4. Second Law, Entropy, and Exergy

Introduction and Context

The first law of thermodynamics establishes that energy is conserved. It does not, however, determine whether a process is possible, in which direction it will proceed, or how much of an energy interaction can be converted into useful work. A hot body can transfer heat to a cold body spontaneously, but the reverse process cannot occur without external work. Similarly, energy is not destroyed when fuel is burned, yet the ability of that energy to produce useful work is degraded.

The second law provides the directionality and quality criterion missing from the first law. It explains why real processes are irreversible, establishes upper limits on the performance of heat engines and refrigerators, and provides the basis for entropy and exergy analysis. In engineering practice, these concepts are central to the evaluation of turbines, compressors, boilers, condensers, heat exchangers, refrigeration systems, combustion devices, and integrated energy systems.

Entropy measures the extent to which energy has become unavailable for conversion into useful work, while exergy measures the maximum useful work obtainable as a system comes into equilibrium with its environment. Entropy generation identifies the internal causes of performance loss; exergy destruction quantifies their economic and technical significance.

Learning Objectives

After studying this chapter, the reader should be able to:

1. Explain the physical meaning and alternative statements of the second law.

2. Apply the Clausius inequality and determine entropy changes for closed and open systems.

3. Distinguish reversible, irreversible, and internally reversible processes.

4. Calculate entropy generation and interpret its relationship to irreversibility.

5. Derive and use Carnot limits for heat engines, refrigerators, and heat pumps.

6. Define exergy and evaluate non-flow and flow availability.

7. Perform entropy and exergy balances for engineering devices.

8. Identify the principal sources of thermodynamic loss in practical systems.

9. Use exergetic efficiency to compare designs and support improvement decisions.

10. Recognize the limitations, measurement issues, and professional implications of second-law analysis.

The Second Law: Direction, Limits, and Quality of Energy

Two classical statements express the second law from different viewpoints. The Kelvin–Planck statement says that no cyclic heat engine can receive heat from a single reservoir and convert it completely into work. Some heat must be rejected to a lower-temperature reservoir. The Clausius statement says that no cyclic device can transfer heat from a cold reservoir to a hot reservoir without receiving work from outside.

These statements are equivalent. Violation of either would violate the other. Together they distinguish energy conversion from energy conservation. A heat engine must operate between at least two temperature levels, whereas a refrigerator must consume work to move heat against its natural direction of transfer.

A process is reversible if the system and surroundings can be restored exactly to their initial states without leaving any net change in the universe. Reversibility is an idealization requiring infinitesimal temperature, pressure, and chemical-potential differences and the absence of friction, turbulence, mixing, electrical resistance, and unrestrained expansion. Real processes are irreversible. Nevertheless, reversible processes provide limiting performance standards against which actual devices can be evaluated.

Irreversibility may be classified as internal or external. Internal irreversibility arises within the system, for example through viscous dissipation, fluid friction, combustion, diffusion, mixing, and finite-rate heat transfer within the device. External irreversibility occurs because of interactions across the system boundary, such as heat transfer through a finite temperature difference. The distinction depends on the chosen system boundary; the total entropy generation of the complete process is independent of that arbitrary division.

Carnot’s Limit and Temperature Scales

The reversible Carnot cycle consists of two reversible isothermal processes and two reversible adiabatic processes. For a heat engine operating between source temperature T_H and sink temperature T_L, its thermal efficiency is

η_Carnot = 1 − T_L/T_H

where temperatures are absolute temperatures. No engine operating between these two reservoirs can have a higher efficiency. The Carnot efficiency depends only on reservoir temperatures, not on the working fluid or the construction of the engine.

For a refrigerator and heat pump, the corresponding coefficients of performance are

COP_R,Carnot = T_L/(T_H − T_L)

COP_HP,Carnot = T_H/(T_H − T_L)

and COP_HP = COP_R + 1. These expressions show why refrigeration becomes difficult when the temperature lift increases. They also demonstrate why reducing condenser temperature or increasing evaporator temperature can be more effective than simply increasing compressor size.

Carnot’s theorem provides the thermodynamic basis for the absolute temperature scale. For reversible heat transfer, the ratio of heat interactions equals the ratio of absolute temperatures:

Q_H/Q_L = T_H/T_L.

The use of absolute temperature is essential. Celsius or Fahrenheit differences may be used for temperature intervals, but not in ratios defining thermodynamic limits.

