Quasity and Exergy: Toward a theory of performance (original) (raw)

Chapter 8 EXERGY: A MEASURE OF WORK POTENTIAL

| 423 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 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 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 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. cen84959_ch08.qxd 4/20/05 4:05 PM Page 423

Analogies from Function, Flow and Performance Metrics

2016

The process of engineering design involves moving through different levels of abstraction of the design problem and solution. During this cycle, the use of analogies has been shown to be a powerful mechanism for the development of a design solution. These design analogies are often drawn from systems that embody a similar function-flow-performance metric combination. Yet, most existing design tools focus not on these abstract representations, but instead focus on functional, linguistic descriptions of the systems. This paper focuses on several significant concepts that are essential to the exploitation of function-flow-performance based comparisons of design analogies.