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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
A concise yet technically authoritative overview of modern marine energy devices with the goal of sustainable electricity generation
With 165 full-colour illustrations and photographs of devices at an advanced stage, the book provides inspiring case studies of today’s most promising marine energy devices and developments, including full-scale grid-connected prototypes tested in sea conditions. It also covers the European Marine Energy Centre (EMEC) in Orkney, Scotland, where many of the devices are assessed.
Topics discussed:
- global resources – drawing energy from the World’s waves and tides
- history of wave and tidal stream systems
- theoretical background to modern developments
- conversion of marine energy into grid electricity
- modern wave energy converters and tidal stream energy converters
This book is aimed at a wide readership including professionals, policy makers and employees in the energy sector needing an introduction to marine energy. Its descriptive style and technical level will also appeal to students of renewable energy, and the growing number of people who wish to understand how marine devices can contribute to carbon-free electricity generation in the 21st century.
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Veröffentlichungsjahr: 2013
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Table of Contents
Title Page
Copyright
Preface
Acknowledgements
Chapter 1: Introduction
1.1 Marine energy and Planet Earth
1.2 Marine resources
1.3 A piece of history
1.4 Power, energy and performance
1.5 Into the future
References
Chapter 2: Capturing marine energy
2.1 Ocean waves
2.2 Wave energy conversion
2.3 Tidal streams
2.4 Tidal stream energy conversion
2.5 Research and development
References
Chapter 3: Generating electricity
3.1 Introductory
3.2 Power take-off
3.3 AC electricity
3.4 Generators
3.5 Connecting to the grid
3.6 Large-scale renewable energy
References
Chapter 4: Case studies: Wave energy converters
4.1 Introductory
4.2 Case studies
References
Chapter 5: Case studies: Tidal stream energy converters
5.1 Introductory
5.2 Case studies
References
Index
This edition first published 2014
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Library of Congress Cataloging-in-Publication Data
Lynn, Paul A.
Electricity from wave and tide : an introduction to marine energy / Paul A. Lynn.
pages cm
Includes bibliographical references and index.
ISBN 978-1-118-34091-2 (hardback)
1. Tidal power-plants. 2. Ocean wave power. 3. Tidal power. I. Title.
TK1081.L95 2014
621.31′2134– dc23
2013019102
A catalogue record for this book is available from the British Library.
ISBN: 9781118340912
Preface
The world's waves and tides are eternal and non-polluting, and the technology for converting their energy into grid electricity has reached an exciting stage. Many ingenious large-scale devices are currently being tested by developers as a prelude to commercialisation, in the confident hope that marine renewable energy will add significantly to conventional power generation in the coming decades.
This book introduces the history, theoretical background and practical development of today's wave and tidal stream devices to a wide readership including professionals, policy makers and employees in the energy sector needing an introduction or quick update. Its style and level also make it suitable background reading for university students and the growing number of thoughtful people who are interested in the contribution marine energy can make to ‘keeping the lights on’ in the twenty-first century. This is probably the first book to introduce wave and tidal stream technologies in a single volume and, although it assumes some basic familiarity with physics and maths, words are used every bit as much as symbols to give a descriptive flavour, enhanced by about 200 colour photographs and illustrations.
In more detail, Chapter 1 covers the historical background and Chapter 2 some of the key concepts underpinning today's practical developments. I have decided to devote Chapter 3 to electricity generation, for readers with little or no background in electrical engineering. Large-scale wave and tidal energy converters feed electricity into AC grid networks for the benefit of us all; yet electrical generation, grid connection and distribution are hardly ever explained in the context of marine energy and their terminology is mysterious to many people. I hope the account given here, which is very similar to that in my recent book on wind energy (also published by Wiley), will prove helpful.
Chapters 4 and 5 present case studies of modern wave and tidal stream devices, selected for their advanced state of development, including the testing of full-scale, or near full-scale, prototypes in sea conditions. I have relied heavily on the various developers for information about their devices, many of which have been, or are being, assessed at the internationally famous European Marine Energy Centre (EMEC) in Orkney, Scotland.
My interest in renewable energy goes back over 30 years, but I no longer have links with academia or industry and the selection and presentation of topics is my own. I claim no originality for the technical material, which has been gathered, sifted, and sorted from many websites, books, technical papers and articles. I see my role as hunter-gatherer, not master chef, and hope the menu will help advertise the remarkable developments currently taking place in the international quest for ‘Electricity from Wave and Tide’.
Paul A. LynnButcombe, Bristol, EnglandSummer 2013
Acknowledgements
I am very grateful to the following companies and organisations for information about their activities and devices, and for permission to use the excellent colour photographs and illustrations they have provided:
Andritz Hydro Hammerfest, Hammerfest, Norway:
Figures 5.1
and
5.2
.
