Electricity Production from Renewable Energies E-Book

Benoît Robyns

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 352

Veröffentlichungsjahr: 2012

Beschreibung

Energy and environmental issues have caused a marked increase in electricity production from renewable energy sources since the beginning of the 21st Century. The concept of sustainable development and concern for future generations challenge us every day to produce new technologies for energy production, and new patterns of use for these energies. Their rapid emergence can make the understanding and therefore the perception of these new technologies difficult. This book aims to contribute to a better understanding of the new electricity generation technologies by addressing a diverse audience. It presents the issues, sources and means of conversion into electricity using a general approach and develops scientific concepts to understand their main technical characteristics. Systems of electricity generation from renewable energy resources of small to medium powers are presented. The basic electrical concepts necessary for understanding the operating characteristics of these energy converters are introduced, and the constraints and problems of integration in the electrical networks of those means of production are discussed. Several exercises are provided to the reader for evaluation purposes. Contents 1. Decentralized Electricity Production from Renewable Energy, Benoît Robyns. 2. Solar Photovoltaic Power, Arnaud Davigny. 3. Wind Power, Bruno Francois and Benoît Robyns. 4. Terrestrial and Marine Hydroelectricity: Waves and Tides, Benoît Robyns and Antoine Henneton. 5. Thermal Power Generation, Jonathan Sprooten. 6. Integration of the Decentralized Production into the Electrical Network, Benoît Robyns.

Leseprobe

Table of Contents

Foreword

Introduction

Chapter 1. Decentralized Electricity Production from Renewable Energy

1.1. Decentralized production

1.2. The issue of renewable energies

1.3. Renewable energy sources

1.4. Production of electricity from renewable energies

1.5. Bibliography

Chapter 2. Solar Photovoltaic Power

2.1. Introduction

2.2. Characteristics of the primary resource

2.3. Photovoltaic conversion

2.4. Maximum electric power extraction

2.5. Power converters

2.6. Adjustment of the active and reactive power

2.7. Solar power stations

2.8. Exercises

2.9. Bibliography

Chapter 3. Wind Power

3.1. Characteristic of the primary resource

3.2. Kinetic wind energy

3.3. Wind turbines

3.4. Power limitation by varying the power coefficient

3.5. Mechanical couplings between the turbine and the electric generator

3.6. Generalities on induction and mechanical electric conversion

3.7. “Fixed speed” wind turbines based on induction machines

3.8. Variable speed wind turbine

3.9. Wind turbine farms

3.10. Exercises

3.11. Bibliography

Chapter 4. Terrestrial and Marine Hydroelectricity: Waves and Tides

4.1. Run-of-the-river hydraulics

4.2. Hydraulic power of the sea

4.3. Bibliography

Chapter 5. Thermal Power Generation

5.1. Introduction

5.2. Geothermal power

5.3. Thermodynamic solar power generation

5.4. Cogeneration by biomass

5.5. Bibliography

Chapter 6. Integration of the Decentralized Production into the Electrical Network

6.1. From a centralized network to a decentralized network

6.2. Connection voltage

6.3. Connection constraints

6.4. Limitations of the penetration level

6.5. Perspectives for better integration into the networks

6.6. Bibliography

First published 2012 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

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John Wiley & Sons, Inc.111 River Street Hoboken, NJ 07030 USA

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© ISTE Ltd 2012

The rights of Benoît Robyns, Arnaud Davigny, Bruno François, Antoine Henneton, Jonathan Sprooten to be identified as the author of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988.

Library of Congress Cataloging-in-Publication Data

Electricity production from renewables energies / Benoît Robyns … [et al.].     p. cm.   Includes bibliographical references and index.   ISBN 978-1-84821-390-6   1. Renewable energy sources. 2. Energy development. 3. Geothermal resources. 4. Ocean energy resources. 5. Electric power distribution. I. Robyns, Benoit.    TJ808.E44 2012    621.31--dc23

2011051810

British Library Cataloguing-in-Publication Data A CIP record for this book is available from the British Library

Foreword

Writing the foreword of a book you have initially “commissioned” as series editor (originally for the French Hermes series “Electrical Energy Sciences and Technologies”) is a somewhat unusual exercise…

I was aware of the extent of the task, when I approached Benoit Robyns in 2008 to write a book for educational purposes, which would gather together in a single volume the summarized knowledge about means of electricity production from renewable energies. But I also knew that the region of Lille was a resourceful environment. He proved me right, by bringing together a competent team made up of five lecturers/researchers who have internationally recognized experience and practice: himself, Benoit Robyns, Arnaud Davigny, Bruno François, Antoine Henneton and Jonathan Sprooten.

