Ocean Wave Energy Conversion: Resource, Technologies and Performance

By Aurelien Babarit

The waves that animate the surface of the oceans represent a deposit of renewable energy that for the most part is still unexploited today. This is not for lack of effort, as for more than two hundred years inventors, researchers and engineers have struggled to develop processes and systems to recover the energy of the waves. While all of these efforts have failed to converge towards a satisfactory technological solution, the result is a rich scientific and technical literature as well as extensive and varied feedback from experience.

For the uninitiated, this abundance is an obstacle. In order to facilitate familiarization with the subject, we propose in this work a summary of the state of knowledge on the potential of wave energy as well as on the processes and technologies of its recovery (wave energy converters). In particular, we focus on the problem of positioning wave energy in the electricity market, the development of wave energy conversion technologies from a historical perspective, and finally the energy performance of the devices. This work is aimed at students, researchers, developers, industry professionals and decision makers who wish to acquire a global perspective and the necessary tools to understand the field.

• Reviews the state of knowledge and developments on wave energy recovery
• Presents the history of wave energy recovery
• Classifies the various systems for recovering this type of energy

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 17, 2017
• ISBN: 9780081023907

Aurelien Babarit

Aurélien Babarit is head of the Marine and Ocean Energies team at the LHEEA laboratory of the École Centrale de Nantes - CNRS and director of the GDR Énergies Marines Renouvelables (CNRS) in France. Since 2002, his research has focused on the modeling and optimization of wave energy converters.

Book preview

Ocean Wave Energy Conversion

Resource, Technologies and Performance

Aurélien Babarit

Series Editor

Alain Dollet

Table of Contents

Cover

Title page

Copyright

Foreword

Introduction

1: Potential: Energy Resource and Markets

Abstract

1.1 Ocean wave energy resource

1.2 Market considerations

1.3 Conclusion

2: Wave Energy Conversion Historical Perspective

Abstract

2.1 Before 1973: prehistory

2.2 1973–1985: the modern age

2.3 1985–1998: the trough of the wave

2.4 Since 1998: the wave of technologies

2.5 Summary

2.6 2012–2016: a crisis of confidence

2.7 Conclusion: are we at a turning point in the history of wave energy?

3: Working Principles and Technologies of Wave Energy Conversion

Abstract

3.1 Classifications of wave energy converters

3.2 Working principles

3.3 New trends

3.4 Conclusion

4: Energy Performance of Wave Energy Converters

Abstract

4.1 Capture width and capture width ratio

4.2 Wave energy conversion as wave interference

4.3 Capture width ratio theoretical maxima in the two-dimensional case

4.4 Theoretical maxima for the capture width ratio in the three-dimensional case

4.5 Energy performance and capture width ratios observed under realistic conditions

4.6 Technical and economic comparison of a representative selection of wave energy converters

4.7 Conclusion

Conclusion

Appendix 1: Additional List of Intermediate-Scale Prototypes of Wave Energy Converters, Tested Offshore Between 2001 and 2016

A1.1 In Europe

A1.2 In the rest of the world

A1.3 Summary

Appendix 2: Refresher About the Linearized Free-Surface Potential Flow Theory

A2.1 Problem statement

A2.2 Assumptions

A2.3 The boundary value problem for the velocity potential

A2.4 Integral equation for the velocity potential

A2.5 Far-field coefficients, Kochin function

A2.6 Wave energy flux in the far field

A2.7 Energy balance

Bibliography

Index

Copyright

First published 2017 in Great Britain and the United States by ISTE Press Ltd and Elsevier Ltd

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Notices

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein.

For information on all our publications visit our website at http://store.elsevier.com/

© ISTE Press Ltd 2017

The rights of Aurélien Babarit to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.

British Library Cataloguing-in-Publication Data

A CIP record for this book is available from the British Library

Library of Congress Cataloging in Publication Data

A catalog record for this book is available from the Library of Congress

ISBN 978-1-78548-264-9

Printed and bound in the UK and US

Foreword

Over several years, the National Renewable Energy Laboratory (NREL) and Sandia National Laboratories have been collaborating along with leading world experts on a wave energy technology innovation project known as structured innovation. Aurélien Babarit is one of these experts. Thus, we had the opportunity of working closely together. I have known Aurélien for many years through his significant contributions in the wave energy field and through his invaluable support during tank testing at Central Nantes. However, during the 6 months that Aurélien spent with us at NREL in early 2016, I had the pleasure of getting a real sense of his energetic and astute style of conducting research and achieving progress. This is characterized by curiosity, intellect, scientific rigour, purpose of application and most importantly, the joy he derives from making valuable contributions to society.