Aquamarine Power Ltd, Edinburgh, Scotland:
Figures 1.1
,
2.15
c,
4.6
–
4.12
.
Atlantis Resources Corporation, London, England: Figures
1.2
,
2.33
b,
2.38
,
5.6
–
5.8
.
Datawell BV, Haarlem, The Netherlands:
Figure 2.14
.
European Marine Energy Centre (EMEC), Orkney, Scotland:
Figures 1.4
,
2.43
–
2.45
,
2.47
–
2.54
.
Marine Current Turbines Ltd, Bristol, England:
Figures 2.33
a,
5.9
–
5.13
.
Ocean Power Technologies Inc, New Jersey, USA:
Figures 2.15
b,
4.21
,
4.23
,
4.24
.
OpenHydro Ltd, Dublin, Ireland:
Figures 5.14
–
5.19
.
Pelamis Wave Power Ltd, Edinburgh, Scotland:
Figures 2.15
a,
4.1
–
4.5
.
Pulse Tidal Ltd, Sheffield, England:
Figures 5.20
–
5.25
.
Scotrenewables Tidal Power Ltd, Orkney, Scotland:
Figures 2.41
,
5.26
–
5.33
.
Tidal Generation Ltd, Bristol, England:
Figures 2.33
c,
5.34
–
5.37
.
Voith Hydro Ocean Current Technologies GmbH, Heidenheim, Germany:
Figure 1.19
.
Voith Hydro Wavegen Ltd, Inverness, Scotland:
Figures 2.15
d,
4.13
,
4.15
,
4.16
.
Wave Dragon ApS, Copenhagen, Denmark:
Figures 2.15
e,
4.17
–
4.20
.
Wello Oy, Espoo, Finland:
Figures 4.25
–
4.29
.
The publishers acknowledge use of the above photographs and illustrations, which are reproduced by permission of the copyright holders.
I also acknowledge the following photos and illustrations obtained from Wikipedia: Figures 1.16–1.18, 2.21, 2.23, 2.24, 2.26–2.28, 2.35, 3.3.
The writer of an introductory book covering a wide field inevitably draws on many sources for information and inspiration. I make no claims for technical originality in the material presented and have tried to give an adequate list of references at the end of each chapter.
I would particularly like to thank staff at EMEC in Orkney for their enthusiastic cooperation and advice. I am also indebted to two books that have proved invaluable for clear explanations of difficult concepts which I have attempted to summarise. They are: Ocean Wave Energy: Current Status and Future Perspectives by Joao Cruz (editor), published by Springer in 2008; and The Analysis of Tidal Stream Power by Jack Hardisty, published by Wiley-Blackwell in 2009. Both are more comprehensive and advanced than my own offering, and I recommend them to anyone wishing to learn more about marine renewable energy.
Among the many figures in this book are 70 technical illustrations by David Thompson, who worked closely with me on two previous books, Electricity from Sunlight (Wiley, 2010), and Onshore and Offshore Wind Energy (Wiley, 2012). It has been a pleasure to repeat the collaboration.
Paul A. Lynn
Chapter 1
Introduction
1.1 Marine energy and Planet Earth
For over a century most of the electricity used in our homes, offices and factories has been generated in large power plants based on fossil fuels and, in some countries, nuclear reactors and hydroelectric turbines. But as the new millennium gets into its stride important changes are taking place in the how, where and why of electricity generation due to increasing concerns about climate change, fossil fuel depletion and the risks of nuclear power. Terms such as renewable, sustainable and carbon-free have entered the popular imagination and most experts and politicians now accept that a major redirection of energy policy is essential if Planet Earth is to survive the twenty-first century in reasonable shape.
For the last few hundred years humans have been using up fossil fuels that nature took around 400 million years to form and store underground. A huge effort is now under way to develop energy systems that make use of natural energy flows in the environment – including those produced by waves and tidal streams. This is not simply a matter of fuel reserves, for it is becoming clearer by the day that, even if those reserves were unlimited, we could not continue to burn them with impunity. Today's scientific consensus assures us that the resulting carbon dioxide emissions would very likely lead to a major environmental crisis. So the danger is now seen as a double-edged sword: on the one side, fossil fuel depletion; on the other, the increasing inability of the natural world to absorb emissions caused by the burning of what fuel remains, leading to accelerated global warming.
Things were not always like this. Back in the 1970s there was little public discussion about energy sources and engineering courses in universities paid little attention to them. The environmental movement was in its infancy, far removed from the mainstream political agenda, and its proponents were often dismissed as eccentric busybodies. Few people had any idea how the electricity they took for granted was produced, or that the burning of coal, oil and gas might be building up global environmental problems. Those who were aware tended to assume that the advent of nuclear power would prove a panacea, a few even claiming that nuclear electricity would be so cheap that it would not be worth metering!
