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Ebook Thermodynamics an engineering (5th edition): Part 2

(BQ) Part 2 book "Thermodynamics an engineering" has contents: Gas power cycles, vapor and combined power cycles, refrigeration cycles, thermodynamic property relations, gas mixtures, chemical reactions, chemical and phase equilibrium, compressible flow,.and other contents. | cen84959_ch08.qxd 4/20/05 4:05 PM Page 423 Chapter 8 EXERGY: A MEASURE OF WORK POTENTIAL T he increased awareness that the world’s energy resources are limited has caused many countries to reexamine their energy policies and take drastic measures in eliminating waste. It has also sparked interest in the scientific community to take a closer look at the energy conversion devices and to develop new techniques to better utilize the existing limited resources. The first law of thermodynamics deals with the quantity of energy and asserts that energy cannot be created or destroyed. This law merely serves as a necessary tool for the bookkeeping of energy during a process and offers no challenges to the engineer. The second law, however, deals with the quality of energy. More specifically, it is concerned with the degradation of energy during a process, the entropy generation, and the lost opportunities to do work; and it offers plenty of room for improvement. The second law of thermodynamics has proved to be a very powerful tool in the optimization of complex thermodynamic systems. In this chapter, we examine the performance of engineering devices in light of the second law of thermodynamics. We start our discussions with the introduction of exergy (also called availability), which is the maximum useful work that could be obtained from the system at a given state in a specified environment, and we continue with the reversible work, which is the maximum useful work that can be obtained as a system undergoes a process between two specified states. Next we discuss the irreversibility (also called the exergy destruction or lost work), which is the wasted work potential during a process as a result of irreversibilities, and we define a second-law efficiency. We then develop the exergy balance relation and apply it to closed systems and control volumes. Objectives The objectives of Chapter 8 are to: • Examine the performance of engineering devices in light of the second law