This book is a clear and strong reflection of these characteristics, making it an interesting and refreshing read while dealing with the subject matter in a thorough and well-structured way. It provides a comprehensive overview of the field of wave energy, covering energy resource and market potential, historic eras and the current status of research and technology development, principles and technologies of wave energy conversion and their theoretical and realistic energy performance. It combines rigorous theory, insightful interpretation and results from practical implementation. Many points of interest, clarification and reflection are incorporated through the abundance of elucidating footnotes while maintaining the composition and flow of the book’s content. The author does not hold back from casting a critical eye over the field of his passion and drawing conclusions on the potential of wave energy exploitation, identifying challenges as well as promising trends and making recommendations in support of a successful implementation of wave energy technology. The book is of interest to newcomers as well as students and experts of the field of wave energy. It provides a comprehensive introduction to the foundations, history and state of the art of wave energy to those who are new to the domain, and, for those who have been active in the field, offers a reflective overview of the plethora of wave energy concepts and technology development eras as well as the associated challenges and opportunities.

In Chapter 1, the book opens with the cause and drive of the field by considering the wave energy resource and energy markets, and identifying the potential for wave energy as a renewable energy form. Following the mathematical formulation of ocean waves, the wave resource at a given site is characterized and the global resource distribution is considered. Features of the resource are directly linked to implications and requirements of the wave energy converter technology. For instance, with the observation that incident wave power levels vary by two orders of magnitude between operational and survival conditions, comes the design recommendation to implement a transparency to extreme energy levels in order to better cope with the associated loads, rather than trying to resist them. Considerations of the modification of wave energy resource characteristics offshore to nearshore provide valuable insight into the exploitable energy resource in these domains. Based on the electricity market requirements and the cost of energy projections from current state of the art wave energy converter prototypes, an economical assessment of learning effects from mass production is made. The required volume of installed capacity, the related investment and the associated portion of the resource are clearly recognized to be too large to consider production and commercial rollout of current technologies. Thus, the need for a disruptive innovation step change during the precommercial technology development phase to reduce the cost of energy projections by a factor of two is identified.

Chapter 2 provides a historic review of wave energy technology development and reaches right up to the current state of the art including the newest developments in the field. The descriptive and insightful titles of these research and technology development eras include prehistory, the modern age, the trough of the wave, the wave of technologies, a crisis of confidence. Given the huge variety of functional wave energy converter concepts and associated long- or short-term temporary operational technology prototypes, the wave energy utilization challenge is identified not as technical but as economic. Chronological tables of the large-scale prototype testing campaigns at sea provide an overview of developments. The overall investment in research, technology development and demonstration is estimated to be over one billion euros in the last 10 years. Given these circumstances along with recent technical and corporate failures, the author highlights the need for a strong shift in technology development methodology toward early emphasis on economic performance potential prior to maturing technologies to higher readiness. Posing the question whether wave energy technology development is at a critical turning point, the author emphasises the significance of untapped opportunities, highlights some encouraging development examples, underlines the continued need for learning from the past and concludes with the likelihood of a positive outlook and successful deployment of wave energy.

Chapter 3 provides an overview and descriptions of working principles and technologies of wave energy conversion along with a number of classification schemes. Classical systems as well as new trends in the form of flexible or deformable converters, hybrid systems and systems focusing on niche and alternative markets are presented.

The theoretical foundations, as well as their practical constraints, and limitations of the energy performance of wave energy converters are presented in Chapter 4. Key quantities for the expression of wave energy absorption and conversion performance are identified and theoretical optima in two-dimensional and three-dimensional wave field cases are derived, insightfully illustrated and subjected to upper bounds and practical limits. Finally, energy capture observed under realistic conditions is reported and several prominent technology types are compared with respect to characteristic economic performance drivers.

The comprehensive findings in this book are supported by an extensive bibliography and made accessible through an elaborate index. This book has the potential to further existing interest and attract new attention to the fascinating field of wave energy research and technology development through providing an introduction, overview, analysis, interpretation and future outlook. This work makes a very valuable contribution by concentrating and sharing knowledge and experience and thus supporting progress in the development of wave energy technology to eventually provide a real contribution to the renewable energy mix, an imperative for a sustainable society.

Jochem Weber, Chief Engineer, National Renewable Energy Laboratory

Introduction

Since the beginning of the 2000s, renewable energies have experienced a spectacular development. In 2016, the global installed wind power capacity reached 487 GW, while it was only 24 GW in 2001. For photovoltaic solar energy, it reached 295 GW (whereas it was only 1.6 GW in 2001). These indicators are a positive sign given the pressing need for transformation of our energy system.

Wave energy constitutes another source of renewable energy. Unlike wind or solar power, it is still untapped nowadays. This is not for want of trying. For 40 years, inventors, engineers and researchers have devised numerous systems to capture wave energy. Although all these efforts have failed to converge to a satisfying technological solution, an abundant scientific and technical literature has resulted therefrom as well as numerous and diverse lessons learned.

To the outside reader, this proliferation may be an obstacle. In order for this subject to be more easily uncovered, we propose in this book a synthesis and a discussion of the notions and basic concepts of wave energy. This work is intended for students, researchers, developers, industrialists, and decision makers wishing to acquire a global vision and the essential elements for an understanding of the field. This is the result of 15 years of research and 7 years of experience teaching this topic, especially lecturing students at Ecole Centrale de Nantes.

This book consists of four chapters and two appendices. Chapters 2 and 3 as well as Appendix 1 are accessible to a wide audience. Chapters 1 and 4, and Appendix 2, are more technical and require a basic mathematical background.