Yet even in those years a few brave voices suggested that all was not well. In his famous book Small is Beautiful [1], first published in 1973, E.F. Schumacher poured scorn on the idea that the problems of production in the industrialised world had been solved. Modern society, he claimed, does not experience itself as part of nature, but as an outside force seeking to dominate and conquer it. And it is the illusion of unlimited powers deriving from the undoubted successes of much of modern technology that is the root cause of our present difficulties, in particular because we are failing to distinguish between capital and income components of the earth's resources. We use up capital, including coal, oil and gas reserves, as if they were steady and sustainable income, but they are actually once-and-only capital. Schumacher's heartfelt plea encouraged us to start basing industrial and energy policy on what we now call sustainability, recognising the distinction between capital and income and the paramount need to respect the planet's finite ability to absorb the polluting products of industrial processes – including electricity production.
Schumacher's message, once ignored or derided by the majority, is now seen as mainstream. For the good of Planet Earth and future generations we have started to distinguish between capital and income and to invest heavily in renewable technologies that produce electricity free of carbon emissions. In recent years the message has been powerfully reinforced by former US Vice President Al Gore, whose inspirational lecture tours and video presentation An Inconvenient Truth [2] have been watched by many millions of people around the world.
Into this melting pot of hopes and concerns fall a number of promising renewable technologies based on the immense natural energy flows in Planet Earth's environment. These include winds and the ocean waves they produce (see Figure 1.1), tides and tidal streams (see Figure 1.2), and sunlight falling on the Earth's surface. All are eternal and inexhaustible; nothing is ‘wasted’ if we ignore them because they are there anyway. They are income, not capital, and we should surely regard them as precious gifts of nature to be harnessed in ways that are technically efficient, economic and environmentally sensitive. All this represents a hugely challenging and inspiring agenda for engineers and scientists – now and for the rest of the century.
Figure 1.1 Harnessing wave energy (Aquamarine Power Ltd).
Figure 1.2 Transporting a tidal stream turbine (Atlantis Resources; Mike Roper (photographer)).
Perhaps we should consider the meaning of renewable energy a little more carefully. It implies energy that is sustainable in the sense of being available in the long term without significantly depleting the Earth's capital resources, or causing environmental damage that cannot readily be repaired by nature itself. In his excellent book A Solar Manifesto [3], German politician Hermann Scheer considered Planet Earth in its totality as an energy conversion system. He noted how, in its early stages, human society was itself the most efficient energy converter, using food to produce muscle power and later enhancing this with simple mechanical tools. Subsequent stages – releasing relatively large amounts of energy by burning wood; focussing energy where it was needed by building sailing ships for transport and windmills to grind grain and pump water – were still essentially renewable activities in the above sense.
What really changed things was the nineteenth century development of the steam engine for factory production and steam navigation. Here, almost at a stroke, the heat energy locked in coal was converted into powerful and highly concentrated motion. The industrial society was born and ever since we have continued burning coal, oil and gas in ways which pay no attention to the natural rhythms of the earth and its ability to absorb wastes and by-products, or to keep providing energy capital. Our approach has become the opposite of renewable and it is high time to change priorities.
Since the reduction of carbon emissions is a principal advantage of wave, tidal and other renewable technologies, we should recognise that this benefit is also proclaimed by supporters of nuclear power. But frankly they make strange bedfellows, in spite of sometimes being lumped together as ‘carbon-free’. It is true that all offer electricity generation without substantial carbon emissions, but in almost every other respect they are poles apart. The renewables, including wave and tidal stream energy, give us the option of widespread, relatively small-scale electricity generation, but nuclear must, by its very nature, continue the practice of building huge centralised power stations. Waves and tides give us ‘free fuel’ and produce no waste in operation; the nuclear industry is beset by problems of radioactive waste disposal. On the whole renewable technologies pose no serious problems of safety or susceptibility to terrorist attack – advantages which nuclear power can hardly claim. Finally, there is the issue of nuclear proliferation and the difficulty of isolating civil nuclear power from nuclear weapons production. Taken together these factors amount to a profound divergence of technological expertise and political attitudes, even of philosophy. It is not surprising that most environmentalists are unhappy with the continued development and spread of nuclear power, even though some accept that it is proving hard to avoid. In part, of course, they claim that this is the result of policy failures to invest sufficiently in the benign alternatives over the past 30 or 40 years.
However, we must be careful not to assume that renewable energy is an easy answer. For a start it is generally diffuse and intermittent. Quite often, it is unpredictable. The design and manufacture of efficient machines to harness natural energy flows pose big technical problems, and although the ‘fuel’ may be free and the waste products minimal, up-front investment costs tend to be large. There are certainly major challenges to be faced and overcome as we develop a new energy mix for the twenty-first century.
Our story now moves on to modern wave and tidal stream technology, currently enjoying rapid progress and poised to make a significant contribution to electricity generation in the coming decades. But before getting involved in the details, we should consider the natural resources that promise to help wean us away from our addiction to fossil fuels.
