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2622 results in Civil and geotechnical engineering

Page 35 of 132

  • 3 - Hydro Power

  • Nicholas Jenkins, Cardiff University, Janaka Ekanayake, University of Peradeniya, Sri Lanka
  • Book:
    Renewable Energy Engineering
    Published online:
    28 May 2018
    Print publication:
    06 April 2017, pp 72-119

  • Summary

    INTRODUCTION

    Hydro power produces around 16% of the world's electrical energy (~3500 TWh annually) from more than 900 GW of generating plant. The power that can be generated from a hydro power scheme depends on the flow of water and the height through which the water falls. In Europe most of the suitable sites for large hydro power schemes have been exploited and any future developments are likely to be small. In less populated parts of the world (e.g. parts of Asia, South America and Africa) there remain sites suitable for large hydro power schemes that have yet to be developed. The technical resource worldwide is estimated to be more than 16 400 TWh annually (4½ times present output) although much of this resource will never be developed because of environmental and cost constraints.

    The flow of water into a hydro scheme varies season by season and the output of hydro schemes is not constant but varies throughout the year. Although water can be stored in the reservoirs of large schemes its extraction has to be managed to maintain suitable environmental conditions. Flows in rivers and the height of water in reservoirs must be maintained in order to support fish, plants and wildlife. Worldwide the annual capacity factor of hydro schemes is ~44% and they often operate at only part of their capacity or are shut down. The hydro schemes in Great Britain have an overall capacity factor of ~39% depending on the rainfall that year. This rather low utilisation of the generating plant is partly due to the availability of water but also because the output power of a hydro generator can be altered very quickly and so hydro power stations are often used to balance the varying electrical load demand of the power system.

    Several very large hydro power schemes have been developed, such as the 22 GW Three Gorges scheme in China and the 14 GW Itaipu scheme on the border of Brazil and Paraguay. Around 10% of the worldwide capacity of hydro power is from a large number of small hydro schemes (less than 5–10 MW).

  • 7 - Marine Energy

  • Nicholas Jenkins, Cardiff University, Janaka Ekanayake, University of Peradeniya, Sri Lanka
  • Book:
    Renewable Energy Engineering
    Published online:
    28 May 2018
    Print publication:
    06 April 2017, pp 225-276

  • Summary

    INTRODUCTION

    The oceans cover more than 70% of the earth's surface and energy from the tides and waves is an important potential source of electrical power. However, the marine environment is extremely demanding, not least because of the wide range of forces that any marine energy conversion device will experience. In normal operating conditions a marine device must extract energy effectively from relatively small and benign movements of water caused by the waves and tides but it must also survive damaging extreme storms. In addition, installing and gaining access to an offshore marine energy device for maintenance is difficult while transmitting electrical power to the shore using submarine cables is expensive. Seawater is corrosive and so any materials used must be carefully selected and protected. Hence although there has been considerable research and development effort over many years and a number of demonstration projects have been installed, marine energy has yet to achieve a commercial breakthrough and there is at present less than 1 GW of marine energy electricity generating capacity in service worldwide.

    Marine energy describes three distinct technologies that, because of the very different characteristics of the resource and stages of maturity of their technical and commercial development, are considered separately in this book. These are generation by:

  • • tidal range

  • • tidal stream

  • • wave power.

    • The tides are caused by a very low frequency water wave that is created by the gravitational attraction of the moon and the sun and the rotation of the earth. This wave with a period of around 12½ hours leads to variations in the height of the water and in the creation of marine currents as it approaches the shoreline. In contrast, ocean waves are created by winds, which are ultimately due to the heating of the earth by solar energy and contain a range of higher frequencies. Both tidal range and tidal stream generation have the most desirable attribute that their power output can be predicted years in advance. This is not the case with wave energy.

Renewable Energy Engineering
  • Nicholas Jenkins, Janaka Ekanayake
  • Published online:
    28 May 2018
    Print publication:
    06 April 2017
  • This book provides a quantitative yet accessible overview of renewable energy engineering practice and the technologies that will transform our energy supply system over the coming years. Covering wind, hydro, solar thermal, photovoltaic, ocean and bioenergy, the text is suitable for engineering undergraduates as well as graduate students from other numerate degrees. The technologies involved, background theory and how projects are developed, constructed, and operated are described. Worked examples of the simple techniques used to calculate the output of renewable energy schemes engage students by showing how theory relates to real applications. Tutorial chapters provide background material, supporting students from a range of disciplines and ensuring they receive the broad understanding essential for a successful career in the field. Over 150 end-of-chapter problems are included with answers to the problems available in the book and full solutions at www.cambridge.org/jenkins, password-protected for instructors.

