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Chapter_8

# Chapter_8 - Student Study Guide for 5th edition of...

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Student Study Guide for 5 th edition of Thermodynamics by Y. A. Ç engel & M. A. Boles 8-1 Chapter 8-1 Chapter 8: Exergy: A Measure of Work Potential The energy content of the universe is constant, just as its mass content is. Yet at times of crisis we are bombarded with speeches and articles on how to “conserve” energy. As engineers, we know that energy is already conserved. What is not conserved is exergy, which is the useful work potential of the energy. Once the exergy is wasted, it can never be recovered. When we use energy (to heat our homes, for example), we are not destroying any energy; we are merely converting it to a less useful form, a form of less exergy. Exergy and the Dead State The useful work potential of a system is the amount of energy we extract as useful work. The useful work potential of a system at the specified state is called exergy. Exergy is a property and is associated with the state of the system and the environment. A system that is in equilibrium with its surroundings has zero exergy and is said to be at the dead state. The exergy of the thermal energy of thermal reservoirs is equivalent to the work output of a Carnot heat engine operating between the reservoir and the environment. Exergy Forms Now let’s determine the exergy of various forms of energy. Exergy of kinetic energy Kinetic energy is a form of mechanical energy and can be converted directly into work. Kinetic energy itself is the work potential or exergy of kinetic energy independent of the temperature and pressure of the environment.

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Student Study Guide for 5 th edition of Thermodynamics by Y. A. Ç engel & M. A. Boles 8-2 Chapter 8-2 Exergy of kinetic energy: 2 ke (kJ/kg) 2 V xk e == G Exergy of potential energy Potential energy is a form of mechanical energy and can be converted directly into work. Potential energy itself is the work potential or exergy of potential energy independent of the temperature and pressure of the environment. Exergy of potential energy: pe (kJ/kg) xp e g z Useful Work The work done by work producing devices is not always entirely in a useable form. Consider the piston-cylinder device shown in the following figure.
Student Study Guide for 5 th edition of Thermodynamics by Y. A. Ç engel & M. A. Boles 8-3 Chapter 8-3 The work done by the gas expanding in the piston-cylinder device is the boundary work and can be written as 00 b, useful 0 () WP d V P P d V P d V d V δ == + =+ The actual work done by the gas is b, useful 0 b, useful 0 2 1 WW P d V V V The word done on the surroundings is surr 0 0 2 1 d V P V V Any useful work delivered by a piston-cylinder device is due to the pressure above the atmospheric level. us u r r W =−

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Student Study Guide for 5 th edition of Thermodynamics by Y. A. Ç engel & M. A. Boles 8-4 Chapter 8-4 Reversible Work Reversible work W rev is defined as the maximum amount of useful work that can be produced (or the minimum work that needs to be supplied) as a system undergoes a process between the specified initial and final states.
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Chapter_8 - Student Study Guide for 5th edition of...

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