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Exergy
Energy, Environment And Sustainable Development
By Ibrahim Dincer, Marc A. Rosen Elsevier
Copyright © 2013 Ibrahim Dincer and Marc A. Rosen
All rights reserved.
ISBN: 978-0-08-097090-5
CHAPTER 1
Thermodynamic Fundamentals
Chapter Outline 1.1 Introduction 1
1.2 Energy 2
1.2.1 Applications of Energy 2
1.2.2 Concept of Energy 2
1.2.3 Forms of Energy 3
1.2.3.1 Macroscopic 3
1.2.3.2 Microscopic 3
1.2.4 The First Law of Thermodynamics 4
1.2.5 Energy and the FLT 4
1.2.6 Economic Aspects of Energy 4
1.2.7 Energy Audit Methods 5
1.2.8 Energy Management 5
1.2.8.1 Maintenance Opportunities 5
1.2.8.2 Low-Cost Opportunities 6
1.2.8.3 Retrofit Opportunities 6
1.3 Entropy 6
1.3.1 Order and Disorder and Reversibility
and Irreversibility 6
1.3.2 Characteristics of Entropy 7
1.3.3 Significance of Entropy 8
1.3.4 Carnot's Contribution 8
1.3.5 The SLT 9
1.3.6 SLT Statements 9
1.3.7 The Clausius Inequality 10
1.3.8 Useful Relationships 10
1.4 Exergy 10
1.4.1 The Quantity Exergy 10
1.4.2 Exergy Analysis 10
1.4.3 Characteristics of Exergy 11
1.4.4 The Reference Environment 11
1.4.5 Exergy versus Energy 12
1.4.6 Exergy Efficiencies 13
1.4.7 Solar Exergy and the Earth 13
1.5 Illustrative Examples 14
1.5.1 Illustrative Example 1 14
1.5.2 Illustrative Example 2 14
1.5.3 Illustrative Example 3 15
1.5.3.1 Further Discussion on Entropy Generation
Associated with Heat Transfer 16
1.5.4 Illustrative Example 4 16
1.6 Closing Remarks 19
Problems 19
ABSTRACT
This chapter provides background for understanding energy, entropy, and exergy concepts. Also, basic thermodynamic principles and general definitions are explained, and various practical applications and implications of these thermodynamic quantities are discussed. The first and second laws of thermodynamics are described, as are energy audits and energy management. The chapter focuses on the portion of the field of thermodynamics at the intersection of the energy, entropy, and exergy fields.
KEYWORDS
Energy; Entropy; Exergy; First law of thermodynamics; Second law of thermodynamics; Energy audit; Energy management.
1.1 INTRODUCTION
Energy, entropy, and exergy concepts stem from thermodynamics and are applicable to all fields of science and engineering. This chapter provides the necessary background for understanding these concepts, as well as basic principles, general definitions, practical applications, and implications. Illustrative examples are provided to highlight the important aspects of energy, entropy, and exergy.
The scope of this chapter is partly illustrated in Figure 1.1, where the domains of energy, entropy, and exergy are shown. This chapter focuses on the portion of the field of thermodynamics at the intersection of the energy, entropy, and exergy fields. Note that entropy and exergy are also used in other fields (such as statistics and information theory); therefore they are not subsets of energy. Also, some forms of energy (such as shaft work) are entropy free, and thus entropy subtends only part of the energy field. Likewise, exergy subtends only part of the energy field because some systems (such as air at atmospheric conditions) possess energy but no exergy. Most thermodynamic systems (such as steam in a power plant) possess energy, entropy, and exergy, and thus appear at the intersection of these three fields.
1.2 ENERGY
Energy comes in many forms. Thermodynamics play a key role in the analysis of processes, systems, and devices in which energy transfers and energy transformations occur. The implications of thermodynamics are far reaching and applications span the range of the human enterprise. Throughout our technological history, our ability to harness energy and use it for society's needs has improved. The industrial revolution was fueled by the discovery of how to exploit energy on a large scale and how to convert heat into work. Nature allows the conversion of work completely into heat, but heat cannot be entirely converted into work, and doing so requires a device (e.g., a cyclic engine). Engines attempt to optimize the conversion of heat to work.