  • 9 - Development and Appraisal of Renewable Energy Projects

  • Nicholas Jenkins, Cardiff University, Janaka Ekanayake, University of Peradeniya, Sri Lanka
  • Book:
    Renewable Energy Engineering
    Published online:
    28 May 2018
    Print publication:
    06 April 2017, pp 317-332

  • Summary

    INTRODUCTION

    Compared with modern fossil fuelled power stations, many renewable energy projects to generate electricity are relatively small (typically less than 50 MWe) with limited construction budgets (usually less than £100M). Hence a large number of schemes are needed to meet a nation's electricity needs. Before contracts are finally awarded and any physical work done, the development phase of a project can take a significant fraction of the overall budget and not all proposals will progress to construction. Therefore there is a great demand for engineers and other professionals to work on project development and appraisal of renewable energy projects. This short chapter addresses this area of work and describes three important aspects of renewable energy project development and appraisal.

  • • Project development and assessment of the renewable energy resource.

  • • Economic appraisal.

  • • Environmental impact assessment.

    • These topics are discussed in terms of a general renewable energy project as, although the characteristics of each form of renewable energy are different, the process of project development has many aspects that are common across a range of resources and technologies. Details of some particular aspects of the development of projects using individual technologies have been addressed within the earlier chapters. This chapter is written from the perspective of a project developer who would buy in equipment and services to construct the project. The project developer can either be an independent company or the specialist arm of a larger organization.

    Renewable energy projects differ from other power projects in that a good understanding of the renewable resource is essential and the plants are often located in areas of great landscape and ecological value. Thus detailed and expensive investigations are needed to quantify the renewable resource and identify potential impacts on the environment and local communities. During the feasibility phase and before planning permission is received it is particularly important to strike a balance that limits expenditure (which might be wasted) while at the same time doing the work necessary to increase the chances of a successful outcome of the planning process, the project being commissioned on time and then working profitably over its life.

  • 4 - The Solar Energy Resource

  • Nicholas Jenkins, Cardiff University, Janaka Ekanayake, University of Peradeniya, Sri Lanka
  • Book:
    Renewable Energy Engineering
    Published online:
    28 May 2018
    Print publication:
    06 April 2017, pp 120-137

  • Summary

    INTRODUCTION

    The sun is essential for life on earth; plant and animal life in their current form would not exist without its power. Radiation from the sun ultimately drives all the forms of renewable energy discussed in this book (the only exception being tidal energy). On earth, the sun is a diffuse energy source with a power at the earth's surface of less than 1000 W/m2 and the natural processes that create the earth's wind and waves, rainfall and biomass act to concentrate the energy from the sun so making its subsequent conversion into mechanical or electrical power easier.

    An understanding of the solar energy resource is essential for the study of both photovoltaic electricity generation and solar thermal processes. This short chapter gives a simple engineering description of the solar resource and its content is a pre-requisite for the study of Chapters 5 and 6. In Sections 4.1 and 4.2 the solar resource is described with examples. Section 4.3 presents a simple view of the sun–earth geometry. Section 4.4 discusses the orientation of solar panels. Section 4.5 describes the solar spectrum and air mass. Finally in Section 4.6, a brief introduction is given to the wave–particle description of light.

    The Solar Resource

    The sun is a spherical collection of hot gases with a diameter of 1.4 × 106 km and an internal temperature of up to 20 × 106 K. The earth orbits the sun in a slightly elliptical orbit at a distance of approximately 1500 × 106 km. The centre of the sun can be thought of simply as a nuclear fusion reactor converting hydrogen into helium and so releasing energy. The energy generated by the fusion reaction at the centre of the sun is transmitted through its hot gases to the surface with increasing wavelength. Finally radiation is emitted from the surface of the sun with a spectrum similar to that of a black body at a temperature of 6000 K. This radiation then travels through space to arrive at the outer surface of the earth's atmosphere. At the outer surface of the earth's atmosphere the intensity of the radiation is around 1367 W/m2.

