Introduction

I.1. Presentation of the book

This book approaches mechanical conversion by proposing a series of exercises and by presenting their solutions. These are generally scaling exercises. They implement methods which determine the characteristics and dimensions of installations and machines which, for the different studied processes, provide the desired power or quantity of energy.

The fields of application affect a variety of marine energies: tidal and hydrokinetic energy, wave energy, marine thermal energy and osmotic energy. They also include wind energy and hydroelectricity. The different areas of application are discussed in separate chapters. For each type of energy, the energy resource is described, a process for recovering it is presented and its dimensioning is carried out for a fixed production objective.

Proposing an exercise is a simplification when compared to the real situation with which the engineer is confronted. We are not required to comply with the constraints of a site or a real situation. Nevertheless, the conditions of the proposed exercises were not chosen by chance. For many of them, we had in view a specific installation and, in return, it was sometimes possible to compare the scaling carried out with known data on these installations. Reference facilities are named when they were used to develop an exercise. The information available is often incomplete. Considering dimensioning as an exercise allows you to keep a distance from a situation that is only partially known. In doing so, the exercise makes it possible to focus on the principles of mechanical conversion which constitute the subject of this book.

The scientific discipline to which these exercises belong is mainly fluid mechanics. This is why Chapter 1 offers reminders in fluid mechanics. It should be seen as a memorandum rather than a lesson. The reader can refer to fluid mechanics books to read the missing demonstrations. This chapter basically provides the elements necessary for the resolution of the exercises. Chapter 2 presents course reminders on hydraulic turbine engines. Two exercises are included in it. Other exercises directly related to the methods exposed in Chapter 2 are found in the following chapters, devoted to wind energy, hydroelectricity or tidal energy.

The fields of application of the exercises fall under renewable energies. This is why the term renewable energies appears in the subtitle of this book. For each sector, a short inventory is drawn up in the chapter devoted to it. The objective is for the reader to know the degree of maturity and to have a notion of the powers and energies delivered by the installations currently built. Some of these sectors are rapidly changing. The figures given are those known at the time of publication of this work. They can quickly become obsolete. This is the reason why this inventory remains succinct. This is not the objective of this book, which aims to contribute to the acquisition of dimensioning methods. The reader will find an up-to-date inventory of a sector in the reports published in France by the Renewable Energies Syndicate1 or by ADEME.

The operation of the electricity production facilities studied in this work, which use renewable energies, do not produce CO2. Nevertheless, the manufacture of the equipment necessary for the operation of the factories involves processes which in themselves emit CO2. An analysis of the lifecycle of the installations is necessary to establish the true carbon footprint of these units. This aspect is not covered in this book.

I.2. Power, energy and load factor

Energy data are quantified by energy and power. Power is energy per unit time. In steady state, if an energy system delivers power ℘ at a constant rate, the energy transferred during a duration T is calculated by the formula E = ℘T.

In the international unit system, power is measured in Watts. The Watt is converted into international units (kg, m, s) according to:

1 W = 1 kg m2s–3

Energy is measured in Joules (J), with the equivalence in units (kg, m, s):

1 J = 1 kg m2s–2

Energy data vary over wide ranges. This is why kiloW (1 kW = 103 W), MegaW (1 MW = 106 W), GigaW (1 GW = 109 W) and TeraW (1 TW = 1012 W) are commonly used.

To quantify energy, the kWh is also used. Note that 1 Wh is the energy delivered for 1 h by a system with a power of 1 W. As a result:

1 Wh = 3,600 J

Similarly, 1 kWh = 103 Wh, 1 MWh = 106 Wh, 1 GWh = 109 Wh, 1 TWh = 1012 Wh.

To assess an energy system, it is necessary to know both its power and the energy delivered during a reference period. If, for example, we know the nominal power of a hydroelectric plant ℘ = 50 MW and the energy it produces in one year, E = 200 GWh, these two pieces of information tell us that the installation has supplied the energy in one year as if it had operated at its rated speed for the duration:

The time unit of the result is in seconds if the energy and the power are given in J and in W. It is in hours if the energy and the power are given in Wh and in W.

A year has 8,760 h. A duration of operation at rated power of 4,000 h means that the installation has operated at the rated power for 46% of the duration of one year. We use the load factor:

to assess the use of an installation. Tyear is the duration of a year, expressed in the unit corresponding to the units used to express power and energy. It is easily understood that it is economically more advantageous to design an installation of lower power if the load factor is too small due to a limited energy resource. It is possible to make such a choice for a hydroelectric installation. On the other hand, for wind energy for example, the load factor is fixed by the wind conditions, which are random by nature.

The powers and energies vary considerably depending on the dimensions of the systems considered. It is essential to have a few orders of magnitude in mind to judge the relevance of a dimensioning.

Table I.1. Orders of magnitude of powers and energies

Table I.1 indicates orders of magnitude of power and energy. The data presented are essentially consumption data. In terms of production, only the power of an onshore wind turbine and the power of a nuclear power plant unit are shown in the table. More precise figures on production are provided in the chapters of this book for the different technologies studied. Table I.1 also indicates the order of magnitude of the price of domestic electricity. This is a price that includes distribution, taxes and VAT. It obviously differs from the electricity negotiation prices between producers and distributors, which vary according to supply and demand.

I.3. The sources of this book

The content of this book was developed as part of the teaching provided by its author within the international master’s degree in Simulation and Optimization of Energy Systems (SIMOS) awarded at the École nationale supérieure en génie des technologies industrielles, an engineering school belonging to the University of Pau and Pays de l’Adour.

This master’s degree emphasizing energy optimization, we find in many exercises of this book an optimization point of view for dimensioning equipment.

The pedagogical approach of this book is built around the exercises and their resolution. The author is aware that the theoretical bases presented in Chapters 1 and 2 will be indigestible for some readers. However, he did not want to develop them any further. This book is not a textbook. It suggests that the reader approaches the resolution of the exercises directly in order to come gradually to the theory and master its implementation by going back and forth between the exercises and the theoretical parts.

I.4. Energy conversion

Energy comes in several forms: electrical energy, thermal energy, mechanical energy, chemical energy, etc. Converting energy means changing one form of energy into another. The emphasis in this book is on mechanical conversion....