The development of the book was long (more than 3 years), and thus shows the difficulty and the extent of the work. However, today the result is in front of us, and we now have an overview covering the wide spectrum of the sciences and technologies of the conversion of renewable energy resources into electricity.

A contextual introduction presents the great potential of renewable resources; without a doubt the only resources able to provide humanity with a sustainable future. The authors then concisely discuss the subject with a clarity that will enable people with an academic scientific background to understand it. They shed light on the conversion principles and the following associated technologies: photovoltaics, wind energy, hydraulics (land and maritime solutions, including wave-power generators and underwater turbines) and thermodynamics (biomass, geothermal energy, ocean thermal energy). By way of a conclusion, the last chapter discusses the integration of a very decentralized production into the network.

I would like to thank the authors for their tenacity and goodwill, particularly in view of my requirements (my scrupulous proofreading, etc.) that could have discouraged them! We have to admit that in the context of these new subjects, the pedagogy adapted to a public of electricians is not yet established. Therefore, this is probably the aspect upon which they had to work the most.

This book is a valuable addition to ISTE and John Wiley and Sons publications, and is one whose influence will hopefully measure up to its ambition. I am convinced that it will be a reference for the electrical engineering community, and I hope above all that it will increase the penetration rate of these technologies. An improved training of lecturers and students is an inevitable vector, so that the entire world takes without delay the (high speed) train of sustainable energy, to ensure the sustainability of its economy and environment!

Bernard MULTON Ecole Normale Supérieure de Cachan SATIE-CNRS Brittany Campus January 2012

Introduction

From the beginning of the 21st Century energy and environmental challenges have led to increasing electricity production from renewable energies. The concept of sustainable development and the concern for future generations challenge us daily, leading to the emergence of new energy production technologies and new behavior usage for these energies. The quick emergence of new technologies can make its understanding and perception difficult. The purpose of this book is to contribute to a better understanding of these new electricity production technologies by targeting a large audience. It presents the challenges, sources and their conversion process into electricity by following a general approach. It also develops basic scientific notions to comprehend their main technical characteristics with a global view.

The objectives of this book are:

– to present electricity production systems from renewable energy sources from small to mean powers (up to 100 to 200 MW);

– to introduce basic electrical notions that are necessary for the understanding of the operational characteristics of these energy converters;

– to discuss integration constraints and issues in the electrical networks of these production units;

– to set a few exercises for self-assessment.

Chapter 1 introduces the concept of decentralized electricity production from renewable energy resources. It presents the challenges that have led to the development of electricity production, not only just from the 20th Century centralized approach, but also dispersed throughout the territories. After all, the available resources that are managed by various actors in competition are also dispersed. This chapter also presents the challenges that led to the development of electricity production from renewable resources. It introduces the various exploitable energies and describes the basic principles of their conversion into electrical energy.

Chapter 2 presents the direct production (photovoltaics) of electricity from solar energy. It describes the characteristics of photovoltaic cells and panels. It explains the operational principles of power electronic converters, which help to control the energy extracted from solar radiation and to transform it into the form required by consumers. The chapter ends with some exercises.

Chapter 3 develops the conversion principles of converting wind kinetic energy into electrical energy. It describes the main wind turbine technologies. Is also explains electro-mechanical conversion from synchronous and induction generators, at fixed and variable speed. Examples of the characteristics of effective high and low power wind turbines are also provided. Exercises concerning various types of wind turbines at fixed and variable speed, the characterization of a wind turbine and the estimate of the generated power are also proposed.

Chapter 4 introduces electrical energy production from the potential or kinetic energy of water, whether in a terrestrial or marine environment. At first, the principles of hydroelectricity (the first renewable source producing electricity, which has been implemented for more than a century) are developed, by focusing more specifically on the running of river hydraulics. Secondly, water power coming from marine sources are presented: wave, marine current and tidal power. The exploitation of these energies is still not very developed and most of the associated technologies are just emerging, except for tidal power production, which is quite mature but still marginal. A few examples of these technologies will be described in this chapter. Some exercises in the context of a small river hydroelectric power plant and a tidal power plant are also proposed.

Chapter 5 introduces the concept of thermal electricity production, in which heat is produced from renewable resources. This is the case for geothermal power, for concentrated thermodynamic solar power and for cogeneration, whose principles are described. The operational principles and characteristics of the synchronous alternators directly coupled to the electrical network are also presented.

