INTRODUCTION TO 2nd LAW and EXERGY

Yunus Çengel

Yildiz Technical University, Istanbul

University of Nevada, Reno

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Outline• Introduce the 2nd law of thermodynamics, and Describe

the Kelvin–Planck and Clausius statements 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 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, and apply it to processes encountered in practice.

1st and 2nd Laws• The 1st law of thermodynamics is mundane since it deals with

conserved quantities.

• The 2nd law is quite exciting and sometimes even bizarre since it baffles the mind and intrigues the imagination by dealing with quantities that are created and destroyed.

• Many people, including some engineers, have difficulty grasping the 2nd law concepts such as entropy, exergy, and 2nd law efficiency, and question the utility of the 2nd law analysis. As a result, they tend to limit thermodynamics to an energy analysis only.

• Energy analysis provides a one-sided view of a process or system, and a study is not complete without an accompanying 2nd -law analysis, which enables one to examine the system or process from a different angle.

• The 1st law (or energy) analysis provides a map for energy flow and conversion for a process, whereas the 2nd law provides a map of inefficiencies and waste occurring throughout the process.

• Mass balance: Mass change = Mass transfer

• Energy balance: Energy change = Energy transfer

• Entropy balance: Entropy change = Entropy transfer + Entropy generation

• Exergy balance: Exergy change = Exergy transfer - Exergy destruction

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Entropy Generation and Exergy destruction

associated with heat transfer

Energy is conserved,

Entropy is generated,

Exergy is destroyed.

The 2nd Law of Thermodynamics:

The 2nd Dimension of Energy

1st Law: Quantity Based

• Energy is energy.

• All energies are equal.

• Quantity is always conserved; it cannot be destroyed.

2nd law: Quality Based

• There is difference from energy to energy.

• All energies are not equal.

• Quality diminishes in all processes, except the ‘perfect’ ones.

• Is the theoretical upper limit in energy conversion %100? If not, what is it?

• How close are we to Perfection? Or, how much room do we have for improvement?

Basic Questions

ANOTHER QUESTION: From a thermodynamic point of view, which is a better

energy conversion/transfer process?

1st law efficiency: The level of performance achieved compared to the resources provided.

2nd law efficiency: The level of performance achieved compared to the best possible performance under the circumstances.

Process A: 1st Law efficiency=100%; 2nd Law efficiency <100%.Process B: 1st Law efficiency<100%; 2nd Law efficiency =100%..

2nd law efficiency – A measure of perfection

• A process with a 2nd law efficiency of 100%:

- Is perfect (even if its 1st law efficiency is less than 100%).

- Entropy generation = 0

- Exergy destruction = 0

- Waste = 0

• Something cannot be more perfect than perfect. The 2nd law defines the upper limit.

• Perfection is good and beautiful.

• Beauty and perfection are liked for what they are.

• GOAL: Perfection and zero waste.

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1st LAW vs. 2nd LAW: Matter vs. Non-matter• 1st law deals with Matter and Energy - Their existence is certain

- Physical quantities - Conserved (subject to conservation laws)- Can be perceived by 5 senses- Conforms to the matter-energy universe that started with big-bang

• 2nd law deals with Entropy and Exergy- Their existence is certain- Non-physical quantities (beyond physics)- Non-matter (or meaning)- Non-Conserved (not subject to conservation laws)- Can NOT be perceived by 5 senses- Outside the matter-energy universe that started with big-bang

• We are blinded by matter, and conditioned with conserved quantities. • Some have difficulty grasping Entropy and Exergy since they are invisible non-matter quantities. • They are like spirits working behind the scenes and governing physical phenomena. Entropy is like the bad spirit, and exergy the good spirit.

Comparison of mass and energy with entropy and exergy

Mass, energy Entropy, exergy

Associated with 1st law 2nd law

Existence Yes Yes

Physical property Yes Yes

Material quantity Yes No

Conserved Yes No

Matter, energy Yes No

Part of big-bang universe Yes No

Perceived by 5 senses Yes No

Valuable engineering tool Yes Yes

Practical utility of the 2nd law • The 2nd law (or exergy) analysis serves as a mirror to see the

effectiveness of each segment of the process, and to identify waste and inefficiencies.

• The 2nd law efficiency is a measure of the perfection of a process.

• The destruction of exergy is a measure of imperfections associated with a process, and it shows the room we have for improvement.

• Global warming and the associated climate change that pose the greatest risk for the future of planet earth are closely related to the 2nd-law concepts.

