Energy Conservation and Exergy Concepts

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Questions and Answers

What is exergy primarily associated with in a thermodynamic context?

  • The total energy of a system
  • The heat transfer between two bodies
  • The equilibrium state of two systems
  • The maximum theoretical value of work that can be developed (correct)

In the context of two systems reaching equilibrium, what is a necessary condition for work to be developed?

  • The systems must be at different states (correct)
  • Only one system can perform work
  • The systems must be isolated from one another
  • Both systems must have the same temperature

What happens to the ability to develop work once the two systems reach equilibrium?

  • Work potential decreases (correct)
  • Work can still be developed with external energy
  • Work potential remains unchanged
  • Work potential increases

If a body at a lower temperature than the surrounding atmosphere is warmed up, what occurs?

<p>Work can be developed as the body warms to the temperature of the atmosphere (C)</p> Signup and view all the answers

What defines the 'dead state' in thermodynamic analysis?

<p>The point where all systems are at uniform temperature and pressure (C)</p> Signup and view all the answers

What conditions define the environment described in the content?

<p>Large, uniform temperature at $T_0$ and pressure at $p_0$ (B)</p> Signup and view all the answers

What does the term 'dead state' refer to?

<p>The point at which there is no interaction between the system and the environment (D)</p> Signup and view all the answers

Which statement accurately describes 'exergy'?

<p>Exergy measures the theoretical work obtainable as a system reaches dead state equilibrium (C)</p> Signup and view all the answers

Which of the following correctly describes an important aspect of exergy?

<p>Exergy is a measure of how far the system state deviates from the environment (D)</p> Signup and view all the answers

What can be inferred about interactions that lead to developing work?

<p>Maximum theoretical work is obtained in the absence of irreversibilities (B)</p> Signup and view all the answers

What happens to exergy during a spontaneous change to the dead state?

<p>It is destroyed by irreversibilities. (C)</p> Signup and view all the answers

What is the value of exergy when a system is at the dead state?

<p>Zero, as the system is in thermal and mechanical equilibrium. (B)</p> Signup and view all the answers

How can exergy be viewed when assessing work input requirements?

<p>As the minimum theoretical work input required to bring the system from the dead state to a given state. (A)</p> Signup and view all the answers

In the context of thermodynamic evaluations, what distinguishes thermomechanical exergy from chemical exergy?

<p>Chemical exergy includes reactions with environmental components for work development. (A)</p> Signup and view all the answers

Which statement accurately reflects the relationship between energy and exergy?

<p>Energy is conserved while exergy is destroyed by irreversibilities. (B)</p> Signup and view all the answers

What is the primary reason the initial fuel-air combination is considered more useful than the final warm mixture?

<p>The initial fuel has a greater economic value for generating electricity. (D)</p> Signup and view all the answers

Which statement accurately reflects the behavior of exergy compared to energy?

<p>Exergy can be destroyed by irreversibilities. (C)</p> Signup and view all the answers

What does the term 'exergy destruction' imply in this context?

<p>The ability of a system to perform useful work diminishes. (D)</p> Signup and view all the answers

How can improved energy resource utilization be achieved according to the material?

<p>By reducing exergy losses and destructions. (B)</p> Signup and view all the answers

What happens to the economic value of resources throughout the discussed process?

<p>It decreases as the potential for use is lost. (D)</p> Signup and view all the answers

Flashcards

Exergy

The maximum theoretical work that can be obtained from a system as it comes to equilibrium with its surroundings.

Dead State

The state of a system where no more work can be extracted because it has reached thermal equilibrium with the surroundings.

Environment

The surroundings of a system that are considered to be at a constant temperature and pressure. It acts as a reference state for calculating exergy.

Power Cycle

The process of transferring heat from a high-temperature body to a lower-temperature body to produce work. This work is obtained due to the difference in temperatures.

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Spontaneous Cooling

The spontaneous flow of heat from a hot object to a cold object. No work is produced in this scenario.

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What is exergy?