Chapter 6 raises the question of the integration of renewable energy sources and more, generally, of the decentralized production into electrical network. The latter are indeed confronted with a new paradigm, because of the random and unpredictable nature of some of these sources, due to their scattering on the territory, and because of the rules of a liberalized electricity market. The main connection constraints of these sources are also briefly described. Perspectives for a better integration into the networks of these sources are identified by considering actions on the levels of the sources, networks and consumers. Developments and incentives are initiated, so that the future electrical networks become smarter.

Chapter 1Decentralized Electricity Production fromRenewable Energy1

1.1. Decentralized production

There is no clear official definition of decentralized production. Generally, decentralized production is defined as the opposite of centralized production [CRA 08, JEN 00]. To simplify, let us say at first that decentralized units:

– are not planned in a centralized way;

– are not controlled (or dispatched) in a centralized manner;

– have a power, which does not exceed 50 to 100 MW;

– are generally connected to the distribution network and not to the transportation network.

Another characteristic of decentralized production is that it is scattered over a territory, in contrast to conventional production, which is concentrated on a limited number of well-defined sites.

The development of decentralized production over the last few years has been especially favored by the opening of the electricity markets (which has spread in Europe from the beginning of the 2000s) and the development of renewable energies, especially wind energy, driven by a real commitment to the environment on a European scale. Decentralized production is thus developing in many countries on the basis of cogeneration units, renewable energy systems or conventional productions, which have been installed by independent producers.

The development of this type of production can contribute to solving technical, economic and environmental problems [CRA 08, JEN 00], even if it is not the only answer to these multiple challenges.

Let us make a list of elements favoring decentralized production:

– the desire to reduce greenhouse gas emissions (mainly CO2) encourages the development of renewable energies;

– the energy efficiency increase, which has been obtained thanks to cogeneration systems;

– the opening up of the electricity market enabling the emergence of independent producers;

– the desire to widen the range of energy supply, in order to limit the energy dependence of the European Union, which results from the use of fossil fuels;

– technological progresses contributing to the reliability and availability of 100 kW to 150 MW units;

– the greater facility to find sites able to accommodate a reduced power production;

– shorter construction periods and lower investments than for large conventional power plants;

– a production that can be carried out at the proximity of its use, thus reducing transportation costs.

Depending on the profile of the historical generation system of each country, the structure of their transport and distribution network and the organization of the electrical system, these various points can be more or less important, depending on the countries, especially within Europe.

1.2. The issue of renewable energies

1.2.1. Observations

The growing interest in the development of renewable energies is caused by several elements: climate change, increasing energy demand, limits of fossil fuel reserves, low global efficiency of the energy system and energy dependence, especially in the case of Western countries [CHA 04].

Climate change

The growing “greenhouse effect” leads to the increase of the global temperature at the surface of the planet. And yet, because of human activities, the concentration of greenhouse gases has soared since the pre-industrial era (1750–1800). Carbon dioxide concentration (CO2) (the main greenhouse gas) has increased by 30% since the pre-industrial era. The combined effects of all the greenhouse gases (CO2, methane, ozone, etc.) nowadays amounts to a 50% CO2 increase compared with this period.

Since 1860, the mean temperature at the surface of the Earth has risen by 0.6°C. Several prospective scenarios are predicting that by 2100, this temperature will increase further between 1.5 and 6°C, if energy systems and current consumption habits do not change. This significant increase would be accompanied by a sea level rise from 20 cm to 1 m. If the climate change seems non-reversible, this evolution can however be slowed down, by significantly reducing greenhouse gas emissions.

The natural CO2 wells, such as land, trees and oceans, would only be able to absorb a little less than half of the CO2 production resulting from human activities (produced in 2000). In order to stabilize the CO2 concentration at its current level, we thus would have to immediately reduce the gas emissions from 50 to 70%. This drastic reduction is clearly impossible. However, it is urgent to start acting, because this is a cumulative issue. Indeed, the carbon dioxide lifespan in the atmosphere is of about one century and, therefore, the stabilization of the CO2 concentrations to an acceptable level will take several generations.

CO2 is produced by the combustion of all fossil fuels: oil, gas and coal. CO2 emissions from coal are twice as high as the emissions from natural gas. Oil emissions are in-between.

At the beginning of the 21st Century, the distribution by sectors of CO2 emissions in the world was as follows: electricity production 39%, transport 23%, industry 22%, residential 10%, service sector 4% and agriculture 2%. This distribution varies however from one country to another. For example, in France where only one tenth of the electricity is produced from fossil fuels, the transport sector is responsible for more than 40% of the CO2 emissions into the atmosphere.

