Thermal Energy

Sources, Recovery, and Applications

Yatish T. Shah

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781315305936
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Fossil Fuels
• Index: Technology & Engineering

The book details sources of thermal energy, methods of capture, and applications. It describes the basics of thermal energy, including measuring thermal energy, laws of thermodynamics that govern its use and transformation, modes of thermal energy, conventional processes, devices and materials, and the methods by which it is transferred. It covers 8 sources of thermal energy: combustion, fusion (solar) fission (nuclear), geothermal, microwave, plasma, waste heat, and thermal energy storage. In each case, the methods of production and capture and its uses are described in detail. It also discusses novel processes and devices used to improve transfer and transformation processes.

1	Basics of Macrothermal Energy

1.1 INTRODUCTION

Heat is one of the oldest and the most basic energy sources known to man. Cavemen have utilized the power of heat to create light, cook food, and keep warm. When man began to create machines to harness the energy for other purposes, steam engines were among the first, appearing in various forms as early as ancient Alexandria and featuring prominently in the Industrial Revolution.

Heat is defined in physics as the transfer of thermal energy across a well-defined boundary around a thermodynamic system. The thermodynamic free energy is the amount of work that a thermodynamic system can perform. Enthalpy is a thermodynamic potential, designated by the letter H, that is the sum of the internal energy of the system (U) plus the product of pressure (P) and volume (V). Joule is a unit to quantify energy, work, or the amount of heat [1, 2, 3, 4].

Heat transfer is a process function (or path function), as opposed to functions of state; therefore, the amount of heat transferred in a thermodynamic process that changes the state of a system depends on how that process occurs, not only the net difference between the initial and final states of the process. Thermodynamic and mechanical heat transfer is calculated with the heat transfer coefficient, the proportionality between the heat flux, and the thermodynamic driving force for the flow of heat. Heat flux is a quantitative, vectorial representation of the heat flow through a surface [1, 2, 3, 4]. In engineering contexts, the term heat is taken as synonymous to thermal energy. This usage has its origin in the historical interpretation of heat as a fluid (caloric) that can be transferred by various causes [1, 2, 3, 4], and that is also common in the language of laymen and everyday life.

The transport equations for thermal energy (Fourier’s law), mechanical momentum (Newton’s law for fluids), and mass transfer (Fick’s laws of diffusion) are similar [1, 2, 3, 4], and analogies among these three transport processes have been developed to facilitate prediction of conversion from any one to the others [1, 2, 3, 4]. Thermal engineering concerns the generation, use, conversion, and exchange of heat transfer. As such, heat transfer is involved in almost every sector of the economy [1, 2, 3, 4]. Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation, and transfer of energy by phase changes [1, 2, 3, 4].

On account of its high relevance to societal needs, heat-transfer research has had a long history spanning more than 300 years. The formulation of Newton’s law of cooling and Fourier’s law [1, 2, 3, 4] in 1701 and 1822, respectively, established the foundations for studying convection and conduction, two of the three heat transfer modes. In comparison, comprehensive understanding of the third heat-transfer mode, thermal radiation, began with the discovery of Planck’s law in 1900 [1, 2, 3, 4], which played an important role in the development of quantum physics, a scientific revolution that has profoundly impacted both basic science and modern technologies.

The assumptions made in the mechanisms of heat transfer and associated thermodynamics are only valid in macrosystems in which the systems are considered as continuous and the processes are not examined at atomic or molecular level. In this chapter, thus, we examine basics of macrothermal energy. In recent years, significant work has been carried out at micro- and nanolevels, in which thermal processes discussed here do not apply. Understanding of nanolevel thermal processes has allowed the development of new and improved heat-transfer materials and devices [1, 2, 3, 4]. These advances in materials, devices, and thermal processes at micro- and nanolevels are discussed in Chapter 2.

1.2 TYPES OF HEAT

Fundamentally, there are two types of heat: sensible heat and latent heat.

1.2.1 SENSIBLE HEAT

Sensible heat is heat exchanged by a thermodynamic system that changes the temperature of the system without changing some variables such as volume or pressure. As the name implies, sensible heat is the heat that you can feel. The sensible heat possessed by an object depends on its temperature. As temperature increases, the sensible heat content also increases. However, for a given change in sensible heat content, all objects do not change temperature by the same amount. Each substance has its own characteristics relationship between heat content and temperature. The proportionality constant between temperature rise and change in heat content is called the specific heat, measured in calories per gram per degree Celsius or joules per kilogram per kelvin. Water, for example, has a specific heat of 1 Cal/g/°C. In general, the gain in heat is accompanied by either a change in volume or in pressure (e.g., the water in the pot swells somewhat as you heat it; if you heat gas in a fixed volume, its pressure goes up).

The sensible heat term is used in contrast to a latent heat, which is the amount of heat exchanged that is hidden, meaning it occurs without change of temperature. For example, during a phase change, such as the melting of ice, the temperature of the system containing the ice and the liquid is constant until all ice has melted. The terms latent and sensible are correlative. That means that they are defined as a pair, depending on which other macroscopic variables are held constant during the process.

The sensible heat of a thermodynamic process may be calculated as the product of the body’s mass (m) with its specific heat capacity (c) and the change in temperature ($\text{Δ}T$):

$Q_{\text{sensible}}=mc\text{Δ}T$

(1.1)

Here m is mass and c is the specific heat of material. In the writings of the early scientists who pr...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Series Preface
• Preface
• About the Author
• Chapter 1 Basics of Macrothermal Energy
• Chapter 2 Advances in Micro- and Nanolevel Thermal Processes, Materials, and Devices
• Chapter 3 Geothermal Heat
• Chapter 4 Fusion/Solar Heat
• Chapter 5 Fission/Nuclear Heat
• Chapter 6 Combustion—Chemical Heat
• Chapter 7 Electrical Heating—Part 1: Joule Heating
• Chapter 8 Electrical Heating—Part 2: Infrared and UV Heating
• Chapter 9 Electrical Heating—Part 3: Induction Heating
• Chapter 10 Electrical Heating—Part 4: Dielectric (Microwave) Heating
• Chapter 11 Electrical Heating—Part 5: Electric Arc Heating
• Chapter 12 Electrical Heating—Part 6: Plasma Heating
• Chapter 13 Electrical Heating—Part 7: Power Beam Heating
• Chapter 14 Waste Heat
• Chapter 15 Thermal Energy Storage
• Index