• Combating climate change and the associated green practices are closely related to avoiding waste and thus minimizing entropy generation or exergy destruction.

• “Sustainability” is also a 2nd law concept, as it involves the practice of best resource utilization and waste elimination.

• Even “reliability” is related to the 2nd law as wasted energy often causes excessive operating temperatures, and thus a higher rate of failure.

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1. The direction of processes can be identified.

2. The second law asserts that energy has quality as well as quantity. The first law is concerned with the quantity of energy and the transformations of energy from one form to another with no regard to its quality.

3. The second law provides the necessary means to determine the quality as well as the degree of degradation of energy during a process.

4. The second law is used in determining the theoretical limits for the performance of commonly used engineering systems.

5. Exergy efficiencies provide a measure of how nearly actual performance approaches the ideal, and identifies the causes and locations of thermodynamic losses.

6. Exergy analysis can assist in improving and optimizing designs.

PRIMARY USES OF THE 2nd LAW AND EXERGY

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INTRODUCTION TO THE 2nd LAW

A cup of hot coffee does not get hotter in a cooler room.

Transferring heat to a wire will not generate electricity.

Transferring heat to a paddle wheel will not cause it to rotate.

These processes cannot occur even though they are not in violation of the first law.

The 2nd Law and Exergy

The Second Law of Thermodynamics:

Kelvin–Planck Statement

A heat engine that violates the Kelvin–Planck statement of the second law.

It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work.

No heat engine can have a thermal efficiency of 100 percent, or as for a power plant to operate, the working fluid must exchange heat with the environment as well as the furnace.

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The Second Law of Thermodynamics:

Clausius Statement

It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher-temperature body.

It states that a refrigerator cannot operate unless its compressor is driven by an external power source, such as an electric motor.

A refrigerator that violates the Clausius statement of the second law.

THE CARNOT HEAT ENGINE

No heat engine can have a higher efficiency than a reversible heat engine operating between the same high- and low-temperature reservoirs.

Any heat engine

Carnot heat engine

The Quality of Energy

The fraction of heat that can be converted to work as a function of source temperature.

The higher the temperature of the thermal energy, the higher its quality.

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THE CARNOT REFRIGERATORAND HEAT PUMP

No refrigerator can have a higher COPthan a reversible refrigerator operatingbetween the same temperature limits.

Any refrigerator or heat pump

Carnot refrigerator or heat pump

How do you increase the COP of a Carnot refrigerator or heat pump? How about for actual ones?

EXERGY: WORK POTENTIAL OF ENERGY

A system that is in equilibrium with its environment is said to be at the dead state.

The useful work potential of a given amount of energy at some specified state is called exergy, which is also called the availability or available energy.A system is said to be in the dead state when it is in thermodynamic equilibrium with the environment it is in.

The atmosphere contains a tremendous amount of energy, but

no exergy.

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REVERSIBLE WORK and EXERGY DESTRUCTION

As a closed system expands, some work needs to be done to push the atmospheric air out of the way (Wsurr).

For constant-volume systems, the total actual and useful works are identical (Wu = W).

Reversible work Wrev: The maximum amount of useful work that can be produced (or the

minimum work that needs to be supplied) as a system undergoes a process between the

specified initial and final states.

The difference between reversible work and

actual useful work is the irreversibility.

Exergy (Work Potential) Associated with Kinetic and Potential Energy

The work potential or exergy of potential

energy is equal to the potential energy itself.

Exergy of kinetic energy:

Exergy of potential energy:

The exergies of kinetic and potential energies are equal to themselves, and they are entirely available for work.

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Exergy of a Fixed Mass: Nonflow (or Closed System) Exergy

The exergy of a specified mass at a specified state is the useful work that can be produced as the mass undergoes a reversible process to the state of the environment.

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Exergy of a Flow Stream: Flow (or Stream) Exergy

Exegy of flow energy

Flow exergy

The exergy associated with flow energy is the useful work that would be delivered by an imaginary piston in the flow section.

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EXERGY TRANSFER BY HEAT, WORK, AND MASS

Exergy by Heat Transfer, Q

Exergy transfer by heat

The Carnot efficiency c=1T0 /T represents the fraction of the energy transferred from a heat source at temperature T that can be converted to work in an environment at temperature T0.

The transfer and destruction of exergy during a heat transfer

process through a finite temperature

difference.

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EXERGY TRANSFER BY WORK, W

There is no useful work transfer associated with boundary work when the pressure of the system is maintained constant at atmospheric pressure.

Exergy Transfer by Mass, m

Mass contains energy, entropy, and exergy, and thus mass flow into or out of a system is accompanied by energy, entropy, and exergy transfer.