Entropy and the Clausius Inequality

For a reversible differential process, entropy is defined by

dS = δQ_rev/T.

Entropy is a state property, so its change between two equilibrium states is independent of the actual path. Although the real process may involve irreversible heat transfer, the entropy change can be calculated by imagining any convenient reversible path connecting the same states.

The Clausius inequality is

∮ δQ/T ≤ 0,

with equality for a reversible cycle and strict inequality for an irreversible cycle. It leads to the entropy balance for a closed system:

ΔS_system = ∫ δQ/T_boundary + S_gen,

where S_gen ≥ 0 is entropy generation. For heat transfer at several boundary temperatures,

ΔS_system = Σ(Q_k/T_k) + S_gen.

For an isolated system, Q = 0 and therefore ΔS_isolated = S_gen ≥ 0. This is the entropy statement of the second law. Entropy of an isolated system never decreases. It remains constant only for an ideal reversible process.

Entropy is an extensive property. Specific entropy, s, is entropy per unit mass, and molar entropy is entropy per mole. Units are commonly kJ/(kg·K) or kJ/(kmol·K). Entropy values may be obtained from property tables, equations of state, caloric relations, or measured data.

For an ideal gas with constant specific heats,

Δs = c_p ln(T_2/T_1) − R ln(p_2/p_1)

for a change in temperature and pressure. Equivalent expressions include

Δs = c_v ln(T_2/T_1) + R ln(v_2/v_1).

For an incompressible substance with approximately constant specific heat,

Δs ≈ c ln(T_2/T_1).

For a phase change at constant saturation temperature,

Δs = h_fg/T_sat.

These relations are computational forms of the state-property definition, not independent laws.

Entropy Balance for Control Volumes

For a control volume, the general entropy-rate balance is

dS_cv/dt = Σ(Q̇_k/T_k) + Σṁ_in s_in − Σṁ_out s_out + Ṡ_gen.

At steady state,

0 = Σ(Q̇_k/T_k) + Σṁ_in s_in − Σṁ_out s_out + Ṡ_gen.

Thus,

Ṡ_gen = Σṁ_out s_out − Σṁ_in s_in − Σ(Q̇_k/T_k),

and physical admissibility requires Ṡ_gen ≥ 0. A negative calculated value usually indicates an incorrect sign convention, inconsistent property data, an inappropriate boundary temperature, or measurement error.

For an adiabatic steady-flow device with one inlet and one outlet,

Ṡ_gen = ṁ(s_2 − s_1) ≥ 0.

An ideal turbine, compressor, or nozzle that is adiabatic and internally reversible is isentropic, meaning s_2 = s_1. An actual adiabatic device has entropy increase, although the entropy change alone does not directly indicate the magnitude of work loss unless the surrounding reference state is also considered.

Entropy Generation and Irreversibility

The Gouy–Stodola theorem relates entropy generation to lost work:

I = T_0 S_gen,

where I is irreversibility or exergy destruction and T_0 is the environmental temperature. For rates,

Ī = T_0 Ṡ_gen.

This result is powerful because it converts an abstract entropy quantity into an energy-equivalent measure of performance loss. At a fixed ambient temperature, the same entropy generation always corresponds to the same destroyed exergy. Entropy generation is therefore a universal indicator of thermodynamic imperfection, while exergy destruction gives a direct measure of lost work potential.

Common mechanisms of entropy generation include:

• Heat transfer across a finite temperature difference.
• Friction and pressure drop in fluid flow.
• Throttling and unrestrained expansion.
• Mixing of fluids or species with different compositions.
• Combustion and chemical reaction away from equilibrium.
• Electrical resistance and mechanical damping.
• Shock waves and turbulent dissipation.

A useful local expression for heat transfer is

Ṡ_gen,heat = Q̇(1/T_c − 1/T_h),

where heat Q̇ flows from T_h to T_c and T_h > T_c. The expression is positive and approaches zero as the temperature difference approaches zero. This explains why a sequence of smaller temperature differences can be thermodynamically superior to one large temperature drop, although it may require greater heat-transfer area and capital cost.

Availability and Exergy

Energy analysis identifies how much energy enters and leaves a system. Exergy analysis asks a more practical question: how much of that energy can theoretically be converted into useful work relative to a specified environment? The environment is characterized by a reference temperature T_0 and pressure p_0, and sometimes by chemical composition. The system is said to reach the dead state when it is in equilibrium with this environment.