In Chapter 1, we focus on the potential of wave energy both from the energy resource and the electricity market perspectives. The first objective is to clarify concepts about characteristics and orders of magnitude of the energy resource that we wish to exploit. We will see in particular that, although the resource is considerable, it is of an order of magnitude that is less than the global energy consumption and therefore would not constitute a majority solution in a decarbonized energy system. Another objective is to raise readers’ awareness, if necessary, of the economic constraints faced by all the technologies for the production of electrical energy from renewable sources.

In Chapter 2, we propose a historical perspective of wave energy conversion. In effect, one notable aspect of this topic is the very large number of inventions it has led to. The first objective of this chapter is to allow the reader, possibly neophyte and eventually bearing in mind the concept of a wave energy converter (WEC), to learn from the past. Moreover, this visit through wave energy history will show that, despite the fact that arguably some demonstration projects of wave energy converters at sea have failed miserably, a larger proportion of projects has been successfully completed. We want to demonstrate to the reader that the wave energy problem is not technical – yes, it is possible that wave energy be recovered to produce electricity – but economic: how can the energy of waves be recovered at an acceptable cost?

Despite the different operating principles of wave energy converters mentioned in Chapter 2, we have preferred to retain their detailed presentations for Chapter 3. Because of the proliferation that can be observed since the beginning of the 2000s, we believe that there is a risk that the reader might become somewhat confused faced with all concepts at the end of Chapter 2. Returning in detail to the operating principles and the different possible classifications for wave energy converters, we will show that they are particularly useful as reference templates so as to not become overwhelmed. In the second part of Chapter 3, we will discuss new trends, which perhaps will result in technological disruption.

In Chapter 4, we will focus on the energy performance of wave energy converters. Analogously to any energy system, it is instructive to quantify their efficiency, that is the ratio between the absorbed energy and the available energy. We will address it at first from a theoretical point of view, then we will compare these theoretical results to the energy performance that has been observed under realistic conditions. Finally, since in absolute terms the decisive criterion is the cost of energy, we will finish with the results from a study comparing a selection of wave energy converters from a technical and economic point of view.

1

Potential: Energy Resource and Markets

Abstract

In this chapter, we focus on the potential of ocean wave energy both from the energy resource and the electricity market standpoints. The issues are as follows: can ocean wave energy satisfy a significant part of the energy needs of humanity? Where are the main energy sources and what is the objective cost of energy so as to enable market penetration?

Keywords

Airy regular wave model; Directionality; Electricity market; Energy resource and markets; JONSWAP spectrum; Nearshore resource; Ocean wave energy; Sea state; Shoaling effects; Wave measurement buoy

In this chapter, we focus on the potential of ocean wave energy both from the energy resource and the electricity market standpoints. The issues are as follows: can ocean wave energy satisfy a significant part of the energy needs of humanity? Where are the main energy sources and what is the objective cost of energy so as to enable market penetration?

First, we include an ocean engineering refresher on ocean waves which is necessary to address the characterization of the energy resource. Next, we discuss the energy source and its distribution around the globe, and then the nearshore resource. This discussion leads us to recommend that preference should be given by developers of wave energy converters to systems deployed in finite depth of the order of 10–20 m. Finally, the energy cost of current technical solutions is weighed against market retail energy price in order to discuss development potentials from an economic point of view.

1.1 Ocean wave energy resource

1.1.1 The Airy regular wave model

In the context of wave energy conversion, the basic element to describe ocean waves is the Airy wave model (also called regular wave). This model provides a means to mathematically describe the flow corresponding to the propagation of a wave of amplitude A0 and with a direction making an angle β with axis x (Figure 1.1). This flow is fully characterized by its velocity potential Φ0(M, t) [ARD 16]

Figure 1.1 Illustration of the free-surface deformation for the Airy wave model. The wave period is 5 sec in this example

   [1.1]

with M(x, y, z) any point in the fluid, g the wavenumber and λ the wavelength, φ0 the phase reference and f0(z) a function depending on the vertical coordinate z.z = 0 defines the free surface with z with h in deep water (infinite water depth).

The free-surface elevation η(x, y, t) is obtained using the linearized version of the kinematic free-surface kinematic condition:

   [1.2]

Figure 1.1 shows the free-surface deformation corresponding to this wave model. It can be seen that it is a regular and cylindrical wave.

The dynamic component of the pressure is obtained using the linearized version of Bernoulli’s equation:

   [1.3]

with ρ the density of water. It can be observed that the dynamic component of the pressure is proportional to the free-surface elevation. The coefficient of proportionality is the function f0(z). In the context of wave energy conversion, we can thus already make the following remarks:

in practice), the effect of wave propagation is maximal at the free surface and dampens when moving down the water column. A wave energy converter¹ thus benefits from being located close to the interface to best take advantage of the resource;

, the effect of wave propagation can be observed in the whole water column. A wave energy converter can therefore be placed at any depth inside the water column, including at the bottom, while maintaining access to the resource.

In the Airy