  • Contents

  • Nicholas Jenkins, Cardiff University, Janaka Ekanayake, University of Peradeniya, Sri Lanka
  • Book:
    Renewable Energy Engineering
    Published online:
    28 May 2018
    Print publication:
    06 April 2017, pp v-xiii
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  • Preface

  • Nicholas Jenkins, Cardiff University, Janaka Ekanayake, University of Peradeniya, Sri Lanka
  • Book:
    Renewable Energy Engineering
    Published online:
    28 May 2018
    Print publication:
    06 April 2017, pp xix-xx

  • Summary

    The popularity of renewable energy as a subject of study at undergraduate level is growing rapidly, stimulated by the widespread recognition that ways must be found to provide the power, light and heat that society needs while minimising damage to the environment. Many countries throughout the world are adopting policies to support the use of renewable energy as part of their commitment to limit the emission of greenhouse gases and there is a critical shortage of engineers and technologists to develop, construct and operate renewable energy schemes.

    The book has been developed from a number of courses given by the authors to classes of undergraduate engineering students, often together with those following Masters conversion courses who had previously studied a range of science and other numerate subjects. Students from a wide variety of backgrounds wish to study the engineering aspects of renewable energy and this textbook is intended to be accessible to all of them. A general level of high school physics and mathematics is assumed, and examples throughout the text demonstrate the various calculation techniques. Problems are provided at the end of each chapter with their numerical answers. The problems are graded in terms of their difficulty and the early questions of each chapter can be used by the reader to quickly check their understanding of the subject matter. The full solutions of the problems as well as extended exercises for coursework are on the companion website www.cambridge.org/Jenkins that is intended for instructors/teachers or those studying independently.

    The book provides ample material to support the teaching of a one-semester course, giving an introduction to the commonly used renewable energy technologies. It describes the various renewable energy resources, how they can be quantified and the fundamentals of their conversion to useful energy. The material presented to the students can be chosen based on their particular interests and backgrounds. After Chapter 1, ‘Energy in the Modern World’, the chapters can be studied in almost any order to reflect the interests of the reader, with the exceptions that Chapter 4 ‘The Solar Energy Resource’ is a pre-requisite for Chapter 5 ‘Photovoltaic Systems’ and Chapter 6 ‘Solar Thermal Energy’. Chapter 7 ‘Marine Energy’ uses concepts from both Chapter 2 ‘Wind Energy’ and Chapter 3 ‘Hydro Power’ and so should be read after them.

  • 6 - Solar Thermal System

  • Nicholas Jenkins, Cardiff University, Janaka Ekanayake, University of Peradeniya, Sri Lanka
  • Book:
    Renewable Energy Engineering
    Published online:
    28 May 2018
    Print publication:
    06 April 2017, pp 182-224

  • Summary

    INTRODUCTION

    Energy from the sun is radiated across space and enters the earth's atmosphere as solar irradiance to provide both sunlight and heat. Without solar radiation there would be no life on earth. Photovoltaic cells directly convert sunlight into electrical energy and their operation was discussed in Chapter 5. This chapter addresses the use of the sun's energy to provide useful heat. The particular uses of solar energy considered here are for:

  • (1) heating of buildings

  • (2) heating of water

  • (3) supply of high temperature heat for electricity generation and industrial processes.

    • Solar thermal energy may be divided into those processes that use a low temperature (less than ~150 °C) and a high temperature (greater than ~150 °C). Although the use of low temperature solar heat may appear simple it is of great practical importance. More than 40% of the energy used in the UK is consumed as heat with 75% of this as low temperature heat. 80% of the heat energy generated in the UK is from natural gas and a reduction in the volume of gas burnt or substitution of even a fraction of this by solar energy would lead to major financial and environmental benefits.

    The design of low energy buildings is influenced heavily by the need to maximise natural heat gain from the sun and to retain heat in winter while avoiding overheating in summer. Windows are essential for the thermal performance of buildings and increasingly are double or triple glazed. Most governments apply building standards that specify the thermal performance of new buildings in order to save energy and minimise the use of fossil fuels. Good passive solar design of buildings is essential for the successful development of low energy cities. In addition many governments have financial support programmes to improve the thermal performance of existing buildings and so reduce their heat loss. However, improving the energy performance of old buildings while maintaining their appearance and function is both difficult and expensive. Just as with the energy efficiency measures discussed in Chapter 1, improving the energy performance of buildings is not only a technical matter but depends heavily on the behaviour of the occupants.