Exergy is not an intrinsic property independent of surroundings. The exergy of a given quantity of energy changes when T_0 or p_0 changes. This dependence is not a weakness; it reflects the fact that work potential is determined by the contrast between a system and its surroundings.

For a closed system containing a non-flowing substance, the physical exergy relative to an environment may be written as

Ex_ph = (U − U_0) + p_0(V − V_0) − T_0(S − S_0),

when changes in kinetic and potential energy are negligible. A more complete expression includes kinetic and potential terms:

Ex = (U − U_0) + p_0(V − V_0) − T_0(S − S_0) + KE + PE.

For a flowing stream, the specific flow exergy is

ex_ph = (h − h_0) − T_0(s − s_0) + V^2/2 + gz.

The term h − h_0 includes flow-work effects. Chemical exergy must be added when composition differs from that of the reference environment. In combustion systems, chemical exergy can be substantial because fuel contains organized chemical potential relative to the products and ambient air.

Exergy associated with heat transfer Q at boundary temperature T is

Ex_Q = Q(1 − T_0/T).

Heat supplied at a temperature close to the environment has little work potential, whereas high-temperature heat has greater exergy. Work is pure exergy under the usual reference assumptions because it can be completely converted into any other form of work.

Exergy Balances and Exergetic Efficiency

A steady-flow exergy balance may be expressed as

Eẋ_in + Eẋ_Q = Eẋ_out + Ẇ_useful + Ī,

with appropriate separation of work, heat-transfer, and stream exergy terms. More explicitly, for a control volume,

0 = Σ(1 − T_0/T_k)Q̇_k − Ẇ_cv + Σṁ_in ex_in − Σṁ_out ex_out − Ī,

when sign conventions are defined consistently. Since Ī = T_0 Ṡ_gen, exergy destruction is always nonnegative.

Exergetic efficiency is defined as

ε = useful exergy output / exergy input.

The precise definition depends on the device. For a heat engine, useful output is net work and input is the exergy supplied with heat and streams. For a refrigerator, useful output may be the exergy associated with the desired cooling effect, while input is shaft work. For a heat exchanger, the product may be the increase in exergy of the cold stream, depending on the intended function.

Exergetic efficiency should not be confused with energy efficiency. A boiler may have high energy efficiency because most fuel energy appears as steam enthalpy, yet its exergy efficiency may be modest because high-temperature combustion is used to produce heat at a much lower temperature. Exergy analysis reveals this quality mismatch and helps locate opportunities for cascading energy use.

Component-Level Interpretation in Thermal Systems

In a steam power plant, the boiler or combustor commonly has large exergy destruction because chemical reaction and heat transfer occur across substantial temperature differences. The turbine loses exergy through aerodynamic losses, leakage, moisture effects, and non-isentropic expansion. The condenser rejects a large amount of energy, but its exergy destruction may be lower than expected because heat is rejected near the environmental temperature. Pumps usually have smaller absolute losses, although their efficiency remains important.

In gas-turbine systems, combustion irreversibility and exhaust-gas losses are major targets. Recuperators can reduce fuel consumption by recovering exhaust heat, but pressure losses and finite temperature differences may create additional exergy destruction. In combined-cycle plants, the gas turbine exhaust is used to generate steam, improving the overall utilization of high-temperature exergy.

In refrigeration systems, compressor inefficiency, throttling, non-ideal heat transfer, and pressure drops are principal sources of exergy destruction. The expansion valve is highly irreversible because the throttling process is approximately isenthalpic but produces entropy. An expander can recover some work, although its added cost and complexity must be justified.

Heat exchangers illustrate the trade-off between energy recovery and exergy loss. A high effectiveness exchanger may reduce external fuel use, but excessive pressure drop or heat transfer across a large temperature difference can reduce net exergy performance. Design should therefore consider both thermal effectiveness and pumping or compression power.

Worked Example: Entropy Generation in Heat Transfer

Consider 100 kJ of heat transferred from a reservoir at 600 K to another at 300 K. The entropy change of the hot reservoir is

ΔS_hot = −100/600 = −0.1667 kJ/K.

The entropy change of the cold reservoir is

ΔS_cold = 100/300 = 0.3333 kJ/K.

Therefore,

S_gen = 0.3333 − 0.1667 = 0.1666 kJ/K.