Increasing energy demand

Many energy scenarios are developed each year by specialized organizations in the energy field. These scenarios plan an energy demand in 2050 of about 15 to 25 Gtoe. These prospective scenarios are based on various parameters, such as economic growth, increased by world population increase, the progressive access to electricity of the 1.6 billion people still without any access to it at present, the growing needs of developing countries and the implementation of energy-saving policies in order to protect the environment. The uncertainties in relation to the evolution of these parameters explain the significant gap between extreme scenarios.

However, it seems quite reasonable to predict that by the middle of the century, the energy demand will have doubled.

Limits of the fossil fuel reserves

The R/P oil ratio (known reserves to the annual production) is about 40 years. This piece of data (which is equivalent to a period) should not be mixed up with the period during which we will still dispose of oil, nor to the one during which it will still be cheap enough. These two periods are completely unpredictable, because they depend on too many parameters. Let us note that since the 1980s, each year we consume more oil than we discover.

For natural gas, the R/P ratio is about 60 years. But if we wanted to replace oil and coal with gas, in order to reduce greenhouse gas emissions, the R/P ratio would then only be 17 years. When some countries give up nuclear energy for the benefits of gas, it could increase the consumption of resources.

Coal is the fossil fuel with the most significant reserves. Its R/P ratio is estimated at more than 200 years.

The R/P ratio of uranium is about 60 years (on the basis of “reasonably assured resources” added to “recoverable resources” at less than $130 per kg of natural uranium and a conventional fission exploiting isotope 235). Let us also note that nuclear fission only contributes up to 2.7% to the final energy on a global scale and that doubling its production will only have a small impact on the reduction of greenhouse gas emissions.

The energy demand until 2050 (then predicted to be between 15 and 25 Gtoe, compared to 12 Gtoe in 2010) could still be met mainly (at present) by nonrenewable raw energy materials. This would have dramatic consequences for the climate in particular, and for the environment and would not really take into account the needs of future generations.

In order to limit the rise in temperature to a range from 1 to 3°C, the total emissions for centuries would have to be only a third of the current emissions, caused the combustion of the accessible resources of gas, oil and coal. Humanity would then have to stop burning two-thirds of a relatively cheap and accessible energy source. It is thus not reasonable to bank on an early exhaustion of the resources, in order to naturally reduce greenhouse gas emissions. This is particularly true, because the relatively low price of fossil fuels (despite regular explosions) are disrupting the emergence of new technologies, which are inevitably more expensive until they become integrated into a mass production process.

Low global efficiency of the energy system

The global efficiency of our energy system is quite low: for example, in 2008, to satisfy the French requirements for final energy (marketed) of 168 Mtoe, 262 primary Mtoe were needed to produce them, which corresponds to a 63% efficiency, all the while knowing that effectively useful energy is much lower. The energy transformation losses alone when making marketable energy are about 27%. 94 Mtoe have thus been lost in energy transformations (refining, electricity production, etc.). These losses of 94 Mtoe, associated with bad uses of the final energy (bad building insulation, low efficiency of the car heat engines, etc.) are the main item of expenditures and finally the most important cause of CO2 emissions. For example, in 2000, the global efficiency was about 34%.

Energy dependence

About 50% of the energy consumed within the European Union comes from resources located in countries outside the EU. If nothing changes in the energy production field, and taking into account the expected consumption increase, this energy dependence will go up to 70% by 2030.

The global dependence on Middle East countries (which possesses 65% of the known oil reserves) should increase. The dependence is even higher for uranium (100% for France). From 2020–2030, economic and political tensions could arise from the diminution of fossil resources, which are easily exploitable and from their concentration in politically unstable zones. This would question the supply security of the European Union countries.

1.2.2. The sustainable development context

The concept of sustainable development was defined in 1986 as follows: “meeting the needs of the present without compromising the ability of future generations to meet their own needs”.

This concept implies the exploitation of renewable energy sources, which are the only sustainability guarantees, and the minimization of environmental impacts, associated with their conversion and the manufacturing of their converters. Fossil and fissile fuels are appearing as a finite and economically limited resource, which causes emissions affecting the environment and/or contributing to climate change in the case of fossil fuels. A sustainable energy system must include renewable energy sources and/or conversion chains exploiting low emission renewable fuels, which are available at an acceptable price. Despite the fact that the implementation of new energy facilities takes several decades, an increasing number of large companies are involved in the development and marketing of these new technologies.