    Solar thermal water heaters are widely used in countries that have a suitable solar resource, e.g. Greece and Japan, where they provide the domestic hot water needs of dwellings for at least part of the year.

  • 8 - Bioenergy

  • Nicholas Jenkins, Cardiff University, Janaka Ekanayake, University of Peradeniya, Sri Lanka
  • Book:
    Renewable Energy Engineering
    Published online:
    28 May 2018
    Print publication:
    06 April 2017, pp 277-316

  • Summary

    INTRODUCTION

    Biomass supplies some 10% of the world's primary energy, mostly from traditional fuels in developing countries. In these rural societies the traditional biomass fuels of wood, charcoal, crop residues or dried animal dung are widely used for cooking and heating. In the industrialised world, the use of bioenergy to generate electricity and as fuel for road vehicles is growing rapidly although from a relatively low base.

    There are a very large number of sources of biomass and different processes by which it can be converted into useful energy, many of which are still being investigated and developed. This chapter does not deal exhaustively with each bioenergy feedstock and process, but rather describes the main routes by which biomass is converted into useful energy at commercial scale. Biomass processing techniques can be divided into thermochemical and biochemical processes as well as the extraction of oil from plants. Thermochemical processes use heat and catalysts to transform biomass into useful energy by combustion or gasification. Biochemical processes use enzymes and microorganisms in alcoholic fermentation or anaerobic digestion. Vegetable oils are extracted from plant seeds, processed and either used directly in compression ignition engines or converted into biodiesel.

    Bioenergy has a number of very useful attributes. Biomass can be stored as a dry solid or converted into a gaseous or liquid biofuel. Hence although bioenergy is a concentrated form of solar energy it does not depend on the instantaneous irradiance of the sun and can be used when needed. Biomass is often processed into biofuel in small units near where it is grown and so contributes to rural employment. The use of bioenergy can reduce the national requirement for importing fossil fuels so saving foreign exchange.

    The other renewable energy technologies, once the equipment is manufactured and installed, generate electricity with essentially no emissions of greenhouse gas. With bioenergy, the same quantity of CO2 that is absorbed from the atmosphere in photosynthesis is subsequently released when the biomass is converted into useful energy. The process can then be said to be CO2 neutral. In practice fossil energy is needed for the cultivation, fertiliser, harvesting, transport and processing of biomass and so careful analysis is required to determine the lifetime environmental costs and benefits of any biomass scheme.

  • 1 - Energy in the Modern World

  • Nicholas Jenkins, Cardiff University, Janaka Ekanayake, University of Peradeniya, Sri Lanka
  • Book:
    Renewable Energy Engineering
    Published online:
    28 May 2018
    Print publication:
    06 April 2017, pp 1-24
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  • Summary

    INTRODUCTION

    An adequate supply of energy is essential for the working of any modern society. Energy is needed for cooking food, heating or cooling, lighting, materials processing and for transport. At present most energy comes from fossil fuels. Primary energy from fossil fuels – coal, oil and gas – is used either directly or converted into electricity. An increasing quantity of electrical energy is consumed in the operation of computers and communication equipment.

    Over the past 25 years, the world demand for energy, supplied mainly from fossil fuels, has grown continuously at a rate of increase of around 2½% per year. This increase in consumption cannot be sustained indefinitely both because of depletion of reserves and, more urgently, because of the environmental impact of burning fossil fuels. The reserves of coal, oil and gas that are now being used were laid down over millions of years and have been exploited for less than three centuries. At some time in the future the costs of the extraction of oil and gas will become so high as to limit their use. There is clear agreement among climate scientists that burning fossil fuels and the consequent emission of CO2 into the atmosphere is leading to damaging climate change. Most governments are making strenuous efforts to improve the efficiency with which energy is used and to control demand. However, attempts to control demand for energy have had only limited success. As the world population rises and societies grow richer their consumption of energy increases.