If the environmental temperature is 300 K, the exergy destruction is

I = 300(0.1666) ≈ 50 kJ.

The energy transferred is still 100 kJ, but 50 kJ of work potential has been destroyed because the transfer occurred across a finite temperature difference. This example demonstrates why the first-law energy balance alone cannot identify the severity of a thermal mismatch.

Illustrative Case Study: Exergy Improvement in an Industrial Steam System

Background

Consider an illustrative food-processing plant that produces saturated steam for sterilization and uses hot exhaust gases from a furnace to heat boiler feedwater. The plant meets its production requirement, but fuel consumption is higher than expected.

Situation and Problem

An energy audit shows that the boiler has an energy efficiency of 84 percent. Management initially considers replacing the burner. A second-law assessment is conducted to determine whether the dominant loss is incomplete combustion, heat leakage, or poor matching between heat-source and steam temperatures.

Analysis

The exergy balance identifies three major effects. First, combustion generates substantial entropy because chemical energy is converted into hot gases and then transferred to water across finite temperature differences. Second, the furnace exhaust leaves at a high temperature, carrying significant physical exergy. Third, the feedwater enters the boiler at a relatively low temperature, so the heat-exchange process has a large temperature mismatch at the beginning of the heating path.

The condenser-like heat rejection from auxiliary equipment is found to have low exergy content because it occurs near ambient temperature. Therefore, reducing that heat quantity would save energy but yield relatively little improvement in useful-work potential. The high-temperature exhaust and combustion zone are the more important targets.

Approach and Solution

The plant introduces an economizer to preheat feedwater with exhaust gas, improves insulation around the furnace, and adjusts the excess-air ratio using oxygen monitoring. A pinch-based heat-integration study is also used to match hot and cold streams more closely. The design is evaluated with both energy efficiency and exergy efficiency, while accounting for fan power and pressure drop.

Outcome

The revised arrangement reduces fuel demand and lowers exhaust temperature without creating excessive pressure loss. The principal improvement is not merely a reduction in heat loss; it is a reduction in the destruction of high-temperature exergy. The analysis also prevents an expensive investment in a burner replacement that would have addressed a smaller loss mechanism.

Learning Points

1. High energy efficiency does not guarantee high exergy efficiency.

2. Losses should be ranked by exergy destruction, not only by energy quantity.

3. Heat integration is most valuable when it preserves high-temperature work potential.

4. Component improvements must include auxiliary power and pressure-drop penalties.

5. Exergy analysis supports resource allocation by identifying where investment has the greatest thermodynamic value.

Practical Procedure for Second-Law Analysis

A reliable analysis normally follows these steps:

1. Define the system boundary, operating mode, and reference environment.

2. State mass-flow rates, inlet and outlet conditions, heat interactions, and work interactions.

3. Apply the first-law balance to establish energy consistency.

4. Obtain enthalpy and entropy properties from appropriate tables, software, or equations of state.

5. Apply the entropy balance and calculate entropy generation.

6. Convert entropy generation into exergy destruction using T_0 Ṡ_gen.

7. Perform the exergy balance and determine exergetic efficiency.

8. Rank component losses and examine whether reducing one loss increases another.

9. Evaluate technical feasibility, cost, reliability, safety, and environmental consequences.

10. Report assumptions, uncertainty, reference conditions, and sign conventions.

Challenges, Limitations, and Common Errors

The most common error is using Celsius temperatures in entropy or Carnot expressions. Absolute temperature is required in all ratios and in the Gouy–Stodola relation. Another error is treating entropy as energy. Entropy has units of energy per temperature and cannot be added directly to energy terms.

A second difficulty is selecting the boundary temperature for heat transfer. When heat crosses a finite temperature range, using an arbitrary average temperature can produce misleading entropy transfer. Heat-transfer paths should be divided into identifiable interactions or evaluated using local temperature data.

Property inconsistency is also serious. Entropy values from different reference states may be combined only when the reference conventions are compatible. Measured plant data often contain uncertainty, and a small negative entropy generation may result from instrumentation error rather than a violation of the second law.

Exergy results depend on the reference environment. Comparisons are meaningful only when the same T_0, p_0, and chemical reference state are used. Exergetic efficiency also depends on how the product and fuel are defined. A poorly chosen definition can make two otherwise identical systems appear incomparable.

