1.2: Preview of Topics

This book is intended to both illustrate individual energy conversion technologies and illuminate the relationship between them. For this reason, it is organized in two parts. The first part is a survey of energy conversion processes. The second part introduces calculus of variations and uses it as a framework to relate energy conversion processes.

Due to the wide variety, it is not possible to discuss all energy conversion devices, even all direct devices, in detail. However, by studying the example direct energy conversion processes, we can gain an understanding of indirect processes and other applications. The devices discussed in this book involve energy conversion between electrical form and another form. Additionally, devices involving magnets and coils will not be discussed. Many useful devices, including motors, generators, wind turbines, and geothermal power plants, convert energy electromagnetically using magnets and coils. Approximately 90% of power supplied to the electrical grid in the United States comes from generators that use magnets and coils [1]. Also, about 2/3 to 3/4 of energy used by manufacturing facilities goes towards motors [2, ch. 1]. However, plenty of good resources discussing these topics exist. Furthermore, this book emphasizes device that operate near room temperature and at relatively low power (<1 kW). Many interesting devices, such as nuclear power plants, operate at high temperatures. One reason not to discuss more powerful devices is that the vast majority of large electrical generators in use today involve turbines with coils and magnets. Another reason is that these devices are often limited by material considerations. Finding materials to construct high temperature devices is a challenging problem, but it is not the purpose of this book. Additionally, only technologies commercially available on the market today are discussed in this book. Also, many quality texts exist on the topics of renewable and alternative energy sources. For this reason, this book will not focus on renewable or alternative energy technologies. Topics like wind turbines, which involve electromechanical energy conversion with magnets and coils, are not discussed. Solar cells, piezoelectric devices, and other direct energy conversion devices are discussed and can be considered both direct energy conversion devices and renewable energy devices.

While a few books on direct energy conversion exist, there are few things which set this book apart. First, many of the books on direct energy conversion, including [3] and [4], are written at the graduate level while this book is aimed at a more general audience. This book is used for the course Direct Energy Conversion taught at Trine University, which is a junior undergraduate level course for electrical engineers. This book is not intended only for electrical engineering students. It is also aimed at researchers who are interested in how energy conversion is studied by scientists and engineers in other disciplines. The idea of energy conversion is fundamental to physics, chemistry, mechanical engineering, and multiple other disciplines. This book discusses fundamental physics behind energy conversion processes, introduces terminology used, and relates concepts of material science used for building devices. The chapters were written so that someone who is not an antenna designer, for example, can read the relevant chapter as an introduction and gain insights into some of the terminology and key concepts used by electromagnetics researchers. Second, a number of good books on the topic, including [3] and [5] were written decades ago. The concepts of these books remain relevant, and these books often predicted which technologies would be of interest. However, there is a need for a book which discusses the most accessible and commonplace direct energy conversion technologies in use today. Additionally, many of these classic texts are out of print, and contemporary texts are needed.

The reader is assumed to be familiar with introductory chemistry and physics. Background in electrical circuits and materials may also be helpful. Math through Calculus I is used in the first part of the book, and math through Calculus III (including partial derivatives) is used in the second part. Many topics in this text are discussed qualitatively. No attempt is made to be mathematically rigorous, and proofs are not given. The physics of devices is emphasized over excessive mathematics. Additionally, all physical systems will be discussed semiclassically, which means that explanations will involve electrons and electromagnetic fields, but the waveparticle duality of these quantities will not be discussed. While quantum mechanical, quantum field theoretical, and other more precise theories exist to describe many physical situations, semiclassical discussions will be used to make this book more easily accessible to readers without a background in quantum mechanics.

Chapters 2 - 10 comprise the first part of this book. As mentioned above, they survey various direct energy conversion processes which convert to or from electricity and which do not involve magnets and coils. Table \(\PageIndex{1}\) lists many of the processes studied along with where in the text they are discussed, and Table \(\PageIndex{2}\) lists some of the devices detailed. This text is not intended to be encyclopedic or complete. Instead, it is intended to highlight the physics behind some of the most widely available and accessible energy conversion devices which convert to or from electrical energy. One way to understand energy conversion devices used to convert to or from electricity is to classify them as most similar to capacitors, inductors, resistors, or diodes. While not all devices fit neatly in these categories, many do. The second column of Table \(\PageIndex{2}\) lists the category for various devices. Similarly, energy conversion processes may be capacitive, inductive, resistive, or diode-like.