1.2 Marine resources
1.2.1 Waves of the world
Surface waves on the world's oceans are generated by the wind. They are not formed instantly but build up over time and with distance, known as the fetch. Waves produced by a storm, arriving from afar over deep water, produce a regular swell which may take hours or days to form and travel hundreds or even thousands of kilometres across an ocean with very little loss of energy (see Figure 1.3). But as waves approach the shore and move into shallow water, they slow down, increase in height and start to break, dissipating lots of energy (see Figure 1.4). Wave characteristics close to shore can be very different from those of a regular, deep-water, swell.
Figure 1.3 Wind-generated ocean waves.
Figure 1.4 Waves approaching a shoreline (EMEC).
From the engineer's point of view wind-generated waves represent a valuable source of renewable energy for generating electricity using wave energy converters [4, 5]. The design of effective machines depends on how far they are to be placed from the shore: offshore, in deep water; near-shore, anchored or fixed to the sea bed; or installed on land at the shoreline (see Figure 1.3). The great oceans cover about 70% of the world's surface and the total wave resource is huge; but it is diffuse, variable, somewhat unpredictable, and occasionally destructive. The engineering challenge is to develop robust machines that capture wave energy efficiently and reliably, not too far from land, while at the same time surviving the worst that angry seas can throw at them.
The global wave resource, expressed as an equivalent amount of electrical power, is around 2 terawatts (TW), or 2 million million watts. This is equivalent to the output of 2000 large conventional electricity plants, each generating 1 gigawatt (GW), and is comparable with global electricity production. However, wave resources are distributed very unevenly across the world's oceans and countries with strong prevailing winds and exposed coastlines are the most favoured. A good example is the UK's coastal waters which receive, on average, wave power roughly equivalent to the nation's electricity demand. Although the exploitable resource in terms of practicality and economics is only a small percentage of the total, there is no doubt that ocean waves could make a significant contribution to an energy mix based increasingly on renewables.
Why do some maritime nations receive much more wave energy than others? The answer to this question is closely related to the world's major wind patterns, set up as the earth spins on its axis. Variations in atmospheric pressure caused by differential solar heating propel air from high pressure to low pressure regions, generating winds that are greatly affected by the earth's rotation and tend to occupy certain latitudes.
The investigation of latitudinal wind belts has a long history. For centuries the captains of sailing ships depended on reliable north-east and south-east trade winds to speed them on their way, and tried to avoid the horse latitudes that could becalm them. They also had to contend with strong but variable westerlies that blow in the mid-latitudes between about and , north and south (see Figure 1.5). It is hardly surprising that wind meteorology exercised some famous minds throughout the great age of sail. Edmond Halley (1656–1742), an English astronomer best known for computing the orbit of Halley's comet, published his ideas on the formation of trade winds in 1686, following an astronomical expedition to the island of St Helena in the South Atlantic. The atmospheric mechanism proposed by George Hadley (1685–1768), a lawyer who dabbled productively in meteorology, attempted to include the effects of the Earth's rotation – a theory that was subsequently corrected and refined by American meteorologist William Ferrel (1817–1891).
Figure 1.5 Atmospheric cells and latitudinal wind belts.
The contributions of Hadley and Ferrel to our understanding of latitudinal wind belts, and the waves they generate, are acknowledged in the names given to the atmospheric ‘cells’ shown in Figure 1.5. Essentially these are produced by the steady reduction in solar radiation from the equator to the poles. The associated winds, rather than flowing northwards or southwards as we might expect, deflect to the east or west in line with the Coriolis effect, named after French engineer Gaspard Coriolis (1792–1843), who showed that a mass (in this case, of air) moving in a rotating system (the Earth) experiences a force acting perpendicular to both the direction of motion and the axis of rotation.
The Hadley cells, closed loops of air circulation, begin near the equator as warm air is lifted and carried towards the poles. At around latitude, north and south, they descend as cool air and return to complete the loop, producing the north-east and south-east trade winds. A similar mechanism produces polar cells in the arctic and antarctic regions, giving rise to polar easterlies.
The Ferrel cells of the mid-latitudes, sandwiched between the Hadley and polar cells, are less well defined and far less stable. Meandering high-level jet streams tend to form at their boundaries with the Hadley cells, generating localised passing weather systems. This makes the coastal wind patterns – and ocean climates – of countries such as Norway, Denmark, Britain and Ireland strong but famously variable. So although the prevailing winds are westerlies, they are quite often displaced by flows from other points of the compass, especially during the winter and spring months.
We can now summarise the significance of latitudinal wind belts for wave energy technology:
The strong but variable
westerlies
that dominate global wind patterns between latitudes of about and (north and south) produce most of the world's exploitable wave energy. In the southern hemisphere they are famously referred to as the
Roaring Forties
. Countries with west-facing coastlines are especially favoured.