Finally, the largest exergy destruction is not always the easiest or most economical target. A component with large destruction may be difficult to modify, while a smaller loss may be inexpensive to eliminate. Thermodynamic analysis should therefore support, not replace, economic, environmental, safety, and reliability assessments.

Professional, Environmental, and Digital Applications

Exergy analysis is increasingly used in industrial energy management, district heating, cogeneration, waste-heat recovery, sustainable refrigeration, and process integration. It is particularly useful when several energy streams of different temperatures are present and when fuel quality matters more than total energy quantity.

Digital twins and plant historians can provide time-resolved temperatures, pressures, flow rates, and compositions for automated entropy and exergy monitoring. Optimization algorithms can rank loss mechanisms under changing loads. Machine-learning models may detect abnormal increases in entropy generation, such as fouling in heat exchangers or degradation in turbomachinery. However, data-driven tools do not replace thermodynamic constraints. Sensor bias, incomplete measurements, model extrapolation, and opaque optimization objectives must be controlled through validation and engineering review.

Professional practice also requires transparent reporting. Analysts should state environmental reference conditions, property models, boundary definitions, uncertainty ranges, and whether chemical exergy has been included. Decisions based on exergy should not encourage unsafe operating temperatures, excessive equipment complexity, or environmentally damaging fuel substitution merely because a numerical efficiency improves.

Best Practices for Design and Operation

Good second-law practice begins with temperature matching: use high-temperature energy where high-temperature demand exists, and cascade residual heat to lower-temperature applications. Avoid unnecessary throttling, excessive pressure drops, and mixing of streams at widely different temperatures or compositions. Maintain heat-transfer surfaces to limit fouling, because fouling increases temperature differences and pumping requirements.

For reporting, present energy efficiency and exergetic efficiency together. A loss diagram or Grassmann diagram can show where exergy enters, is productively used, is destroyed, and is rejected. Component-level balances are generally more useful than a single plant-wide efficiency. Sensitivity analysis should be used to identify whether conclusions depend strongly on ambient temperature, operating load, or uncertain measurements.

Chapter Summary

The second law establishes the direction of natural processes and the limits on energy conversion. The Kelvin–Planck and Clausius statements are equivalent expressions of this law, while the Carnot cycle defines the maximum performance of engines, refrigerators, and heat pumps between specified temperatures. Entropy is a state property whose balance includes entropy generation; the latter is zero only for a reversible process and positive for every real process.

For control volumes, entropy balances quantify irreversibility in turbines, compressors, heat exchangers, boilers, condensers, and refrigeration systems. The Gouy–Stodola theorem converts entropy generation into destroyed exergy. Exergy measures the maximum useful work relative to a specified environment and provides a quality-based assessment of energy resources. Exergy balances and exergetic efficiency reveal losses that energy efficiency alone may conceal.

Effective engineering decisions require ranking exergy destruction, improving temperature matching, reducing friction and pressure losses, recovering appropriate waste heat, and considering cost, safety, reliability, environmental impact, and measurement uncertainty. Digital monitoring can extend these methods, but thermodynamic definitions and professional judgment remain essential.

Key Terms

Entropy: A thermodynamic state property that quantifies energy dispersal and appears in the second-law balance.

Entropy generation: Entropy produced by irreversible processes; it is never negative.

Irreversibility: Lost work potential caused by entropy generation.

Reversible process: An ideal process that can be exactly reversed without net change in system and surroundings.

Irreversible process: A real process involving entropy generation.

Clausius inequality: The cyclic relation ∮δQ/T ≤ 0.

Carnot cycle: An ideal reversible cycle consisting of two isothermal and two adiabatic processes.

Carnot efficiency: The maximum heat-engine efficiency between two reservoir temperatures.

Coefficient of performance: Useful refrigeration or heating effect divided by work input.

Availability: Another term for exergy, meaning maximum useful work relative to an environment.

Exergy: Maximum theoretical useful work obtainable as a system reaches the dead state.

Flow exergy: Exergy carried by a flowing stream, including enthalpy, entropy, kinetic, and potential terms.

Chemical exergy: Work potential associated with chemical composition relative to the environment.

Dead state: Equilibrium state with the reference environment.

Reference environment: Specified surroundings characterized by quantities such as T_0 and p_0.

Gouy–Stodola theorem: Relation I = T_0S_gen between irreversibility and entropy generation.