Table \(\PageIndex{1}\)

Process	Forms of Energy	Example Devices	Discussed in Section
Piezoelectricity	Electricity \(\updownarrow\) Mechanical Energy	Piezoelectric Vibration Sensor, Electret Microphone	2.3
Pyroelectricity	Electricity \(\updownarrow\) Heat	Pyroelectric Infrared Detector	3
Electro-optic Effect	Optical Electromagnetic Energy \(\updownarrow\) Material Polarization	Controllable Optics, Liquid Crystal Displays	3.2
Electromagnetic Transmission and Reception	Electricity \(\updownarrow\) Electromagnetic Energy	Antenna	4
Hall Effect	Electricity \(\updownarrow\) Magnetic Energy	Hall Effect Device	5
Magnetohydrodynamic Effect	Electricity \(\updownarrow\) Magnetic Energy	Magnetohydrodynamic Device	5.2
Absorption	Optical Electromagnetic Energy \(\downarrow\) Electricity	Solar cell, Semiconductor Optical Photodetector	6
Spontaneous Emission	Electricity \(\downarrow\) Optical Electromagnetic Energy	Lamp, LED	7.2
Stimulated Emission	Electricity \(\downarrow\) Optical Electromagnetic Energy	Laser, Optical Amplifier	7.3
Thermoelectric Effects (Incl. Seebeck, Peltier and Thomson)	Electricity \(\updownarrow\) Heat	Thermoelectric cooler, Peltier device, Thermocouple	8.7
(Battery or Fuel Cell) Discharging	Chemical Energy \(\downarrow\) Electricity	Battery, Fuel Cell	9
(Battery or Fuel Cell) Charging	Electricity \(\downarrow\) Chemical Energy	Battery, Fuel Cell	9
Thermionic Emission	Heat \(\downarrow\) Electricity	Thermionic Device	10.1
Electrohydrodynamic Effect	Electricity \(\updownarrow\) Fluid flow	Microfluidic Pump, Valve	10.5

Table \(\PageIndex{2}\)

Device	Similar to Component	Forms of Energy	Discussed Section
Piezoelectric Device	Capacitor	Electricity \(\updownarrow\) Mechanical Energy	2.3
Pyroelectric Device	Capacitor	Electricity \(\updownarrow\) Heat	3
Electro-optic Device	Capacitor	Optical Energy \(\updownarrow\) Material Polarization	3.2
Antenna	Inductor	Electricity \(\updownarrow\) Electromagnetic	4
Hall Effect Device	Inductor	Electricity \(\updownarrow\) Magnetic Energy	5
Magnetohydrodynamic Device	Inductor	Electricity \(\updownarrow\) Magnetic Energy	5.2
Solar Cell	Diode	Optical Energy \(\downarrow\) Electricity	6
LED, Laser	Diode	Electricity \(\downarrow\) Optical Energy	7
Thermoelectric Device	Diode	Electricity \(\updownarrow\) Heat	8.7
Geiger Counter	Diode	Radiation \(\downarrow\) Electricity	10.2
Resistance Temp. Detector	Resistor	Heat \(\downarrow\) Electricity	10.4
Potentiometer	Resistor	Electricity \(\downarrow\) Heat	10.4
Strain Gauge	Resistor	Mechanical Energy \(\downarrow\) Electricity	10.4

Capacitive energy conversion processes are discussed in Chapters 2 and 3. Capacitors, piezoelectric devices, pyroelectric devices, and electro-optic devices are discussed. A piezoelectric device is a device which converts mechanical energy directly to electricity or converts electricity directly to mechanical energy [6] [3]. A material polarization and voltage develop when the piezoelectric device is compressed. A pyroelectric device converts a temperature differential into electricity [6]. The change in temperature induces a material polarization and a voltage in the material. Electro-optic devices convert an optical electromagnetic field to energy of a material polarization. In these devices, an external optical field typically from a laser induces a material polarization and a voltage across the material. Chapters 4 and 5 discuss inductive energy conversion devices including antennas, Hall effect devices, and magnetohydrodynamic devices. An antenna converts electrical energy to an electromagnetic field or vice versa. A Hall effect device converts a magnetic field to or from electricity. A magnetohydrodynamic device converts kinetic energy of a conducting material in the presence of a magnetic field into electricity.

Optical devices are discussed in Chapters 6 and 7. These chapters discuss devices made from diode-like pn junctions such as solar cells, LEDs, and semiconductor lasers as well as other types of devices such as incandescent lamps and gas lasers. Thermoelectric devices convert a temperature differential into electricity [3, p. 146]. They are also made from junctions of materials in which heat and charges flow at different rates, and they are discussed in Chapter 8. Batteries and fuel cells are discussed in Chapter 9. A battery is a device which stores energy as a chemical potential. Batteries range in size from tiny hearing aid button sized batteries which store tens of milliamp-hours of charge to large car batteries which can store 10,000 times as much energy. A fuel cell is a device which converts chemical energy to electrical energy through the oxidation of a fuel [3]. During battery operation, the electrodes are consumed, and during fuel cell operation, the fuel and oxidizer are consumed instead. A variety of resistor-like energy conversion devices, among other devices, are discussed briefly in Chapter 10.

Chapters 11 - 14 comprise the second part of this book. These chapters are more theoretical, and they establish a mathematical framework for understanding energy conversion. This mathematics allows relationships to be studied between energy conversion devices built by electrical engineers, mechanical engineers, chemists, and scientists of other disciplines. Chapters 11 and 12 introduce the idea of calculus of variations and apply it to a wide variety of energy conversion processes. Chapter 13 applies the idea of calculus of variations to energy conversion within an individual atom. Chapter 14 shows how a study of the symmetries of the equations produced from calculus of variations can provide further insights into energy conversion processes.