Available Energy, Availability and Irreversibility topics include: Reversible process, energy quality, useful work, availability, gouy stondola theorem, second law efficiency and comments on exergy. Availability and irreversibility are two key concepts in thermodynamics. Availability is the maximum amount of useful work that can be obtained from a system. It is also known as exergy.  Irreversibility is the inevitable loss of energy that occurs during a thermodynamic process. It is also known as energy destruction.  Irreversible processes lead to a decrease in the availability of energy.... Show more

Available Energy, Availability and Irreversibility topics include: Reversible process, energy quality, useful work, availability, gouy stondola theorem, second law efficiency and comments on exergy.

Availability and irreversibility are two key concepts in thermodynamics. Availability is the maximum amount of useful work that can be obtained from a system. It is also known as exergy. 

Irreversibility is the inevitable loss of energy that occurs during a thermodynamic process. It is also known as energy destruction. 
Irreversible processes lead to a decrease in the availability of energy. This is because some of the energy that enters the system is lost as waste heat. 

Unavailable energy 
Unavailable energy is the energy that cannot be utilized for doing useful work. It is also known as anergy. 

The second law of thermodynamics states that it is not possible to convert all the heat absorbed by a system into work.

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Thermodynamics Practice Test: Available Energy, Availability and Irreversibility

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25 Questions

1. When considering a finite energy source, its working fluid expands,

reversibly and adiabaticallyreversiblyadiabaticallynone of these

2. Which of the following is true?

rate of loss of exergy is directly proportional to the rate of entropy generationrate of loss of exergy does not depend on the rate of entropy generationnone of theserate of loss of exergy is inversely proportional to the rate of entropy generation

3. A heat exchanger increases the availability of 3 kg/s water by 1650 kJ/kg by using 10 kg/s air which comes in at 1400 K and leaves with 600 kJ/kg less availability. What is the irreversibility?

1030 kW1050 kW1020 kW1040 kW

4. The available energy is known as ____ and the unavailable energy is known as ____

both are called energyenergy, exergyboth are called exergyexergy, energy

5. A reversible adiabatic air compressor takes in air at 100 kPa, 25°C and delivers it at 1 MPa. Assuming the specific heat is constant, calculate the specific work.

145.6 kJ/kg278.6 kJ/kg178.6 kJ/kg324.6 kJ/kg

6. Since irreversibility > 0, the only processes allowed by the second law are those for which the exergy of the isolated system

decreasesnone of theseincreasesremains constant

7. The exergy of a fluid at a higher temperature is ___ than that at a lower temperature and ___ as the temperature decreases.

more, decreasesmore, increasesless, increasesless, decreases

8. The unavailable energy is the product of the lowest temperature of heat rejection and the change of entropy of system during the process of supplying heat.

Don't KnowTrueMaybeFalse

9. Find the specific reversible work for a steam turbine with inlet at 4 MPa, 500°C and an actual exit state of 100 kPa, x = 1.0 with a 25°C ambient temperature.

550.0 kJ/kg850.0 kJ/kg750.0 kJ/kg650.0 kJ/kg

10. Which of the following is an example of low grade energy?

all of the mentionedheat from combustion of fossil fuelheat from nuclear fission or fusionheat or thermal energy

11. Which of the following clearly defines availability or exergy?

none of theseit is the maximum useful work obtainable from a system as it reaches the dead stateit is the minimum work required to bring the closed system from the dead state to the given stateboth of the mentioned

12. A heavily insulated cylinder/piston contains ammonia at 60°C, 1200 kPa. The piston is moved, expanding the ammonia in a reversible process until the temperature is −20°C during which 600 kJ of work is given out by ammonia. What was the initial volume of the cylinder?

0.485 m30.385 m30.585 m30.285 m3

13. A rock bed(at 70°C) consists of 6000 kg granite. A house with mass of 12000 kg wood and 1000 kg iron is at 15°C. They are brought to a uniform final temperature. Find the irreversibility of the process, assuming an ambient temperature of 15°C.

19191 kJ17191 kJ18191 kJ20191 kJ

14. A spring loaded piston cylinder contains 1.5 kg air at 160 kPa and 27°C. It is heated in a process where pressure is linear in volume, P = A + BV, to twice the initial volume where it reaches 900 K. Find the work assuming a source at 900 K.

161.4 kJ61.4 kJ261.4 kJ361.4 kJ

15. Ammonia at 10°C with a mass of 10 kg is in a piston-cylinder arrangement with an initial volume of 1 m³. The piston initially resting on the stops has a mass such that a pressure of 900 kPa will float it. The ammonia is now slowly heated to 50°C. Find the work in the process.

783.2 kJ583.2 kJ483.2 kJ683.2 kJ

16. A steam turbine receives steam at 6 MPa, 800°C. It has a heat loss of 49.7 kJ/kg and an isentropic efficiency of 90%. For an exit pressure of 15 kPa and surroundings at 20°C, find the actual work.

2483.95 kJ/kg2423.95 kJ/kg1648.79 kJ/kg2233.79 kJ/kg

17. The force needed to compress a non-linear spring is given by F = 200x + 30x^2, where F is force in Newton and x is displacement of the spring in meter. Determine the work needed to compress the spring a distance of 0.6 m.

32.46 J38.16 J48.87 J23.43 J

18. A steam turbine inlet is at 1200 kPa, 500°C. The actual exit is at 300 kPa, 300°C with an actual work of 407 kJ/kg. What is its reversible work output if ambient temperature is at 25°C?

314.9 kJ/kg214.9 kJ/kg614.9 kJ/kg414.9 kJ/kg

19. The actual work done by a system is always ____ than the reversible work, and the difference between the two is called ____ of the process.

less, irreversibilityless, reversiblemore, reversibilitymore, irreversibility

20. A piston cylinder contains air at 600 kPa, 290 K and volume of 0.01 m³. A constant pressure process gives out 54 kJ of work. Find the final temperature of the air.

2900 K2700 K2800 K3000 K

21. A mass of 1 kg of air contained in a cylinder at 1000 K, 1.5 MPa, expands in a reversible isothermal process to a volume 10 times larger. Calculate the heat transfer during the process.

560.84 kJ660.84 kJ460.84 kJ760.84 kJ

22. Evaluate the steady state exergy flux due to a heat transfer of 250 W through a wall with 400 K on one side and 600 K on the other side. Also find the exergy destruction in the wall.

82 W62 W52 W72 W

23. When a system exchanges heat with a thermal energy reservoir in addition to the atmosphere, the maximum useful work

remains constantnone of theseincreasesdecreases

24. The exergy of an isolated system can ____

never increaseincreasenever decreasedecrease

25. Consider a small air pistol with a cylinder volume of 1 cm3 at 27°C, 250 kPa. The bullet acts as a piston and is released so the air expands in an adiabatic process. If the pressure should be 100 kPa as the bullet leaves the cylinder find the work done by the air.

0.115 J0.135 J0.125 J0.145 J