Exergy destruction: Work potential destroyed by irreversibility.

Exergetic efficiency: Useful exergy output divided by exergy input.

Isentropic process: A constant-entropy process, usually associated with an internally reversible adiabatic process.

Review Questions

Short Answer Questions

1. State the Kelvin–Planck and Clausius statements of the second law.

2. Why must absolute temperature be used in the Carnot efficiency expression?

3. Define entropy generation and state its permissible sign.

4. What is meant by the dead state in exergy analysis?

5. Distinguish energy efficiency from exergetic efficiency.

Long Answer Questions

1. Derive the entropy balance for a steady-flow control volume and explain the physical meaning of each term.

2. Explain the Carnot cycle and derive the efficiency of a reversible heat engine.

3. Discuss the principal sources of entropy generation in heat exchangers, turbines, compressors, and combustion systems.

4. Derive the expression for flow exergy and explain the significance of its individual terms.

5. Explain how exergy analysis can guide the design and operation of an industrial steam system.

Analytical / Application Questions

1. A heat engine receives 1200 kJ from a reservoir at 800 K and rejects heat to a reservoir at 300 K. Determine the maximum possible work and the minimum rejected heat.

2. Heat is transferred at a rate of 50 kW from a surface at 500 K to surroundings at 300 K. Calculate the entropy generation rate and exergy destruction rate.

3. An adiabatic turbine has an inlet entropy of 6.8 kJ/(kg·K) and an outlet entropy of 7.1 kJ/(kg·K), with a mass flow rate of 4 kg/s. For T_0 = 298 K, calculate the entropy generation rate and lost-work rate, and explain what design features might reduce the loss.

22. Multiple Choice Questions with Answers

Question 1

A heat engine operates reversibly between 900 K and 300 K. Its maximum possible thermal efficiency is

A. 33.3%

B. 50.0%

C. 66.7%

D. 75.0%

Correct Answer: C. 66.7%

Question 2

For a real cyclic process, the Clausius integral ∮δQ/T is

A. Always positive

B. Zero only for an irreversible cycle

C. Less than or equal to zero

D. Equal to one

Correct Answer: C. Less than or equal to zero

Question 3

A closed system receives 400 kJ of heat reversibly at 500 K. Its entropy increase is

A. 0.8 kJ/K

B. 1.25 kJ/K

C. 200 kJ/K

D. 500 kJ/K

Correct Answer: A. 0.8 kJ/K

Question 4

An adiabatic steady-flow device has one inlet and one outlet. If the outlet entropy is greater than the inlet entropy, the device has

A. Negative entropy generation

B. Positive entropy generation

C. Zero entropy generation necessarily

D. A violation of the first law

Correct Answer: B. Positive entropy generation

Question 5

The exergy destroyed in a process with entropy generation of 0.20 kJ/K at an environmental temperature of 300 K is

A. 0.00067 kJ

B. 0.60 kJ

C. 60 kJ

D. 1500 kJ

Correct Answer: C. 60 kJ

Question 6

Which process is inherently highly irreversible under ordinary operating conditions?

A. Reversible isothermal expansion

B. Reversible adiabatic compression

C. Throttling through a valve

D. Quasi-static phase change

Correct Answer: C. Throttling through a valve

Question 7

A heat exchanger transfers heat between streams with a very large temperature difference. The principal second-law consequence is

A. Zero entropy generation

B. Increased entropy generation

C. A decrease in total energy

D. A violation of the Carnot theorem

Correct Answer: B. Increased entropy generation

Question 8

Why can a boiler have high energy efficiency but comparatively low exergetic efficiency?

A. Energy is not conserved in boilers

B. The boiler produces no enthalpy increase

C. High-quality fuel exergy is degraded while producing lower-temperature steam

D. Entropy is always zero during boiling

Correct Answer: C. High-quality fuel exergy is degraded while producing lower-temperature steam

Question 9

For a refrigerator operating between fixed temperatures, increasing the temperature difference between the evaporator and condenser generally causes the ideal COP to

A. Increase without limit

B. Decrease

C. Remain exactly unchanged

D. Become equal to thermal efficiency

Correct Answer: B. Decrease

Question 10

In an exergy balance, the term T_0 S_gen represents

A. Useful work output

B. Heat supplied by the environment

C. Exergy destruction

D. Flow kinetic energy

Correct Answer: C. Exergy destruction