Sustainable development requires good management of the balance between economic development, social equity and environmental protection in all the regions of the world. This concept can only become effective with real political will from an increasing number of countries.

1.2.3. Commitments and perspectives

The concept of sustainable development is an answer to the observations above. To implement it, several decisions and associated objectives have been progressively made on the European and international level.

Kyoto Protocol

In 1997, the Kyoto protocol set the objective to have reduced global greenhouse gas emissions by 5.2% around 2010, in comparison to 1990 levels. The European Union promised an 8% reduction of its emissions by 2010, and each member was allocated their own reduction quota of emissions, by taking into account the specificities of each country. More than half of the countries had to reduce their emissions (Germany, Austria, Belgium, Denmark, Italy, Luxemburg, Netherlands), some others needed to stabilize their emissions (France, Finland), while other countries were authorized to increase their emissions (Greece, Ireland, Portugal, Spain, Sweden).

To stop the increase of the carbon dioxide concentration in the atmosphere by 2050, our current emissions will have to be halved worldwide, and thus reduced to between 1/3 to 1/5 in developed countries. The Bali conference of December 2007 re-launched the negotiations between the States, in order to increase commitments to countries reducing CO2 emissions.

European Union and sustainable energy development

At the beginning of the 21st Century, the European Commission made the development of renewable energies a political priority, this is described in the White paper “Energy for the future: renewable sources of energy” and the Green paper “Towards a European strategy for the security of energy supply”.

The objective set by the Commission was to double the proportion of renewable energies in the global energy consumption, from 6% in 1997 to 12% in 2010. This doubling objective fits within a strategy of supply security and sustainable development; a particularly significant effort has to be made in the electrical field. Within the European Union, the proportion of electricity produced from renewable energy sources should reach 22.1% in 2010, compared to 14.2% in 1999. This objective was defined for the EU-15, and was significantly lowered for the EU-27, in order to reach 21%.

In 2007, the European Council promised to reduce CO2 emissions by 20% within the European Union by 2020. The objective was that 20% of the final energy consumption should be ensured by renewable energies, with a 10% biofuel use in transport, and a 20% energy efficiency improvement.

Electricity market opening

Since the beginning of the 2000s, the electricity sector has been the scene of a deep restructuring, resulting from the European Directive CE 96-92. This directive imposes a management of the activities inherent in the transport of electricity, which is independent from the electric energy production activities. The backbone of the electrical supply remains the transport network, which is managed in each state by one or a reduced number of managers appointed by the government involved.

One of the consequences of the opening of the electricity market is the development of a decentralized production, on the basis of cogeneration units, renewable energy sources or traditional production, which is installed by independent producers.

The integration to the electrical networks of renewable energy sources, more specifically those subjected to climate vagaries, such as wind and solar power, and more generally of decentralized production, will require significant network upgrading, as well as the implementation of new equipment and new management methods. The challenge is then to maintain the reliability and quality of the power supply for private individuals and businesses, despite market liberalization and the growing use of random renewable energy sources.

Technological prospects

It is quite difficult to identify the technologies that will play a crucial role in the future for the fight against the greenhouse effect. A future energy system with low greenhouse gas emissions will probably rely on a combination of energies, of energy vectors and converters, which will take on various forms in the different regions of the world.

It is however possible to determine five trends of our energy future:

– The proportion of renewable energies is progressively increasing, but this progress will notably depend on the reduction of their costs and on the progress made in terms of massive energy storage, which could integrate important quantities of intermittent and scattered production into electrical networks. In the long run, it seems unlikely that each of these renewable energy sources would exceed 10% of the global energy supply. However, according to the most optimistic predictions, their combination could enable them to reach 30 to 50% of the market around the middle of the century (at the beginning of the 2000s, all renewable energies represented about 10% of the energy production).

– Fossil energies will still be used for several decades, all the while favoring energies with a low carbon content, such as gas. However, the capture and storage of carbon dioxide in economically bearable conditions seem to be the only technological option, which is likely to authorize the use of fossil resources, all the while limiting the CO2 concentration in the atmosphere, while waiting for significant technological developments.

– Nuclear power does not generate CO2, except for the CO2 emissions during the plant construction and during uranium enrichment. This type of energy will continue to be developed in some countries, such as France, by means of a well developed waste management process, the development of a new generation of safer reactors, knowing that fissile resources are also limited, and then in the long-term by the development of nuclear fusion. However, nuclear fusion is only considered on the horizon of time much after 2050.

