume of such radioactive rocks. This condition is satisfied by some, but not all, granitic igneous intrusions, whose original magma became ch
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

2 High- to medium-enthalpy steam fields

When the geothermal gradient heats water above the temperature at which it boils at atmospheric pressure, at a depth accessible to drilling, conditions can favour using natural geothermal steam to generate electricity. Typically, the pressure can be several tens to hundreds of times that of the atmosphere. Even at 200 °C, high pressure can ensure that much of the fluid in a geothermally heated aquifer remains in the liquid state. Author(s): The Open University

1 Geothermal energy

Although energy from the Earth's interior that flows though the surface is on average very low — about a thousand times less than the solar energy that falls on the surface — it is sufficiently abundant worldwide to make it locally worth exploiting. The top 3 km of the Earth's crust stores an estimated 4.3 × 107 EJ of thermal energy by virtue of the temperature of rocks and their thermal capacity. Because global consumption of energy during 2002 was 451 EJ heat stored within t
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

Learning outcomes

By the end of this unit you should be able to:

  • explain the principles that underlie the ability of geothermal energy to deliver useable energy;

  • outline the technologies that are used to harness the power of geothermal energy;

  • discuss the positive and negative aspects of geothermal energy in relation to natural and human aspects of the environment.


Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

Introduction

Energy from sources other than fossil and nuclear fuels is to a large extent free of the concerns about environmental effects and renewability that characterise those two sources. Each alternative source supplies energy continually, whether or not we use it. And many alternative sources of energy have been used in simple ways for millennia, e.g. wind and water mills, sails, wood burning — but only in the last two centuries has their potential begun to be exploited on an industrial scale. Ex
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

Acknowledgements

Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit:

Except for third party materials and otherwise stated (see terms and conditions), this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Licence.

Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

6 Summary

Nuclear power generation results from fission of uranium isotopes when bombarded by neutrons. Conventional burner reactors require relatively scarce uranium-235, whereas fast breeder reactors (which have not yet been developed on any significant scale) would exploit more abundant uranium-238.

In the early 21st century over 400 nuclear — mainly burner — reactors produced 16% of global electricity demand.

The UK played a leading role in nuclear power developments during the 1950
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

4.5 Geological criteria for safe radioactive waste disposal

Even in the best of circumstances, containers such as the one shown in Figure 19 will survive for only 100–1000 years, although the glass itself may inhibit the migration of radioactive isotopes for a further 1000 years. So, in view of the long decay times (Author(s): The Open University

4.4 Radioactive waste disposal

Most fission products from nuclear reactors are solid at ordinary temperatures. They cluster around atomic mass numbers 90 and 140 (see, for example, Equation 2). From the point of view of waste disposal, the problem is that most of them are highly radioactive. The common radioactive isotopes produced in nuclea
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

4.1 Introduction

Nuclear power generation involves concentrated fissionable fuels which, after fission, leave significant quantities of fission-product isotopes, some of which are highly radioactive. Much of the criticism levelled against the industry falls under four main headings to which we have alluded in preceding sections:

  1. the operational safety of nuclear reactors;

  2. the biological effects of abnormal radiation levels arising from fuel transport,
    Author(s): The Open University

    License information
    Related content

    Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

3.3 Uranium production and economics

Table 3 lists the major uranium-producing countries. Currently, Canada (with 29% of global supply in 2003) is the world's largest producer of uranium, followed by Australia (21%), both having increased production since about 1980, whereas production from the USA, France, and South Africa has declined (
Author(s): The Open University

3.2 Uranium occurrence and ore deposits

In igneous rocks, uranium is more abundant in granites (~3.5 ppm) than in basalts (~1 ppm). The large size of the uranium atom prevents it from easily entering the structures of common rock-forming minerals, so it is an incompatible element that tends to remain in magmas until a late stage of crystallisation, when it enters minor minerals, or even the uranium oxide, uraninite (UO2). In suitable circumstances, following fractional crystallisation of uranium-rich granitic magm
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

3.1 Introduction

Just how readily available are uranium resources, and do their distribution and cost impose restrictions on nuclear power generation? Compared to a coal-fired power station a nuclear power station requires far less fuel in terms of mass. You have seen that a 1 GW burner reactor requires 5000 t of natural uranium over 30 years, whereas a comparable modern coal-fired power station needs 10 000 t of coal every day. However, uranium does not occur naturally in metallic form, nor in the concentrat
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

2.5 The growth, decline and future of nuclear power

The Calder Hall Magnox reactor near Sellafield fired the UK's first commercial nuclear power station in 1956, and launched an early UK lead in global nuclear developments. By 1960 six commercial reactors were operating, and Magnox technology had been exported to Italy and Japan. The UK Magnox building programme was complete in 1971 with eleven stations, each producing between 245 MW and 840 MW. Author(s): The Open University

2.3.2 Fast breeder reactors

If fast neutrons produced in the chain reactions are not moderated or absorbed, the rate of conversion of uranium-238 into plutonium-239 (Equation 3) can exceed the fission rate of plutonium-239. Reactors that use fast neutrons in this way are called fast breeder reactors.

Their main fuel is
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share