Lesson test: Exergy

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Exergy

1.[2p]
What is the dead state?

The state a system reaches at equilibrium with its environment, where no work can be extracted

Correct

The state at absolute zero, where entropy vanishes

The answer is: The state a system reaches at equilibrium with its environment, where no work can be extracted

The state at which a working fluid stops circulating

The answer is: The state a system reaches at equilibrium with its environment, where no work can be extracted

The state at which a cycle's efficiency reaches the Carnot bound

The answer is: The state a system reaches at equilibrium with its environment, where no work can be extracted

2.[2p]
Heat of $2921$ kJ/kg is supplied at $1600$ K with surroundings at $298.15$ K. What is its exergy, in kJ/kg?

CorrectNot quite: 2377

3.[2p]
The same $2921$ kJ/kg supplied from a geothermal source at $500$ K. What is its exergy, in kJ/kg?

CorrectNot quite: 1179

4.[3p]
A turbine raises the steam's entropy from $6.7450$ to $7.1444$ kJ/(kg K) while delivering $852.6$ kJ/kg. With $T_{0} = 298.15$ K, how much exergy does it destroy, in kJ/kg?

CorrectNot quite: 119

5.[3p]
For that turbine, what is the second-law efficiency?

CorrectNot quite: 0.877

6.[2p]
The condenser of a steam plant carries away most of the plant's rejected energy and destroys most of its exergy.

True

The answer is: False

False

Correct

7.[3p]
In the audit of the plant, why does the boiler destroy over half the exergy while losing no energy at all?

Combustion gas at $1600$ K is handing heat to water between $46$ and $350$ degrees Celsius, across a huge temperature difference

Correct

The boiler tubes are poorly insulated

The answer is: Combustion gas at $1600$ K is handing heat to water between $46$ and $350$ degrees Celsius, across a huge temperature difference

The feed water enters below its saturation temperature

The answer is: Combustion gas at $1600$ K is handing heat to water between $46$ and $350$ degrees Celsius, across a huge temperature difference

The steam leaving is superheated rather than saturated

The answer is: Combustion gas at $1600$ K is handing heat to water between $46$ and $350$ degrees Celsius, across a huge temperature difference

8.[3p]
Match each destination of the plant's exergy to its share.

Net work out
Destroyed in the boiler
Destroyed in the condenser
Destroyed in the turbine

5.7 per cent
53.6 per cent
35.7 per cent
5.0 per cent

Show the answer

Net work out: 35.7 per cent Destroyed in the boiler: 53.6 per cent Destroyed in the condenser: 5.7 per cent Destroyed in the turbine: 5.0 per cent

9.[3p]
Which are honest limits on an exergy analysis?

Select all that apply

The result depends on the chosen dead state, which must be stated

Correct

Chemical exergy is not the same as a fuel's heating value

Correct

It prices thermodynamic loss, not capital cost

Correct

It cannot be applied to devices that consume rather than produce work

The answer is: The result depends on the chosen dead state, which must be stated, Chemical exergy is not the same as a fuel's heating value, It prices thermodynamic loss, not capital cost

10.[3p]
A domestic gas boiler makes hot water at $333$ K from a fuel whose exergy is $1.04$ times its heating value, at $90$ per cent first-law efficiency, with $T_{0} = 298.15$ K. Roughly what second-law efficiency does that give?

CorrectNot quite: 0.09

1.[2p] What is the dead state?

2.[2p] Heat of 2921 kJ/kg is supplied at 1600 K with surroundings at 298.15 K. What is its exergy, in kJ/kg?

3.[2p] The same 2921 kJ/kg supplied from a geothermal source at 500 K. What is its exergy, in kJ/kg?

4.[3p] A turbine raises the steam's entropy from 6.7450 to 7.1444 kJ/(kg K) while delivering 852.6 kJ/kg. With T0=298.15 K, how much exergy does it destroy, in kJ/kg?

5.[3p] For that turbine, what is the second-law efficiency?

6.[2p] The condenser of a steam plant carries away most of the plant's rejected energy and destroys most of its exergy.

7.[3p] In the audit of the plant, why does the boiler destroy over half the exergy while losing no energy at all?

8.[3p] Match each destination of the plant's exergy to its share.

9.[3p] Which are honest limits on an exergy analysis?

10.[3p] A domestic gas boiler makes hot water at 333 K from a fuel whose exergy is 1.04 times its heating value, at 90 per cent first-law efficiency, with T0=298.15 K. Roughly what second-law efficiency does that give?

1.[2p]
What is the dead state?

2.[2p]
Heat of $2921$ kJ/kg is supplied at $1600$ K with surroundings at $298.15$ K. What is its exergy, in kJ/kg?

3.[2p]
The same $2921$ kJ/kg supplied from a geothermal source at $500$ K. What is its exergy, in kJ/kg?

4.[3p]
A turbine raises the steam's entropy from $6.7450$ to $7.1444$ kJ/(kg K) while delivering $852.6$ kJ/kg. With $T_{0} = 298.15$ K, how much exergy does it destroy, in kJ/kg?

5.[3p]
For that turbine, what is the second-law efficiency?

6.[2p]
The condenser of a steam plant carries away most of the plant's rejected energy and destroys most of its exergy.

7.[3p]
In the audit of the plant, why does the boiler destroy over half the exergy while losing no energy at all?

8.[3p]
Match each destination of the plant's exergy to its share.

9.[3p]
Which are honest limits on an exergy analysis?

10.[3p]
A domestic gas boiler makes hot water at $333$ K from a fuel whose exergy is $1.04$ times its heating value, at $90$ per cent first-law efficiency, with $T_{0} = 298.15$ K. Roughly what second-law efficiency does that give?