– The spreading of fuel cells could lead to the development of a “hydrogen economy”. The production of hydrogen does not generate CO2, if hydrogen is produced from renewable, nuclear or fossil energies with CO2 sequestration. The USA did not ratify the Kyoto protocol, because they consider it to be too restrictive for their economy. In 2003 they launched an ambitious research program aiming to reduce the hydrogen production cost, while controlling greenhouse gas emissions, mastering hydrogen storage and reducing the fuel cell cost.

– Finally, controlling greenhouse gas emissions will not be possible without significant progress in energy efficiency in the construction, industrial and transport sectors. The challenge is to use less energy to satisfy the same needs, all the while knowing that the potential energy savings are huge.

1.3. Renewable energy sources

In our time scale, renewable energies are those continuously provided by nature. They come from solar radiation, the core of the Earth and from the gravitational forces of the Moon and the Sun with oceans. We can distinguish several types of renewable energies: wind power, solar power, biomass, hydraulics and geothermics [MUL 04, RSS].

1.3.1. Wind energy

The available wind resources are evaluated on a global scale at 57,000 TWh/year. The contribution of offshore wind power (at sea) is estimated at 25,000 to 30,000 TWh/year, if we limit ourselves to sites, whose depth does not exceed 50 m. The global production of electricity in 2008 was about 20,000 TWh (which corresponds to a primary consumed energy of about 50,000 TWh, related to the low efficiency of the thermo-mechanical cycles, often ranging between 30 and 40%). In theory, wind energy could satisfy the global electricity demand. However, the main disadvantage of this energy source is its instability. There is often not much or no wind during very cold or very hot periods; and yet there is an increased energy demand during these periods. This is why we could envisage an important development of wind power, all the while associating it with other renewable energy sources, which would be less random or complementary, or have thermal sources or electrical energy storage devices. However, if there are many ideas to store electrical power in large quantities (notably pumped storage power stations), their implementation still needs technological progression, in order to extend their possibilities and reduce costs.

Europe represents 9% of the wind energy potential available in the world. It produced 131 TWh of electricity from wind energy in 2009. The wind energy technically available in Europe, not including offshore, would be 5,000 TWh/year.

1.3.2. Solar energy

The projected lifespan of the sun is 5 billion years, which makes it in our time scale an inexhaustible and thus renewable energy. The total energy received at the surface of the Earth is 720 million TWh/year, i.e. 6,000 times the current primary consumption of all human activities. But the availability of this energy depends on the day-night cycle, on the latitude of where this energy is captured, on the seasons and on the cloud cover.

Solar thermal energy consists of producing hot water usable in construction or enabling the operation of turbines, by exploiting concentration phenomena to increase temperatures, in thermal power stations with thermodynamic cycles, in order to produce electricity. This electricity generation technique has been the subject of experimental power stations, whose 15% net efficiency turns out to be quite low. Sea surfaces are naturally heated by the sun and there is thus a gigantic energy reservoir in the tropical zone. Projects for the extraction of this “ocean thermal energy” have been carried out by implementing thermodynamic machines, which operate on the small difference found between the surface (25 to 30°C) and the depth (5°C at 1,000 m). In order for this solution to be exploitable, the temperature difference has to be higher than 20°C. However, the obtained efficiency (around 2%) is very low. Let us note that the low level of these efficiencies results mainly in higher machine sizes, but does not have the seriousness associated with the consumption of non-renewable raw materials, which are irreparably consumed.

Photovoltaic solar energy consists of directly producing electricity via silicon cells. When the sun shines and weather conditions are favorable, the sun supplies a peak force of 1 kW/m². Marketed photovoltaic panels help to directly convert 10 to 15% of this power into electricity. The productivity of a photovoltaic panel varies with the level of sunshine: about 100 kWh/m²/year in Northern Europe and twice this amount in the Southern Mediterranean region. A photovoltaic roof of 5 × 4 meters has a power of 3 kW and produces from 2 to 6 MWh/year, depending on the sunshine. If the 10,000 km2 of roof in France were used as photovoltaic generators, the production would be of 1,000 TWh/year, i.e. more than double the yearly final electricity consumption in France at the beginning of the 2000s (450 TWh).

The main “brakes” to the massive use of photovoltaic solar (and thermal) energy are the intermittence of the supplied power (which requires electricity storage for an autonomous use or the use of additional energy sources) on the one hand and the economic competitiveness on the other hand. Outside the zones not connected to the network, where it is already profitable, the parity between photovoltaic production costs and electricity sale prices starts to be found in countries where electricity is the most expensive and where sunshine levels are at the highest. It should spread to all territories before 2050.