1.2.1 Applications of Energy
Most of our daily activities involve energy transfer and energy change. The human body is a familiar example of a biological system in which the chemical energy of food or body fat is transformed into other forms of energy such as heat and work. Engineering applications of energy processes are wide ranging and include power plants to generate electricity, engines to run automobiles and aircraft, refrigeration and air conditioning systems, and so forth.
Many examples of such systems are discussed here. In a hydroelectric power system, the potential energy of water is converted into mechanical energy through the use of a hydraulic turbine. The mechanical energy is then converted into electric energy by an electric generator coupled to the shaft of the turbine. In a steam power-generating plant, chemical or nuclear energy is converted into thermal energy in a boiler or a reactor. The energy is imparted to water, which vaporizes into steam. The energy of the steam is used to drive a steam turbine, and the resulting mechanical energy is used to drive a generator to produce electric power. The steam leaving the turbine is then condensed, and the condensate is pumped back to the boiler to complete the cycle. Breeder reactors use uranium-235 as a fuel source and can produce more fuel in the process. A solar power plant uses solar concentrators (parabolic or flat mirrors) to heat a working fluid in a receiver located on a tower where a heated fluid expands in a turbogenerator, as in a conventional power plant. In a spark-ignition internal combustion engine, chemical energy of fuel is converted into mechanical work. An air–fuel mixture is compressed and combustion is initiated by a spark device. The expansion of the combustion gases pushes against a piston, which results in the rotation of a crankshaft. Gas turbine engines, commonly used for aircraft propulsion, convert the chemical energy of fuel into thermal energy that is used to run the turbine. The turbine is directly coupled to a compressor that supplies the air required for combustion. The exhaust gases, upon expanding in a nozzle, create thrust. For power generation, the turbine is coupled to an electric generator and drives both the compressor and the generator. In a liquid-fuel rocket, a fuel and an oxidizer are combined, and combustion gases expand in a nozzle, creating a propulsive force (thrust) to propel the rocket. A typical nuclear rocket propulsion engine offers a higher specific impulse when compared to chemical rockets. A fuel cell converts chemical energy into electric energy, directly making use of an ion-exchange membrane. When a fuel such as hydrogen is ionized, it flows from the anode through the membrane toward the cathode. The released electrons at the anode flow through an external load. In a magnetohydrodynamic generator, electricity is produced by moving a high-temperature plasma through a magnetic field. A refrigeration system utilizes work supplied by an electric motor to transfer heat from a refrigerated space. Low-temperature boiling fluids such as ammonia and refrigerant-12 absorb thermal energy as they vaporize in the evaporator, causing a cooling effect in the region being cooled.
These are only a few of the numerous engineering applications. Thermodynamics is relevant to a much wider range of processes and applications not only in engineering, but also in science. A good understanding of this topic is required to improve the design and performance of energy-transfer systems.
1.2.2 Concept of Energy
The concept of energy was first introduced in mechanics by Newton when he hypothesized about kinetic and potential energies. However, the emergence of energy as a unifying concept in physics was not adopted until the middle of the nineteenth century and is considered one of the major scientific achievements in that century. The concept of energy is so familiar to us today that it seems intuitively obvious to understand, yet we often have difficulty defining it precisely.
Energy is a scalar quantity that cannot be observed directly, but can be recorded and evaluated by indirect measurements. The absolute value of the energy of a system is difficult to measure, whereas the energy change is relatively easy to evaluate.
Examples of energy use in life experiences are endless. The sun is the major source of the earth's energy. It emits a spectrum of energy that travels across space as electromagnetic radiation. Energy is also associated with the structure of matter and can be released by chemical and atomic reactions. Throughout history, the emergence of civilizations has been characterized by the discovery and effective application of energy to help meet society's needs.
1.2.3 Forms of Energy
Energy manifests itself in many forms, which are either internal or transient. Energy can be converted from one form to another. In thermodynamic analysis, the forms of energy can be classified into two groups: macroscopic and microscopic.
1.2.3.1 Macroscopic
Macroscopic forms of energy are those which an overall system possesses with respect to a reference frame, for example, kinetic and potential energies. The macroscopic energy of a rising object changes with velocity and elevation. The macroscopic energy of a system is related to motion and the influence of external effects such as gravity, magnetism, electricity, and surface tension.
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Excerpted from Exergy by Ibrahim Dincer, Marc A. Rosen. Copyright © 2013 Ibrahim Dincer and Marc A. Rosen. Excerpted by permission of Elsevier.
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