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EXERGY DESTRUCTION

The exergy change of a system can be negative, but the exergy destruction cannot.

Exergy destroyed is a positive quantity for any actual process and becomes zero for a reversible process.

The exergy of an isolated system during a process always decreases or, in the limiting case of a reversible process, remains constant. In other words, it never increases and exergy is destroyed during an actual process. This is known as the decrease of exergy principle.

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EXERGY BALANCE

Mechanisms of exergy transfer.

The exergy change of a system during a process is equal to the difference between the net exergy transfer through the system boundary and the exergy destroyed within the system boundaries as a result of irreversibilities.

2nd Law (Exergetic) Efficiency, II

SECOND-LAW EFFICIENCY, II

Second-Law Efficiency of Resistance HeatersA dealer advertises that he has just received a shipment

of electric resistance heaters for residential buildings that have an efficiency of 100%. Assuming an indoor temperature of 21°C and outdoor temperature of 10°C, determine the second-law efficiency of these heaters.

Second-law efficiency of all reversible devices and processes is100%.

2nd Law efficiency of Reversible devices

Q: Can the 2nd-law efficiency be greater than 1st-law efficiency?

SECOND-LAW EFFICIENCY, II

2nd-Law Efficiency of Steady-Flow Devices

Heat Exchangers:

Mixing Chambers:

Compressors:

Turbines:

EXAMPLE: Heating with a hot iron block

Actual process

Reversible process

500 kg

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EXAMPLE: Minimum work input to a R-134a compressor

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EXAMPLE : Heating of a gas by stirring vs. a heat pump

1st and 2nd law views of energy flows

• Energy flow = 1 kW• Entropy flow = 0.002 kW/K• Exergy flow = 0.4 kW

“IMPURE ENERGY”

• Energy flow = 1 kW• Entropy flow = 0• Exergy flow = 1 kW

“PURE ENERGY”

HEAT ENERGY FLOW, 1 kJ/s

ELECTRİCAL ENERGY FLOW, 1 kW

T0 = 300 K

500 K

2nd Law as a Guide for Green Practices and Sustainability

Green Engineering and Thermodynamics• Green engineering is the design, commercialization, and use

of processes and products that are feasible and economical while reducing the generation of pollution at the source and minimizing the risk to human health and the environment.

• Green thermodynamics is a subcategory of green engineering related to energy.

• In thermodynamics, the concept of green can be associated with an energy source, an energy interaction or transfer, and energy conversion or use.

• Green thermodynamic practices are closely related to minimizing waste or:

- Minimizing entropy generation,

- Minimizing exergy destruction (2nd law)

• A process with a higher second-law efficiency is a greener thermodynamic process.

Sustainability and Ethics • Sustainability is usually concerned with long-term impact on resources,

environment, and processes on macro scale whereas Green is also concerned with short-term effects, such as the indoor air quality, on micro scale.

• Green/sustainable practices allow humans to exploit nature, but to do so without inflicting irreversible damage to the environment and without disturbing the ecological balance.

• Sustainable development: “Development that meets the needs of the present without compromising the ability of future generations to meet their own needs”. (World Commission on Environment and Development, 1987).

• This definition is closely tied to engineering ethics. Ethical practices require being considerate of the needs of future generations since the world’s resources belong to them as much as they belong to us.

• Green practices involve doing the right thing and doing it right.

• The terms “Green” and “Sustainability” are loosely defined. They are used to promote ‘green thinking’, and they should be viewed as goals to strive to reach.

• “Green” and “Sustainable” are often used interchangeably.

• A process with a higher second-law efficiency is a greener process.

A 2nd Law Application: ASHRAE GreenGuide

• Guidance to HVAC&R system designers involved in green or sustainable building design.

• A step-by-step manual for the entire building lifecycle.

• Includes 29 Green Tips, specific measures for improving sustainability, such as Ground-Source Heat Pumps.

• Covers green design techniques applicable to related technical disciplines, such as plumbing and lighting.

Conservation: A 2nd Law Concept

Minimization of exergy

destruction is the

greenest thermodynamic practice!

• Conservation implies conserving the “exergy content” or “usefulness” of energy.

• 1st law: Energy is always conserved, even when heat is lost from a building (conservation of energy principle).

• 2nd law: Degraded energy is wasted energy. Conserving energy is preserving it at the most useful form. Energy converted to a useless form is lost forever.

Last word from the 2nd law point of view:

“ZERO WASTE”

Thank you!