cen84959_ch08 - Chapter 8 EXERGY A MEASURE OF WORK...

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Chapter 8 EXERGY: A MEASURE OF WORK POTENTIAL | 433 T he increased awareness that the world’s energy resources are limited has caused many countries to reexamine their energy policies and take drastic mea- sures in eliminating waste. It has also sparked interest in the scientific community to take a closer look at the energy con- version 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 can- not be created or destroyed. This law merely serves as a nec- essary 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 thermody- namic systems. In this chapter, we examine the performance of engineering devices in light of the second law of thermody- namics. 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 reversi- ble 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 of thermodynamics. Define exergy , which is the maximum useful work that could be obtained from the system at a given state in a specified environment. Define reversible work , which is the maximum useful work that can be obtained as a system undergoes a process between two specified states. Define the exergy destruction, which is the wasted work potential during a process as a result of irreversibilities. Define the second-law efficiency . Develop the exergy balance relation. Apply exergy balance to closed systems and control volumes.
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8–1 EXERGY: WORK POTENTIAL OF ENERGY When a new energy source, such as a geothermal well, is discovered, the first thing the explorers do is estimate the amount of energy contained in the source. This information alone, however, is of little value in deciding whether to build a power plant on that site. What we really need to know is the work potential of the source—that is, the amount of energy we can extract as useful work. The rest of the energy is eventually discarded as waste energy and is not worthy of our consideration. Thus, it would be very
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