Trade winds
blowing between about and (north and south) may also be significant for wave energy conversion. Although less energetic on average than the westerlies, they are more consistent.
Polar easterlies
are much less important because the swells they produce tend to be relatively small (and may be hampered by sea ice).
The dominance of wave energy produced by prevailing westerly winds is emphasised in Figure 1.6, which shows coastlines that receive heavy swells generated over long fetches of ocean. Not surprisingly, they lie in countries presently showing great interest in wave energy, principally:
In Europe:
UK, Norway, Denmark, Ireland, France, Spain and Portugal.
In North America:
USA and Canada.
In the Southern Hemisphere:
Australia, New Zealand, Chile and South Africa.
Figure 1.6 Coastlines with large wave energy resources.
We now come to a very important question: how much power do ocean waves possess as they travel across an ocean and approach a coastline? The first point to make is that it depends on the distance from the shoreline. Wave power is greatest well offshore in deep water but, as the waves move into shallower water, friction with the sea bed and their tendency to break cause energy losses. The usual way of expressing power levels is in terms of average kilowatts per metre length of wave front . Figure 1.7 shows typical values well off the coastlines of Western Europe favoured with some of the world's best wave resources. We see that, for example, the average power of waves approaching the west coast of Portugal is around ; off the west coast of Scotland, one of the most productive areas in the world, around ; and along the coast of Norway, around and diminishing steadily towards the Arctic Circle. Out in the Atlantic ocean it can reach . Elsewhere in the world such values are only matched along certain coastlines in Australia, New Zealand and Chile, especially those facing the Roaring Forties that blow unhindered from west to east across the Southern Ocean at latitudes around (see Figure 1.6). It is hardly surprising that the most powerful wave climate in the world, averaging some , is found at latitude in the Southern Ocean – but so far from civilisation that it is extremely unlikely ever to be harnessed!
Figure 1.7 Average values of wave power off the coasts of Western Europe, expressed in kilowatts per metre of wave front.
Such values are certainly impressive. It is sometimes said that is enough to interest wave energy enthusiasts, but the found off the coastlines of Western Europe is clearly a great deal better and represents a power concentration rarely found in natural energy flows. For example, it is far greater than that of the airstreams harnessed by wind turbines. Essentially this is because wave power is built up and concentrated gradually over long stretches of ocean; and because seawater is far denser than air – a point we will return to in the next chapter.
However, power concentration is not the only criterion of interest to designers of wave energy converters. It is certainly important because the more power a machine of given size can capture the better but variability is also a major issue. It is all very well to choose a location with a high average wave power, promising high annual energy capture, but it is likely to produce occasional peaks that place great mechanical stresses on wave machines and may even threaten their survival. Reliability and survivability are crucial to economic justification and designers sleep better at night if their devices are located in somewhat calmer, less variable, waters. This makes ocean waves off the coastlines of countries such as Japan, Peru and Ecuador potential candidates for wave energy conversion even though average power concentrations rarely exceed 30 kW m–1.
The variability of wave power at a particular location occurs over widely differing time scales:
Short term.
Successive waves in an ocean swell are not all equal in size, but vary in a somewhat random fashion. A wave energy converter must cope with variable power levels over time scales from seconds to minutes, even when the sea state is nominally steady. In heavy seas there may be occasional ‘rogue’ waves. The situation close to shore, where waves start to break, is even less predictable.
Medium term.
Ocean swells caused by storms at sea build up and decay over time scales from hours to days. Such variability is especially strong in the mid-latitudes, for example off the western coasts of the UK and Ireland.
Long term.
The average wave power in most locations varies substantially according to the season of the year. In the mid-latitudes of the northern hemisphere this
seasonality
may easily result in a 3 : 1 ratio between wave resources in the winter and summer months. In the southern hemisphere, seasonal variations tend to be considerably smaller.
Wind-generated waves are an important energy resource but their variability presents major challenges to designers and engineers. In Chapter 4 we will meet a number of wave energy converters that illustrate current approaches to harnessing this powerful but unruly gift of nature.
1.2.2 Tides of the world
The main influences on ocean tides are the gravitational attractions of the moon and sun, and the earth's rotation. If you stand on a seashore you will likely see the water rising and falling twice a day or, more precisely, twice every 24 hours, 50 minutes and 28 seconds, the moon's apparent period of rotation about the earth. Such tides are referred to as semi-diurnal. In some locations there is only one high and one low tide each day, referred to as diurnal.