1.3.3. Hydraulics

Hydraulics is currently the first exploited renewable source of electricity. The global potential could however be better exploited. Global production at the beginning of the 2000s was 2,700 TWh/year, with an installed capacity of 740 GW. It could go up to 8,100 TWh by 2050 with a competitive economic doubling of the installed capacity. 14,000 TWh would technically be exploitable and the theoretical potential would be 36,000 TWh.

Large hydraulics (with a power higher than 10 MW) are exploited almost at the maximum of their potential in industrialized countries. Dams store the energy and supply it in peak energy demand. In some cases, high and low storage pools enable actual energy storage by using groups of reversible turbo-generators, which are pumping in off-peak periods. This form of storage is frequently used around the world. In France, 4,200 MW are installed for this function.

Small hydraulics (of a power lower than 10 MW) are partly made from run-of-river power plants, which depend highly on the river flow rate. The small power plants are quite interesting for decentralized production. The global production is estimated at 85 TWh. In France, while large hydraulics has almost reached saturation, the development potential of small hydraulics remains, which is estimated at 4 TWh/year, 1/3 of which comes from of the improvement of the existing facilities and the remainder from new facilities.

Tidal power can be used to produce electricity. In France, the Rance Tidal Power Station (240 MW) has proved the feasibility of this electricity production technique. Other significant projects are currently studied in Canada and England; however, whether or not these studies will be put into practice remains uncertain, because of the considerable changes that would occur in local ecosystems.

Wave motion is an important source of energy. The average annual power on the Atlantic coast ranges between 15 and 80 kW/m of coast. However the marine environment is very restrictive and wave energy recuperators are not yet wellestablished: they are not exploitable on a large scale. Prototypes of wave-energy power plants are however in the testing stages.

1.3.4. Geothermal energy

The temperature of our planet increases considerably as we get closer to the center. In some zones of our planet we can find, at depth, water at a high temperature. High temperature geothermal energy (150 to 300°C) consists of pumping this water towards the surface, producing vapor via exchanges, then turbining this vapor as in conventional thermal power stations and producing electricity.

Low temperature geothermal resources (lower than 100°C) are upgraded with heat pumps for heating requirements.

The potential of natural geothermal energy is however limited, because there are many sites where the temperature is high (higher than 200°C), but where there is no water. This thermal resource might be exploited with the help of the so-called “hotdry rock” technology, which is currently under development. It consists of injecting, into a well, pressurized water in in-depth zones (deeper than 3,000 m) of fractured rocks. This reheated water returns to the surface up by a second well and helps to produce electricity, as in conventional thermal power stations. However, the proportion of this potential, which would be technically accessible, has not yet been specified.

1.3.5. Biomass

Provided a sustainable exploitation of resources, biomass is a renewable energy that supplies biofuels, which are generally solid or liquid.

Wood covers more than 10% of the primary energy demand in many countries of Asia, Africa and Latin America and in some European countries (Sweden, Finland and Austria). The use of wood in developing countries has strongly increased in the last decades, but this resource has not always been exploited sustainably and has thus led to deforestation. Emissions coming from wood combustion in a modern industrial boiler are advantageous in comparison to fossil fuels. If the forests where wood comes from are sustainably dealt with, the CO2 emissions from the wood energy chain are only those corresponding to the gas and oil used during plantation, crop and marketing operations. This represents about 5% of the fuel sold. We can note that a renewable energy is not necessarily a completely non-polluting energy.

The consumption of biomass in France in primary energy is 10–11 Mtoe (at the beginning of the 2000s); this is mainly wood. Without any specific energy crop, the biomass potential could be doubled by a systematic repercussion of all organic waste: non-recyclable household and industrial waste, methanization process of the sewage sludge and agriculture waste, which generates biogas. The energy potential is 60 TWh/year, i.e. 15% of the final electricity consumption in France.

Biomass is frequently used in cogeneration systems, which produce electricity such as conventional power stations, all the while upgrading the heat that is usually lost in various applications: heating of the facilities, industrial needs, agriculture, etc. This technology helps to increase the efficiency of energy conversion.

Liquid biofuels are more expensive to obtain and are industrially produced from energy crops (rape, sunflower, beet, wheat, barley, corn, etc.), and are better upgraded in transport applications. They are currently mainly used in engines and are mixed in small quantities in conventional fuels, in order to improve their characteristics.