    The use of renewable energy has an important part to play in the future supply of energy and in the transition to a more sustainable economy, but renewable energy brings its own challenges. In general, the initial capital cost of renewable energy schemes is high and their output depends completely on the resource and so varies with the strength of the sun and wind.

    This book examines the various renewable sources of energy and how they can be used effectively. It focuses on those technologies that can make a significant contribution to energy supply over the next 30 years or so and pays particular attention to the renewable energy resource. Without a good energy resource it is impossible to develop a cost-effective renewable energy scheme.

  • 5 - Photovoltaic Systems

  • Nicholas Jenkins, Cardiff University, Janaka Ekanayake, University of Peradeniya, Sri Lanka
  • Book:
    Renewable Energy Engineering
    Published online:
    28 May 2018
    Print publication:
    06 April 2017, pp 138-181

  • Summary

    INTRODUCTION

    The direct generation of electricity from solar energy by photovoltaic panels is one of the most attractive and rapidly growing of the renewable energy technologies. In 2015, 60 GW of photovoltaic electricity generating capacity was added worldwide bringing the total installed capacity to more than 320 GW. The rapid growth of installed capacity of around 30% per year in the past few years has been due partly to the financial support measures that are offered by a number of governments for the installation of photovoltaic systems but also because in recent years there has been a dramatic reduction in the price of photovoltaic modules with increased manufacturing volumes.

    The majority of solar panels that are installed use mono- or poly-crystalline solar cells (often described as first-generation cells) and export their power to the electricity grid. The panels are mounted either on the roofs of buildings or are supported on ground mounted structures in large solar farms. Some innovative buildings have photovoltaic modules integrated into their facades or roof. Large solar farms are usually located in areas of high solar radiation where land is cheap but are now being constructed in many parts of the world including the UK. In addition to grid-connected systems, photovoltaic systems play an important role in supplying electrical power in remote areas where there is no grid electricity supply. Photovoltaic modules have no moving parts and are extremely robust. They can operate for more than 20 years with minimal maintenance and their installation has limited environmental impact.

    In addition to the present first-generation photovoltaic cells that use crystalline silicon, second-generation cells made from thin film devices are offered by a number of manufacturers using a range of materials. Thin film cells use reduced volumes of semiconductor material on an inert substrate and have the potential for significant cost reduction compared to the bulk silicon first-generation cells. There are also exciting new developments of new materials for third-generation cells but these have yet to be deployed commercially.

    The output from a photovoltaic system depends completely on the solar energy resource. In temperate latitudes in the winter the solar irradiance drops to very low levels resulting in low capacity factors and poor utilisation of the photovoltaic equipment. Of course, at night a photovoltaic system produces no output.

  • 10 - Electrical Energy Systems

  • Nicholas Jenkins, Cardiff University, Janaka Ekanayake, University of Peradeniya, Sri Lanka
  • Book:
    Renewable Energy Engineering
    Published online:
    28 May 2018
    Print publication:
    06 April 2017, pp 333-374

  • Summary

    INTRODUCTION

    The world's reliance on electricity to meet its needs for heat, light, motive power and the exchange of information continues to increase. World consumption of electrical energy grew from 17 500 TWh in 2004 to 24 000 TWh in 2015, an annual growth rate of around 3.4%. Reducing emissions of CO2 to limit climate change will require a transition from fossil fuels to electricity for many services including for heating (e.g. from gas boilers to heat pumps) and for transport (e.g. from internal combustion engines to electric vehicles). This will further increase the demand for electricity.

    A traditional power system is supplied by large fossil fired or hydro units with synchronous generators. The operation of these units is well understood and, because of the energy stored in the fossil fuel or in large reservoirs, they are relatively easy to control. In contrast, many renewable generators, e.g. wind and solar power units, respond to the instantaneous renewable resource and are connected to the network through static power electronic interfaces which have quite different operating characteristics to those of large rotating generators.

    The power generated by a large renewable generator such as a wind farm or hydro generator must be transported through transmission and distribution networks to the load. Smaller generators, e.g. domestic PV systems, are connected close to the customers but rely on the central power system to provide a stable reference of voltage and frequency. Depending on their location and power output with respect to the local load, renewable generators that are distributed over the network will have a greater or lesser impact on the voltages and the frequency of the power system.