The highest and lowest points reached by a tide are known as high water and low water, and the vertical difference between them as the tidal range. At a given location on an ocean shore the tidal range varies over the course of each month according to the relative positions of sun, moon and earth. The sun's attraction is only about half as great as the moon's because, although the sun is massive, it is much further away than the moon and gravitation is governed by an inverse-square law of distance. When the sun and moon are in line (see Figure 1.8), giving a full moon or a new moon, their gravitational attractions act together and the tidal range is greatest. This is referred to as a spring tide. But when the sun and moon are at right angles, the moon is said to be in its first or third quarter and the tidal range is a minimum. This is known as a neap tide. In most locations the dominant cyclic variation in tidal range repeats twice a month.
Figure 1.8 Relative positions of sun, moon and earth giving rise to spring and neap tides.
The moon's orbit around the earth is slightly elliptical rather than circular, and the earth is not at the centre of the ellipse. This means that there is a time in each month when the moon is nearest to the earth (perigee), and another when it is furthest away (apogee). In a few locations tidal ranges are more influenced by this effect than by the more familiar spring-to-neap variations, and the resulting tides are referred to as anomalistic.
A further factor affecting tidal range is the moon's declination – its angular offset with respect to the earth's equatorial plane – which tends to make the two tides of a day unequal in range. When this effect is especially pronounced the tides are referred to as declinational.
We see that subtleties in the moon's orbit affect tidal ranges in a number of ways. The situation is further complicated by the influence of coastal geography and seabed geometry (bathymetry) as tidal undulations or ‘bulges’ make their daily journey around the earth's surface. In the open ocean tidal ranges are less than 1 m, but close to land they can be greatly increased by the way huge volumes of water work their way round continents, build up against coastlines, force their way through channels, or enter bays and estuaries. Tidal ranges in Canada's Bay of Fundy can reach 17 m, and in England's Bristol Channel 14 m, but in parts of the Mediterranean, Baltic and Caribbean seas they are close to zero. Coastal geography also affects the temporal patterns of tides, which may depart dramatically from those experienced in deep water offshore. But whatever the details, the tides in a particular location go through a monthly cycle.
So far we have discussed tidal ranges, the regular ‘ups and downs’ of sea level which, in a few locations, are used to generate electricity with tidal barrages [6]. However our focus in this book is on tidal streams [7], the oscillating horizontal currents that accompany the rise and fall of tides. Tidal stream technology captures moving water's kinetic energy, whereas tidal barrages make use of stored water's potential energy. The global tidal stream resource is comparable to that of wave energy (say ), of which perhaps 3% is reasonably accessible for electricity generation. But, as we shall see, it is very unevenly distributed.
Tidal power is more reliable than wave power because it depends on highly predictable movements of the earth, moon and sun. This makes the technology attractive to energy planners who like to know well in advance how much electricity they will be offered, and when it will arrive! Another advantage of tidal streams is their comparative docility – although we must be careful to emphasise the word comparative because fast streams are generally turbulent and can place very high stresses on turbines and other devices. The marine environment is invariably tough, at and below the sea surface.
Tidal streams are reasonably predictable, but they are certainly not constant. A stream consists of a flow phase as the tide rises, alternating with an ebb phase as it falls, and a good stream for energy generation achieves high peak velocities in both phases. The times when the current ceases are referred to as slack water, and in most locations they occur close to high and low water. Efficient devices must generate electricity on both ebb and flow, in other words they are bi-directional, unlike land-based run-of-river turbines which extract energy from water flowing in one direction only.
Figure 1.9 shows the flow pattern of a strong tidal stream over an 11-day period encompassing spring and neap tides, in a location where the tides exhibit a straightforward semi-diurnal pattern (two similar tides per day). At spring tides the peak flow rate exceeds in both directions (positive for the flow phase, negative for the ebb phase), but at neap tides it falls to about . In many locations the basic pattern is modified by turbulence, which can affect peak flows considerably. Also, there are variations with water depth; flow rates are greatest near the surface and reduce as the seabed is approached.
Figure 1.9 Flow rates of a tidal stream.
In this book we generally quote tidal flow rates in metres per second. However alternative units such as kilometres per hour, miles per hour and knots (nautical miles per hour) are sometimes used – the latter especially by sailors and mariners. The relationship between them is:
For convenience it is helpful to remember that the speed in knots is very close to twice its value in metres per second.
The flow pattern of a tidal stream correlates closely with the rise and fall of local tides, although their influence may be hard to analyse. Many tides produce far more complex flow patterns than the straightforward spring–neap variation shown in Figure 1.9. The flow volume along a curved or irregular channel remains constant, even though its depth, speed and direction may vary continuously. The formation of large eddies depends on the channel's shape upstream, not downstream. Generally speaking, flow magnitudes vary over the lunar cycle, peaking a few days after each new or full moon and increasing still further around the time of the equinoxes in March and September. All these effects are very significant for the output of a tidal turbine, which oscillates in sympathy with the complex flow pattern.