1.3.6. Contribution of the various renewable energies

In 2009, the proportion of the various renewable sectors in the production of primary renewable energies in the European Union was as shown in Table 1.1 [EUR 10].

Table 1.1.Proportion of the various renewable sectors in the production of renewable primary energy of the European Union in 2009

Biomass

66.6%

Hydraulics

19.7%

Wind power

7.2%

Geothermal energy

4.8%

Solar power

1.7%

The contribution of each renewable energy source in the production of renewable electricity within the European Union in 2009 is shown in Table 1.2, for a total of 584.1 TWh.

Table 1.2.Contribution of each renewable energy source in the production of renewable electricity within the European Union in 2009

Hydraulics

55.8%

Wind power

22.4%

Biomass

18.3%

Solar power

2.5%

Geothermal energy

1%

The growing rates of these sectors are really high, which contributes to an improvement in the penetration rate from one year to another.

1.4. Production of electricity from renewable energies

1.4.1. Electricity supply chains

To carry out energy conversions to produce electricity, several supply chains can be considered, depending on the use or not of electronic power converters.

The most frequently used electricity generation cycle requires a heat source to heat the water, in order to obtain vapor under pressure. By expanding in a turbine, this vapor drives an alternator, which generates electricity. After passing through the turbine, this vapor is condensated with the help of a cold source, which is generally water (river, sea) or air in cooling towers. Figure 1.1 represents the conventional cycle of electricity generation.

Figure 1.1.Conventional cycle of electricity generation

When the heat generated by the vapor condensation is recovered for heating needs, we can speak about cogeneration.

The heat source is generally obtained by the combustion of fossil fuels (oil, gas, coal) or by a nuclear fission reaction in reactors designed to control the extent of this reaction.

The fossil fuels or uranium used in conventional cycles can be replaced by some renewable energy sources. The heat source can then be obtained by:

– biomass combustion (wood, biogas, organic waste);

– the heat found in the depths of our planet, either by directly pumping hot water towards the surface or by exploiting the high temperature of the deep rocks by injecting them with water from the surface;

– the sun by concentrating its rays with the help of mirrors or by exploiting the water heated at the surface of the seas in tropical zone.

With some renewable energies, the electricity supply chain does not require a heat source. This is the case for wind power, hydraulics and photovoltaic solar energy.

In the case of wind power and hydraulics, the wind or water pressure drives the rotation of a turbine, which in its turn drives an alternator, which produces electricity. Figure 1.2 represents this energy conversion chain.

Figure 1.2.Wind power or hydraulics electricity supply chain

Wind pressure results from its kinetic energy. Water pressure results from its potential energy and its kinetic energy.

The electricity generated by the alternator can be directly sent along the electrical network without going through power converters, as indicated in Figure 1.2. However, in this case, in order to maintain the frequency of the voltages and the constant generated currents at 50 or 60 Hz, the alternator speed must be maintained as constant by acting on the direction of the turbine blades, or in the case of hydraulics, by winnowing upstream of the turbine.

The interesting aspect of power converters is that they enable alternators to operate at variable speed and thus to increase the energy conversion efficiency, all the while reducing the need for turbine mechanical control and for winnowing in the case of hydraulics. This variable speed operation is developing in the field of hydraulics (especially in small hydraulics), and tends to impose itself in wind powers, where this type of operation seems natural, because of the strong variations in the wind speed.

Electronic converters help us to convert power from one form to another. They can include rectifiers, inverters and choppers, or just a single inverter. The converter must be compatible with the frequency of the network and is equipped with filters, in order to satisfy power quality standards. Power electronics also ensures the protection functions of the production unit and the local network to which it is connected.

In the case of photovoltaic solar power, electricity is produced directly with the help of silicon cells using the energy from solar radiation. Power converters are generally used to ensure the optimization of energy conversion. Figure 1.3 illustrates this energy conversion chain.

Figure 1.3.Photovoltaic solar power chain of production generation

Electricity can also be produced via a diesel engine or a gas turbine (derived from a jet engine) driving an alternator. The source of primary energy is generally made up of fossil fuels, but we can consider replacing them with biofuel or biogas.

1.4.2. Efficiency factor

The key factor for the competitiveness of energy production systems, based on renewable energies, is the cost of the kWh product. This cost is calculated from the investment cost of the generation system, its lifespan, the interest rate of the loan that may have been required and the operating costs related to maintenance and primary energy – which is free when it is the sun, wind– and not free in the case of fossil fuels, nuclear power–