    With an increasing penetration of renewables the operation of the power system becomes more difficult. However, many smaller national electricity systems (e.g. Ireland) operate routinely with more than 50% of their electrical power being supplied from wind energy. Traditionally a power system has attempted to supply any load when electricity is demanded. With a high fraction of renewable generation operating on the system it becomes necessary to take control of the load, particularly in the event of an unexpected failure of a generator or an error in forecasting the renewable resource.

  • 2 - Wind Energy

  • Nicholas Jenkins, Cardiff University, Janaka Ekanayake, University of Peradeniya, Sri Lanka
  • Book:
    Renewable Energy Engineering
    Published online:
    28 May 2018
    Print publication:
    06 April 2017, pp 25-71

  • Summary

    INTRODUCTION

    Generating electricity from the wind is one of the more cost effective ways of harnessing renewable energy and increasing numbers of wind turbines are being installed in many countries. By 2015 there was more than 430 GW of wind turbine capacity installed worldwide. For comparison, peak electricity demand in Great Britain is around 55 GW and in the continental USA around 800 GW.

    Modern wind turbines can be very large with rotor diameters greater than 100 m and similar tower heights. Advanced composite materials are used to resist the forces of the wind as well as those created by gravity acting on these large rotating structures. The most important loads are caused by extreme wind speeds and fatigue loads that are created either by turbulence in the wind or the very large number of rotations and hence stress reversals experienced by a wind turbine blade during its life. All large modern wind turbines operate at variable rotational speed and are connected to the electrical grid though power electronic converters. As wind turbines become larger their structures are increasingly dynamic using active control to manage the loads through sophisticated control systems.

    As with all renewable energy technologies, the resource determines if a scheme will be profitable and this chapter includes a description of the wind resource and how energy from a wind farm can be estimated.

    In many countries the environmental impact of a wind farm, particularly visual impact and noise, will determine if permission for its construction will be obtained from the civil authorities. In response to concerns over environmental impact some wind turbines are being sited offshore, particularly around the coasts of Europe. Locating turbines offshore leads to a higher cost of the electricity but very large offshore wind farms can be built with limited environmental impact.

    This chapter describes the wind energy resource, wind turbines and wind farms.

  • 2 - Classification and Analysis of LongWaves

  • Jurjen A. Battjes, Technische Universiteit Delft, The Netherlands, Robert Jan Labeur, Technische Universiteit Delft, The Netherlands
  • Book:
    Unsteady Flow in Open Channels
    Published online:
    09 February 2017
    Print publication:
    16 February 2017, pp 13-26

  • Summary

    The category of long waves encompasses different wave types, each with a different origin and with different dynamics, in the sense that the relative importance of the various physical processes, as expressed by the different terms in the equation of motion, can vary. In this chapter we will first give an overview of the various types of long wave and their generation, and we will verify formally that they can indeed be classified as ‘long’. Next, we will estimate the relative magnitudes of terms in the equation of motion, which provides an ordering of wave types based on the importance of inertia relative to the resistance.

    Types of Long Waves

    In general, one can say that the faster the flow varies, the more important will be the inertia relative to the resistance, and the more it will be in balance with the net driving force. Before dealing with these dynamics we give short descriptions of the origin and typical characteristics of the different types of long waves:

    1. • translatory waves (transient variations in discharge and water level, usually caused by operation of controls)

    2. • tsunamis (sea waves generated by subsea earthquakes, volcanic eruptions etc.)

    3. • seiches (standing oscillations in lakes, bays, harbours etc.)

    4. • tides in oceans, shelf seas, estuaries and lowland rivers

    5. • flood waves in rivers

    Translatory Waves

    As a result of manipulation (or breakdown!) of pumps or valves in the operation of locks, weirs, evacuation sluices, hydropower plants, etc., variations in discharge (δQ) can occur. These are accompanied by variations in water surface elevation (δh). Such disturbances travel as so-called translatory waves into the adjacent reaches of the conduit. The passage of such wave induces a rise in elevation in the case of an increase in discharge, and a lowering in the case of a decrease in discharge; see Figure 2.1. The particle velocities in a translatory wave are in one direction only (either forward or backward), which explains the name ‘translatory waves’, as opposed to ‘oscillatory waves’, in which the particles move back and forth.

Page 35 of 132