Exergy is a thermodynamic property that measures a system's potential for useful work. It represents the maximum amount of work that can be obtained from a system as it reaches equilibrium with its environment.

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How is exergy different from energy?

Unlike energy, exergy is not conserved. It can be destroyed due to irreversibilities, which are processes that are not reversible and result in energy dissipation. For example, friction and heat transfer are irreversible processes that lead to exergy destruction.

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Can exergy be transferred?

Exergy can be transferred to and from systems. Exergy transferred to the surroundings without being used represents a loss, as the potential for useful work is wasted.

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What is the purpose of exergy analysis?

Exergy analysis helps identify points in a system where exergy destruction and losses occur. This allows us to prioritize improving these areas, focusing on the aspects that offer the greatest potential for cost-effective efficiency gains.

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How can we improve energy resource utilization?

Improved energy resource utilization can be achieved by reducing exergy destruction within a system and/or reducing exergy losses to the surroundings.

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Exergy Reference Environment

The reference point used to determine exergy. It is a large, uniform system with typical ambient conditions, like 1 atm and 25°C.

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Exergy is a system and environment property

Exergy is a property of the system and the environment. Once the environment is known, we can calculate exergy based on the system's properties.

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Exergy cannot be negative

Exergy is always positive because a system will spontaneously change to reach the dead state. This change leads to a potential to do work.

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Irreversibility

Any process that creates entropy and prevents a system from fully realizing its theoretical work potential.

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Chemical Exergy

A measure of the maximum amount of useful work that can be extracted from a system undergoing a chemical reaction, like combustion.

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Key Difference: Energy vs. Exergy

Although energy and exergy are both measured in the same units, they represent profoundly different concepts. Energy is conserved, meaning it can change forms but is never lost. Exergy, however, is destroyed by irreversibilities, meaning it can be lost as a result of processes like friction or heat transfer.

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Study Notes

Energy Conservation and Exergy

  • Energy is conserved in every process; it cannot be destroyed.
  • Accounting for energy input (fuel, electricity) in a system is possible in its products and by-products.
  • Energy conservation alone isn't sufficient to account for all resource utilization aspects.
  • Figure 7.1a shows an isolated system with fuel and abundant air initially.
  • Burning the fuel results in a slightly warm mixture of combustion products (Fig. 7.1b and 7.1c).
  • The total energy within the system is constant because no energy transfers happen across the boundary.
  • The initial fuel-air mix is inherently more useful than the final mixture.
  • The initial mixture has greater potential for use, and it is largely wasted in the process, because of irreversibility.
  • Exergy quantifies potential for use, unlike energy, which is not conserved, but destroyed by irreversibilities.
  • Exergy can be transferred in/out of systems.
  • Loss occurs when exergy is transferred from a system to its surroundings without use.
  • Improving resource utilization reduces exergy destruction in a system.

Exergy and Economic Value

  • Exergy is linked to economic value.
  • The initial fuel has high economic value, with the final warm mixture having low value.
  • Exergy destruction results in a decrease in economic value.
  • Chapter 5 explains the exergy concept.

Defining Exergy

  • Exergy is the maximum theoretical work obtainable from a total system (system + environment) as the system comes to equilibrium with the environment.
  • Systems often interact, needing auxiliary devices like a power cycle, to conduct work, utilizing a heat source and sink.
  • Maximum work is attained when there are no irreversibilities, as explored in the next section.
  • Exergy is a system property that is the departure from the environment.
  • Exergy is an extensive property and cannot be negative.
  • Exergy turns zero when a system reaches equilibrium with the environment.

Thermoeconomics and Costing

  • Thermal systems frequently encounter work and heat interactions with their surroundings, often involving reactive mixtures.
  • Thermal systems represent important instances in everyday life.
  • Their design and operation integrate thermodynamic principles with fluid mechanics, heat transfer, materials science, and design.
  • Thermoeconomics considers the economical aspects for evaluating performance improvements in thermal systems in relation to exergy improvement.

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Related Documents

Exergy: A Detailed Explanation PDF

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