It is important to realise that strong tidal streams do not necessarily accompany large tidal ranges. For example you might be standing on a shore watching the tide rise and fall through many metres, yet see no horizontal currents strong enough to work a turbine. Conversely, a relatively small tidal range may produce a vigorous tidal stream through a narrow channel between an island and nearby coastline, or through the inlet of a large bay or estuary. The relationship between tidal ranges and tidal streams depends on complex interactions between the movement of large volumes of water around continents and islands, and local effects including coastal features and the shape of the seabed (bathymetry). In addition, when sea enters a bay or estuary the tidal patterns may be greatly influenced by its depth, length and area [6].
So where does all this complexity lead us in terms of the regions of the world which have most potential for tidal stream technology? Figure 1.10 summarises the global scene, and it is interesting to compare it with Figure 1.6 which showed the distribution of wave energy potential. For a start, the powerful wave resources off the west-facing coasts of Portugal, Chile and Australia are not matched by comparable tidal stream resources. Conversely, ocean areas to the north of Australia and east of China with little wave energy potential have some very powerful tidal streams. With few exceptions, powerful wave and tidal stream resources do not coincide.
Figure 1.10 Coastal regions of the world with good tidal stream resources.
One of the few nations in the world to be doubly blessed is the UK, and especially Scotland, whose waves and tidal streams, backed up by technical innovation and political will, are producing world leadership in marine renewable energy. We may use some of Scotland's prime tidal stream locations for illustrative purposes. Figure 1.11 shows three areas of coastal water and two narrow channels formed by the complex geography of the west coast and inner Hebridean islands:
Figure 1.11 Important tidal stream locations in Scottish waters.
Figure 1.12 A fast-flowing tidal stream in the Sound of Islay (Paul A. Lynn).
These five sites represent a good selection for testing and proving today's tidal stream machines. Relatively small, sheltered, locations such as the Sound of Islay and Kyle Rhea are ideal for installing and proving the viability of prototype arrays. Operational experience gained from such sites will give developers confidence to move into larger, more exposed, sea areas including the Pentland Firth where the dream of making a significant contribution to national electricity supplies can become a reality.
1.3 A piece of history
1.3.1 Working with waves
The history of wave energy conversion goes back over 200 years and may conveniently be divided into two phases. The first ended with the 1973 ‘oil shock’, when oil-producing nations in the Middle East showed their determination to exercise greater control over the price and availability of their ‘black gold’. This acted as a wake-up call for western governments to consider alternative energy sources, including ocean waves. The second phase, from 1973 to the present day, is very much the modern history of wave energy, with its moments of optimism and setback, culminating in sustained interest and commitment by governments to support a fledgling industry moving towards commercialisation.
The first patent covering the design of a wave machine was granted to a Monsieur Girard and his son in Paris in 1799. Their idea was startling: to dock naval warships and use their bobbing up and down on the waves to rock long wooden beams. The heaving motion of the beams, acting as levers with their fulcrums on the shore, would generate mechanical power to drive pumps, saws and other machinery. In a moment of optimism the patent declared that ‘with a vessel suspended at the extremity of a lever, one may conceive the idea of the most powerful machine that has ever existed’. However the Girard plan was never realised. Perhaps it was considered too outlandish; or maybe the naval authorities in France had more pressing duties for their warships. After all, the year before had seen a heavy defeat of Napoleon's navy at the Battle of the Nile, and six years later came the battle of Trafalgar.
The nineteenth century spawned many new proposals for transmitting the oscillating motion of waves to pumps and other machinery using various types of mechanical linkage. By 1973 well over 1000 wave energy patents had been registered in Western Europe, North America and Japan, including 340 in the UK. Among various early efforts to translate paper designs into practical machines, we will consider two that are widely considered of historical importance [4]. Their operating principles are illustrated in Figure 1.13.
Figure 1.13 Two early examples of wave machines: (a) P. Wright's ‘wave motor’ of 1898 and (b) the Bochaux-Praceique ‘oscillating water column’ of 1910.
The ‘wave motor’ shoreline system invented by P. Wright was patented in the USA in 1898. Essentially it consisted of one or more hinged floats (F) which rode the approaching waves. Each float transmitted vertical motion to a connecting rod (C) operating a hydraulic pump (H) that could be used to power a wide variety of machinery. This was one of many devices proposed for the wave-rich beaches of Southern California in the 1890s. Only a few designs made it to full-scale, and only the Wright example still exists – buried, presumably in a very sad condition, beneath the sand of Manhattan Beach.
Among European efforts at working with waves, the system built by Monsieur Bochaux-Praceique at Royan, near Bordeaux, is historically important both for its overall design and because it successfully generated up to of electricity to power and light his home (Figure 1.13b). A shaft (S), sunk in a nearby cliff, was sealed at the top by a pressure cap (P) and connected to the sea by a short tunnel below low water level. Waves caused the water level in the shaft to oscillate, producing pressure fluctuations in the air column above and driving a turbine (T) connected to a generator (G). This was an early example of an oscillating water column – a very different approach to wave energy conversion from that taken by Wright and most other early inventors. By today's standards it was, of course, hopelessly uneconomic; just imagine carrying out major earthworks in order to generate a mere 1 kW of intermittent electricity!
