Book cover for Thermodynamics: An Engineering Approach

Thermodynamics: An Engineering Approach

Yunus A. Cengel, Michael A. Boles

ISBN #9781259822674

9th Edition

2,694 Questions

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Homework Questions

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

Chapter 8 provides a comprehensive overview of exergy – the work potential of energy – and examines its application in gas power cycles. By distinguishing between closed and open systems, and employing first-law and second-law analyses, the chapter illustrates how engineers can evaluate system performance, identify irreversibility, and optimize modifications to enhance efficiency. The methodology of calculating reversible work and understanding exergy destruction is central to designing more efficient engines and power systems.

Learning Objectives

1

Explain the concept of exergy and its significance as a measure of work potential in thermal systems.

2

Differentiate between closed (nonflow) and open (flow) systems in the context of gas power cycles.

3

Apply first-law and second-law analyses to evaluate performance and identify irreversibilities in gas power cycles.

4

Analyze various cycle modifications such as regeneration, intercooling, and reheating, and their effects on efficiency.

5

Describe the methods to calculate reversible work and diagnose exergy destruction in steady-flow devices.

Key Concepts

CONCEPT

DEFINITION

Exergy

The maximum useful work obtainable from a system as it comes into equilibrium with a reference environment; it reflects the quality or work potential of energy.

Gas Power Cycles

Cycles in which working fluids remain in the gas phase throughout the process, converting thermal energy into mechanical work.

Closed System (Nonflow) Exergy

Exergy analysis applied to a fixed-mass (nonflow) system where changes occur without mass entering or leaving the system.

Flow (Stream) Exergy

Exergy associated with a mass flow stream, relevant for open systems where mass enters and exits the control volume.

Reversible Work

The work extracted from a system undergoing an ideal, reversible process, representing the maximum possible work without irreversibilities.

Second-Law Efficiency

A measure of how effectively a system converts available exergy into useful work, taking into account the losses due to irreversibility.

Irreversibility

Loss of work potential (exergy destruction) due to entropy generation in processes, typically arising from inefficiencies such as friction, unrecoverable heat transfer, and other non-idealities.

Example Problems

Example 1

What final state will maximize the work output of a device?

Example 2

Is the exergy of a system different in different environments?

Example 3

Under what conditions does the reversible work equal irreversibility for a process?

Example 4

How does useful work differ from actual work? For what kinds of systems are these two identical?

Example 5

How does reversible work differ from useful work?
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Step-by-Step Explanations

QUESTION

How do you determine the exergy of a fixed mass system undergoing a process?

STEP-BY-STEP ANSWER:

Step 1: Identify the system’s initial state properties (internal energy, pressure, temperature) and the corresponding properties of the reference environment.
Step 2: Use the definition of exergy for a fixed mass by applying the formula: Exergy = (U - U0) + P0(V - V0) - T0(S - S0), where U, V, and S are the system's internal energy, volume, and entropy, respectively, and the subscript 0 denotes the reference state.
Step 3: Calculate the changes in each property relative to the environment to obtain the work potential.
Step 4: Sum the contributions from energy, work (P·V), and entropy terms, considering the environmental (dead state) conditions.
Final Answer: The system’s exergy quantifies the maximum useful work that can be extracted, accounting for the differences between the system’s state and the environment.

Calculation of Exergy in a Closed System

QUESTION

What is the procedure to calculate the reversible work for a gas power cycle?

STEP-BY-STEP ANSWER:

Step 1: Define the cycle process parameters including temperatures, pressures, and specific heats relative to the working fluid.
Step 2: Establish the ideal reversible process path (typically isentropic processes for compression and expansion) for the cycle.
Step 3: Compute the work output for each stage using thermodynamic relations appropriate for the reversible processes.
Step 4: Sum the work contributions from all stages to determine the total reversible work.
Final Answer: The reversible work is the sum of individual work outputs calculated along the ideal reversible process path, representing the maximum potential work of the cycle.

Analyzing Reversible Work in a Gas Power Cycle

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Common Mistakes

  • Assuming energy and exergy are equivalent and neglecting the quality or work content of energy.
  • Overlooking the impact of irreversibilities when analyzing thermodynamic cycles, leading to overestimated efficiencies.
  • Mixing up closed (nonflow) exergy and flow (stream) exergy, which have distinct analysis methods.
  • Ignoring environmental reference conditions, which are critical for accurate exergy evaluation.
  • Failing to correctly apply the second-law analysis, thereby not fully identifying all sources of exergy destruction.