As the twentieth century got into its stride the world's energy industries became more and more focussed on oil and its supreme usefulness for powering internal combustion engines. Coal was the main source of energy for generating electricity in large centralised power plants. Curious machines for extracting power from ocean waves faded into the background and although many patents continued to be granted the costs and engineering difficulties of constructing and installing viable devices seemed, to most people, insurmountable. A notable exception was provided by navigation lights for buoys at sea which need only a small amount of electricity. Starting in 1945, Japanese inventor Yoshio Masuda pioneered various wave-activated devices which he mounted and tested on an 80 m long ship specially adapted for the purpose [4]. One of his most successful designs was based on a float and long vertical tube which pumped air through a small turbine-generator and charged a battery. From 1965 onwards hundreds of Masuda devices provided electricity for navigation lights at sea. However, such a low-power application hardly addressed the question whether large wave energy converters could generate substantial amounts of electricity for use onshore.
As already noted, the first ‘oil shock’ of 1973 acted as a turning point. In the USA, President Carter gave serious support to renewable energy technologies, principally wind and solar power, and although President Reagan's subsequent administration proved far less enthusiastic, the die had been cast and international attention was now focussed on the increasing price and eventual depletion of the world's oil supplies. It is only fair to add that large-scale wave machines had few supporters at this stage – at least, not until the publication in 1974 of an article [8] in the scientific journal Nature by Professor Stephen Salter of the University of Edinburgh.
It would be hard to overestimate the effect of the Salter article on wave energy research and development, both in the UK where it was born, and internationally. Here was a talented engineer and researcher who re-examined wave energy conversion from first principles, discarding the widespread assumption that waves are only effective at generating up-and-down motion, and able to back up his theoretical insights with a splendid series of practical experiments [9]. And so the Edinburgh Duck, popularly known as the Nodding Duck, was born. Although it subsequently progressed as far as a 1/10th scale model, its early promise was never realised, partly because politicians and energy policymakers tended to slip temporarily back into denial about the oil problem, and partly – as many would claim – because the UK government under Margaret Thatcher had a love of nuclear power and an irrational dislike of anything renewable. In any case, funding for the UK's wave energy programme was severely cut in the late 1970s and the Nodding Duck was a principal casualty. Yet it has had a profound and lasting influence on the wave energy community, stimulating innovation in design and an appreciation of the challenges ahead.
The Nodding Duck is illustrated in Figure 1.14. Essentially, it consists of a device shaped like a large cam, able to ‘nod’ backwards and forwards about a horizontal axis under the action of incoming waves. Its oscillatory motion is converted into electricity by a hydraulic-electric subsystem. The highly original design concept is based on the way water particles in an ocean swell actually move as waves approach the device – not just up and down as might be expected, but in a circular path. We shall have more to say about this in Chapter 2. Nodding is caused partly by the dynamic pressure of moving water on the Duck's ‘paunch’, and partly by changing hydrostatic pressure on its buoyant ‘beak’. Such devices can achieve remarkable efficiencies, up to 90%, in converting wave energy to mechanical energy. To generate substantial amounts of power, an array of nodding ducks can be mounted on a common shaft set parallel to the incoming wave front.
Figure 1.14 Nodding Ducks: (a) a single device and (b) an array.
The surge of interest in wave energy in the 1970s generated other ideas [5] that influence designers and engineers to this day, including:
Contouring raft [10].
British engineer Christopher Cockerell, best known for his invention of the Hovercraft, was also active in wave energy research and suggested using a set of hinged rafts to follow the ups and downs of waves as they passed (see
Figure 1.15
). The relative motion between each pair of rafts at the hinges would supply power to a hydraulic subsystem. In 1978 his company tested a 1/10th scale, 3-raft model in waters off the Isle of Wight in southern England and, for a time, the contouring raft was seen as a strong competitor of the nodding duck.
Wave focussing [4].
This term is used to describe various techniques for concentrating the energy of long wave fronts onto relatively small power conversion devices. They include the
antenna effect
pioneered by Johannes Falnes and colleagues at the University of Trondheim in Norway who have made major contributions to wave energy research and development over many years [11]. Successful focussing of surging waves in a narrow tapering channel was achieved by the Norwegian
Tapchan
system installed in 1985. The action of such a system is illustrated in
Figure 1.15
b. Incoming waves increase in height as they move up the channel, finally ‘overtopping’ the lip of a man-made or natural reservoir (R) set a few metres above mean sea level (L). This converts the kinetic energy of the waves into potential energy that drives a turbine (T) and generator (G).
Figure 1.15 Tapping wave energy using (a) contouring rafts and (b) a tapering channel.
